Current Articles
2025, Volume 43, Issue 1
Display Method:
Advances and Trends in Sedimentological Research in Oil and Gas Exploration and Development in China
2025,
43(1):
1-19.
doi: 10.14027/j.issn.1000-0550.2024.075
Abstract:
Significance Exploration for mineral resources such as oil and gas depends to a large extent on the development of the science of sedimentology. Worldwide advances in knowledge of the sedimentology of carbonates and deposition associated with both deep-water gravity flow and fine-grained materials have been instrumental in accumulating data enabling the location of oil and gas reserves, as well as for increased production from carbonate, deep-water and unconventional reservoirs. Progress Studies of source-to-sink deposition systems, and research into stratigraphic forward modeling based on sedimentary constraints, have increased the ability to assess the properties of non-marine shale sediments and the mechanisms of deep-to-ultra-deep carbonate reservoir development and preservation, and the effect of these on hydrocarbon production potential. Genesis analysis and the evaluation of low-permeability and tight reservoir heterogeneity provide guidelines for evaluating and optimizing favorable zones, including the deployment of wells and developing oil and gas exploration and production programs. Conclusions and Prospects The continuous development of oil and gas exploration practices pushes forward and expands the parameters of research into deep- to ultra-deep, unconventional, and complex reservoirs. Future trends in sedimentology will include research into source-to-sink systems, innovative sedimentary and stratigraphic forward modeling techniques, non-marine shale sedimentology, quantitative evaluation of reservoir heterogeneity, and artificial intelligence analysis systems. It is expected that these will boost the level of high-quality development of the international and domestic oil and gas industries and make new contributions to the ‘non-marine shale revolution’.
Significance Exploration for mineral resources such as oil and gas depends to a large extent on the development of the science of sedimentology. Worldwide advances in knowledge of the sedimentology of carbonates and deposition associated with both deep-water gravity flow and fine-grained materials have been instrumental in accumulating data enabling the location of oil and gas reserves, as well as for increased production from carbonate, deep-water and unconventional reservoirs. Progress Studies of source-to-sink deposition systems, and research into stratigraphic forward modeling based on sedimentary constraints, have increased the ability to assess the properties of non-marine shale sediments and the mechanisms of deep-to-ultra-deep carbonate reservoir development and preservation, and the effect of these on hydrocarbon production potential. Genesis analysis and the evaluation of low-permeability and tight reservoir heterogeneity provide guidelines for evaluating and optimizing favorable zones, including the deployment of wells and developing oil and gas exploration and production programs. Conclusions and Prospects The continuous development of oil and gas exploration practices pushes forward and expands the parameters of research into deep- to ultra-deep, unconventional, and complex reservoirs. Future trends in sedimentology will include research into source-to-sink systems, innovative sedimentary and stratigraphic forward modeling techniques, non-marine shale sedimentology, quantitative evaluation of reservoir heterogeneity, and artificial intelligence analysis systems. It is expected that these will boost the level of high-quality development of the international and domestic oil and gas industries and make new contributions to the ‘non-marine shale revolution’.
2025,
43(1):
20-38.
doi: 10.14027/j.issn.1000-0550.2023.072
Abstract:
Objective Strong heterogeneity is the main feature of continental shale strata. Its characteristics are caused by the fact that the sedimentary environment of lacustrine fine grained rocks is close to the source area and the water depth is shallow, which is clearly controlled by hydrodynamic, climatic, structural, bioturbation, and other factors. In particular, the vertical composition, morphology, and structure of laminated fine-grained sedimentary rocks can exhibit significant differences at the millimeter scale. According to published literature statistics and analysis of previous research results, the reservoir physical properties and hydrocarbon accumulation ability of laminated fine-grained rocks are superior to the undeveloped/weakly developed laminated fine-grained rocks, exhibiting an excellent positive correlation with the development degree as well. However, the summary of relevant differences is still unclear and needs to be revealed. Methods Focusing on the relevant literature from the past ten years, the types and genesis of pores and fractures of laminated fine-grained sedimentary rocks were reviewed, the laminae sequence was divided into sandy and muddy laminae sequences, and the reservoir physical properties and hydrocarbon accumulation characteristics and mechanism of different laminae sequences were described. Conclusions and Prospects The laminae structure has three advantages, frequent vertical change of material composition, abnormal development of internal micro-fractures, and concentrated distribution of organic matter in the laminae structure, further improving the ability of hydrocarbon accumulation on the basis of fine-grained sedimentary rocks. Five main controlling factors are summarized： laminae composition； laminae continuity； laminae sequence； abundance of organic matter； degree of thermal evolution of organic matter. In the future, it is necessary to combine the inorganic genetic mechanism of lamellar fine grained rock with the organic evolution model and establish the corresponding relationship between micro-lamellar types and combinations and the evaluation of macro-oil and gas sweet spots and high-quality reservoirs.
Objective Strong heterogeneity is the main feature of continental shale strata. Its characteristics are caused by the fact that the sedimentary environment of lacustrine fine grained rocks is close to the source area and the water depth is shallow, which is clearly controlled by hydrodynamic, climatic, structural, bioturbation, and other factors. In particular, the vertical composition, morphology, and structure of laminated fine-grained sedimentary rocks can exhibit significant differences at the millimeter scale. According to published literature statistics and analysis of previous research results, the reservoir physical properties and hydrocarbon accumulation ability of laminated fine-grained rocks are superior to the undeveloped/weakly developed laminated fine-grained rocks, exhibiting an excellent positive correlation with the development degree as well. However, the summary of relevant differences is still unclear and needs to be revealed. Methods Focusing on the relevant literature from the past ten years, the types and genesis of pores and fractures of laminated fine-grained sedimentary rocks were reviewed, the laminae sequence was divided into sandy and muddy laminae sequences, and the reservoir physical properties and hydrocarbon accumulation characteristics and mechanism of different laminae sequences were described. Conclusions and Prospects The laminae structure has three advantages, frequent vertical change of material composition, abnormal development of internal micro-fractures, and concentrated distribution of organic matter in the laminae structure, further improving the ability of hydrocarbon accumulation on the basis of fine-grained sedimentary rocks. Five main controlling factors are summarized： laminae composition； laminae continuity； laminae sequence； abundance of organic matter； degree of thermal evolution of organic matter. In the future, it is necessary to combine the inorganic genetic mechanism of lamellar fine grained rock with the organic evolution model and establish the corresponding relationship between micro-lamellar types and combinations and the evaluation of macro-oil and gas sweet spots and high-quality reservoirs.
2025,
43(1):
39-49.
doi: 10.14027/j.issn.1000-0550.2022.163
Abstract:
Objective The grain-size parameter plays a vital role in characterizing the basic features of sediments, and it is a crucial reference for distinguishing sedimentary environments. For a long time, the particle size, Φ, and particle-size parameters based on the equal ratio system and negative logarithm conversion have been widely used in the field of geology and marine science, and have become the national standard. Among them, the mathematical connotation of the sorting coefficient is closely related to the standard deviation, so whether the sorting coefficient has the same dimension as the particle size, and what units the sorting coefficient should have, have become a controversial issues. Methods In order to clarify this problem, this paper reviews the Udden-Wentworth equal ratio system and the origin of the Φ value, and provides modifications to the currently used calculation formulas. We discuss the evolutionary process of the calculation formulas used for determining particle-size parameters, such as the sorting coefficient used in the graphic and the moment methods, and analyze the usage habits of the terms and units of the sorting coefficient used by Chinese scholars since 1970. Results （1） In 1963, McManus modified the calculation formula of Φ and established it as a dimensionless parameter, making Φ a convenient index to express the particle size； however, Φ does not have the inherent property of a unit of measurement, and it cannot be used as a unit of length dimension （i.e., particle size）. （2） Although the sorting coefficient is derived from the statistical standard deviation, it has been independently developed as an important parameter to describe the sorting of detrital sediments, which is parallel with the skewness coefficient, kurtosis coefficient, etc. It is recommended that units should not be added when Φ is used, because there are still units in the results of the existing sorting coefficient calculation formula. Therefore, the formula of the sorting coefficient is improved by referring to McManus’ method of removing its dimension. （3） The results of literature statistics show that most domestic scholars tend to use the term “sorting coefficient” rather than the statistical term “standard deviation”, and do not use units when describing the sorting results. In all statistical kinds of literature, with the shortening of time, the proportion of literature using the term "sorting coefficient" and describing the sorting results without using units has increased.（4） The current national standard GB/T 12763.8-2007 states that the unit of the sorting coefficient is Φ. We find that there is a typographical error in the description of the particle size parameter calculation formula, and it is suggested that this needs to be corrected when next revising the standard. Conclusions In this paper, the origin and evolution of Φ and the existing commonly used sorting coefficient calculation formulas are reviewed based on various definitions, and the current situation of the usage of terms and units of the sorting coefficient in China is analyzed. Finally, on the basis of previous studies, some suggestions are put forward to modify the sorting coefficient calculation formula. The above work can provide suggestions for the revision and improvement of marine investigation norms, and provide suggestions for promoting the unification and standardization of terms.
Objective The grain-size parameter plays a vital role in characterizing the basic features of sediments, and it is a crucial reference for distinguishing sedimentary environments. For a long time, the particle size, Φ, and particle-size parameters based on the equal ratio system and negative logarithm conversion have been widely used in the field of geology and marine science, and have become the national standard. Among them, the mathematical connotation of the sorting coefficient is closely related to the standard deviation, so whether the sorting coefficient has the same dimension as the particle size, and what units the sorting coefficient should have, have become a controversial issues. Methods In order to clarify this problem, this paper reviews the Udden-Wentworth equal ratio system and the origin of the Φ value, and provides modifications to the currently used calculation formulas. We discuss the evolutionary process of the calculation formulas used for determining particle-size parameters, such as the sorting coefficient used in the graphic and the moment methods, and analyze the usage habits of the terms and units of the sorting coefficient used by Chinese scholars since 1970. Results （1） In 1963, McManus modified the calculation formula of Φ and established it as a dimensionless parameter, making Φ a convenient index to express the particle size； however, Φ does not have the inherent property of a unit of measurement, and it cannot be used as a unit of length dimension （i.e., particle size）. （2） Although the sorting coefficient is derived from the statistical standard deviation, it has been independently developed as an important parameter to describe the sorting of detrital sediments, which is parallel with the skewness coefficient, kurtosis coefficient, etc. It is recommended that units should not be added when Φ is used, because there are still units in the results of the existing sorting coefficient calculation formula. Therefore, the formula of the sorting coefficient is improved by referring to McManus’ method of removing its dimension. （3） The results of literature statistics show that most domestic scholars tend to use the term “sorting coefficient” rather than the statistical term “standard deviation”, and do not use units when describing the sorting results. In all statistical kinds of literature, with the shortening of time, the proportion of literature using the term "sorting coefficient" and describing the sorting results without using units has increased.（4） The current national standard GB/T 12763.8-2007 states that the unit of the sorting coefficient is Φ. We find that there is a typographical error in the description of the particle size parameter calculation formula, and it is suggested that this needs to be corrected when next revising the standard. Conclusions In this paper, the origin and evolution of Φ and the existing commonly used sorting coefficient calculation formulas are reviewed based on various definitions, and the current situation of the usage of terms and units of the sorting coefficient in China is analyzed. Finally, on the basis of previous studies, some suggestions are put forward to modify the sorting coefficient calculation formula. The above work can provide suggestions for the revision and improvement of marine investigation norms, and provide suggestions for promoting the unification and standardization of terms.
2025,
43(1):
50-62.
doi: 10.14027/j.issn.1000-0550.2023.054
Abstract:
Objective We analyzed the sedimentary environment of manganese ore in the Datangpo Formation of the Nanhua System in the Guzhang area, northwest Hunan province. Methods Based on the geochemical analysis of major and trace elements, rare earth elements and carbon and oxygen isotopes, the oxidation-reduction environment and the source of ore-forming materials during the deposition of manganese ore in the Datangpo Formation were studied. Results （1） There is a positive correlation between SiO2, TiO2, K2O, and Al2O3, weak negative correlation with MgO and CaO, and weak correlation with MnO, Na2O, TFe, and P2O5 for the main elements of manganese ore in the Guzhang area. The deposit is similar to the Gucheng manganese ore in Hubei, Xiangtan manganese ore in Hunan, and Zhaodong manganese ore in Hunan, with an overall high Si, low Fe, and P-bearing manganese ore. （2） The redox-sensitive elemental indicators, Fe/Mn<1, 2<V/Cr<4.5, V/（V+Ni）>0.57, Ni/Co<5, and U/ Th<0.75, of the depositional environment indicate that the water column was in a normoxic-poor environment when the manganese ore was formed in the Datangpo Formation. The enrichment pattern of the medium rare earth elements, depletion of light and heavy rare earth elements, and the weak positive Ce anomaly indicate that Mn was first precipitated in the form of manganese oxides or hydroxides and then changed into Mn carbonate during digenesis. （3） A variety of metallogenic material source identification indicators, SiO2/Al2O3>3.6, Y/Ho close to that of the hydrothermal fluid on the seafloor, results of Fe-Mn-（Ni+Cu+Co）×10 triangulation, and Co/Zn-（Cu+Ni+Co） diagrammatic show that the formation of Mn elements from the manganese ores in the Datangpo Formation in the Guzhang area is closely related to submarine hydrothermal action and has characteristics of hot-water deposition. Conclusions The manganese ore of the Datangpo Formation was formed in a normoxic-poor environment in the Guzhang area, northwest Hunan province. The formation of manganese is closely related to submarine hydrothermal action.
