2024 Vol. 42, No. 5
Display Method:
2024, 42(5): 1479-1493.
doi: 10.14027/j.issn.1000-0550.2023.048
Abstract:
Objective Solid bitumen is commonly distributed in petroliferous basins worldwide, and it has been of great interest to the oil and gas exploration. Solid bitumen is particularly important for the oil and gas exploration in deep and ultra-deep carbonate reservoirs. However, a number of problems exist in the concrete application of solid bitumen in oil and gas exploration such as its reflectance is hard to determine under the microscope and the applicability of oil/gas-source correlation parameters of solid bitumen is constrained by its genesis type. If these problems are not addressed, achieved results may be incorrect. As a result, it is important to effectively analyze and summarize the multiple application of solid bitumen in the oil and gas exploration. Methods Based on extensive research, the morphological characteristics and genetic types of solid bitumen that are closely related to its application in the oil and gas exploration are systematic analyzed, and the multiple applications of solid bitumen in the oil and gas exploration are summarized. Finally, the advantages and disadvantages of the applications of solid bitumen in the oil and gas exploration are identified. Results The results are as follows. Solid bitumen is characterized by complex and diverse optical structures, such as isotropic, fine-grained mosaic, medium-grained mosaic, coarse-grained mosaic, coarse flow mosaic, domain, and fibrous optical structures, which are controlled by factors such as formation environment and parent material composition. Solid bitumen also has complex and diverse ultramicroscopic morphology, such as massive, finger-like, vesicular, warty, sheet-like, thin-skinned globular, botryoidalis, and vermicular ultramicroscopic morphology, which may be affected by factors such as migration and the generation and escalation of natural gas. Solid bitumen has multiple genetic types, such as thermal chemical alteration, anaerobic or aerobic biodegradation, deasphalting, and thermochemical sulfate reduction, which have significant differences in organic element composition, carbon and sulfur isotope values, and biomarker composition. Furthermore, solid bitumen can be used to indicate the generation, migration, and accumulation of oil and gas, and its reflectivity and laser Raman parameters can be used to indicate the thermal evolution maturity. In addition, the carbon isotopes, biomarkers, trace and rare earth elements, and rhenium and osmium isotopes of solid bitumen can be applied to trace the origins of oil and gas. However, owing to the various formation mechanisms, the geochemical characteristics of solid bitumen with different genetic types derived from same parent rock are different, resulting in significant differences in the applicability of the oil/gas-source correlation parameters for solid bitumen with different genetic types. As a result, it is necessary to determine the genetic types of solid bitumen prior to the application of its oil/gas-source correlation parameters. Conclusions This study can provide strong support for the effective application of solid bitumen in oil and gas exploration, playing an important guiding role in deep and ultra-deep oil and gas exploration.
Objective Solid bitumen is commonly distributed in petroliferous basins worldwide, and it has been of great interest to the oil and gas exploration. Solid bitumen is particularly important for the oil and gas exploration in deep and ultra-deep carbonate reservoirs. However, a number of problems exist in the concrete application of solid bitumen in oil and gas exploration such as its reflectance is hard to determine under the microscope and the applicability of oil/gas-source correlation parameters of solid bitumen is constrained by its genesis type. If these problems are not addressed, achieved results may be incorrect. As a result, it is important to effectively analyze and summarize the multiple application of solid bitumen in the oil and gas exploration. Methods Based on extensive research, the morphological characteristics and genetic types of solid bitumen that are closely related to its application in the oil and gas exploration are systematic analyzed, and the multiple applications of solid bitumen in the oil and gas exploration are summarized. Finally, the advantages and disadvantages of the applications of solid bitumen in the oil and gas exploration are identified. Results The results are as follows. Solid bitumen is characterized by complex and diverse optical structures, such as isotropic, fine-grained mosaic, medium-grained mosaic, coarse-grained mosaic, coarse flow mosaic, domain, and fibrous optical structures, which are controlled by factors such as formation environment and parent material composition. Solid bitumen also has complex and diverse ultramicroscopic morphology, such as massive, finger-like, vesicular, warty, sheet-like, thin-skinned globular, botryoidalis, and vermicular ultramicroscopic morphology, which may be affected by factors such as migration and the generation and escalation of natural gas. Solid bitumen has multiple genetic types, such as thermal chemical alteration, anaerobic or aerobic biodegradation, deasphalting, and thermochemical sulfate reduction, which have significant differences in organic element composition, carbon and sulfur isotope values, and biomarker composition. Furthermore, solid bitumen can be used to indicate the generation, migration, and accumulation of oil and gas, and its reflectivity and laser Raman parameters can be used to indicate the thermal evolution maturity. In addition, the carbon isotopes, biomarkers, trace and rare earth elements, and rhenium and osmium isotopes of solid bitumen can be applied to trace the origins of oil and gas. However, owing to the various formation mechanisms, the geochemical characteristics of solid bitumen with different genetic types derived from same parent rock are different, resulting in significant differences in the applicability of the oil/gas-source correlation parameters for solid bitumen with different genetic types. As a result, it is necessary to determine the genetic types of solid bitumen prior to the application of its oil/gas-source correlation parameters. Conclusions This study can provide strong support for the effective application of solid bitumen in oil and gas exploration, playing an important guiding role in deep and ultra-deep oil and gas exploration.
2024, 42(5): 1494-1511.
doi: 10.14027/j.issn.1000-0550.2023.045
Abstract:
Significance Kerogen is the most abundant source of natural organic matter in the world. It is important to study the pyrolysis of kerogen for the exploitation and utilization of oil shales. The traditional pyrolysis experiment cannot easily reveal the mechanism of kerogen pyrolysis, but the molecular simulation method can expose the microscopic mechanism of kerogen pyrolysis at the atomic and molecular level, which is an important research method. [Progress] In this study, the research progress of molecular simulation of kerogen pyrolysis was systematically reviewed. Combined with the experimental results, the effects of temperature, heating rate, water, pressure, and shale mineral composition on kerogen pyrolysis were described. The results showed： （1） The most commonly used method for constructing a kerogen molecular structure model is based on the elements, functional groups, and structural parameters obtained from experimental analysis methods. （2） Temperature, heating rate, water, pressure, and shale mineral components all have varying degrees of influence on the molecular number and components of kerogen pyrolysis products. In general, the number of molecules undergoing kerogen pyrolysis increases with the increase in tempera-ture. High temperature is not conducive to the direct pyrolysis process but helps to fully conduct the reaction in the hydropyrolysis process. An increase in heating rate increases the temperature at which kerogen begins to pyrolyze. The number of molecules produced by the pyrolysis of kerogen increases with the increase in heating rate, but a high heating rate reduces the number of molecules produced by pyrolysis. Choosing the appropriate temperature and heating rate can maximize the yield of shale oil. （3） Water molecules can provide more hydrogen radicals to participate in the reaction, thereby promoting the cracking of kerogen and heavy shale oil, hindering the formation of C-C crosslinking structure, and improving the formation of light shale oil and gas yields. （4） Compared with the experimental results, molecular simulation has significant advantages in the quantitative description, whereas it is slightly insufficient in the qualitative description. （5） Molecular simulation increases the simulation temperature to shorten the reaction time to compensate for the geological thermal evolution time, which is one of the shortcomings of molecular simulation at present. （6） The high-temperature simulation of kerogen produces a large amount of C2H4, which is not consistent with the experimental facts and geological situation, which is also a deficiency of the current molecular simulation. [Prospects] Future studies on molecular simulation of kerogen pyrolysis concentrating on the following aspects will be beneficial. （1） The machine learning method is used to quickly construct a kerogen molecular structure model with a relative molecular weight of millions, reflecting both the chemical structure and pore structure of kerogen. （2） A multi-scale and complex kerogen model rich in shale mineral components, formation water, organic acids, and inorganic salts is established. （3） The effects of heating rate, water phase, formation water, pressure, mineral composition and different thermal evolution degree on kerogen pyrolysis are studied. （4） Combined with the actual geological situation, the mechanism of kerogen pyrolysis to generate hydrocarbon at low temperatures is explored to bridge the gap between experiment, geology, and theory, providing important reference information and theoretical guidance for the exploration and development of shale oil and gas.
Significance Kerogen is the most abundant source of natural organic matter in the world. It is important to study the pyrolysis of kerogen for the exploitation and utilization of oil shales. The traditional pyrolysis experiment cannot easily reveal the mechanism of kerogen pyrolysis, but the molecular simulation method can expose the microscopic mechanism of kerogen pyrolysis at the atomic and molecular level, which is an important research method. [Progress] In this study, the research progress of molecular simulation of kerogen pyrolysis was systematically reviewed. Combined with the experimental results, the effects of temperature, heating rate, water, pressure, and shale mineral composition on kerogen pyrolysis were described. The results showed： （1） The most commonly used method for constructing a kerogen molecular structure model is based on the elements, functional groups, and structural parameters obtained from experimental analysis methods. （2） Temperature, heating rate, water, pressure, and shale mineral components all have varying degrees of influence on the molecular number and components of kerogen pyrolysis products. In general, the number of molecules undergoing kerogen pyrolysis increases with the increase in tempera-ture. High temperature is not conducive to the direct pyrolysis process but helps to fully conduct the reaction in the hydropyrolysis process. An increase in heating rate increases the temperature at which kerogen begins to pyrolyze. The number of molecules produced by the pyrolysis of kerogen increases with the increase in heating rate, but a high heating rate reduces the number of molecules produced by pyrolysis. Choosing the appropriate temperature and heating rate can maximize the yield of shale oil. （3） Water molecules can provide more hydrogen radicals to participate in the reaction, thereby promoting the cracking of kerogen and heavy shale oil, hindering the formation of C-C crosslinking structure, and improving the formation of light shale oil and gas yields. （4） Compared with the experimental results, molecular simulation has significant advantages in the quantitative description, whereas it is slightly insufficient in the qualitative description. （5） Molecular simulation increases the simulation temperature to shorten the reaction time to compensate for the geological thermal evolution time, which is one of the shortcomings of molecular simulation at present. （6） The high-temperature simulation of kerogen produces a large amount of C2H4, which is not consistent with the experimental facts and geological situation, which is also a deficiency of the current molecular simulation. [Prospects] Future studies on molecular simulation of kerogen pyrolysis concentrating on the following aspects will be beneficial. （1） The machine learning method is used to quickly construct a kerogen molecular structure model with a relative molecular weight of millions, reflecting both the chemical structure and pore structure of kerogen. （2） A multi-scale and complex kerogen model rich in shale mineral components, formation water, organic acids, and inorganic salts is established. （3） The effects of heating rate, water phase, formation water, pressure, mineral composition and different thermal evolution degree on kerogen pyrolysis are studied. （4） Combined with the actual geological situation, the mechanism of kerogen pyrolysis to generate hydrocarbon at low temperatures is explored to bridge the gap between experiment, geology, and theory, providing important reference information and theoretical guidance for the exploration and development of shale oil and gas.