Objective We analyzed the sedimentary environment of manganese ore in the Datangpo Formation of the Nanhua System in the Guzhang area, northwest Hunan province. Methods Based on the geochemical analysis of major and trace elements, rare earth elements and carbon and oxygen isotopes, the oxidation-reduction environment and the source of ore-forming materials during the deposition of manganese ore in the Datangpo Formation were studied. Results （1） There is a positive correlation between SiO2, TiO2, K2O, and Al2O3, weak negative correlation with MgO and CaO, and weak correlation with MnO, Na2O, TFe, and P2O5 for the main elements of manganese ore in the Guzhang area. The deposit is similar to the Gucheng manganese ore in Hubei, Xiangtan manganese ore in Hunan, and Zhaodong manganese ore in Hunan, with an overall high Si, low Fe, and P-bearing manganese ore. （2） The redox-sensitive elemental indicators, Fe/Mn<1, 2<V/Cr<4.5, V/（V+Ni）>0.57, Ni/Co<5, and U/ Th<0.75, of the depositional environment indicate that the water column was in a normoxic-poor environment when the manganese ore was formed in the Datangpo Formation. The enrichment pattern of the medium rare earth elements, depletion of light and heavy rare earth elements, and the weak positive Ce anomaly indicate that Mn was first precipitated in the form of manganese oxides or hydroxides and then changed into Mn carbonate during digenesis. （3） A variety of metallogenic material source identification indicators, SiO2/Al2O3>3.6, Y/Ho close to that of the hydrothermal fluid on the seafloor, results of Fe-Mn-（Ni+Cu+Co）×10 triangulation, and Co/Zn-（Cu+Ni+Co） diagrammatic show that the formation of Mn elements from the manganese ores in the Datangpo Formation in the Guzhang area is closely related to submarine hydrothermal action and has characteristics of hot-water deposition. Conclusions The manganese ore of the Datangpo Formation was formed in a normoxic-poor environment in the Guzhang area, northwest Hunan province. The formation of manganese is closely related to submarine hydrothermal action.
2025,
43(1):
63-75.
doi: 10.14027/j.issn.1000-0550.2023.014
Abstract:
Objective River sediments record abundant information about their source, which is significant for exploring weathering, the transport-sedimentation cycle and parent rock composition in the source region of the basin. Unlike large watersheds that may integrate a range of weathering mechanisms and contain complex rock types, small rivers have similar climatic and lithological backgrounds, which provides valuable information when studying sediment composition. Methods In order to explore the influence of fluvial processes on sediment composition, eight samples were obtained from the upper and lower of the Balan River for granular processing （<63 μm, 63-125 μm, 125-250 μm）. The <63 μm samples were subjected to geochemical testing （major, trace, rare earth and Sr-Nd isotopes）. The 63-125 μm and 125-250 μm samples were tested for heavy minerals. Results It was found that the sediments in the Balan River have undergone low chemical weathering, are of low maturity, and are in the primary cycle of deposition. The provenance is mainly from felsic parent rock. The provenance indices （La/Sc, Co/Th, Cr/Th, Th/Sc） and Sr-Nd isotopic compositions showed little change overall, which represents the overall characteristics of the Balan River basin. Conclusions The Balan River samples showed consistent chemical weathering, maturity levels, recycling characteristics and host rock properties, indicating that the fluvial process had not significantly affected their geochemical composition. The heavy minerals were diluted and mixed after river transport； those from different river reaches and with different grain size fractions showed obviously different compositional content. This indicates that short-distance transport is unlikely to evenly distribute heavy minerals within different grain size fractions. Therefore, when studying river source-sink systems, a small number of samples does not necessarily represent the heavy mineral composition of the entire basin, even in small rivers. This study also emphasizes that the combination of Sr-Nd isotope and inactive element ratio can be used effectively for river provenance tracing.
Objective River sediments record abundant information about their source, which is significant for exploring weathering, the transport-sedimentation cycle and parent rock composition in the source region of the basin. Unlike large watersheds that may integrate a range of weathering mechanisms and contain complex rock types, small rivers have similar climatic and lithological backgrounds, which provides valuable information when studying sediment composition. Methods In order to explore the influence of fluvial processes on sediment composition, eight samples were obtained from the upper and lower of the Balan River for granular processing （<63 μm, 63-125 μm, 125-250 μm）. The <63 μm samples were subjected to geochemical testing （major, trace, rare earth and Sr-Nd isotopes）. The 63-125 μm and 125-250 μm samples were tested for heavy minerals. Results It was found that the sediments in the Balan River have undergone low chemical weathering, are of low maturity, and are in the primary cycle of deposition. The provenance is mainly from felsic parent rock. The provenance indices （La/Sc, Co/Th, Cr/Th, Th/Sc） and Sr-Nd isotopic compositions showed little change overall, which represents the overall characteristics of the Balan River basin. Conclusions The Balan River samples showed consistent chemical weathering, maturity levels, recycling characteristics and host rock properties, indicating that the fluvial process had not significantly affected their geochemical composition. The heavy minerals were diluted and mixed after river transport； those from different river reaches and with different grain size fractions showed obviously different compositional content. This indicates that short-distance transport is unlikely to evenly distribute heavy minerals within different grain size fractions. Therefore, when studying river source-sink systems, a small number of samples does not necessarily represent the heavy mineral composition of the entire basin, even in small rivers. This study also emphasizes that the combination of Sr-Nd isotope and inactive element ratio can be used effectively for river provenance tracing.
2025,
43(1):
76-87.
doi: 10.14027/j.issn.1000-0550.2023.004
Abstract:
Objective The Chinese Loess Plateau （CLP） preserves the most widely distributed, thickest, and continuous aeolian dust deposits in the world. Studying the evolution of loess provenance is important for deducing climate change in inland Asia. Predecessors have made a series of achievements, but the Quaternary climate changed quickly during the glacial-interglacial period accompanied by alternating winter-summer monsoon. Therefore, the source of loess is likely to change with the monsoon； however, there is still controversy regarding the change of loess sources in the glacial-interglacial period due to the lack of evidence of continuous strata. Methods Therefore, in this study, we collected nine loess-paleosol layers from Luochuan （S0, L1, S1, L2, and S2 ） and Lingtai （S0, L1, L2 and S2） for detrital zircon U-Pb dating study. Zircon U-Pb ages were determined at the State Key Laboratory of Continental Dynamics, Northwest University. The U-Pb ages of detrital zircons of the samples are displayed in PDP （Probability Density Plot）, KDE （Kernal Density Estimate）, and CAD （Cumulative Age Distribution） diagrams. By comparing the zircon age combinations between the different layers an potential source regions, we used MDS （multi-dimensional scaling） technology to analyze the data correlation. Results Based on the results, the correlation analysis of the data using MDS showed that nine samples had better correlation with the loess-paleosol samples, including the Ruoshui River, Western Mu Us Desert, Yellow River, and Songpan and Qaidam Basins. However, we found several differences in the KDE and PDP diagrams and indicating that the main age distribution of the samples was 200-600 Ma and the secondary age distributions were between 900-1 200 Ma, 1 800-2 100 Ma, and>2 700 Ma. The U-Pb age characteristics of the Lingtai and Luochuan samples were similar to those of the Songpan Basin, Ruoshui River, Western Mu Us Desert, and Yellow River samples. The Qaidam Basin and Eastern Mu Us Desert showed different age characteristics with our samples. Thus, the Qaidam Basin and Eastern Mu Us Desert were not the main source area of the loess, and we concluded that the dust material was mainly derived from the northwest mixture source, including northeast of the Tibetan Plateau, Alxa region, and Western Mu Us Desert. In addition, no significant difference was found between the loess-paleosol layers during the glacial-interglacial period. To avoid errors caused by data volume, we divided the sample data into two units, namely loess and paleosol, for analysis. The cumulative growth trend of the sample age was analyzed using the CAD diagram, and the results showed that there were not enough data differences, suggesting the provenance remained stable between the glacial and interglacial periods. Conclusions Based on the above research and analysis, we draw the following conclusions： （1） Affected by Asian climate drought and Tibetan Plateau uplift, the dust material is mainly derived from the northwest mixture source, including northeast of the Tibetan Plateau, Alxa region, and Western Mu Us Desert； （2） The source of loess indicating the glacial-interglacial period did not change significantly.
Objective The Chinese Loess Plateau （CLP） preserves the most widely distributed, thickest, and continuous aeolian dust deposits in the world. Studying the evolution of loess provenance is important for deducing climate change in inland Asia. Predecessors have made a series of achievements, but the Quaternary climate changed quickly during the glacial-interglacial period accompanied by alternating winter-summer monsoon. Therefore, the source of loess is likely to change with the monsoon； however, there is still controversy regarding the change of loess sources in the glacial-interglacial period due to the lack of evidence of continuous strata. Methods Therefore, in this study, we collected nine loess-paleosol layers from Luochuan （S0, L1, S1, L2, and S2 ） and Lingtai （S0, L1, L2 and S2） for detrital zircon U-Pb dating study. Zircon U-Pb ages were determined at the State Key Laboratory of Continental Dynamics, Northwest University. The U-Pb ages of detrital zircons of the samples are displayed in PDP （Probability Density Plot）, KDE （Kernal Density Estimate）, and CAD （Cumulative Age Distribution） diagrams. By comparing the zircon age combinations between the different layers an potential source regions, we used MDS （multi-dimensional scaling） technology to analyze the data correlation. Results Based on the results, the correlation analysis of the data using MDS showed that nine samples had better correlation with the loess-paleosol samples, including the Ruoshui River, Western Mu Us Desert, Yellow River, and Songpan and Qaidam Basins. However, we found several differences in the KDE and PDP diagrams and indicating that the main age distribution of the samples was 200-600 Ma and the secondary age distributions were between 900-1 200 Ma, 1 800-2 100 Ma, and>2 700 Ma. The U-Pb age characteristics of the Lingtai and Luochuan samples were similar to those of the Songpan Basin, Ruoshui River, Western Mu Us Desert, and Yellow River samples. The Qaidam Basin and Eastern Mu Us Desert showed different age characteristics with our samples. Thus, the Qaidam Basin and Eastern Mu Us Desert were not the main source area of the loess, and we concluded that the dust material was mainly derived from the northwest mixture source, including northeast of the Tibetan Plateau, Alxa region, and Western Mu Us Desert. In addition, no significant difference was found between the loess-paleosol layers during the glacial-interglacial period. To avoid errors caused by data volume, we divided the sample data into two units, namely loess and paleosol, for analysis. The cumulative growth trend of the sample age was analyzed using the CAD diagram, and the results showed that there were not enough data differences, suggesting the provenance remained stable between the glacial and interglacial periods. Conclusions Based on the above research and analysis, we draw the following conclusions： （1） Affected by Asian climate drought and Tibetan Plateau uplift, the dust material is mainly derived from the northwest mixture source, including northeast of the Tibetan Plateau, Alxa region, and Western Mu Us Desert； （2） The source of loess indicating the glacial-interglacial period did not change significantly.
2025,
43(1):
88-98.
doi: 10.14027/j.issn.1000-0550.2022.165
Abstract:
Objective The Middle-Upper Ordovician Saergan Formation in the Tarim Basin represents an important hydrocarbon source rock deposited in a marine environment. However, the current model of the deposition of the Saergan Formation remains nebulous. High-precision geochronologic constraints may shed new light on the deposition history of the Saergan Formation. Methods The identification of Milankovitch cycles, in particular, a 405 ka long orbital eccentricity cycle, can be used to construct precise time scales for the sedimentary succession of the Paleozoic Era. The magnetic susceptibility （MS） is a measure of the degree of magnetization of a material in response to an applied magnetic field, which can be a powerful tool for cyclostratigraphy and paleoclimate studies. Here, a total of 701 MS measurements from the 9 m Saergan Formation in the Subashigou section of the Kalpin area, Aksu city, Xinjiang were obtained to conduct a cyclostratigraphic analysis, including a multitaper method spectral analysis, an evolutive harmonic analysis, and bandpass filtering. Results The spectral analyses show significant peaks at 0.74-1.28 m, 0.2-0.27 m, 0.057-0.071 m, and 0.035-0.039 m with ratios of 21.14∶1, 7.71∶1, 5.71∶1, 1.86∶1, and 1.11∶1, which are close to those of the Middle-Late Ordovician astronomical cycles. Although there is no reliable astronomical solution for the Ordovician period, astronomical calibration can be achieved using a stable 405 ka eccentricity cycle. After the 405 ka long eccentricity calibration, the spectral analysis shows periods with 100 ka （short eccentricity）, 30.6-31.2 ka （obliquity）, and 20-21.8 ka （precession）. Ten eccentricity cycles are obtained using bandpass filtering and a “floating” astronomical time scale with a duration of ~3.9 Ma is established for the Saergan Formation of the Subashigou section. An amplitude modulation （AM） envelope of the interpreted 30.6 ka band of the 405 ka calibrated MS time series reveals long-term cycles with a period of ~1.2 Ma for the Ordovician, which is identical with that at present. The first appearance datum （FAD） of the graptolite Nemagraptus. gracilis is used as a tie point for building the correlation between the Subashigou section and Dawangou section （Global Stratotype Section and Point auxiliary Stratotype of the Sandbian）. The results show that the initiation of deposition of the Saergan Formation in the Subashigou section was ~1.03 Ma later than that in the Dawangou section. This supports the assertion that the Saergan black shales developed during a strong sea-level transgression process, which drowned the platform, and organic-rich sediments were deposited on the carbonate platform. Based on a further comparison among the MS series, 1.2 Ma obliquity cycle, and global sea-level curve during the Middle-Late Ordovician, we suggest that the 1.2 Ma long obli-quity cycle may have controlled the sea-level changes during the time of the Saergan Formation deposition. Ice accumulation at the million year scale can be explained by a sustained reduction of ice melting in cool high-latitude summers during an interval of a series of reduced obliquity variations （minima of the long term obliquity cycle）. Conclusions In summary, the orbitally forced formation of the Saergan Formation is confirmed and a possible model of the deposition process of the Saergan shale is proposed based on the high-resolution cyclostratigraphy study in this study.
Objective The Middle-Upper Ordovician Saergan Formation in the Tarim Basin represents an important hydrocarbon source rock deposited in a marine environment. However, the current model of the deposition of the Saergan Formation remains nebulous. High-precision geochronologic constraints may shed new light on the deposition history of the Saergan Formation. Methods The identification of Milankovitch cycles, in particular, a 405 ka long orbital eccentricity cycle, can be used to construct precise time scales for the sedimentary succession of the Paleozoic Era. The magnetic susceptibility （MS） is a measure of the degree of magnetization of a material in response to an applied magnetic field, which can be a powerful tool for cyclostratigraphy and paleoclimate studies. Here, a total of 701 MS measurements from the 9 m Saergan Formation in the Subashigou section of the Kalpin area, Aksu city, Xinjiang were obtained to conduct a cyclostratigraphic analysis, including a multitaper method spectral analysis, an evolutive harmonic analysis, and bandpass filtering. Results The spectral analyses show significant peaks at 0.74-1.28 m, 0.2-0.27 m, 0.057-0.071 m, and 0.035-0.039 m with ratios of 21.14∶1, 7.71∶1, 5.71∶1, 1.86∶1, and 1.11∶1, which are close to those of the Middle-Late Ordovician astronomical cycles. Although there is no reliable astronomical solution for the Ordovician period, astronomical calibration can be achieved using a stable 405 ka eccentricity cycle. After the 405 ka long eccentricity calibration, the spectral analysis shows periods with 100 ka （short eccentricity）, 30.6-31.2 ka （obliquity）, and 20-21.8 ka （precession）. Ten eccentricity cycles are obtained using bandpass filtering and a “floating” astronomical time scale with a duration of ~3.9 Ma is established for the Saergan Formation of the Subashigou section. An amplitude modulation （AM） envelope of the interpreted 30.6 ka band of the 405 ka calibrated MS time series reveals long-term cycles with a period of ~1.2 Ma for the Ordovician, which is identical with that at present. The first appearance datum （FAD） of the graptolite Nemagraptus. gracilis is used as a tie point for building the correlation between the Subashigou section and Dawangou section （Global Stratotype Section and Point auxiliary Stratotype of the Sandbian）. The results show that the initiation of deposition of the Saergan Formation in the Subashigou section was ~1.03 Ma later than that in the Dawangou section. This supports the assertion that the Saergan black shales developed during a strong sea-level transgression process, which drowned the platform, and organic-rich sediments were deposited on the carbonate platform. Based on a further comparison among the MS series, 1.2 Ma obliquity cycle, and global sea-level curve during the Middle-Late Ordovician, we suggest that the 1.2 Ma long obli-quity cycle may have controlled the sea-level changes during the time of the Saergan Formation deposition. Ice accumulation at the million year scale can be explained by a sustained reduction of ice melting in cool high-latitude summers during an interval of a series of reduced obliquity variations （minima of the long term obliquity cycle）. Conclusions In summary, the orbitally forced formation of the Saergan Formation is confirmed and a possible model of the deposition process of the Saergan shale is proposed based on the high-resolution cyclostratigraphy study in this study.
2025,
43(1):
99-107.
doi: 10.14027/j.issn.1000-0550.2022.141
Abstract:
Objective Seabed pockmarks are not only active records and instructions of the seabed fluid but also key factors affecting oilfield and offshore platforms. Therefore, their study is extremely important. The Niger Delta Basin is extremely rich in deep-water oil and gas reserves and is an important oil and gas production area globally. Therefore, studies of their seabed pockmark characteristics have particular value. Methods Based on sedimentary principles and methods and high resolution three-dimensional （3D） seismic data conducted on the sedimentary characteristics, genetic mechanism, and research significance of seabed pockmarks near West Africa, we identified and illustrated the sedimentary characteristics and size and genetic mechanism of seabed pockmarks in the study areas, exploring their significance. Results （1） Based on the plane morphological characteristics of pockmarks, there are mainly two types of pockmarks in the study area： striped pockmarks and isolated pockmarks, which are circular, elliptical and crescent in shape on the plane. The length of the striped pockmarks is 0.5-3.0 km, the width is 0.5-1.0 km, and the width of the isolated pockmarks is generally 10-300 m； （2） According to the development characte-ristics and sedimentary characteristics, Pockmark distribution characteristics of pockmark were found to be isolated and banded in the study area, and the genesis of pockmarks is classified into five types： dip structure, fault, mud volcano, ancient channel, and sedimentary boundary. The profile is U-shaped or W-shaped, and the seismic reflection is disordered； （3） They are primarily caused by plastic fluid and oil and gas leakage. One is caused by plastic fluid, the other is caused by oil and gas spilling along the fault. The high pressure fluid is squeezed into the overlying sedimentary layer along the fault plane, and the reduction of local stress causes the submarine surface sediment to undergo plastic deformation, forming an upward migration channel. The finally liquefied sediment is transported by the underflow, forming a pockmark. Conclusions The results of pockmark can indicate the best locations for oil and gas exploration and development, including drilling and engineering facilities in the future, we should be paid attention to exploration and development In the process oil and gas of field.
Objective Seabed pockmarks are not only active records and instructions of the seabed fluid but also key factors affecting oilfield and offshore platforms. Therefore, their study is extremely important. The Niger Delta Basin is extremely rich in deep-water oil and gas reserves and is an important oil and gas production area globally. Therefore, studies of their seabed pockmark characteristics have particular value. Methods Based on sedimentary principles and methods and high resolution three-dimensional （3D） seismic data conducted on the sedimentary characteristics, genetic mechanism, and research significance of seabed pockmarks near West Africa, we identified and illustrated the sedimentary characteristics and size and genetic mechanism of seabed pockmarks in the study areas, exploring their significance. Results （1） Based on the plane morphological characteristics of pockmarks, there are mainly two types of pockmarks in the study area： striped pockmarks and isolated pockmarks, which are circular, elliptical and crescent in shape on the plane. The length of the striped pockmarks is 0.5-3.0 km, the width is 0.5-1.0 km, and the width of the isolated pockmarks is generally 10-300 m； （2） According to the development characte-ristics and sedimentary characteristics, Pockmark distribution characteristics of pockmark were found to be isolated and banded in the study area, and the genesis of pockmarks is classified into five types： dip structure, fault, mud volcano, ancient channel, and sedimentary boundary. The profile is U-shaped or W-shaped, and the seismic reflection is disordered； （3） They are primarily caused by plastic fluid and oil and gas leakage. One is caused by plastic fluid, the other is caused by oil and gas spilling along the fault. The high pressure fluid is squeezed into the overlying sedimentary layer along the fault plane, and the reduction of local stress causes the submarine surface sediment to undergo plastic deformation, forming an upward migration channel. The finally liquefied sediment is transported by the underflow, forming a pockmark. Conclusions The results of pockmark can indicate the best locations for oil and gas exploration and development, including drilling and engineering facilities in the future, we should be paid attention to exploration and development In the process oil and gas of field.
2025,
43(1):
108-120.
doi: 10.14027/j.issn.1000-0550.2022.148
Abstract:
Objective Large-scale lacustrine carbonate sediments were developed in the upper Fourth member of the Shahejie Formation （Es4 s 4 s
Objective Large-scale lacustrine carbonate sediments were developed in the upper Fourth member of the Shahejie Formation （Es
2025,
43(1):
121-138.
doi: 10.14027/j.issn.1000-0550.2023.007
Abstract:
Purpose During the Late Carboniferous to Middle Permian at the northwestern margin of the Ordos Basin, the sedimentary environment changed from marine to continental facies, accompanied by the advance and retreat of North China and Qilian seawaters. The spatial and temporal configuration and evolution of the sedimentary system are complex, and the understanding of paleogeographic pattern differences under diverse sedimentary backgrounds has been relatively limited. Methods Analysis of outcrop drilling logging data and micro-thin-section identification for main elements, trace elements and rare earth elements in more than 150 samples has clarified the sedimentary environment and source area characteristics within the study area. The constraints of the sedimentary background on the paleogeographic reconstruction are also discussed. Results The northwestern margin of the Ordos Basin was generally warm and humid during the Late Carboniferous-Middle Permian. The water rapidly deepened and then remained at a relatively stable level during the period from the Yanghugou Formation to the Taiyuan Formation. The sedimentary period of the Taiyuan Formation shows a reducing background, with the transitional environment being dominated by salt water. For the Shihezi Formation within the Shanxi Formation, the sedimentary period indicates an oxidizing transitional environment dominated by fresh water. The provenance system from the Yanghugou Formation to the Shanxi Formation is complex, and the supply tended to be stable during the Shihezi Formation. Conclusions The paleo-geography of the study area differs from that to the north and south at different times. Mainly sand-poor sedimentary systems are evident in the south （e.g., lagoon-tidal flat, bay-shallow water shelf and semi-deep water shelf）, while predominantly sand-rich sedimentary systems of river-controlled and tidal deltas are developed in the north. With the closure of the paleo-Asian ocean and the transformation of the parent rock lithology in the provenance area, the delta system in the north continued to expand, and the water area in the south decreased sharply and developed into a bay/offshore lake.
Purpose During the Late Carboniferous to Middle Permian at the northwestern margin of the Ordos Basin, the sedimentary environment changed from marine to continental facies, accompanied by the advance and retreat of North China and Qilian seawaters. The spatial and temporal configuration and evolution of the sedimentary system are complex, and the understanding of paleogeographic pattern differences under diverse sedimentary backgrounds has been relatively limited. Methods Analysis of outcrop drilling logging data and micro-thin-section identification for main elements, trace elements and rare earth elements in more than 150 samples has clarified the sedimentary environment and source area characteristics within the study area. The constraints of the sedimentary background on the paleogeographic reconstruction are also discussed. Results The northwestern margin of the Ordos Basin was generally warm and humid during the Late Carboniferous-Middle Permian. The water rapidly deepened and then remained at a relatively stable level during the period from the Yanghugou Formation to the Taiyuan Formation. The sedimentary period of the Taiyuan Formation shows a reducing background, with the transitional environment being dominated by salt water. For the Shihezi Formation within the Shanxi Formation, the sedimentary period indicates an oxidizing transitional environment dominated by fresh water. The provenance system from the Yanghugou Formation to the Shanxi Formation is complex, and the supply tended to be stable during the Shihezi Formation. Conclusions The paleo-geography of the study area differs from that to the north and south at different times. Mainly sand-poor sedimentary systems are evident in the south （e.g., lagoon-tidal flat, bay-shallow water shelf and semi-deep water shelf）, while predominantly sand-rich sedimentary systems of river-controlled and tidal deltas are developed in the north. With the closure of the paleo-Asian ocean and the transformation of the parent rock lithology in the provenance area, the delta system in the north continued to expand, and the water area in the south decreased sharply and developed into a bay/offshore lake.
2025,
43(1):
139-153.
doi: 10.14027/j.issn.1000-0550.2022.152
Abstract:
Objective Multi-types of shore-parallel sandbodies are ubiquitous in the delta to shallow water area on the shelf, and their morphologies, scales, and spatial-temporal patterns were comprehensively responding to hydrodynamic processes, sediment supply, and climate changes thus recording plenty of geological information. Meanwhile, they are favorable reservoir sandbodies of stratigraphic trap exploration. This study intended to finely interpretate the high amplitude and wedge-shaped shore-parallel sandbodies of the Huizhou area in the Pearl River Mouth Basin of the South China Sea and discuss their controlling evolutionary factors. Methods Based on well and seismic data, detailed studies were conducted on the high-resolution stratigraphic framework of the Hanjiang Formation and Zhujiang Formation, Lower-Middle Miocene. Combined with 3D seismic interpretation, spectral decomposition, and other analysis tools of seismic sedimentology, analysis based on core and logging data, and comparisons with modern deposits, the characteristics of the shelf-parallel sandbodies were discussed in detail. Results Amounts of high-amplitude and shore-parallel sandbodies were identified in the forced regressive systems tracts within several sequences. The shore-parallel sandbodies could be identified as mainly ridge-shaped on seismic sections and primarily included a high-amplitude seismic reflection in the head, high-angle shingling progradational reflections in the middle, and low-amplitude reflections in the tail； Meanwhile, these shore-parallel sandbodies tended to widen from northeast to southwest, with significant densely arranged striated internal structures； Coarsening upward succession and wave erosion surface can be observed on core； Sharp-based, boxed-shape log response can be found on wireline log. Conclusions In conclusion, these shore-parallel sandbodies were identified to ancient beach ridges and mainly controlled by forced regression and stable high sediment supply. Based on the analysis of ancient beach ridges’ planar distribution characteristics, it was speculated that their formation and evolution might be influenced by the southwest-oriented paleocurrent environment of the study area. Moreover, according to the development time of the earliest ancient beach ridge, it was proposed that the southwest-oriented paleocurrent should exist since ca. 21 Ma and affected the development of the Pearl River delta.