2024, 42(5): 1512-1529.
doi: 10.14027/j.issn.1000-0550.2022.149
Abstract:
Objective The Pearl River Delta front is a complicated sedimentary system controlled by rivers, waves, and tides. Most previous studies focused on the physical and chemical sedimentary characteristics. However, biogenic sedimentary structures are extremely sensitive to the environment； therefore, comprehensive and high resolution research should be conducted. Methods Based on sedimentary and ichnological methods, salinity, turbidity, total organic carbon （TOC）, grain size analysis, X-ray scans, and three-dimensional （3D） reconstruction were applied to the modern biogenic sedimentary structures in different microenvironments of the Pearl River Delta front. Results The main results are as follows： （1） The main trace makers are Bivalve clams, Arthropoda hermit crabs, Crustacean crabs, Annelid bitoothed berbera, and chordate mudskippers. （2） The main biogenic traces include climbing tracks, foot tracks, bird tracks and excretion tracks, and burrows, and the main morphology of burrows include Y, L, U, and I-shape. （3） The diversity, abundance, and bioturbation in the tidal channel and island are higher than the interdistributary bay. Conclusions This study perfected the ichnology of the delta front and modern delta sedimentary environment. Moreover, it resolved the high resolution identification of a paleo-delta front sedimentary system.
Objective The Pearl River Delta front is a complicated sedimentary system controlled by rivers, waves, and tides. Most previous studies focused on the physical and chemical sedimentary characteristics. However, biogenic sedimentary structures are extremely sensitive to the environment； therefore, comprehensive and high resolution research should be conducted. Methods Based on sedimentary and ichnological methods, salinity, turbidity, total organic carbon （TOC）, grain size analysis, X-ray scans, and three-dimensional （3D） reconstruction were applied to the modern biogenic sedimentary structures in different microenvironments of the Pearl River Delta front. Results The main results are as follows： （1） The main trace makers are Bivalve clams, Arthropoda hermit crabs, Crustacean crabs, Annelid bitoothed berbera, and chordate mudskippers. （2） The main biogenic traces include climbing tracks, foot tracks, bird tracks and excretion tracks, and burrows, and the main morphology of burrows include Y, L, U, and I-shape. （3） The diversity, abundance, and bioturbation in the tidal channel and island are higher than the interdistributary bay. Conclusions This study perfected the ichnology of the delta front and modern delta sedimentary environment. Moreover, it resolved the high resolution identification of a paleo-delta front sedimentary system.
2024, 42(5): 1530-1540.
doi: 10.14027/j.issn.1000-0550.2022.145
Abstract:
Objective To study particle size characteristics of the burst flood sediment in barrier. Methods Composition of flood sediment particle samples from the “11· 03” Baige dammed lake outburst flood on the Jinsha River were analyzed, and size parameters of sediment were calculated. Changes in sediment characteristics from upstream to downstream and its causes were analyzed. Then, the granularity characteristics of this flood sediment were compared with those of other primary river basins in China, while sand particle size characteristics of ancient flood sediment were discussed. Results and Discussions The sediment types of the Baige barrier lake outburst flood were silt, sandy silt, and silty sand, with 23.65%, 64.19% and 12.16%, respectively. All particle sizes of sediment in barrier lake were less than 2.0 mm. The proportion of silt was 55.18%, sand of 32.86%, and clay of 11.97%. The silt content gradually increased, while the sand content gradually decreased from the upstream Maoding village to the downstream Shigu town. Moreover, the clay content always fluctuated around 12%. As the flow rate of the flood was reduced gradually, the ability of hydrodynamic to carry particulate matter was weakened by degrees. The clay was less affected by the hydrodynamic conditions as a result of the small particles and the electric charge, that’s why the clay had a relatively stable proportion during the flood peak process. The median particle size was 41.34 μm, and the average particle size was 31.73 μm. Both of the value decreased gradually in pace with the increasing distance from the White River barrier lake. The skewness value was 0.27, which is a positive deviation. The peak value was 0.94 and was moderately sharp. The sorting coefficient was 0.57, indicating good sorting performance. Bimodal distribution accounted for 52% of the grain size distribution curve of sediment. The main peak was high and narrow, located at 100 μm. And the secondary peak is low and wide, the peak value located at 10 μm. The single peak accounted for 48%, with a narrow peak at 50 μm. All sediment sizes in the profile were less than 2.0 mm. There were very small differences in composition. The predominant component was the sand. The silty content was slightly less than the fine sand, and the proportion of clay content was approximately 11.0%. The median value of particle size was larger than the average, both have a tendency to increase from the surface layer to the bottom. The sorting coefficient was less than 0.6, with the extremely positive deviation and the moderate peak state. Compared with other floods, the "11·03" outburst flood had a finer sediment particle size of mainly silt, followed by a smaller median and average value of particle size, with wider peak state and positive skewness, indicating better sorting performance by this flood. Conclusions The research results have important reference value for understanding the sediment characteristics in barrier lake outburst floods, burst flood hydrodynamic transport process and mechanism in the future. It will promote the development of water conservancy and hydropower projects and ecological civilization construction in river basins.
Objective To study particle size characteristics of the burst flood sediment in barrier. Methods Composition of flood sediment particle samples from the “11· 03” Baige dammed lake outburst flood on the Jinsha River were analyzed, and size parameters of sediment were calculated. Changes in sediment characteristics from upstream to downstream and its causes were analyzed. Then, the granularity characteristics of this flood sediment were compared with those of other primary river basins in China, while sand particle size characteristics of ancient flood sediment were discussed. Results and Discussions The sediment types of the Baige barrier lake outburst flood were silt, sandy silt, and silty sand, with 23.65%, 64.19% and 12.16%, respectively. All particle sizes of sediment in barrier lake were less than 2.0 mm. The proportion of silt was 55.18%, sand of 32.86%, and clay of 11.97%. The silt content gradually increased, while the sand content gradually decreased from the upstream Maoding village to the downstream Shigu town. Moreover, the clay content always fluctuated around 12%. As the flow rate of the flood was reduced gradually, the ability of hydrodynamic to carry particulate matter was weakened by degrees. The clay was less affected by the hydrodynamic conditions as a result of the small particles and the electric charge, that’s why the clay had a relatively stable proportion during the flood peak process. The median particle size was 41.34 μm, and the average particle size was 31.73 μm. Both of the value decreased gradually in pace with the increasing distance from the White River barrier lake. The skewness value was 0.27, which is a positive deviation. The peak value was 0.94 and was moderately sharp. The sorting coefficient was 0.57, indicating good sorting performance. Bimodal distribution accounted for 52% of the grain size distribution curve of sediment. The main peak was high and narrow, located at 100 μm. And the secondary peak is low and wide, the peak value located at 10 μm. The single peak accounted for 48%, with a narrow peak at 50 μm. All sediment sizes in the profile were less than 2.0 mm. There were very small differences in composition. The predominant component was the sand. The silty content was slightly less than the fine sand, and the proportion of clay content was approximately 11.0%. The median value of particle size was larger than the average, both have a tendency to increase from the surface layer to the bottom. The sorting coefficient was less than 0.6, with the extremely positive deviation and the moderate peak state. Compared with other floods, the "11·03" outburst flood had a finer sediment particle size of mainly silt, followed by a smaller median and average value of particle size, with wider peak state and positive skewness, indicating better sorting performance by this flood. Conclusions The research results have important reference value for understanding the sediment characteristics in barrier lake outburst floods, burst flood hydrodynamic transport process and mechanism in the future. It will promote the development of water conservancy and hydropower projects and ecological civilization construction in river basins.
2024, 42(5): 1541-1552.
doi: 10.14027/j.issn.1000-0550.2022.111
Abstract:
Results The grain shape in exposed, intermediate and sheltered parts of the Baoding Bay beach show little difference longshore, but there is a trend of gradually decreasing size from land to sea cross-shore. When the size of the grains in different parts of the beach is less than 2.5 Φ, their shape gradually shows a downward trend longshore from the exposed section to the sheltered section. The grain-shape increase direction indicates the transport trend of the sediment. The grain-shape trend analysis model is highly accurate and useful for calculating the transport trend of any grain size range, and works well in combination with other grain-size analysis methods for different research purposes. Conclusions The results of the study provide theoretical support for beach evolution mechanism analysis and beach stability research. [Objective and Methods] The basic properties of sediment particles （grain size and shape） provide information about their transport history, mode, and sedimentary environment. The grain size and shape of 253 sediments from 52 beach profiles in Baoding Bay were analyzed using the dynamic image method. The cross-shore and longshore distribution characteristics of the grain size and grain shape are analyzed and discussed. A trend analysis model was established for grain size and shape.