Objective Multi-types of shore-parallel sandbodies are ubiquitous in the delta to shallow water area on the shelf, and their morphologies, scales, and spatial-temporal patterns were comprehensively responding to hydrodynamic processes, sediment supply, and climate changes thus recording plenty of geological information. Meanwhile, they are favorable reservoir sandbodies of stratigraphic trap exploration. This study intended to finely interpretate the high amplitude and wedge-shaped shore-parallel sandbodies of the Huizhou area in the Pearl River Mouth Basin of the South China Sea and discuss their controlling evolutionary factors. Methods Based on well and seismic data, detailed studies were conducted on the high-resolution stratigraphic framework of the Hanjiang Formation and Zhujiang Formation, Lower-Middle Miocene. Combined with 3D seismic interpretation, spectral decomposition, and other analysis tools of seismic sedimentology, analysis based on core and logging data, and comparisons with modern deposits, the characteristics of the shelf-parallel sandbodies were discussed in detail. Results Amounts of high-amplitude and shore-parallel sandbodies were identified in the forced regressive systems tracts within several sequences. The shore-parallel sandbodies could be identified as mainly ridge-shaped on seismic sections and primarily included a high-amplitude seismic reflection in the head, high-angle shingling progradational reflections in the middle, and low-amplitude reflections in the tail； Meanwhile, these shore-parallel sandbodies tended to widen from northeast to southwest, with significant densely arranged striated internal structures； Coarsening upward succession and wave erosion surface can be observed on core； Sharp-based, boxed-shape log response can be found on wireline log. Conclusions In conclusion, these shore-parallel sandbodies were identified to ancient beach ridges and mainly controlled by forced regression and stable high sediment supply. Based on the analysis of ancient beach ridges’ planar distribution characteristics, it was speculated that their formation and evolution might be influenced by the southwest-oriented paleocurrent environment of the study area. Moreover, according to the development time of the earliest ancient beach ridge, it was proposed that the southwest-oriented paleocurrent should exist since ca. 21 Ma and affected the development of the Pearl River delta.
2025,
43(1):
154-168.
doi: 10.14027/j.issn.1000-0550.2022.107
Abstract:
Objective Deep-water gravity flow deposits are widely developed in the Chang 7 oil member of the Triassic Yanchang Formation in the Ordos Basin. However, the research on the distribution pattern of lithofacies, sedimentary evolution process, and depositional model is relatively weak, which restricts the deployment of deep-water oil and gas exploration in this area. Methods Taking the Chang 7 oil member in the southwestern Ordos Basin as the research object, based on a large number of cores, well logging, thin section identification, electron microscope scanning, particle size date analysis, X-ray diffraction whole rock analysis, mineral identification analysis, and other data, the rock types, lithofacies characteristics, lithofacies assemblages, distribution characteristics, and sedimentary facies division of the deep-water gravity flow deposits were analyzed. Then, the gravity flow depositional model was established. Results （1） The lithology of deep-water gravity flow deposits of the Chang 7 oil member is mainly composed of gray and brown-gray fine-grained sandstone, gray and dark gray ultrafine-powder sandstone, and dark and black mud shale, with a small amount of tuff； the mineral components are mainly composed of quartz, feldspar, and clay minerals； （2） the main lithofacies types include massive bedding fine sandstone facies （Sm）, positive-order silt-fine sandstone facies （Sa）, banded （silty） mudstone facies （Mh）, laminated shale facies （Sh）, block like mudstone facies （Mm）, and tuff facies （T）. According to the development of different sedimentary environments, four lithofacies assemblages are summarized： Sm-Mh （Sh）, Sm-Sa-Mh （Sh）, Sa-Mh （Sh）, and T-Sh （Mm）. This reflects sandy debris flow -deep water in situ deposition, sandy debris flow -turbidity currents-deep water in situ deposition, turbidity currents -deep water in situ deposition, and volcaniclastic deposits-deep water in situ deposition in the lacustrine environments； （3） based on the lithofacies assemblages and plane distribution characteristics developed by different types of gravity flow deposits, from below the slope break zone to the center of the lake basin, the gravity flow presents the evolution characteristics of sandy debris flow, sandy debris flow- turbidity currents, and turbidity currents. （4） According to the sandstone proportion of sandy debris flow in gravity flow, lithofacies types, lithofacies assemblages, GR form, and channel scale, the gravity flow deposits are divided into three subfacies： restricted channel, non-restricted channel, and front end lobes. Conclusions This study helps to clarify the development pattern and predict the distribution of high-quality sand bodies in the Chang 7 oil member, providing guidance for the exploration and development of unconventional oil and gas in the study area.
Objective Deep-water gravity flow deposits are widely developed in the Chang 7 oil member of the Triassic Yanchang Formation in the Ordos Basin. However, the research on the distribution pattern of lithofacies, sedimentary evolution process, and depositional model is relatively weak, which restricts the deployment of deep-water oil and gas exploration in this area. Methods Taking the Chang 7 oil member in the southwestern Ordos Basin as the research object, based on a large number of cores, well logging, thin section identification, electron microscope scanning, particle size date analysis, X-ray diffraction whole rock analysis, mineral identification analysis, and other data, the rock types, lithofacies characteristics, lithofacies assemblages, distribution characteristics, and sedimentary facies division of the deep-water gravity flow deposits were analyzed. Then, the gravity flow depositional model was established. Results （1） The lithology of deep-water gravity flow deposits of the Chang 7 oil member is mainly composed of gray and brown-gray fine-grained sandstone, gray and dark gray ultrafine-powder sandstone, and dark and black mud shale, with a small amount of tuff； the mineral components are mainly composed of quartz, feldspar, and clay minerals； （2） the main lithofacies types include massive bedding fine sandstone facies （Sm）, positive-order silt-fine sandstone facies （Sa）, banded （silty） mudstone facies （Mh）, laminated shale facies （Sh）, block like mudstone facies （Mm）, and tuff facies （T）. According to the development of different sedimentary environments, four lithofacies assemblages are summarized： Sm-Mh （Sh）, Sm-Sa-Mh （Sh）, Sa-Mh （Sh）, and T-Sh （Mm）. This reflects sandy debris flow -deep water in situ deposition, sandy debris flow -turbidity currents-deep water in situ deposition, turbidity currents -deep water in situ deposition, and volcaniclastic deposits-deep water in situ deposition in the lacustrine environments； （3） based on the lithofacies assemblages and plane distribution characteristics developed by different types of gravity flow deposits, from below the slope break zone to the center of the lake basin, the gravity flow presents the evolution characteristics of sandy debris flow, sandy debris flow- turbidity currents, and turbidity currents. （4） According to the sandstone proportion of sandy debris flow in gravity flow, lithofacies types, lithofacies assemblages, GR form, and channel scale, the gravity flow deposits are divided into three subfacies： restricted channel, non-restricted channel, and front end lobes. Conclusions This study helps to clarify the development pattern and predict the distribution of high-quality sand bodies in the Chang 7 oil member, providing guidance for the exploration and development of unconventional oil and gas in the study area.
2025,
43(1):
169-181.
doi: 10.14027/j.issn.1000-0550.2023.006
Abstract:
Objective To clarify the multi-stage dolomitization of the Lower-Middle Ordovician carbonate rocks in the Maigaiti Slope. Methods On the basis of lithofacies analysis and in⁃situ trace and rare earth element （REEs） tests using laser ablation inductively coupled plasma mass spectroscopy （LA-ICP-MS）, we analyzed the origin of dolomites and its control on the reservoir. Results The Lower-Middle Ordovician dolomites primarily include silt-crystalline dolomite, grain dolomite, calcite dolomite, medium-crystalline dolomite, and dolomite cement in fractures and vugs. The sedimentary environment is interpreted as a shoal and tidal flat in a restricted platform. The composition and patterns of REEs in the dolomites and abundant natrium （Na） content indicate that the dolomitization fluid was of seawater origin and locally influenced by hydrothermal fluid. The dolomitization of the Lower-Middle Ordovician carbonates is characterized by a multi-stage process. During the deposition of the Penglaiba Formation, penecontemporaneous dolomitization caused tidal deposits to form silt-crystalline dolomite, and seepage-reflux dolomitization caused shoal deposits to form grain dolomite. During the deposition of the Yingshan Formation, burial dolomitization produced the continuous growth of dolomite crystals, and grain-supported textures were partially destroyed, resulting in medium-crystalline dolomite. In addition, fractures and vugs were continuously filled with dolomite. Lime-grainstones in the Yingshan Formation was partially dolomitized during burial, resulting in the patchy distribution of dolomite crystals. Conclusions Multi-stage dolomitization controlled the quality of Lower-Middle Ordovician dolomite reservoirs in the study area. In the shoal and tidal flat system in a restricted platform, the favorable type of reservoir is grain dolomite formed by seepage-reflux dolomitization during early-stage diagenesis.
Objective To clarify the multi-stage dolomitization of the Lower-Middle Ordovician carbonate rocks in the Maigaiti Slope. Methods On the basis of lithofacies analysis and in⁃situ trace and rare earth element （REEs） tests using laser ablation inductively coupled plasma mass spectroscopy （LA-ICP-MS）, we analyzed the origin of dolomites and its control on the reservoir. Results The Lower-Middle Ordovician dolomites primarily include silt-crystalline dolomite, grain dolomite, calcite dolomite, medium-crystalline dolomite, and dolomite cement in fractures and vugs. The sedimentary environment is interpreted as a shoal and tidal flat in a restricted platform. The composition and patterns of REEs in the dolomites and abundant natrium （Na） content indicate that the dolomitization fluid was of seawater origin and locally influenced by hydrothermal fluid. The dolomitization of the Lower-Middle Ordovician carbonates is characterized by a multi-stage process. During the deposition of the Penglaiba Formation, penecontemporaneous dolomitization caused tidal deposits to form silt-crystalline dolomite, and seepage-reflux dolomitization caused shoal deposits to form grain dolomite. During the deposition of the Yingshan Formation, burial dolomitization produced the continuous growth of dolomite crystals, and grain-supported textures were partially destroyed, resulting in medium-crystalline dolomite. In addition, fractures and vugs were continuously filled with dolomite. Lime-grainstones in the Yingshan Formation was partially dolomitized during burial, resulting in the patchy distribution of dolomite crystals. Conclusions Multi-stage dolomitization controlled the quality of Lower-Middle Ordovician dolomite reservoirs in the study area. In the shoal and tidal flat system in a restricted platform, the favorable type of reservoir is grain dolomite formed by seepage-reflux dolomitization during early-stage diagenesis.
2025,
43(1):
182-197.
doi: 10.14027/j.issn.1000-0550.2023.011
Abstract:
Objective Marine carbonates of the Permian Wujiaping Formation commonly occur in tuffs in northwest Sichuan. The tuff beds vary in thickness, and therefore have different natural gas storage capacities. The Wujiaping Formation at the Daping section is 45.02 m thick, and subdivided into two members. Individual tuff beds are 5⁃55 cm thick. Measurements of geochemical parameters were conducted on tuff samples, taking their reservoir types into account. Methods A synthetic database of microfacies was developed for major and trace elements and zircon U-Pb using XRD and scanning electron microscopy （SEM）. Results The tuffs are mainly volcanic dust composed of glass chips smaller than 0.05 mm. The mineral composition of the tuffs is mainly illite and illite/montmorillonite mixtures, and they contain small amounts of quartz and gypsum. The Wu 1 member contains a high Al2O3/TiO2 ratio （23.36-34.56）, a low SiO2/Al2O3 ratio （1.84-2.75） and obvious negative anomalies of Eu, Ba, Sr and Ti. The Wu 2 member has a high Al2O3/TiO2 ratio （21.140-53.049）, a low SiO2/Al2O3 ratio （1.829-2.682） and a moderately negative Eu anomaly, with strong depletion of elements such as Nb and Ti. Conclusions These element profiles indicate that the volcanic debris has undergone strong secondary alteration. The tuffs were sourced from multiple acid volcanic eruptions in Emeishan large igneous province, and were formed in an intraplate extensional environment. The Wu 2 member tuffs were also affected by crust-mantle mixing. Reservoir space types include cavities, irregular dissolution pores, intra- and intergranular dissolution pores, organic matter pores, fractures, etc. The U-Pb age of hydrothermal zircon is dated at 132.37±0.53 Ma. The tuff reservoirs have also been influenced by various other factors such as Lower Cretaceous hydrothermal fluid and tectonic activity.
Objective Marine carbonates of the Permian Wujiaping Formation commonly occur in tuffs in northwest Sichuan. The tuff beds vary in thickness, and therefore have different natural gas storage capacities. The Wujiaping Formation at the Daping section is 45.02 m thick, and subdivided into two members. Individual tuff beds are 5⁃55 cm thick. Measurements of geochemical parameters were conducted on tuff samples, taking their reservoir types into account. Methods A synthetic database of microfacies was developed for major and trace elements and zircon U-Pb using XRD and scanning electron microscopy （SEM）. Results The tuffs are mainly volcanic dust composed of glass chips smaller than 0.05 mm. The mineral composition of the tuffs is mainly illite and illite/montmorillonite mixtures, and they contain small amounts of quartz and gypsum. The Wu 1 member contains a high Al2O3/TiO2 ratio （23.36-34.56）, a low SiO2/Al2O3 ratio （1.84-2.75） and obvious negative anomalies of Eu, Ba, Sr and Ti. The Wu 2 member has a high Al2O3/TiO2 ratio （21.140-53.049）, a low SiO2/Al2O3 ratio （1.829-2.682） and a moderately negative Eu anomaly, with strong depletion of elements such as Nb and Ti. Conclusions These element profiles indicate that the volcanic debris has undergone strong secondary alteration. The tuffs were sourced from multiple acid volcanic eruptions in Emeishan large igneous province, and were formed in an intraplate extensional environment. The Wu 2 member tuffs were also affected by crust-mantle mixing. Reservoir space types include cavities, irregular dissolution pores, intra- and intergranular dissolution pores, organic matter pores, fractures, etc. The U-Pb age of hydrothermal zircon is dated at 132.37±0.53 Ma. The tuff reservoirs have also been influenced by various other factors such as Lower Cretaceous hydrothermal fluid and tectonic activity.
2025,
43(1):
198-211.
doi: 10.14027/j.issn.1000-0550.2023.013
Abstract:
Objective In order to clarify the dissolution mechanism of karstification in the Fourth member of Deng-ying Formation, the distribution law of karst reservoir and the controlling factors. The results increase the understanding of the development mechanism of eogenetic karst reservoirs and provide a basis for the prediction of favorable reservoirs in the study area. Methods The data obtained from scanning, logging of formation micro-resistivity, core inspection, thin sections and conventional logging were used to identify and analyze the karst facies and types,characte-ristics of karst reservoirs and their development. Results Formation micro-scanner imagery indicated seven types of karst facies in Deng4： fracture and cave facies （F1）, cave-filling facies （F2）, breccia facies （F3）, lamination-related vug facies （F4）, scattered vug facies （F5）, high resistance massive dense phase facies（F6） and Low resistance layer dense facies.,. The distribution of karst facies and favorable reservoirs is closely related to the structure of high-frequency sequences. The vug reservoirs （F4 and F5） are mainly found in mound-shoal complex deposits in the upper part of the high-frequency sequence. The thick fracture and cave reservoirs （F1-F4） are present in the thick mound-beach bodies near the third-order sequence interface at the top of Deng4, a result of superposed supergene karsts displaying early karstification. Conclusions Sedimentary facies and sequence-influenced karst types and intensity affect the development of karst reservoirs. Shoal facies is the basic material of karst development. The high-frequency drop in sea level benefited karst development at the early diagenetic stage and affected the scale of karst reservoir development. Supergene karstification near the third-order sequence interface at the top of Deng4 is strong and is the main factor for the formation of high-quality reservoirs. The different hydrodynamic conditions in the karst paleogeomorphic belt affected the intensity of supergenetic karst. The karst slope area is superior to the high karst area in terms of total reservoir thickness, reservoir type and energy storage coefficient.
Objective In order to clarify the dissolution mechanism of karstification in the Fourth member of Deng-ying Formation, the distribution law of karst reservoir and the controlling factors. The results increase the understanding of the development mechanism of eogenetic karst reservoirs and provide a basis for the prediction of favorable reservoirs in the study area. Methods The data obtained from scanning, logging of formation micro-resistivity, core inspection, thin sections and conventional logging were used to identify and analyze the karst facies and types,characte-ristics of karst reservoirs and their development. Results Formation micro-scanner imagery indicated seven types of karst facies in Deng4： fracture and cave facies （F1）, cave-filling facies （F2）, breccia facies （F3）, lamination-related vug facies （F4）, scattered vug facies （F5）, high resistance massive dense phase facies（F6） and Low resistance layer dense facies.,. The distribution of karst facies and favorable reservoirs is closely related to the structure of high-frequency sequences. The vug reservoirs （F4 and F5） are mainly found in mound-shoal complex deposits in the upper part of the high-frequency sequence. The thick fracture and cave reservoirs （F1-F4） are present in the thick mound-beach bodies near the third-order sequence interface at the top of Deng4, a result of superposed supergene karsts displaying early karstification. Conclusions Sedimentary facies and sequence-influenced karst types and intensity affect the development of karst reservoirs. Shoal facies is the basic material of karst development. The high-frequency drop in sea level benefited karst development at the early diagenetic stage and affected the scale of karst reservoir development. Supergene karstification near the third-order sequence interface at the top of Deng4 is strong and is the main factor for the formation of high-quality reservoirs. The different hydrodynamic conditions in the karst paleogeomorphic belt affected the intensity of supergenetic karst. The karst slope area is superior to the high karst area in terms of total reservoir thickness, reservoir type and energy storage coefficient.
2025,
43(1):
212-225.
doi: 10.14027/j.issn.1000-0550.2023.015
Abstract:
Objective To improve the quantitative methods of evaluating the degree of natural fracture development in shale and to clarify the main influences on fracture development. Methods Taking the Wufeng Formation-Longmaxi Formation shale in the Luzhou region as an example, the classification, development index evaluation and influences on natural fracture development in shale reservoirs were systematically studied. Methods included fine core description, organic geochemical testing, triaxial rock mechanics experiments and logging mechanics interpretation. Results Natural fractures in Wufeng Formation - S1l1 1 1 1 1 1
Objective To improve the quantitative methods of evaluating the degree of natural fracture development in shale and to clarify the main influences on fracture development. Methods Taking the Wufeng Formation-Longmaxi Formation shale in the Luzhou region as an example, the classification, development index evaluation and influences on natural fracture development in shale reservoirs were systematically studied. Methods included fine core description, organic geochemical testing, triaxial rock mechanics experiments and logging mechanics interpretation. Results Natural fractures in Wufeng Formation - S1l
2025,
43(1):
226-236.
doi: 10.14027/j.issn.1000-0550.2022.164
Abstract:
Objective The oil sands of the Cretaceous McMurray Formation in the Athabasca region of the Alberta Basin, Canada, are abundant, but the origin and source of the bitumen in the McMurray Formation oil sands remain controversial. Methods A total of 25 oil sand samples from the Cretaceous McMurray Formation in the Mackay River area of Athabasca were analyzed through soluble organic matter extraction, group component separation, saturated hydrocarbon gas chromatography, and saturated and aromatic hydrocarbon chromatographic mass spectrometry. Based on the analysis results of group components and biomarker parameters, the maturity, organic matter depositional environment, and biological origin of the bitumen from the oil sands in the Mackay River area were identified. Results The n-alkanes and acyclic isoprenoids in the samples are below the detection limit, while hopanes and steranes have suffered partial degradation. In contrast, tricyclic terpanes and triaromatic steroids remain relatively intact, exhibiting characteristic features of C23 tricyclic terpane dominance, C27 regular sterane dominance, and C28 triaromatic steroid dominance, indicating that the samples belong to the same group. The hydrocarbon source material is primarily derived from lower aquatic organisms and algae, with a depositional environment of marine facies. The equivalent vitrinite reflectance of the bitumen from the McMurray Formation oil sands ranges from 0.55% to 0.77%, with slight vertical variation in maturity, indicating a low maturity stage. Conclusions The oil sands of the McMurray Formation are the result of severe biodegradation, with a biodegradation level approximately at PM 7. Based on a comparative analysis of the biomarker composition characteristics of source rocks in the study area, it is inferred that the bitumen in the McMurray Formation oil sands may have been supplied by multiple sets of source rocks.
Objective The oil sands of the Cretaceous McMurray Formation in the Athabasca region of the Alberta Basin, Canada, are abundant, but the origin and source of the bitumen in the McMurray Formation oil sands remain controversial. Methods A total of 25 oil sand samples from the Cretaceous McMurray Formation in the Mackay River area of Athabasca were analyzed through soluble organic matter extraction, group component separation, saturated hydrocarbon gas chromatography, and saturated and aromatic hydrocarbon chromatographic mass spectrometry. Based on the analysis results of group components and biomarker parameters, the maturity, organic matter depositional environment, and biological origin of the bitumen from the oil sands in the Mackay River area were identified. Results The n-alkanes and acyclic isoprenoids in the samples are below the detection limit, while hopanes and steranes have suffered partial degradation. In contrast, tricyclic terpanes and triaromatic steroids remain relatively intact, exhibiting characteristic features of C23 tricyclic terpane dominance, C27 regular sterane dominance, and C28 triaromatic steroid dominance, indicating that the samples belong to the same group. The hydrocarbon source material is primarily derived from lower aquatic organisms and algae, with a depositional environment of marine facies. The equivalent vitrinite reflectance of the bitumen from the McMurray Formation oil sands ranges from 0.55% to 0.77%, with slight vertical variation in maturity, indicating a low maturity stage. Conclusions The oil sands of the McMurray Formation are the result of severe biodegradation, with a biodegradation level approximately at PM 7. Based on a comparative analysis of the biomarker composition characteristics of source rocks in the study area, it is inferred that the bitumen in the McMurray Formation oil sands may have been supplied by multiple sets of source rocks.
2025,
43(1):
237-253.
doi: 10.14027/j.issn.1000-0550.2023.023
Abstract:
Objective This study clarified the origin of the sandstone and conglomerate reservoir in the Third member of the Shahejie Formation of the Chezhen Sag and explored the influence of the intensity and occurrence time of overpressure on the development of high-quality reservoirs. Methods Based on restoring the formation pressure of sandy-conglomerate reservoirs in the Third member of the Shahejie Formation in the steep slope belt of Chezhen Sag using the inclusion PVTx method, the logging curve combination and acoustic velocity-density cross-plot methods were used to analyze the origin of overpressure in mudstones. Then, the occurrence of overpressure and oil saturation were synthetically used to analyze the origin of overpressure in reservoirs and compare the differences in reservoir space and diagenesis among different reservoirs with inconsistent histories of pressure evolution. Results The mudstones in the Third member of the Shahejie Formation in the steep slope of the Chezhen Sag have developed medium-strong overpressure caused by hydrocarbon generation pressurization. The overpressures, undergoing two periods of supercharging, in the conglomerate reservoirs are caused by the pressure transition from the overpressure in the source rocks from the Third member of the Shahejie Formation. The first and second phases correspond to the oil emplacements in yellow and blue fluorescence, respectively. However, there were differences in the various regions. The sandstones and conglomerates in the Third member of the Shahejie Formation in the steep slope belt of the Dawangbei Sag underwent two periods of pressurization. The first period occurred at 28.3-27.8 Ma, and the coeval formation pressure coefficient was 1.34-1.35. The second period occurred at 11.6-9.0 Ma, and the coeval formation pressure coefficient was 1.56. The current formation pressure coefficient is 0.8-1.2. The conglomerate reservoirs in the Third member of the Shahejie Formation in the steep slope belt of the Chezhen Sag experienced only one period of pressurization, occurring from 6 Ma to the present, with a coeval formation pressure coefficient of 1.2-1.4. The present-day formation pressure coefficient is 1.0-1.3. The sandstones and conglomerates in the Third member of the Shahejie Formation in the steep slope of the Taoerhe Sag experienced two periods of pressurization. The first period occurred at 35.7-27.2 Ma, and the formation pressure coefficient was 1.54-2.08, while the second period occurred from 12.9 Ma to the present, with formation pressure coefficient of 1.57-2.10. The current formation pressure coefficient is 1.2-2.1 The weaker compaction and cementation, increased number of primary pores, and improved physical properties can be identified in the conglomerate reservoirs in the steep slope belt of Dawangbei Sag with normal pressure-weak overpressure rather than in the Chezhen Sag with normal pressure-medium overpressure. The main reason is that two periods of paleo-overpressure with higher intensity occurred in the Dawangbei Sag, while only one period of paleo-overpressure with relatively low intensity occurred in the Chezhen Sag. Conclusions The development of high-quality clastic rock reservoirs in the middle and deep layers are not only controlled by the overpressure strength in the current formations, but also closely related to the occurrence and strength of ancient overpressure. The earlier and stronger overpressure develops, the more significant inhibitions of compaction and cementation occur, and the better the reservoir physical property is.
Objective This study clarified the origin of the sandstone and conglomerate reservoir in the Third member of the Shahejie Formation of the Chezhen Sag and explored the influence of the intensity and occurrence time of overpressure on the development of high-quality reservoirs. Methods Based on restoring the formation pressure of sandy-conglomerate reservoirs in the Third member of the Shahejie Formation in the steep slope belt of Chezhen Sag using the inclusion PVTx method, the logging curve combination and acoustic velocity-density cross-plot methods were used to analyze the origin of overpressure in mudstones. Then, the occurrence of overpressure and oil saturation were synthetically used to analyze the origin of overpressure in reservoirs and compare the differences in reservoir space and diagenesis among different reservoirs with inconsistent histories of pressure evolution. Results The mudstones in the Third member of the Shahejie Formation in the steep slope of the Chezhen Sag have developed medium-strong overpressure caused by hydrocarbon generation pressurization. The overpressures, undergoing two periods of supercharging, in the conglomerate reservoirs are caused by the pressure transition from the overpressure in the source rocks from the Third member of the Shahejie Formation. The first and second phases correspond to the oil emplacements in yellow and blue fluorescence, respectively. However, there were differences in the various regions. The sandstones and conglomerates in the Third member of the Shahejie Formation in the steep slope belt of the Dawangbei Sag underwent two periods of pressurization. The first period occurred at 28.3-27.8 Ma, and the coeval formation pressure coefficient was 1.34-1.35. The second period occurred at 11.6-9.0 Ma, and the coeval formation pressure coefficient was 1.56. The current formation pressure coefficient is 0.8-1.2. The conglomerate reservoirs in the Third member of the Shahejie Formation in the steep slope belt of the Chezhen Sag experienced only one period of pressurization, occurring from 6 Ma to the present, with a coeval formation pressure coefficient of 1.2-1.4. The present-day formation pressure coefficient is 1.0-1.3. The sandstones and conglomerates in the Third member of the Shahejie Formation in the steep slope of the Taoerhe Sag experienced two periods of pressurization. The first period occurred at 35.7-27.2 Ma, and the formation pressure coefficient was 1.54-2.08, while the second period occurred from 12.9 Ma to the present, with formation pressure coefficient of 1.57-2.10. The current formation pressure coefficient is 1.2-2.1 The weaker compaction and cementation, increased number of primary pores, and improved physical properties can be identified in the conglomerate reservoirs in the steep slope belt of Dawangbei Sag with normal pressure-weak overpressure rather than in the Chezhen Sag with normal pressure-medium overpressure. The main reason is that two periods of paleo-overpressure with higher intensity occurred in the Dawangbei Sag, while only one period of paleo-overpressure with relatively low intensity occurred in the Chezhen Sag. Conclusions The development of high-quality clastic rock reservoirs in the middle and deep layers are not only controlled by the overpressure strength in the current formations, but also closely related to the occurrence and strength of ancient overpressure. The earlier and stronger overpressure develops, the more significant inhibitions of compaction and cementation occur, and the better the reservoir physical property is.