Results The grain shape in exposed, intermediate and sheltered parts of the Baoding Bay beach show little difference longshore, but there is a trend of gradually decreasing size from land to sea cross-shore. When the size of the grains in different parts of the beach is less than 2.5 Φ, their shape gradually shows a downward trend longshore from the exposed section to the sheltered section. The grain-shape increase direction indicates the transport trend of the sediment. The grain-shape trend analysis model is highly accurate and useful for calculating the transport trend of any grain size range, and works well in combination with other grain-size analysis methods for different research purposes. Conclusions The results of the study provide theoretical support for beach evolution mechanism analysis and beach stability research. [Objective and Methods] The basic properties of sediment particles （grain size and shape） provide information about their transport history, mode, and sedimentary environment. The grain size and shape of 253 sediments from 52 beach profiles in Baoding Bay were analyzed using the dynamic image method. The cross-shore and longshore distribution characteristics of the grain size and grain shape are analyzed and discussed. A trend analysis model was established for grain size and shape.
2024, 42(5): 1553-1567.
doi: 10.14027/j.issn.1000-0550.2023.064
Abstract:
Objective The latest three-dimensional （3D） seismic data displays a large amount of strong reflection seismic event progradation in the Yanchang Formation； this is significantly different from the traditional stratigraphic scheme of "flat rise flat fall and equal thickness distribution." This difference provides a new perspective for understanding the stratigraphic framework of the Yanchang Formation. Methods The seismic facies, sequence stratigraphy, and sedimentary system in the Longdong area of the Ordos Basin are systematically studied through core observation, well seismic calibration, and 3D seismic interpretation combined with seismic inversion technology. Results The continuous seismic events of strong reflections in the Yanchang Formation can indicate the deposition of condensed layers during lake flooding. The seismic facies along and across provenance are extremely different. The seismic profiles along provenance can be divided into three types： sub-parallel structures with medium amplitude and medium continuity, progradational reflections with strong amplitude and strong continuity, and disordered structures with low amplitude and low continuity, among which the progradational type are the most evident. The seismic profiles across provenance can also be divided into three types： parallel structures with strong amplitude and strong continuity, mound structure with medium-strong amplitude and strong continuity, and disordered structures with low amplitude and low continuity. The lake basin evolution of the Yanchang Formation in the Longdong area with rapid lake transgression and slow lake regression fluctuating processes can be divided into a third-order sequence unit consisting of multi-stage transgressive-regressive （T-R） system tracts. The sequence units prograded and overlapped as they formed wedges toward the center of the lake, developing the sedimentary sequence of "slope rich in mudstone and both top-set and bottom-set rich in sand bodies." Conclusion This region differs from the traditional sedimentary model in three aspects. First, the mudstone marker layer has not changed, and the corresponding relationship between individual wells has changed. Second, the extent of the lake in the Yanchang Formation has not changed； however, the evolution of basin filling has changed. Third, the overall distribution of sandstone has not changed, whereas the connectivity between sand layers has changed.
Objective The latest three-dimensional （3D） seismic data displays a large amount of strong reflection seismic event progradation in the Yanchang Formation； this is significantly different from the traditional stratigraphic scheme of "flat rise flat fall and equal thickness distribution." This difference provides a new perspective for understanding the stratigraphic framework of the Yanchang Formation. Methods The seismic facies, sequence stratigraphy, and sedimentary system in the Longdong area of the Ordos Basin are systematically studied through core observation, well seismic calibration, and 3D seismic interpretation combined with seismic inversion technology. Results The continuous seismic events of strong reflections in the Yanchang Formation can indicate the deposition of condensed layers during lake flooding. The seismic facies along and across provenance are extremely different. The seismic profiles along provenance can be divided into three types： sub-parallel structures with medium amplitude and medium continuity, progradational reflections with strong amplitude and strong continuity, and disordered structures with low amplitude and low continuity, among which the progradational type are the most evident. The seismic profiles across provenance can also be divided into three types： parallel structures with strong amplitude and strong continuity, mound structure with medium-strong amplitude and strong continuity, and disordered structures with low amplitude and low continuity. The lake basin evolution of the Yanchang Formation in the Longdong area with rapid lake transgression and slow lake regression fluctuating processes can be divided into a third-order sequence unit consisting of multi-stage transgressive-regressive （T-R） system tracts. The sequence units prograded and overlapped as they formed wedges toward the center of the lake, developing the sedimentary sequence of "slope rich in mudstone and both top-set and bottom-set rich in sand bodies." Conclusion This region differs from the traditional sedimentary model in three aspects. First, the mudstone marker layer has not changed, and the corresponding relationship between individual wells has changed. Second, the extent of the lake in the Yanchang Formation has not changed； however, the evolution of basin filling has changed. Third, the overall distribution of sandstone has not changed, whereas the connectivity between sand layers has changed.
2024, 42(5): 1568-1577.
doi: 10.14027/j.issn.1000-0550.2022.131
Abstract:
Objective In the Early Permian, Junggar Basin was an intracontinental successive rift basin formed after the closure of the Paleo-Asian Ocean. During the syn-rifting stage, the most important source rock, the Lower Permian Fengcheng Formation alkaline lake source rock, filled in the Mahu Sag on the northwestern margin of the Junggar Basin, but the exact depositional age remains unclear. Methods A detailed petrological analysis of a piece of tuffaceous lithic sandstone was conducted by scanning electron microscopy and energy dispersive analysis. [Results and Conclusions] It showed that the sample contained a large amount of tuffaceous clasts and volcanic glass （52%）, intermediate-felsic lithic clasts （38%）, and a small amount of quartz, feldspar, and hornblende minerals （10%）. The published LA-ICP-MS zircon U-Pb dating of this sample shows that the average age of the five youngest detrital zircons and the youngest age peak are consistent, with an age of 282±4 Ma, suggesting that the Early Permian syn-sedimentary volcanic rocks are the major source area, and the mountains in west Junggar and the Zhongguai-Luliang uplifts also have been source areas. Combined with the published zircon U-Pb results of two tuffaceous samples at the bottom and top of the alkaline lake deposition, the average ages of the five youngest detrital zircons were 284±4 and 279±4 Ma, respectively. The age gradually becomes younger from the bottom to top, which is the syn-sedimentary record of episodic volcanic eruptions in the syn-rift stage of the basin, indicating that the sedimentary age of the Lower Permian Fengcheng Formation alkaline lake is 284-279 Ma.
Objective In the Early Permian, Junggar Basin was an intracontinental successive rift basin formed after the closure of the Paleo-Asian Ocean. During the syn-rifting stage, the most important source rock, the Lower Permian Fengcheng Formation alkaline lake source rock, filled in the Mahu Sag on the northwestern margin of the Junggar Basin, but the exact depositional age remains unclear. Methods A detailed petrological analysis of a piece of tuffaceous lithic sandstone was conducted by scanning electron microscopy and energy dispersive analysis. [Results and Conclusions] It showed that the sample contained a large amount of tuffaceous clasts and volcanic glass （52%）, intermediate-felsic lithic clasts （38%）, and a small amount of quartz, feldspar, and hornblende minerals （10%）. The published LA-ICP-MS zircon U-Pb dating of this sample shows that the average age of the five youngest detrital zircons and the youngest age peak are consistent, with an age of 282±4 Ma, suggesting that the Early Permian syn-sedimentary volcanic rocks are the major source area, and the mountains in west Junggar and the Zhongguai-Luliang uplifts also have been source areas. Combined with the published zircon U-Pb results of two tuffaceous samples at the bottom and top of the alkaline lake deposition, the average ages of the five youngest detrital zircons were 284±4 and 279±4 Ma, respectively. The age gradually becomes younger from the bottom to top, which is the syn-sedimentary record of episodic volcanic eruptions in the syn-rift stage of the basin, indicating that the sedimentary age of the Lower Permian Fengcheng Formation alkaline lake is 284-279 Ma.
2024, 42(5): 1578-1591.
doi: 10.14027/j.issn.1000-0550.2022.119
Abstract:
[Objective and Methods] Triassic strata are considered absent in the Yungang Basin in northcentral North China, while the Shihezi Formation at the top of the Yangquantou section within the Yungang Basin belongs to the Lower Triassic Liujiagou Formation based on the regional geological survey results of the 1：50 000 Shijiatun map. To settle the dispute and determine the reliability of this new understanding, this study conducted lithostratigraphic correlation and detrital zircon dating of the Shihezi Formation from the Madaotou and Yangquantou sections within the Yungang Basin, and compared with other contemporaneous stratigraphic sections in northcentral North China to determine its deposition age. [Results and Conclusions] It shows that the ‘so-called’ Liujiagou Formation identified in the upper part of Yangquantou section has similar lithofacies associations and gravel compositions with the Shihezi Formation in the Madaotou section, and they are both characterized by grey-green or yellow-green medium- to thick-bedded medium- to coarse-grained sandstone interbedded with thin-bedded purple-red siltstone and sandy mudstone, as well as several layers of thick conglomerates. This differs from the diagnostic characteristics of the Liujiagou Formation in the Fugu, Ningwu, Taiyuan Xishan, and Qinshui Basins in North China, which are characterized by abundant light to fresh red thick-bedded medium- to fine-grained arkose. Moreover, the youngest detrital zircons ages （288.0±4.7 Ma） constrained the maximum depositional age to the Middle Permian. Therefore, the strata at the top of the Yangquantou section belongs to the Middle Permian Shihezi Formation, not the Liujiagou Formation. It is inferred that the orogenic belts in the northern margin of North China likely experienced intense compression and uplift during the Late Triassic, resulting in the denudation of the Triassic strata within the Yungang Basin, which was then unconformable overlain by the Lower Jurassic strata.
[Objective and Methods] Triassic strata are considered absent in the Yungang Basin in northcentral North China, while the Shihezi Formation at the top of the Yangquantou section within the Yungang Basin belongs to the Lower Triassic Liujiagou Formation based on the regional geological survey results of the 1：50 000 Shijiatun map. To settle the dispute and determine the reliability of this new understanding, this study conducted lithostratigraphic correlation and detrital zircon dating of the Shihezi Formation from the Madaotou and Yangquantou sections within the Yungang Basin, and compared with other contemporaneous stratigraphic sections in northcentral North China to determine its deposition age. [Results and Conclusions] It shows that the ‘so-called’ Liujiagou Formation identified in the upper part of Yangquantou section has similar lithofacies associations and gravel compositions with the Shihezi Formation in the Madaotou section, and they are both characterized by grey-green or yellow-green medium- to thick-bedded medium- to coarse-grained sandstone interbedded with thin-bedded purple-red siltstone and sandy mudstone, as well as several layers of thick conglomerates. This differs from the diagnostic characteristics of the Liujiagou Formation in the Fugu, Ningwu, Taiyuan Xishan, and Qinshui Basins in North China, which are characterized by abundant light to fresh red thick-bedded medium- to fine-grained arkose. Moreover, the youngest detrital zircons ages （288.0±4.7 Ma） constrained the maximum depositional age to the Middle Permian. Therefore, the strata at the top of the Yangquantou section belongs to the Middle Permian Shihezi Formation, not the Liujiagou Formation. It is inferred that the orogenic belts in the northern margin of North China likely experienced intense compression and uplift during the Late Triassic, resulting in the denudation of the Triassic strata within the Yungang Basin, which was then unconformable overlain by the Lower Jurassic strata.
2024, 42(5): 1592-1606.
doi: 10.14027/j.issn.1000-0550.2022.126
Abstract:
Objective Continental slopes have become a major focus in petroleum exploration because of their enormous sediment and hydrocarbon potential. Research on the geomorphic evolution of continental slopes contributes to deep-water sedimentology and petroleum exploration. This paper focused on the abundant terrigenous supply of a continental slope in a delta reformed passive continental margin basin. A new methodology for the restoration of ancient slope geomorphic evolution based on the principle of depositional architecture was discussed. Methods Taking a study area on a continental slope in Niger Delta Basin as an example, seismic, logging, and core data were comprehensively applied to reveal the geomorphic evolution characteristics of the continental slope based on the spatio-temporal evolution of depositional architecture in 13 sequences. Results The geomorphic evolution of the study area can be divided into 2 stages. In stage 1,study area transformed from an abyssal plain to continental slope,and the landform was unrestricted. The geomorphic evolution in this stage was driven by the progradation of large deltas. The geomorphic evolution in stage 2 included 3 periods dominated by thrust faults,mud-diapirs,and sedimentation. The landform evolved gradually from restricted to semi-restricted and then to the present non-restricted type. The geomorphic evolution in stage 2 was controlled by gravity sliding. During the thrust faulting,the activity intensity of the underlying flowing mudstone was limited,and the landform was controlled by thrust faults. The study area was located in the remote thrust province of a gravity sliding system. During the mud-diapir period,the intensity of diaper activity increased significantly and was able to change the geomorphic characteristics independently. As a result,the topography of study area began to transform from a thrust to mud-diapir province. Conclusions The gravity sliding structural system was driven by sedimentary processes,and its spatial combination and evolution characteristics were similar to sedimentary facies. The adjacent extensional,mud-diapir and thrust province were closely related in genesis,which was similar to a sequential sedimentary facies. Therefore,the vertical superposition relationship of different structural activities indicated the macro trend of the continental slope advancing seaward contemporaneously. In conclusion,the spatio-temporal evolution characteristics of the gravity flow depositional architecture in the typical deep-water study area can provide important evidence for the restoration of paleogeomorphic evolution of a deep-water continental slope.
Objective Continental slopes have become a major focus in petroleum exploration because of their enormous sediment and hydrocarbon potential. Research on the geomorphic evolution of continental slopes contributes to deep-water sedimentology and petroleum exploration. This paper focused on the abundant terrigenous supply of a continental slope in a delta reformed passive continental margin basin. A new methodology for the restoration of ancient slope geomorphic evolution based on the principle of depositional architecture was discussed. Methods Taking a study area on a continental slope in Niger Delta Basin as an example, seismic, logging, and core data were comprehensively applied to reveal the geomorphic evolution characteristics of the continental slope based on the spatio-temporal evolution of depositional architecture in 13 sequences. Results The geomorphic evolution of the study area can be divided into 2 stages. In stage 1,study area transformed from an abyssal plain to continental slope,and the landform was unrestricted. The geomorphic evolution in this stage was driven by the progradation of large deltas. The geomorphic evolution in stage 2 included 3 periods dominated by thrust faults,mud-diapirs,and sedimentation. The landform evolved gradually from restricted to semi-restricted and then to the present non-restricted type. The geomorphic evolution in stage 2 was controlled by gravity sliding. During the thrust faulting,the activity intensity of the underlying flowing mudstone was limited,and the landform was controlled by thrust faults. The study area was located in the remote thrust province of a gravity sliding system. During the mud-diapir period,the intensity of diaper activity increased significantly and was able to change the geomorphic characteristics independently. As a result,the topography of study area began to transform from a thrust to mud-diapir province. Conclusions The gravity sliding structural system was driven by sedimentary processes,and its spatial combination and evolution characteristics were similar to sedimentary facies. The adjacent extensional,mud-diapir and thrust province were closely related in genesis,which was similar to a sequential sedimentary facies. Therefore,the vertical superposition relationship of different structural activities indicated the macro trend of the continental slope advancing seaward contemporaneously. In conclusion,the spatio-temporal evolution characteristics of the gravity flow depositional architecture in the typical deep-water study area can provide important evidence for the restoration of paleogeomorphic evolution of a deep-water continental slope.
2024, 42(5): 1607-1620.
doi: 10.14027/j.issn.1000-0550.2022.114
Abstract:
Objective The investigation of the genetic kinds and sedimentary properties of barrier deposits has novel implications for establishing paleocurbation direction. Gravel barrier deposits can be an essential distinguishing feature for determining the sedimentary microfacies of river channels. Methods The sedimentary model and formation mechanism of gravelly sediment were investigated using sedimentary investigations of present gravelly rivers in the Anjihai and TouTunhe Rivers on the southern fringe of the Junggar Basin. Result The results revealed that the shape of gravelly barrier sediments could be classified into four types： lenticular deposition, fusiform deposition, approximately circular mafic deposition, and lateral ridges deposition. The lenticular deposit consists of a single large gravel obstacle and a fine-grained sediment ridge behind it. The length of the sediment ridge to the scour region in front of the gravel should be 2∶1, and the long axis of the sediment ridge determines the direction of water flow. Fusiform deposition consists of a single large gravel acting as a barrier, with the sheet gravel imbricated in front of the barrier gravel, forming a sediment ridge. Circular mafic deposition is the main body of numerous gravels that are similar in size to the obstruction, with a "U" shape in the middle supported by fine conglomerate particles. Lateral ridges deposition is the buildup of big gravel on the side or middle of a channel, forming a beach bar that serves as an obstruction to control the river boundary. Conclusion The phenomena of developing barrier deposition in the conglomerate of Kalaza Formation in Junggar Basin is finely split, and the identifying mark of gravel barrier deposition is established, providing new fundamental data for river sedimentology.
Objective The investigation of the genetic kinds and sedimentary properties of barrier deposits has novel implications for establishing paleocurbation direction. Gravel barrier deposits can be an essential distinguishing feature for determining the sedimentary microfacies of river channels. Methods The sedimentary model and formation mechanism of gravelly sediment were investigated using sedimentary investigations of present gravelly rivers in the Anjihai and TouTunhe Rivers on the southern fringe of the Junggar Basin. Result The results revealed that the shape of gravelly barrier sediments could be classified into four types： lenticular deposition, fusiform deposition, approximately circular mafic deposition, and lateral ridges deposition. The lenticular deposit consists of a single large gravel obstacle and a fine-grained sediment ridge behind it. The length of the sediment ridge to the scour region in front of the gravel should be 2∶1, and the long axis of the sediment ridge determines the direction of water flow. Fusiform deposition consists of a single large gravel acting as a barrier, with the sheet gravel imbricated in front of the barrier gravel, forming a sediment ridge. Circular mafic deposition is the main body of numerous gravels that are similar in size to the obstruction, with a "U" shape in the middle supported by fine conglomerate particles. Lateral ridges deposition is the buildup of big gravel on the side or middle of a channel, forming a beach bar that serves as an obstruction to control the river boundary. Conclusion The phenomena of developing barrier deposition in the conglomerate of Kalaza Formation in Junggar Basin is finely split, and the identifying mark of gravel barrier deposition is established, providing new fundamental data for river sedimentology.
2024, 42(5): 1621-1638.
doi: 10.14027/j.issn.1000-0550.2022.120
Abstract:
Objective There are still some disagreement and deficiencies on understanding the sedimentary environment and provenance from the Triassic Yanchang Formation in the middle of the western margin of the Ordos Basin Methods Through geochemical methods,sixteen sand and mudstone samples from different sections of the Late Triassic Yanchang Formation were tested and analyzed,and the sedimentary environment,source rock types,and tectonic background of the Yanchang Formation clastic rocks were evaluated. Results Sandstone types are mainly lithic and feldspathic sandstone, the source areas experienced weak-moderate weathering,and the samples were little affected by the recycling. The source is related to the upper crust material. The Yanchang Formation is in an oxidation-weak reduction freshwater sedimentary environment,with an arid-semi-arid paleoclimate, and the Third member is in a weak oxidation brackish water environment with a warm and humid paleoclimate. The source rocks are primarily felsic,and the provenance of the Third member of the Yanchang Formation is complex,with a little intermediate rock. The source area of the Yanchang Formation has a tectonic background of an active continental margin,while the tectonic background of the Third member source area is complex and is related to both a continental island arc and an active continental margin. Conclusions The provenance and sedimentary environment characteristics traced by elemental geochemistry are in good agreement with the lake basin evolution and regional tectonic environment of the Triassic Yanchang Formation.