2025,
43(1):
254-272.
doi: 10.14027/j.issn.1000-0550.2022.157
Abstract:
Objective The architecture of an alluvial fan affects the development of high quality reservoirs； therefore, it is of great significance for clarifying the architecture units and development pattern of an alluvial fan and predicting the distribution of high quality reservoir. Methods Taking a case study of a modern Poplar River alluvial fan in the northwestern margin of the Junggar Basin, based on a large amount of outcrop data, and on the basis of clarifying the main lithology, lithofacies types and characteristics, the internal different-level reservoir architectures of the fan body are studied in accordance with the reservoir architecture theory. Results The Poplar River alluvial fan was divided into 6th to 9th units of architectures. The 6th units of architectures are composite fans, which are formed by the superposition of the 7th architecture units. The 7th architecture units consist of the microfacies （8th architecture units） formed by single deposition in the alluvial fan. From the proximal to the distal fan, incised channel deposits, sheet-like deposits, unconfined braided streams deposits, and narrow with shallow channels deposits cover the high flow stage. The braided channels with channel bars cover the low flow stage. The 8th architecture units are composed of different types of 9th architecture units, which are supercritical sheet-like flow hyperplasia, unconfined subcritical streamflow hyperplasia, etc. The sediment during the low flow stage was transformed and overlaid by the braided channel in the high flow stage, the spatiotemporal variability in the occurrence of these flow processes controls the types and spatial distribution of different types of architecture units. On this basis, the types and distribution of high quality reservoirs in Poplar River alluvial fan were studied, and the high quality reservoir of the gravely braided-river alluvial fan can be divided into two types： open-framework gravels and clean sandstone, whose distribution is controlled by the 9th architecture units. The open-framework gravels（OFGs） can be subdivided into seven types： OFGs associated with supercritical sheet-like flow （type 1）, OFGs associated with unconfined subcritical stream flows （type 2）, OFGs associated with groundwater-fed channels （type 3）, OFGs associated with trough cross-stratification （type 4）, OFGs associated with sigmoidal cross-stratification （type 5）, OFGs associated with gullies （type 6,） and OFGs associated with aeolian activity （type 7）. Based on the genesis of sandstone, clean sandstone reservoirs can be divided into two types： channel clean sandstone and planar pin-stripe clean sands. Channel sandstones can be subdivided into parallel-bedded sands, trough cross-bedded sands in braided channels and braided channel bars, and ripple cross-laminated sands. Conclusions Through the above research, the spatial distribution model of the 8th architecture units was established： open-framework gravels reservoirs in the study area are distributed in the sheet-like deposits and the braided channels of the outer of the proximal and medial fans. The high-quality sandstone reservoirs are distributed in the braided channels of the outer medial fan. Based on the distribution of high quality reservoirs in the gravely braided-river alluvial fan, the main exploration area in the gravely braided-river alluvial fan should be the sheet-like deposits and the braided channels of the medial fan.
Objective The architecture of an alluvial fan affects the development of high quality reservoirs； therefore, it is of great significance for clarifying the architecture units and development pattern of an alluvial fan and predicting the distribution of high quality reservoir. Methods Taking a case study of a modern Poplar River alluvial fan in the northwestern margin of the Junggar Basin, based on a large amount of outcrop data, and on the basis of clarifying the main lithology, lithofacies types and characteristics, the internal different-level reservoir architectures of the fan body are studied in accordance with the reservoir architecture theory. Results The Poplar River alluvial fan was divided into 6th to 9th units of architectures. The 6th units of architectures are composite fans, which are formed by the superposition of the 7th architecture units. The 7th architecture units consist of the microfacies （8th architecture units） formed by single deposition in the alluvial fan. From the proximal to the distal fan, incised channel deposits, sheet-like deposits, unconfined braided streams deposits, and narrow with shallow channels deposits cover the high flow stage. The braided channels with channel bars cover the low flow stage. The 8th architecture units are composed of different types of 9th architecture units, which are supercritical sheet-like flow hyperplasia, unconfined subcritical streamflow hyperplasia, etc. The sediment during the low flow stage was transformed and overlaid by the braided channel in the high flow stage, the spatiotemporal variability in the occurrence of these flow processes controls the types and spatial distribution of different types of architecture units. On this basis, the types and distribution of high quality reservoirs in Poplar River alluvial fan were studied, and the high quality reservoir of the gravely braided-river alluvial fan can be divided into two types： open-framework gravels and clean sandstone, whose distribution is controlled by the 9th architecture units. The open-framework gravels（OFGs） can be subdivided into seven types： OFGs associated with supercritical sheet-like flow （type 1）, OFGs associated with unconfined subcritical stream flows （type 2）, OFGs associated with groundwater-fed channels （type 3）, OFGs associated with trough cross-stratification （type 4）, OFGs associated with sigmoidal cross-stratification （type 5）, OFGs associated with gullies （type 6,） and OFGs associated with aeolian activity （type 7）. Based on the genesis of sandstone, clean sandstone reservoirs can be divided into two types： channel clean sandstone and planar pin-stripe clean sands. Channel sandstones can be subdivided into parallel-bedded sands, trough cross-bedded sands in braided channels and braided channel bars, and ripple cross-laminated sands. Conclusions Through the above research, the spatial distribution model of the 8th architecture units was established： open-framework gravels reservoirs in the study area are distributed in the sheet-like deposits and the braided channels of the outer of the proximal and medial fans. The high-quality sandstone reservoirs are distributed in the braided channels of the outer medial fan. Based on the distribution of high quality reservoirs in the gravely braided-river alluvial fan, the main exploration area in the gravely braided-river alluvial fan should be the sheet-like deposits and the braided channels of the medial fan.
2025,
43(1):
273-287.
doi: 10.14027/j.issn.1000-0550.2022.150
Abstract:
Objective Clinoform growth is found in lacustrine or marine basins, and its growth mode reveals the changes of sediment supply, lake level, and deep-water sand delivery pattern. The typical clinoform growth styles are identified in the middle-upper Yanchang Formation based on newly obtained three-dimensional （3D） seismic data in the southwestern Ordos Basin. This discovery has changed the traditional understanding of the “flat-filling” deposition of the Yanchang Formation, provided a new idea for analyzing the distribution of sandstone and reservoir prediction from delta to deep-water area. Methods Based on the 3D seismic and drilling data from the Qingcheng area in the southwestern Ordos Basin, the middle-upper Yanchang Formation can be divided into six clinothems named F1⁃F6 respectively, guided by the clinoform growth theory. The characteristics of clinothems F1⁃F4 were described quantitatively, with relatively complete slope form. Results First, the results show that the four clinothems have similar morphological characteristics based on the representative seismic profiles in the study area. All four clinothems have lenticular external morphology and S-shaped internal reflection structure. Second, the sedimentary time span of one clinothem is approximately 4 to 6 million years. Third, clinothems F1⁃F4 can be quantitatively divided into two types, “thick-topset and thin-foreset” and “thin-topset and thick-foreset” based on the strata thickness. Clinothem F1 belongs to the former type, while the other three clinothems belong to the latter type. Fourth, two types of shoreline trajectories were identified, including flat of 0°⁃0.5° and slightly rising of 0.5°⁃1.0°. The trajectory angle of F2 is 0.366°⁃0.489° and the angle of F3 is 0.256°⁃0.275°. While the trajectory angle of F4 is 0.475°⁃0.673°, which has a slight increase compared with the previous period of F2 and F3. Moreover, the shoreline trajectories of F2 and F3 are flat, while the shoreline trajectories of F4 are slightly rising. Conclusions The lacustrine basin reached the largest extent in the F1 period, and aggradation dominated clinothems were formed with the sandstone concentrated in the topset within poor sediment. Owing to the poor sediment supply in the F1 period, the thickness of F1 is thinner than the other three clinothems. As the enhancement of sediment supply and the overall decline of lake level, progradation dominated clinothems were formed during the F2⁃F3 period. The clinothems of F2 and F3 are both progradation dominated clinothems with abundant sandstone in the deep water, which have flat shoreline trajectories. In the F4 period, the clinothems were controlled by aggradation-progradation. Clinothem F4 has slightly rising shoreline trajectories because of the rising lake level, and sandstone is abundant in the topset and deep-water area. We concluded that the sediment supply and lake level change are the main controlling factors of clinoform growth and distribution of sandstone in the study area. With the gradual abundance of sediment supply and the overall regression of lake level, the type of clinothems changed from aggradation dominated to progradation dominated and the sandstone in deep lake area gradually enriched.
Objective Clinoform growth is found in lacustrine or marine basins, and its growth mode reveals the changes of sediment supply, lake level, and deep-water sand delivery pattern. The typical clinoform growth styles are identified in the middle-upper Yanchang Formation based on newly obtained three-dimensional （3D） seismic data in the southwestern Ordos Basin. This discovery has changed the traditional understanding of the “flat-filling” deposition of the Yanchang Formation, provided a new idea for analyzing the distribution of sandstone and reservoir prediction from delta to deep-water area. Methods Based on the 3D seismic and drilling data from the Qingcheng area in the southwestern Ordos Basin, the middle-upper Yanchang Formation can be divided into six clinothems named F1⁃F6 respectively, guided by the clinoform growth theory. The characteristics of clinothems F1⁃F4 were described quantitatively, with relatively complete slope form. Results First, the results show that the four clinothems have similar morphological characteristics based on the representative seismic profiles in the study area. All four clinothems have lenticular external morphology and S-shaped internal reflection structure. Second, the sedimentary time span of one clinothem is approximately 4 to 6 million years. Third, clinothems F1⁃F4 can be quantitatively divided into two types, “thick-topset and thin-foreset” and “thin-topset and thick-foreset” based on the strata thickness. Clinothem F1 belongs to the former type, while the other three clinothems belong to the latter type. Fourth, two types of shoreline trajectories were identified, including flat of 0°⁃0.5° and slightly rising of 0.5°⁃1.0°. The trajectory angle of F2 is 0.366°⁃0.489° and the angle of F3 is 0.256°⁃0.275°. While the trajectory angle of F4 is 0.475°⁃0.673°, which has a slight increase compared with the previous period of F2 and F3. Moreover, the shoreline trajectories of F2 and F3 are flat, while the shoreline trajectories of F4 are slightly rising. Conclusions The lacustrine basin reached the largest extent in the F1 period, and aggradation dominated clinothems were formed with the sandstone concentrated in the topset within poor sediment. Owing to the poor sediment supply in the F1 period, the thickness of F1 is thinner than the other three clinothems. As the enhancement of sediment supply and the overall decline of lake level, progradation dominated clinothems were formed during the F2⁃F3 period. The clinothems of F2 and F3 are both progradation dominated clinothems with abundant sandstone in the deep water, which have flat shoreline trajectories. In the F4 period, the clinothems were controlled by aggradation-progradation. Clinothem F4 has slightly rising shoreline trajectories because of the rising lake level, and sandstone is abundant in the topset and deep-water area. We concluded that the sediment supply and lake level change are the main controlling factors of clinoform growth and distribution of sandstone in the study area. With the gradual abundance of sediment supply and the overall regression of lake level, the type of clinothems changed from aggradation dominated to progradation dominated and the sandstone in deep lake area gradually enriched.
2025,
43(1):
288-300.
doi: 10.14027/j.issn.1000-0550.2022.160
Abstract:
Objective In this study, we clarified the difference and development rule of clastic reservoirs in the middle and deep layers of the No. 4 structural belt in the Nanpu Sag and explored the key factors affecting the quality of middle and deep reservoirs. Methods We used scanning electron microscopy, casting thin section, mercury, injection analysis and other techniques, combined with the empirical calculation formula of diagenetic strength. Reservoir petrology, porosity and permeability, pore structure, and the reason of reservoir quality difference were analyzed in a stratified system. Results The study shows that the sandstones of different strata are similar in lithology, mainly composed of lithic feldspar sandstone and feldspar lithic sandstone, but the reservoir quality obviously differs between layers, showing the "abnormal" phenomenon that the reservoir quality of the First member of the Shahejie Formation （Es1） ,which is the underlying member, improves with depth. And the Second member of Dongying Formation （Ed2） and the Third member of Dongying Formation （Ed3） decreases rapidly with the increase of burial depth. From Es1 to Ed2, the average porosity and permeability increased from16.61% and 9.58×10-3 μm2 to 12.96% and 3.39×10-3 μm2, respectively. Based on the analysis of the high-pressure mercury injection experiment, the study area can be divided into three types of mercury injection curves. The proportions of types Ⅰ, Ⅱ, and Ⅲ in Es1 were 43.75%, 25%, and 31.25%, respectively. The types of mercury injection curves in Ed3 were 28.57%, 64.29%, and 7.14%, respectively. The types of mercury injection curves in Ed2 accounted for 36.36%, 45.45%, and 18.18%, respectively. In general, Es1 has both type Ⅰ and high-quality pore-throat and type Ⅲ with small pore-throat radius, and its physical properties are better than those of other strata. Furthermore, through the calculation of the difference of diagenetic strength, differences are discovered in the diagenesis of different reservoirs. The compaction intensity of Es1 is the weakest （COPL-P = 38.13%）, and the dissolution intensity is the strongest （CRPI-P = 46.47%）. The reservoirs are well reformed and have good physical properties. Ed3 has the strongest compaction （COPL-P = 53.30%）, the weakest dissolution （CRPI-P = 26.03%）, and poor physical properties. Conclusions The difference of sedimentary facies and diagenesis under the control of tectonic evolution and burial is the fundamental reason for the "anomaly" of reservoir quality layers. From Es1 to Ed3 and Ed2, the rifting activity was enhanced, topography steepened, meander river delta evolved into braided river delta, reservoir grain size became coarser, sandstone composition and structure maturity was reduced, sandstone compressive ability decreased, and compaction porosity reduction rate increased, resulting in better preservation of primary pores in Es1. In addition, the rate of dissolution and porosity increase at depth, and high pressure of the formation further increased the interlayer "anomaly" of reservoir quality.