Objective There are still some disagreement and deficiencies on understanding the sedimentary environment and provenance from the Triassic Yanchang Formation in the middle of the western margin of the Ordos Basin Methods Through geochemical methods,sixteen sand and mudstone samples from different sections of the Late Triassic Yanchang Formation were tested and analyzed,and the sedimentary environment,source rock types,and tectonic background of the Yanchang Formation clastic rocks were evaluated. Results Sandstone types are mainly lithic and feldspathic sandstone, the source areas experienced weak-moderate weathering,and the samples were little affected by the recycling. The source is related to the upper crust material. The Yanchang Formation is in an oxidation-weak reduction freshwater sedimentary environment,with an arid-semi-arid paleoclimate, and the Third member is in a weak oxidation brackish water environment with a warm and humid paleoclimate. The source rocks are primarily felsic,and the provenance of the Third member of the Yanchang Formation is complex,with a little intermediate rock. The source area of the Yanchang Formation has a tectonic background of an active continental margin,while the tectonic background of the Third member source area is complex and is related to both a continental island arc and an active continental margin. Conclusions The provenance and sedimentary environment characteristics traced by elemental geochemistry are in good agreement with the lake basin evolution and regional tectonic environment of the Triassic Yanchang Formation.
2024, 42(5): 1639-1652.
doi: 10.14027/j.issn.1000-0550.2022.116
Abstract:
Objective The mixed siliciclastic-carbonate sediments （MSSs） contain both carbonate and detrital components that can concurrently carry aqueous and provenance signatures in their compositions. During the early Cambrian, the MSSs were widely distributed in the Hannan-Micangshan area, western Yangtze Craton, but the factors influencing their development have not been well known. Methods In this study, we performed a combined petrological and sedimentological analysis on the compositions of MSSs in the Xiannüdong Formation of the Zhujiaba section, southern Shaanxi. [Results and Conclusions] The results indicate that MSSs formed from the middle to the inner ramp settings during a regressive depositional cycle, and the highest level of admixture is generally distributed in the middle part of each upward-shoaling sequence compared to that of the lower and upper parts. Fine-grained detrital sediments and low-relief reefs developed in the middle ramp environment, and other carbonate fractions were mainly transported from shallow-water carbonate depositional settings. In contrast, ooid shoals and reefs developed in inner ramp environments with a small quantality of terrigenous sand-sized particles （quartz and lithic fragment）. In addition, the petrological evidence indicates that transported （dolomitized） carbonate grains and fine-grained detritus were concurrently mixed in the lagoon setting. The development of early Cambrian MSSs in the study area may be related to the high carbonate productivity of shallow-water environments, as well as the active onshore-to-offshore transportation mainly induced by storm currents. This study may provide a case for understanding the factors influencing the development of deep-time MSSs and a reference for oil and gas exploration of Cambrian MSSs in the western Yangtze area.
Objective The mixed siliciclastic-carbonate sediments （MSSs） contain both carbonate and detrital components that can concurrently carry aqueous and provenance signatures in their compositions. During the early Cambrian, the MSSs were widely distributed in the Hannan-Micangshan area, western Yangtze Craton, but the factors influencing their development have not been well known. Methods In this study, we performed a combined petrological and sedimentological analysis on the compositions of MSSs in the Xiannüdong Formation of the Zhujiaba section, southern Shaanxi. [Results and Conclusions] The results indicate that MSSs formed from the middle to the inner ramp settings during a regressive depositional cycle, and the highest level of admixture is generally distributed in the middle part of each upward-shoaling sequence compared to that of the lower and upper parts. Fine-grained detrital sediments and low-relief reefs developed in the middle ramp environment, and other carbonate fractions were mainly transported from shallow-water carbonate depositional settings. In contrast, ooid shoals and reefs developed in inner ramp environments with a small quantality of terrigenous sand-sized particles （quartz and lithic fragment）. In addition, the petrological evidence indicates that transported （dolomitized） carbonate grains and fine-grained detritus were concurrently mixed in the lagoon setting. The development of early Cambrian MSSs in the study area may be related to the high carbonate productivity of shallow-water environments, as well as the active onshore-to-offshore transportation mainly induced by storm currents. This study may provide a case for understanding the factors influencing the development of deep-time MSSs and a reference for oil and gas exploration of Cambrian MSSs in the western Yangtze area.
2024, 42(5): 1653-1669.
doi: 10.14027/j.issn.1000-0550.2023.111
Abstract:
Objective The collision of the Arabian-Eurasian Plates led to the formation of the Zagros orogenic belt and related foreland basin. The gradual filling of the Zagros foreland basin resulted in the retreat of the Neo-Tethys Ocean in the Zagros region, which caused the transition from the marine to terrestrial environments. This process provides the chance to explore the demise of the Neo-Tethyan Ocean in the Middle East. Methods In this study, we conducted detailed stratigraphy, sedimentology, sandstone petrography, and detrital zircon U-Pb age determination on the Miocene sedimentary strata in the Lurestan and Khuzestan regions of the Zagros Mountains in southern Iran, which provide new constraints on the filling of the Zagros foreland basin and the regression process of the Neo-Tethys Ocean in the Zagros region. Results The Agha Jari Formation in the Lurestan region is characterized by large-scale, medium-to-thick layered sandstones interbedded with mudstones. Sedimentary environment analysis suggests that it has been formed in a fluvial environment. The sandstone petrography shows a significant number of metamorphic clasts. Combined with the detrital zircon U-Pb age peak of Jurassic ~170 Ma, the Agha Jari Formation in the Lurestan region is found to be derived from the Sanandaj-Sirjan zone to the north. The Agha Jari Formation in the Khuzestan area is a large set of thick mudstone beds, thick mudstone interbedded with sandstone, thick sheeted sandstone interbedded with mudstone, and sand-mudstone interbeds. The sedimentary environment analysis shows that it has been formed in a transitional deltaic environment. The sandstone modal composition indicates that the clasts of the sedimentary rocks occurred. The detrital zircon ages show Mesozoic ages, comparable with the Zagros Fold-and-Thrust Belt. Thus, they have been mainly recycled from the sedimentary strata in Zagros Fold-and-Thrust Belt. Based on the underlying Gachsaran-Mishan Formations, which are composed of shallow marine limestone and evaporite, the Agha Jari Formation represents the regional youngest marine sedimentation, recording the transition from the marine to the terrestrial environments. The calcareous nannofossil analysis from the underlying Mishan Formation yield the youngest age of the Early Miocene. Considering the other biostratigraphic studies, the age of the Agha Jari Formation is constrained to be no earlier than the Miocene. Therefore, based on the sedimentary and biostratigraphic studies on the Agha Jari Formation in the Khuzestan region, the demise of the Neo-Tethyan Ocean in the Zagros region has been constrained by no earlier than the Miocene, with a trend from the northwest to the southeast. Conclusions Combined with regional paleogeography and provenance analyses on the Miocene strata in the Zagros orogenic belt, the gradual uplift of the Zagros orogenic belt occurred after the Arabia-Eurasia collision. The uplifted Zagros orogenic belt has been eroded to provide the clastic materials for the Zagros foreland basin. As a result, the regression of the Neo-Tethys Ocean in the Zagros region is controlled by the continual filling in the foreland basin. This paleogeography of northwest-southeast regression of the Neo-Tethyan Ocean existed no later than the Early Miocene.
Objective The collision of the Arabian-Eurasian Plates led to the formation of the Zagros orogenic belt and related foreland basin. The gradual filling of the Zagros foreland basin resulted in the retreat of the Neo-Tethys Ocean in the Zagros region, which caused the transition from the marine to terrestrial environments. This process provides the chance to explore the demise of the Neo-Tethyan Ocean in the Middle East. Methods In this study, we conducted detailed stratigraphy, sedimentology, sandstone petrography, and detrital zircon U-Pb age determination on the Miocene sedimentary strata in the Lurestan and Khuzestan regions of the Zagros Mountains in southern Iran, which provide new constraints on the filling of the Zagros foreland basin and the regression process of the Neo-Tethys Ocean in the Zagros region. Results The Agha Jari Formation in the Lurestan region is characterized by large-scale, medium-to-thick layered sandstones interbedded with mudstones. Sedimentary environment analysis suggests that it has been formed in a fluvial environment. The sandstone petrography shows a significant number of metamorphic clasts. Combined with the detrital zircon U-Pb age peak of Jurassic ~170 Ma, the Agha Jari Formation in the Lurestan region is found to be derived from the Sanandaj-Sirjan zone to the north. The Agha Jari Formation in the Khuzestan area is a large set of thick mudstone beds, thick mudstone interbedded with sandstone, thick sheeted sandstone interbedded with mudstone, and sand-mudstone interbeds. The sedimentary environment analysis shows that it has been formed in a transitional deltaic environment. The sandstone modal composition indicates that the clasts of the sedimentary rocks occurred. The detrital zircon ages show Mesozoic ages, comparable with the Zagros Fold-and-Thrust Belt. Thus, they have been mainly recycled from the sedimentary strata in Zagros Fold-and-Thrust Belt. Based on the underlying Gachsaran-Mishan Formations, which are composed of shallow marine limestone and evaporite, the Agha Jari Formation represents the regional youngest marine sedimentation, recording the transition from the marine to the terrestrial environments. The calcareous nannofossil analysis from the underlying Mishan Formation yield the youngest age of the Early Miocene. Considering the other biostratigraphic studies, the age of the Agha Jari Formation is constrained to be no earlier than the Miocene. Therefore, based on the sedimentary and biostratigraphic studies on the Agha Jari Formation in the Khuzestan region, the demise of the Neo-Tethyan Ocean in the Zagros region has been constrained by no earlier than the Miocene, with a trend from the northwest to the southeast. Conclusions Combined with regional paleogeography and provenance analyses on the Miocene strata in the Zagros orogenic belt, the gradual uplift of the Zagros orogenic belt occurred after the Arabia-Eurasia collision. The uplifted Zagros orogenic belt has been eroded to provide the clastic materials for the Zagros foreland basin. As a result, the regression of the Neo-Tethys Ocean in the Zagros region is controlled by the continual filling in the foreland basin. This paleogeography of northwest-southeast regression of the Neo-Tethyan Ocean existed no later than the Early Miocene.
2024, 42(5): 1670-1684.
doi: 10.14027/j.issn.1000-0550.2022.155
Abstract:
Objective The Sichuan Basin has abundant oil and gas resources. The breakthrough of the Sinian Deng-ying and Cambrian Longwangmiao Formations reveals the huge exploration potential of deep oil and gas resources in the Sichuan Basin. However, the extent of research and exploration of the Cambrian Gaotai Formation in the eastern Sichuan Basin is typically low. Research between stratigraphy and sedimentary facies characteristics of the Gaotai Formation can provide a basis for the next step of oil and gas exploration and deployment. Methods The basic geological research on the sequence stratigraphy, sedimentary facies characteristics, and distribution of the Gaotai Formation can provide the basis for future oil and gas exploration and deployment. Based on drilling, logging, outcrop profile, core, and seismic data, a comprehensive study on the sequence lithofacies paleogeo.graphy of the Cambrian Gaotai Formation in eastern Sichuan has been conducted. Results The results show that Gaotai Formation can be divided into three third-order sequences （SQ1-SQ3） and six fourth-order sequences （SSQ1-SSQ6）. During the sedimentary period of Gaotai Formation, the carbonate restricted platform sedimentary environment was developed. From west to east, four sedimentary subfacies developed successively： mixed tidal flat, tidal flat, lagoon and inner platform beach. The Gaotai Formation developed with a silty dolomite reservoir, which is characterized by thin thickness, strong heterogeneity, and "medium-low porosity and low permeability". Conclusions The favourable reservoirs in the study area are located in the dorsal spreading area influenced by tectonic activity and hydrothermal fluids. Tidal flat subfacie （dolomite tidal flat microfacies） is a favourable exploration zone in the western of eastern Sichuan Basin.
Objective The Sichuan Basin has abundant oil and gas resources. The breakthrough of the Sinian Deng-ying and Cambrian Longwangmiao Formations reveals the huge exploration potential of deep oil and gas resources in the Sichuan Basin. However, the extent of research and exploration of the Cambrian Gaotai Formation in the eastern Sichuan Basin is typically low. Research between stratigraphy and sedimentary facies characteristics of the Gaotai Formation can provide a basis for the next step of oil and gas exploration and deployment. Methods The basic geological research on the sequence stratigraphy, sedimentary facies characteristics, and distribution of the Gaotai Formation can provide the basis for future oil and gas exploration and deployment. Based on drilling, logging, outcrop profile, core, and seismic data, a comprehensive study on the sequence lithofacies paleogeo.graphy of the Cambrian Gaotai Formation in eastern Sichuan has been conducted. Results The results show that Gaotai Formation can be divided into three third-order sequences （SQ1-SQ3） and six fourth-order sequences （SSQ1-SSQ6）. During the sedimentary period of Gaotai Formation, the carbonate restricted platform sedimentary environment was developed. From west to east, four sedimentary subfacies developed successively： mixed tidal flat, tidal flat, lagoon and inner platform beach. The Gaotai Formation developed with a silty dolomite reservoir, which is characterized by thin thickness, strong heterogeneity, and "medium-low porosity and low permeability". Conclusions The favourable reservoirs in the study area are located in the dorsal spreading area influenced by tectonic activity and hydrothermal fluids. Tidal flat subfacie （dolomite tidal flat microfacies） is a favourable exploration zone in the western of eastern Sichuan Basin.
2024, 42(5): 1685-1698.
doi: 10.14027/j.issn.1000-0550.2022.121
Abstract:
Objective Exploration has shown that the sand body of the Shanxi Formation, the main gas producing layer in the Yan 'an gas field in the southeastern Ordos Basin, is distributed in a long and narrow strip, which is significantly different from its distribution in the north. It is of great significance to clarify the genesis mechanism of the sand body in the 2nd member of Shanxi Formation for understanding the distribution pattern of sand bodies in the area. Methods Taking the 2nd member of the Shanxi Formation in the southeastern basin as the geological prototype, a flume sedimentation simulation experiment was used to investigate the sand formation, evolution process, and control factors. [Results and Conclusions] The gentle slope and low flow intensity of the 2nd member of the Shanxi Formation distributary channel is an important factor in the low sinuosity of the meandering flow state, and the horizontal amplitude rate is a key feature in the longitudinal extension of the sand body, sand size, aspect ratio of the coast, and change of flow control. The extension of the entire distance and length-width ratio of the sand were proportional to the scale and rate of water withdrawal. During the sedimentary period of the Shanxi Formation, the water level dropped significantly, and the shoreline migrated a long distance to the lake area. The water inlet retreated frequently, and the sand in the front of the meandering river delta extended a long distance. The sand superposition style has the characteristics of "vertical superposition", "vertical cutting", and "lateral migration". The results are of great significance to the prediction of sand bodies in the Shanxi Formation and the search for high-quality reservoirs.
Objective Exploration has shown that the sand body of the Shanxi Formation, the main gas producing layer in the Yan 'an gas field in the southeastern Ordos Basin, is distributed in a long and narrow strip, which is significantly different from its distribution in the north. It is of great significance to clarify the genesis mechanism of the sand body in the 2nd member of Shanxi Formation for understanding the distribution pattern of sand bodies in the area. Methods Taking the 2nd member of the Shanxi Formation in the southeastern basin as the geological prototype, a flume sedimentation simulation experiment was used to investigate the sand formation, evolution process, and control factors. [Results and Conclusions] The gentle slope and low flow intensity of the 2nd member of the Shanxi Formation distributary channel is an important factor in the low sinuosity of the meandering flow state, and the horizontal amplitude rate is a key feature in the longitudinal extension of the sand body, sand size, aspect ratio of the coast, and change of flow control. The extension of the entire distance and length-width ratio of the sand were proportional to the scale and rate of water withdrawal. During the sedimentary period of the Shanxi Formation, the water level dropped significantly, and the shoreline migrated a long distance to the lake area. The water inlet retreated frequently, and the sand in the front of the meandering river delta extended a long distance. The sand superposition style has the characteristics of "vertical superposition", "vertical cutting", and "lateral migration". The results are of great significance to the prediction of sand bodies in the Shanxi Formation and the search for high-quality reservoirs.
2024, 42(5): 1699-1710.
doi: 10.14027/j.issn.1000-0550.2022.080
Abstract:
Objective Braided rivers are a common type of river that can form large-scale oil and gas reservoirs. However, their sedimentary architecture is complex. To gain a deeper understanding of the sedimentary architecture patterns of sandy braided rivers, this study focuses on the sedimentary architecture analysis and evolutionary process of a typical gravelly braided river deposits in the Jurassic Yungang Formation in Datong, Shanxi. Methods Observations, measurements, detailed dissections, and analytical tests were conducted on eight sections of the Yungang Formation where braided river deposits are well exposed. This systematic summary covers the geometric morphology, scale, lithological structure and sedimentary structures, lithofacies types, and lithofacies associations of the gravelly braided river depositional bodies. Results In the study area, primarily 10 types of lithofacies, 8 lithofacies associations, and 5 fourth-level architecture elements （including point bars, braided channels, abandoned channels, floodplains, and crevasse splays） as well as fall-out beds have been identified. Conclusion Ten main phases of positive cyclical deposition can be identified in the outcrops of the Yungang Formation. Within each phase, the braided rivers experienced periodic alternation between flood and dry periods. Braid bars interoccur with braiding channels, abandoned channels, floodplains, and crevasse splays, developing and waning cyclically to form "laterally connected" sandy bodies. The braid bar complexes and main braiding channels constitute the main body of the gravelly braided river deposits. Based on this, a sedimentary architecture model for sandy braided rivers has been established. The research findings can provide important references for the analysis of the evolution of gravelly braided river deposits, reservoir characterization and modeling, as well as the selection of quantitative parameters.
Objective Braided rivers are a common type of river that can form large-scale oil and gas reservoirs. However, their sedimentary architecture is complex. To gain a deeper understanding of the sedimentary architecture patterns of sandy braided rivers, this study focuses on the sedimentary architecture analysis and evolutionary process of a typical gravelly braided river deposits in the Jurassic Yungang Formation in Datong, Shanxi. Methods Observations, measurements, detailed dissections, and analytical tests were conducted on eight sections of the Yungang Formation where braided river deposits are well exposed. This systematic summary covers the geometric morphology, scale, lithological structure and sedimentary structures, lithofacies types, and lithofacies associations of the gravelly braided river depositional bodies. Results In the study area, primarily 10 types of lithofacies, 8 lithofacies associations, and 5 fourth-level architecture elements （including point bars, braided channels, abandoned channels, floodplains, and crevasse splays） as well as fall-out beds have been identified. Conclusion Ten main phases of positive cyclical deposition can be identified in the outcrops of the Yungang Formation. Within each phase, the braided rivers experienced periodic alternation between flood and dry periods. Braid bars interoccur with braiding channels, abandoned channels, floodplains, and crevasse splays, developing and waning cyclically to form "laterally connected" sandy bodies. The braid bar complexes and main braiding channels constitute the main body of the gravelly braided river deposits. Based on this, a sedimentary architecture model for sandy braided rivers has been established. The research findings can provide important references for the analysis of the evolution of gravelly braided river deposits, reservoir characterization and modeling, as well as the selection of quantitative parameters.