Objective In this study, we clarified the difference and development rule of clastic reservoirs in the middle and deep layers of the No. 4 structural belt in the Nanpu Sag and explored the key factors affecting the quality of middle and deep reservoirs. Methods We used scanning electron microscopy, casting thin section, mercury, injection analysis and other techniques, combined with the empirical calculation formula of diagenetic strength. Reservoir petrology, porosity and permeability, pore structure, and the reason of reservoir quality difference were analyzed in a stratified system. Results The study shows that the sandstones of different strata are similar in lithology, mainly composed of lithic feldspar sandstone and feldspar lithic sandstone, but the reservoir quality obviously differs between layers, showing the "abnormal" phenomenon that the reservoir quality of the First member of the Shahejie Formation （Es1） ,which is the underlying member, improves with depth. And the Second member of Dongying Formation （Ed2） and the Third member of Dongying Formation （Ed3） decreases rapidly with the increase of burial depth. From Es1 to Ed2, the average porosity and permeability increased from16.61% and 9.58×10-3 μm2 to 12.96% and 3.39×10-3 μm2, respectively. Based on the analysis of the high-pressure mercury injection experiment, the study area can be divided into three types of mercury injection curves. The proportions of types Ⅰ, Ⅱ, and Ⅲ in Es1 were 43.75%, 25%, and 31.25%, respectively. The types of mercury injection curves in Ed3 were 28.57%, 64.29%, and 7.14%, respectively. The types of mercury injection curves in Ed2 accounted for 36.36%, 45.45%, and 18.18%, respectively. In general, Es1 has both type Ⅰ and high-quality pore-throat and type Ⅲ with small pore-throat radius, and its physical properties are better than those of other strata. Furthermore, through the calculation of the difference of diagenetic strength, differences are discovered in the diagenesis of different reservoirs. The compaction intensity of Es1 is the weakest （COPL-P = 38.13%）, and the dissolution intensity is the strongest （CRPI-P = 46.47%）. The reservoirs are well reformed and have good physical properties. Ed3 has the strongest compaction （COPL-P = 53.30%）, the weakest dissolution （CRPI-P = 26.03%）, and poor physical properties. Conclusions The difference of sedimentary facies and diagenesis under the control of tectonic evolution and burial is the fundamental reason for the "anomaly" of reservoir quality layers. From Es1 to Ed3 and Ed2, the rifting activity was enhanced, topography steepened, meander river delta evolved into braided river delta, reservoir grain size became coarser, sandstone composition and structure maturity was reduced, sandstone compressive ability decreased, and compaction porosity reduction rate increased, resulting in better preservation of primary pores in Es1. In addition, the rate of dissolution and porosity increase at depth, and high pressure of the formation further increased the interlayer "anomaly" of reservoir quality.
2025,
43(1):
301-313.
doi: 10.14027/j.issn.1000-0550.2023.002
Abstract:
Objective Thin-layered shale and shell limestone are beneficial for determining the properties of the shale-oil reservoir of the Dongyuemiao member of the Ziliujing Formation （Lower Jurassic） in the northern Sichuan Basin. It is found that the shale and shell limestone have developed various types of fractures, calcite veins, and dissolution. Our aim is to decipher the burial and diagenetic sequence of the shell limestone interlayers, and discuss the relationship between hydrocarbon accumulation and the formation of calcite and reservoir densification. Methods In our study we use detailed core observations, microscopic section observations, and the electron probe composition, trace elements, and paragenetic minerals are determined. Results The results show that seven types of calcite occur in the organic-rich shale of the Dongyuemiao member according to their morphology, staining, and occurrence condition, such as： （1） recrystallized fine-medium crystalline calcite （C1） in the shell limestone； （2） noddle-lenticular fibrous calcite along the edge of the shell limestone （C2）； （3） micro-sparry calcite, which occurs along the edge of shell limestone （C3）； （4） fibrous calcite in the shell mudstone （C4）； （5） microcrystalline granular calcite in the argillaceous shell limestone （C5）； （6） microcrystalline granular calcite in the muddy shell limestone （C6）； and sparry calcite in pores and/ or fractures （C7）. Besides, the microfractures in the shell-limestone are deve-loped, which can be obviously divided into F1 （with low opening degree and reluctantly cut through the shell）, F2 （with curved fracture surface and occurred associated with fibrous calcite）, and F3 （obviously break the shell）. The formation sequence of minerals such as fractures and calcite is： C1+pyrite → F1 → F2→ C2/C4 → C3→ quartz+ kaolinite → F3→ C5/C6+ asphalt → C7. The major elements in the calcite and the division of phases indicate that C1 to C3 calcite, which has a low content of FeO and a high content of SrO, may have formed during an early diagenetic stage. The contact relationship between C2 and interlayer fracture （F2） caused by fluid overpressure indicates that calcite C2 belongs to the first phase of fluid overpressure cement. Calcite C3 is mainly derived from the dissolution and reprecipitation of C1 and C2, and is formed during second phase cementation. Calcite C4 has mainly developed at the edge of the shell mudstone, and the incomplete fibrous crystal morphology and obvious dissolution at the edges imply that C4 should be the product of ion exchange and recrystallization of calcite from the first phase of fluid overpressure. Furthermore, Calcite C5, C6, and C7 have high contents of MnO and FeO and a low content of SrO, which we infer to be the products of middle and late stage diagenesis and belong to fourth stage cementation. Precipitation models of various types of calcite are elucidated by the integration of the regional burial history, diagenetic evolution sequence, and tectonic-thermal evolution history, and four models are pointed out： （1） recrystallization of shell sediments； （2） fibrous calcite formed during stage B of early diagenesis by hydrocarbon generation and expulsion from organic rich shale； （3） calcite formed during stage A of middle diagenesis by dissolution and reprecipitation； and （4） block-sparry calcite formed during stage B of middle diagenesis. The element contents of Fe, Mn, and Sr in the calcite are closely related to the formation time and stage. Conclusions Multi-cementation of calcite is crucial to the densification of limestone reservoirs. Fibrous calcite veins triggered by fluid overpressure led to densification as well as providing a foundation for late dissolution. However, the precipitation of calcite during middle diagenesis eventually decreased the amount of pores. Overall, the precipitation of various types of calcite constrained the distribution of shale-oil in the densified reservoir.
Objective Thin-layered shale and shell limestone are beneficial for determining the properties of the shale-oil reservoir of the Dongyuemiao member of the Ziliujing Formation （Lower Jurassic） in the northern Sichuan Basin. It is found that the shale and shell limestone have developed various types of fractures, calcite veins, and dissolution. Our aim is to decipher the burial and diagenetic sequence of the shell limestone interlayers, and discuss the relationship between hydrocarbon accumulation and the formation of calcite and reservoir densification. Methods In our study we use detailed core observations, microscopic section observations, and the electron probe composition, trace elements, and paragenetic minerals are determined. Results The results show that seven types of calcite occur in the organic-rich shale of the Dongyuemiao member according to their morphology, staining, and occurrence condition, such as： （1） recrystallized fine-medium crystalline calcite （C1） in the shell limestone； （2） noddle-lenticular fibrous calcite along the edge of the shell limestone （C2）； （3） micro-sparry calcite, which occurs along the edge of shell limestone （C3）； （4） fibrous calcite in the shell mudstone （C4）； （5） microcrystalline granular calcite in the argillaceous shell limestone （C5）； （6） microcrystalline granular calcite in the muddy shell limestone （C6）； and sparry calcite in pores and/ or fractures （C7）. Besides, the microfractures in the shell-limestone are deve-loped, which can be obviously divided into F1 （with low opening degree and reluctantly cut through the shell）, F2 （with curved fracture surface and occurred associated with fibrous calcite）, and F3 （obviously break the shell）. The formation sequence of minerals such as fractures and calcite is： C1+pyrite → F1 → F2→ C2/C4 → C3→ quartz+ kaolinite → F3→ C5/C6+ asphalt → C7. The major elements in the calcite and the division of phases indicate that C1 to C3 calcite, which has a low content of FeO and a high content of SrO, may have formed during an early diagenetic stage. The contact relationship between C2 and interlayer fracture （F2） caused by fluid overpressure indicates that calcite C2 belongs to the first phase of fluid overpressure cement. Calcite C3 is mainly derived from the dissolution and reprecipitation of C1 and C2, and is formed during second phase cementation. Calcite C4 has mainly developed at the edge of the shell mudstone, and the incomplete fibrous crystal morphology and obvious dissolution at the edges imply that C4 should be the product of ion exchange and recrystallization of calcite from the first phase of fluid overpressure. Furthermore, Calcite C5, C6, and C7 have high contents of MnO and FeO and a low content of SrO, which we infer to be the products of middle and late stage diagenesis and belong to fourth stage cementation. Precipitation models of various types of calcite are elucidated by the integration of the regional burial history, diagenetic evolution sequence, and tectonic-thermal evolution history, and four models are pointed out： （1） recrystallization of shell sediments； （2） fibrous calcite formed during stage B of early diagenesis by hydrocarbon generation and expulsion from organic rich shale； （3） calcite formed during stage A of middle diagenesis by dissolution and reprecipitation； and （4） block-sparry calcite formed during stage B of middle diagenesis. The element contents of Fe, Mn, and Sr in the calcite are closely related to the formation time and stage. Conclusions Multi-cementation of calcite is crucial to the densification of limestone reservoirs. Fibrous calcite veins triggered by fluid overpressure led to densification as well as providing a foundation for late dissolution. However, the precipitation of calcite during middle diagenesis eventually decreased the amount of pores. Overall, the precipitation of various types of calcite constrained the distribution of shale-oil in the densified reservoir.
2025,
43(1):
314-334.
doi: 10.14027/j.issn.1000-0550.2023.056
Abstract:
Objective The Lower Paleozoic Qiongzhusi Formation and Longmaxi Formation are the key strata for shale gas exploration and development in southern China. The organic matter content in the shale is a key index for determining the degree of gas enrichment. However, there has been a lack of systematic comparative study on the environmental control factors of organic matter enrichment in the two shales. Methods Using core/outcrop, X-ray diffraction, thin section, organic matter content, and element analysis data, the sedimentological and geochemical changes of the two shale layers were compared and analyzed. The differences in the sedimentary paleoenvironment between the two layers for similar geological backgrounds were systematically discussed, and the sedimentary paleoenvironment changes from passive continental margin to cratonic depression were compared and analyzed. The main controlling factors and differences of organic matter enrichment in two sets shale formations were clarified, and the corresponding enrichment models were established. Results （1） Vertically, the bottoms of the Qiongzhusi Formation（Q1 member） and Longmaxi Formation （LM1-LM5） are dominated by organic rich bio siliceous rock siliceous shale deposits, and the total organic carbon （TOC） content of the former is higher than that of the latter. At the top, with the continuous decline of sea level, TOC and biogenic silicon content showed a downward trend, whereas clay or carbonate rock mineral content exhibited an upward trend. Horizontally, under the control of paleogeomorphology, TOC and biogenic silicon content decreased from passive continental margin to cratonic depression in different sedimentary periods. （2） The high TOC shale sections at the bottom of both shales were formed in environments with low terrestrial input, high paleoproductivity, and good preservation conditions. Moreover, the paleoproductivity level and bottom water reduction degree of the Q1 member were higher than those of LM1-LM5, resulting in higher TOC content. With the continuous decline of sea level, both shales exhibited increasing terrigenous input and worsening preservation conditions, but the changes of paleoproductivity are significantly different： in the Qiongzhusi period, ocean current activity weakened with the decline of sea level, and the level of paleoproductivity decreased. In the Longmaxi period, the flexural-migration led to the gradual opening of the northern barrier in the Yangtze Basin, and the scale of ocean current influx gradually expanded. Weakly affected by sea level decline, the level of paleoproductivity increased. Vertically, the TOC content of the Qiongzhusi Formation was primarily controlled by the reduction of paleoproductivity, deterioration of preservation conditions, and increase of terrestrial input, whereas the Longmaxi Formation was controlled by the latter two factors. Horizontally, the two shales transitioned from the passive continental margin to the cratonic depression, and the sea level height and upwelling current activity decreased. The preservation conditions and paleoproductivity level became worse at the same time, and the TOC content decreased accordingly. （3） Controlled by the sedimentary paleogeographic background, the exploration and development of shale gas in the Qiongzhusi Formation should focus on the southwest, whereas Longmaxi Formation should focus on the northwest. The organic matter enrichment environment gradually improved in the southwest direction during the Qiongzhusi stage, and the thickness of the organic matter rich shale increased near the tensile rift center, whereas the organic matter enrichment environment became better in the northwest direction during the the Longmaxi stage, and the sedimentary center gradually migrated to the northwest due to foreland flexus-migration, and the thickness of the organic matter rich shale increased. Conclusions This study can provide a reference for further exploration of the accumulation mechanism for organic matter in Lower Paleozoic Marine shales, guiding shale gas exploration.