2024, 42(5): 1711-1722.
doi: 10.14027/j.issn.1000-0550.2022.102
Abstract:
Objective The interference testing indicates that the sand body connectivity in the lower of He 8 formation, the main gas-producing layer of the Sulige gas field, is poor. Under the condition of the dense well pattern （500 m×600 m） in the SD27-36 block, the distribution characteristics of sand bodies remain unclear.The purpose of this paper is to explore the characterization of sand bodies scale and stacking patterns, so as to clarify the connectivity of sand bodies between wells. Methods Modern river and field outcrop observations were combined with geostatistical inversion to optimize the inversion parameters and establish a characterization model based on the observation results, so as to accurately identify sand body boundaries and their overlapping relationships, and realize three-dimensional quantitative characterization of sand bodies. [Results and Conclusions] The observation shows that the sand body of the braided river single core beach in the lower of He 8 formation has various contact relationships such as isolation, butt and cut stack., and the single sand body has a small plane size （200-600 m long, 50-250 m wide）, and the composite core beach sand body of a specific shape is formed by superposition and composite. The boundary of three-dimensional sand body characterization obtained by geostatistics inversion is clear, the lithology transition between the sand body and the well point is natural, the sand body size is similar to the sediment observation results, and the understanding of sand body connectivity aligns with the results of interference well testing at a rate of 87%. The results and methods of sand body characterization can not only guide the exploration and development planning of the Sulige gas field, but also provide reference for other braided river sand bodies.
Objective The interference testing indicates that the sand body connectivity in the lower of He 8 formation, the main gas-producing layer of the Sulige gas field, is poor. Under the condition of the dense well pattern （500 m×600 m） in the SD27-36 block, the distribution characteristics of sand bodies remain unclear.The purpose of this paper is to explore the characterization of sand bodies scale and stacking patterns, so as to clarify the connectivity of sand bodies between wells. Methods Modern river and field outcrop observations were combined with geostatistical inversion to optimize the inversion parameters and establish a characterization model based on the observation results, so as to accurately identify sand body boundaries and their overlapping relationships, and realize three-dimensional quantitative characterization of sand bodies. [Results and Conclusions] The observation shows that the sand body of the braided river single core beach in the lower of He 8 formation has various contact relationships such as isolation, butt and cut stack., and the single sand body has a small plane size （200-600 m long, 50-250 m wide）, and the composite core beach sand body of a specific shape is formed by superposition and composite. The boundary of three-dimensional sand body characterization obtained by geostatistics inversion is clear, the lithology transition between the sand body and the well point is natural, the sand body size is similar to the sediment observation results, and the understanding of sand body connectivity aligns with the results of interference well testing at a rate of 87%. The results and methods of sand body characterization can not only guide the exploration and development planning of the Sulige gas field, but also provide reference for other braided river sand bodies.
2024, 42(5): 1723-1737.
doi: 10.14027/j.issn.1000-0550.2022.156
Abstract:
Objective In recent years, there has been a continuous exploration of the Canglangpu Formation from the Cambrian Series 2 in the central and northern Sichuan Basin, which exhibits substantial exploration potential. At present, the study of pores is relatively weak, restricting the in-depth understanding of reservoir development characteristics. Methods Based on the observation and identification of cores, cast thin sections, and cathode luminescence, image recognition software was used to obtain quantitative data of rock components and pore parameters. The pore characteristics, genesis, and evolution models of different lithological pores in the First member of the Canglangpu Formation were studied. Results The results show that： the pore type of the First member of Canglangpu Formation was mainly intragranular dissolved pores, which have the best positive correlation with the total face rate, followed by intergranular dissolved pores. The higher oolite/terrigenous sand content ratio is an important material basis for pore development, dissolution is the most favorable factor for pore development, and high energy shelf oolitic beach sedimentation is the key to the development of primary pores. The selective dissolution of atmospheric water in the syngenetic period is the key to the formation of the most important dissolution pores in oolitic sands in the First member of Canglangpu Formation, while the dolomitization in the quasi syngenetic and burial period enhances the compression resistance of rocks, reduces the solid volume, and increases the pores. The dissolution in the burial period is located along the fractures by tectonic disruption and early pores expansion. Terrigenous sand, cementation, compaction, and pressure dissolution are not conducive to the development of pores in the First member of the Canglangpu Formation. Conclusions Therefore, favorable carbonate components and subsequent diagenesis and tectonism have affected the development and evolution of pores and formed the lithologic sequence characteristics of the decreasing pore face rate： bright crystal oolitic dolomite, oolitic dolomite with sandy, sandy oolitic dolomite, fine-grained lithic sandstone, limestone, etc.
Objective In recent years, there has been a continuous exploration of the Canglangpu Formation from the Cambrian Series 2 in the central and northern Sichuan Basin, which exhibits substantial exploration potential. At present, the study of pores is relatively weak, restricting the in-depth understanding of reservoir development characteristics. Methods Based on the observation and identification of cores, cast thin sections, and cathode luminescence, image recognition software was used to obtain quantitative data of rock components and pore parameters. The pore characteristics, genesis, and evolution models of different lithological pores in the First member of the Canglangpu Formation were studied. Results The results show that： the pore type of the First member of Canglangpu Formation was mainly intragranular dissolved pores, which have the best positive correlation with the total face rate, followed by intergranular dissolved pores. The higher oolite/terrigenous sand content ratio is an important material basis for pore development, dissolution is the most favorable factor for pore development, and high energy shelf oolitic beach sedimentation is the key to the development of primary pores. The selective dissolution of atmospheric water in the syngenetic period is the key to the formation of the most important dissolution pores in oolitic sands in the First member of Canglangpu Formation, while the dolomitization in the quasi syngenetic and burial period enhances the compression resistance of rocks, reduces the solid volume, and increases the pores. The dissolution in the burial period is located along the fractures by tectonic disruption and early pores expansion. Terrigenous sand, cementation, compaction, and pressure dissolution are not conducive to the development of pores in the First member of the Canglangpu Formation. Conclusions Therefore, favorable carbonate components and subsequent diagenesis and tectonism have affected the development and evolution of pores and formed the lithologic sequence characteristics of the decreasing pore face rate： bright crystal oolitic dolomite, oolitic dolomite with sandy, sandy oolitic dolomite, fine-grained lithic sandstone, limestone, etc.
2024, 42(5): 1738-1752.
doi: 10.14027/j.issn.1000-0550.2022.143
Abstract:
Objective Quartz is one of the most important minerals in marine shale and can be found in several forms with multiple origins. In addition, different types of quartz may contribute differently to the geomechanical pro-perties and pore evolution of marine shale, and related research is lacking. It restricts a deep understanding of shale reservoir characteristics. Methods This study briefly reviews recent progress in the classification of quartz in typical marine shales. In order to better understand quartz types, silica sources, and the influence of biogenic microcrystalline quartz on geomechanical properties and pore evolution, marine shale samples were collected from the Upper Ordovician Wufeng Formation and the Lower Silurian Gaojiabian Formation, specifically well Gudi-1 of the Lower Yangtze Platform. These samples were analyzed using thin section-, X-ray diffraction-, and geochemical analysis, optical microscopy, nuclear magnetic resonance, and field-emission scanning electron microscopy combined with cathodoluminescence. The aim was to further reveal the pore evolution pattern in the Lower Paleozoic marine shales of the Lower Yangtze Platform from a new perspective and to provide a more reliable geological basis for the storage capacity of shale gas reservoirs.Optical microscope evidence was found of siliceous organism fragments, including biogenic silica accounting for approximately 45.17% of the total silica content, as well as characteristics of major and trace elements, such as a non-hydrothermal area located in the Al-Fe-Mn ternary diagram, negative correlations of Zr and SiO2, an Al/（Al+Fe+Mn） ratio of 0.65 to 0.76, and a cross-plot of Si versus Al. Overall, these indicate that siliceous organisms provided an important silica source for microcrystalline quartz. These siliceous organisms provide a relatively rich silica source for the precipitation of authigenic quartz during early diagenesis. Results and Discussions The quartz in well Gudi-1 is mainly detrital, microcrystalline, and organism skeletal quartz. Detrital quartz is of terrigenous origin and is characterized by bright luminescence. Most detrital quartz particles are silt-sized and range from 10-30 µm. Microcrystalline quartz is one of the most common authigenic quartz types and can be found in several marine shale formations. Microcrystalline quartz has no or low luminescence, indicative of authigenic origin. Furthermore, microcrystalline quartz can be divided into three categories： clay matrix-dispersed microcrystalline quartz, euhedral microcrystalline quartz, and amorphous microcrystalline quartz cements. Organism skeletal quartz is widely distributed in the studied shale samples and was mostly found in fragments of siliceous organisms, such as radiolarians and sponge spicules. Most radiolarians are oval and round with diameters of approximately 100 µm. Conclusions This study indicates that biogenic microcrystalline quartz may have implications for rock mechanics and the evolution of porosity. The positive correlation between the silica content and brittleness index of the studied shale reveals that the development of biogenic microcrystalline quartz enhances the brittleness of the shale to a certain extent. Biogenic microcrystalline quartz can be interconnected to form a rigid siliceous matrix framework, which largely promotes the geomechanical properties of the studied shale. Moreover, the positive correlation between the silica content and porosity indicates that the enrichment of biogenic silica is conducive to pore evolution and preservation. Organic matter and clay minerals in shale are susceptible to ductile deformation by compaction, while biogenic microcrystalline quartz can form a rigid framework to enhance its resistance to compaction, which is conducive to the preser-vation of the internal pore space within shale, especially intergranular- and organic matter pores.