Objective The Lower Paleozoic Qiongzhusi Formation and Longmaxi Formation are the key strata for shale gas exploration and development in southern China. The organic matter content in the shale is a key index for determining the degree of gas enrichment. However, there has been a lack of systematic comparative study on the environmental control factors of organic matter enrichment in the two shales. Methods Using core/outcrop, X-ray diffraction, thin section, organic matter content, and element analysis data, the sedimentological and geochemical changes of the two shale layers were compared and analyzed. The differences in the sedimentary paleoenvironment between the two layers for similar geological backgrounds were systematically discussed, and the sedimentary paleoenvironment changes from passive continental margin to cratonic depression were compared and analyzed. The main controlling factors and differences of organic matter enrichment in two sets shale formations were clarified, and the corresponding enrichment models were established. Results （1） Vertically, the bottoms of the Qiongzhusi Formation（Q1 member） and Longmaxi Formation （LM1-LM5） are dominated by organic rich bio siliceous rock siliceous shale deposits, and the total organic carbon （TOC） content of the former is higher than that of the latter. At the top, with the continuous decline of sea level, TOC and biogenic silicon content showed a downward trend, whereas clay or carbonate rock mineral content exhibited an upward trend. Horizontally, under the control of paleogeomorphology, TOC and biogenic silicon content decreased from passive continental margin to cratonic depression in different sedimentary periods. （2） The high TOC shale sections at the bottom of both shales were formed in environments with low terrestrial input, high paleoproductivity, and good preservation conditions. Moreover, the paleoproductivity level and bottom water reduction degree of the Q1 member were higher than those of LM1-LM5, resulting in higher TOC content. With the continuous decline of sea level, both shales exhibited increasing terrigenous input and worsening preservation conditions, but the changes of paleoproductivity are significantly different： in the Qiongzhusi period, ocean current activity weakened with the decline of sea level, and the level of paleoproductivity decreased. In the Longmaxi period, the flexural-migration led to the gradual opening of the northern barrier in the Yangtze Basin, and the scale of ocean current influx gradually expanded. Weakly affected by sea level decline, the level of paleoproductivity increased. Vertically, the TOC content of the Qiongzhusi Formation was primarily controlled by the reduction of paleoproductivity, deterioration of preservation conditions, and increase of terrestrial input, whereas the Longmaxi Formation was controlled by the latter two factors. Horizontally, the two shales transitioned from the passive continental margin to the cratonic depression, and the sea level height and upwelling current activity decreased. The preservation conditions and paleoproductivity level became worse at the same time, and the TOC content decreased accordingly. （3） Controlled by the sedimentary paleogeographic background, the exploration and development of shale gas in the Qiongzhusi Formation should focus on the southwest, whereas Longmaxi Formation should focus on the northwest. The organic matter enrichment environment gradually improved in the southwest direction during the Qiongzhusi stage, and the thickness of the organic matter rich shale increased near the tensile rift center, whereas the organic matter enrichment environment became better in the northwest direction during the the Longmaxi stage, and the sedimentary center gradually migrated to the northwest due to foreland flexus-migration, and the thickness of the organic matter rich shale increased. Conclusions This study can provide a reference for further exploration of the accumulation mechanism for organic matter in Lower Paleozoic Marine shales, guiding shale gas exploration.
2025,
43(1):
335-348.
doi: 10.14027/j.issn.1000-0550.2023.001
Abstract:
Objective The origin of the micro-nano quartz in Qiongzhusi black shale, the source of the silica therein, and the cause of its differential development in the southwest Upper Yangtze Platform are still unclear. Methods The 36 black shale samples were collected from the bottom of the Qiongzhusi Formation at different sections in the study area. The bulk mineralogy was determined by X-ray diffraction, and the lithology, bulk rock mineral composition, and microstructure of the black shale were determined by light microscopy and field-emission scanning electron microscopy （FE-SEM）. The cathodoluminescence features of the minerals in the shale were determined by SEM-based cathodoluminescence imaging （SEM-CL）. The total organic content was determined by the combustion method. Results The Qiongzhusi black shale in the study area is lithologically variable, consisting of mainly siltstone in Majinzi and well Z1 and mainly mudstone in Songlin. The micro-nano quartz in the Qiongzhusi black shale, which is generally several hundred nanometers to 2 μm （a few are more than 2 μm） in diameter, has an idiomorphic to allomorphic shape, and is low- to non-luminescent. The micro-nano quartz crystals in the shale are either intergrown in clusters or isolated. The content of micro-nano quartz in the black shale varies vertically and from region to region. In general, the quartz content is higher in the black shale in the east （Songlin） than that in the west （Majinzi, well Z1）, and in the lower section of the black shale than in the upper section. Conclusions The micro-nano quartz is authigenic, its silica comes from multiple sources, and the large number of intergrown micro-nano quartz crystals in the black shale may be mainly from the reprecipitation of hydrothermal silica. The content of micro-nano quartz in the Qiongzhusi black shale in the study area is indicative of upwelling： a relatively high content of micro-nano quartz indicates an upwelling sedimentary environment or sedimentary environment in which there have been hydrothermal activities, while a low content of micro-nano quartz may indicate a euxinic and/or anoxic sedimentary environment （or a sedimentary environment that was weakly affected by upwelling）.
Objective The origin of the micro-nano quartz in Qiongzhusi black shale, the source of the silica therein, and the cause of its differential development in the southwest Upper Yangtze Platform are still unclear. Methods The 36 black shale samples were collected from the bottom of the Qiongzhusi Formation at different sections in the study area. The bulk mineralogy was determined by X-ray diffraction, and the lithology, bulk rock mineral composition, and microstructure of the black shale were determined by light microscopy and field-emission scanning electron microscopy （FE-SEM）. The cathodoluminescence features of the minerals in the shale were determined by SEM-based cathodoluminescence imaging （SEM-CL）. The total organic content was determined by the combustion method. Results The Qiongzhusi black shale in the study area is lithologically variable, consisting of mainly siltstone in Majinzi and well Z1 and mainly mudstone in Songlin. The micro-nano quartz in the Qiongzhusi black shale, which is generally several hundred nanometers to 2 μm （a few are more than 2 μm） in diameter, has an idiomorphic to allomorphic shape, and is low- to non-luminescent. The micro-nano quartz crystals in the shale are either intergrown in clusters or isolated. The content of micro-nano quartz in the black shale varies vertically and from region to region. In general, the quartz content is higher in the black shale in the east （Songlin） than that in the west （Majinzi, well Z1）, and in the lower section of the black shale than in the upper section. Conclusions The micro-nano quartz is authigenic, its silica comes from multiple sources, and the large number of intergrown micro-nano quartz crystals in the black shale may be mainly from the reprecipitation of hydrothermal silica. The content of micro-nano quartz in the Qiongzhusi black shale in the study area is indicative of upwelling： a relatively high content of micro-nano quartz indicates an upwelling sedimentary environment or sedimentary environment in which there have been hydrothermal activities, while a low content of micro-nano quartz may indicate a euxinic and/or anoxic sedimentary environment （or a sedimentary environment that was weakly affected by upwelling）.
2025,
43(1):
349-360.
doi: 10.14027/j.issn.1000-0550.2022.159
Abstract:
Objective The extremely low grade metamorphism is in the evolution stage of petroleum, wet gas, dry gas and peat, bituminous coal, and anthracite and is closely related to paleotemperature. Therefore, the study of extremely low grade metamorphism and paleotemperature is of great practical significance for the exploration of energy resources such as oil, gas, and coal. Compared with the Longmaxi Formation shale gas, the study of which has led to exploration breakthroughs, the lower Cambrian Qiongzhusi Formation shale has higher organic matter abundance and thermal evolution in the Upper Yangtze region. Furthermore, the Qiongzhusi Formation has a larger sedimentary thickness and wider distribution area compared with the Longmaxi Formation； therefore, it has huge shale gas exploration potential. It is another key shale gas exploration and development layer in China. At present, there has been no systematic study on the extremely low grade metamorphism and paleotemperature of this high maturity source rock in the Qionghzhusi Formation. It is necessary to reveal the technical parameters such as paleotemperature and its influence on oil and gas accumulation. Methods Based on profile measurements, a study on the ultralow grade metamorphism and paleotemperature of Qiongzhusi Formation in northeast Yunnan was conducted for the first time with the mineral structure order degree, crystallinity degree of illite, and vitrinite reflectance of source rocks. Results （1） The illite content in the illite/Mongolian mixed layer is 95%. The crystallinity of illite in authigenic illite and Illite/Mengzi mixed layer minerals ranges from 0.24 to 0.35. The vitreous reflectance （Ro） of source rocks calculated by bitumen reflectance is between 2.22%⁃2.65%, lower than that of the southeastern margin of the Yangtze region （the vitrinite reflectance, Ro, in this area was calculated by the asphalt reflectance between 3.18%⁃3.43%） and the upper limit of maturity for marine kerogen gas generation （vitrinite reflectance Ro 3%）； （2） The maximum palaeotemperature of the Qiongzhusi Formation in northeastern Yunnan ranged from 200 to 227.57 ℃, lower than that of the southeastern margin of the Yangtze River （211 ℃⁃246 ℃） and the maximum hydrocarbon death temperature （230 ℃⁃300 ℃）. The paleotemperature gradient from the Early Cretaceous to Neogene maximum burial depth ranged from 2.53⁃2.91 ℃ / 100 m, close to the southeastern margin of the Yangtze. （3） Based on the comprehensive classification standard and scheme of metamorphism, the Qiongzhusi Formation in northeastern Yunnan belonged to lower metamorphic zone A⁃B, near metamorphic zone, and lower near metamorphic zone to high near metamorphic zone, and belonged to the Vitiite-chlorospar facies. The Qiongzhusi stage in northeastern Yunnan province was in the favorable transition from sedimentary sag to uplift. Conclusions The Qiongzhusi Formation experienced a lower highest paleotemperature and maturity. In addition, the maturity of organic matter in the Qiongzhusi Formation in northeastern Yunnan province was lower than that in other parts of the Yangtze region, with hydrocarbon formation conditions and material basis. At present, no paleooil reservoir has been found, and the structural preservation conditions were good. The oil and gas prospect of the Qiongzhusi Formation in northeastern Yunnan should be given significant attention.
Objective The extremely low grade metamorphism is in the evolution stage of petroleum, wet gas, dry gas and peat, bituminous coal, and anthracite and is closely related to paleotemperature. Therefore, the study of extremely low grade metamorphism and paleotemperature is of great practical significance for the exploration of energy resources such as oil, gas, and coal. Compared with the Longmaxi Formation shale gas, the study of which has led to exploration breakthroughs, the lower Cambrian Qiongzhusi Formation shale has higher organic matter abundance and thermal evolution in the Upper Yangtze region. Furthermore, the Qiongzhusi Formation has a larger sedimentary thickness and wider distribution area compared with the Longmaxi Formation； therefore, it has huge shale gas exploration potential. It is another key shale gas exploration and development layer in China. At present, there has been no systematic study on the extremely low grade metamorphism and paleotemperature of this high maturity source rock in the Qionghzhusi Formation. It is necessary to reveal the technical parameters such as paleotemperature and its influence on oil and gas accumulation. Methods Based on profile measurements, a study on the ultralow grade metamorphism and paleotemperature of Qiongzhusi Formation in northeast Yunnan was conducted for the first time with the mineral structure order degree, crystallinity degree of illite, and vitrinite reflectance of source rocks. Results （1） The illite content in the illite/Mongolian mixed layer is 95%. The crystallinity of illite in authigenic illite and Illite/Mengzi mixed layer minerals ranges from 0.24 to 0.35. The vitreous reflectance （Ro） of source rocks calculated by bitumen reflectance is between 2.22%⁃2.65%, lower than that of the southeastern margin of the Yangtze region （the vitrinite reflectance, Ro, in this area was calculated by the asphalt reflectance between 3.18%⁃3.43%） and the upper limit of maturity for marine kerogen gas generation （vitrinite reflectance Ro 3%）； （2） The maximum palaeotemperature of the Qiongzhusi Formation in northeastern Yunnan ranged from 200 to 227.57 ℃, lower than that of the southeastern margin of the Yangtze River （211 ℃⁃246 ℃） and the maximum hydrocarbon death temperature （230 ℃⁃300 ℃）. The paleotemperature gradient from the Early Cretaceous to Neogene maximum burial depth ranged from 2.53⁃2.91 ℃ / 100 m, close to the southeastern margin of the Yangtze. （3） Based on the comprehensive classification standard and scheme of metamorphism, the Qiongzhusi Formation in northeastern Yunnan belonged to lower metamorphic zone A⁃B, near metamorphic zone, and lower near metamorphic zone to high near metamorphic zone, and belonged to the Vitiite-chlorospar facies. The Qiongzhusi stage in northeastern Yunnan province was in the favorable transition from sedimentary sag to uplift. Conclusions The Qiongzhusi Formation experienced a lower highest paleotemperature and maturity. In addition, the maturity of organic matter in the Qiongzhusi Formation in northeastern Yunnan province was lower than that in other parts of the Yangtze region, with hydrocarbon formation conditions and material basis. At present, no paleooil reservoir has been found, and the structural preservation conditions were good. The oil and gas prospect of the Qiongzhusi Formation in northeastern Yunnan should be given significant attention.