Objective Quartz is one of the most important minerals in marine shale and can be found in several forms with multiple origins. In addition, different types of quartz may contribute differently to the geomechanical pro-perties and pore evolution of marine shale, and related research is lacking. It restricts a deep understanding of shale reservoir characteristics. Methods This study briefly reviews recent progress in the classification of quartz in typical marine shales. In order to better understand quartz types, silica sources, and the influence of biogenic microcrystalline quartz on geomechanical properties and pore evolution, marine shale samples were collected from the Upper Ordovician Wufeng Formation and the Lower Silurian Gaojiabian Formation, specifically well Gudi-1 of the Lower Yangtze Platform. These samples were analyzed using thin section-, X-ray diffraction-, and geochemical analysis, optical microscopy, nuclear magnetic resonance, and field-emission scanning electron microscopy combined with cathodoluminescence. The aim was to further reveal the pore evolution pattern in the Lower Paleozoic marine shales of the Lower Yangtze Platform from a new perspective and to provide a more reliable geological basis for the storage capacity of shale gas reservoirs.Optical microscope evidence was found of siliceous organism fragments, including biogenic silica accounting for approximately 45.17% of the total silica content, as well as characteristics of major and trace elements, such as a non-hydrothermal area located in the Al-Fe-Mn ternary diagram, negative correlations of Zr and SiO2, an Al/（Al+Fe+Mn） ratio of 0.65 to 0.76, and a cross-plot of Si versus Al. Overall, these indicate that siliceous organisms provided an important silica source for microcrystalline quartz. These siliceous organisms provide a relatively rich silica source for the precipitation of authigenic quartz during early diagenesis. Results and Discussions The quartz in well Gudi-1 is mainly detrital, microcrystalline, and organism skeletal quartz. Detrital quartz is of terrigenous origin and is characterized by bright luminescence. Most detrital quartz particles are silt-sized and range from 10-30 µm. Microcrystalline quartz is one of the most common authigenic quartz types and can be found in several marine shale formations. Microcrystalline quartz has no or low luminescence, indicative of authigenic origin. Furthermore, microcrystalline quartz can be divided into three categories： clay matrix-dispersed microcrystalline quartz, euhedral microcrystalline quartz, and amorphous microcrystalline quartz cements. Organism skeletal quartz is widely distributed in the studied shale samples and was mostly found in fragments of siliceous organisms, such as radiolarians and sponge spicules. Most radiolarians are oval and round with diameters of approximately 100 µm. Conclusions This study indicates that biogenic microcrystalline quartz may have implications for rock mechanics and the evolution of porosity. The positive correlation between the silica content and brittleness index of the studied shale reveals that the development of biogenic microcrystalline quartz enhances the brittleness of the shale to a certain extent. Biogenic microcrystalline quartz can be interconnected to form a rigid siliceous matrix framework, which largely promotes the geomechanical properties of the studied shale. Moreover, the positive correlation between the silica content and porosity indicates that the enrichment of biogenic silica is conducive to pore evolution and preservation. Organic matter and clay minerals in shale are susceptible to ductile deformation by compaction, while biogenic microcrystalline quartz can form a rigid framework to enhance its resistance to compaction, which is conducive to the preser-vation of the internal pore space within shale, especially intergranular- and organic matter pores.
2024, 42(5): 1753-1764.
doi: 10.14027/j.issn.1000-0550.2022.117
Abstract:
Objective The study make clear the main controlling factors of source rock in Shahezi Formation, and provides geologic basis for the optimal selection of gas exploration in deep layer of Songliao Basin. Methods We recovered the sedimentary facies and water property in the Shahazi Formation, based on core observation, organic geochemical analysis, and major and trace elements testing, determining fault activity, sedimentary facies, water properties, and their control on source rock quality. Results Two source rock, mudstone and coal, were identified in the Shahezi Formation. The faulting during the deposition of the first source rock in the Shahezi Formation was characterized by strong activity, generating a small lake with great water depth, high salinity, and strong reducing properties. These source rocks were mudstone in a semi-deep lake. The faulting activity during the deposition of the second source rock gradually weakened, with the lake size increasing while water depth decreased. Salinity and reducibility were also weakened, and the coal in shore lake was the primary source rock. Conclusions The source rock type was mainly controlled by fault activity, sedimentary facies, and water properties. The downthrown side become important storage for mudstone because of the fault activity, which determined the thickness of mudstone. The sedimentary facies controlled the source rock type through their impact on organic matter import. The difference between paleoenvironments, which was determined by paleosalins, paleowater depth, and redox, is the key factor for the enrichment and preservation of organic matter, and the main controlling factor for source rock quality distribution.
Objective The study make clear the main controlling factors of source rock in Shahezi Formation, and provides geologic basis for the optimal selection of gas exploration in deep layer of Songliao Basin. Methods We recovered the sedimentary facies and water property in the Shahazi Formation, based on core observation, organic geochemical analysis, and major and trace elements testing, determining fault activity, sedimentary facies, water properties, and their control on source rock quality. Results Two source rock, mudstone and coal, were identified in the Shahezi Formation. The faulting during the deposition of the first source rock in the Shahezi Formation was characterized by strong activity, generating a small lake with great water depth, high salinity, and strong reducing properties. These source rocks were mudstone in a semi-deep lake. The faulting activity during the deposition of the second source rock gradually weakened, with the lake size increasing while water depth decreased. Salinity and reducibility were also weakened, and the coal in shore lake was the primary source rock. Conclusions The source rock type was mainly controlled by fault activity, sedimentary facies, and water properties. The downthrown side become important storage for mudstone because of the fault activity, which determined the thickness of mudstone. The sedimentary facies controlled the source rock type through their impact on organic matter import. The difference between paleoenvironments, which was determined by paleosalins, paleowater depth, and redox, is the key factor for the enrichment and preservation of organic matter, and the main controlling factor for source rock quality distribution.
2024, 42(5): 1765-1783.
doi: 10.14027/j.issn.1000-0550.2022.144
Abstract:
Objective To determine the maturity and origin of crude oils from the Yingxi area, western Qaidam Basin, Methods the distribution and composition of various biomarkers, alkyl phenanthrenes and diamondoid hydrocarbons were analyzed in detail by gas chromatography-mass spectrometry （GC-MS） and gas chromatography-mass spectrometry-mass spectrometry （GC-MS-MS）. Results and Discussion The analytical results demonstrate that these oils have an even carbon predominance at nC20 and nC22 inthenormal alkane series, the （nC20+nC22）/（nC19+nC21） ratios are more than 1.0. In C13-20 regular isoprenoids, phytane is very abundant, the Pr/Ph ratios are less than 0.60, but the Ph/nC18 ratios are higher than 1.30. Moreover, the content of gammacerane indicating the salinity of depositional environment is relatively high, and the gammacerane index are between 0.90 and 1.36. These characte-ristics are consistent with the geological background of saline depositional environment during the Paleogene in the western Qaidam Basin. Based on the distribution and relative abundance of extended tricyclic terpanes and other biomarkers in different crude oils in the study area, they could be classified into two types. One of them from well Shi 62 and Shixi 58-1 has unusually abundant C19-49 tricyclic terpanes, the C19-30T/C27-35H and C23T/C30H ratios are more than 1.60 and 1.40, respectively. In addition, they have higher sterane/hopane （>4.50） and β-carotane/nC37 ratios （>3.20） and lower nC37/nC36 ratios （<1.0）, suggesting that algae are their main biological input. The other one from well Shi 57 and Shi 61 has the moderate content of extended C19-30 tricyclic terpanes, the C19-30T/C27-35H and C23T/C30H ratios are less than 0.50 and 0.55, respectively. In this king of crude oils, the sterane/hopane and β-carotane/nC37 ratios are less than 2.0 and 1.50, respectively, but the nC37/nC36 ratios are more than 1.0, demonstrating that the algae biological input is relatively low compared to the first one. In the study crude oils, the sterane maturity parameters such as C29 20S/（20S+20R） and ββ/（αα+ββ） have reached their equilibrium values and can not be used to eva-luate their maturity. Furthermore, aromatic maturity parameters related to methyl phenanthrene isomers such as MPI1 and MPR are usually used as maturity indicators, but their distribution and composition in the Paleogene saline source rocks in the study area does not change with increasing the maturity of organic matter, suggesting that they may not be effective maturity parameters in saline environment and can not be used to evaluate the maturity of crude oils. However, the distribution and composition of diamondoid hydrocarbons in source rocks and crude oils are mainly controlled by thermal evolution of organic matter and widely used to evaluate the maturity of organic matter and the cracking extent of crude oils. Based on the maturity parameters MAI and MDI related to diamondoid hydrocarbons, the maturity of these oils from the Yingxi area is very similar, and their Ro values are approximately 1.1%⁃1.3%, suggesting that they were generated after the hydrocarbon generation peak, but before the condensate stage. Conclusions In other words, biological input, not maturity, is the main factor affecting the distribution and abundance of extended tricyclic terpanes in different oils from the Yingxi area. Therefore, the two types of crude oils in the study area should have their own sources, and more attention should be given to them in the future.
Objective To determine the maturity and origin of crude oils from the Yingxi area, western Qaidam Basin, Methods the distribution and composition of various biomarkers, alkyl phenanthrenes and diamondoid hydrocarbons were analyzed in detail by gas chromatography-mass spectrometry （GC-MS） and gas chromatography-mass spectrometry-mass spectrometry （GC-MS-MS）. Results and Discussion The analytical results demonstrate that these oils have an even carbon predominance at nC20 and nC22 inthenormal alkane series, the （nC20+nC22）/（nC19+nC21） ratios are more than 1.0. In C13-20 regular isoprenoids, phytane is very abundant, the Pr/Ph ratios are less than 0.60, but the Ph/nC18 ratios are higher than 1.30. Moreover, the content of gammacerane indicating the salinity of depositional environment is relatively high, and the gammacerane index are between 0.90 and 1.36. These characte-ristics are consistent with the geological background of saline depositional environment during the Paleogene in the western Qaidam Basin. Based on the distribution and relative abundance of extended tricyclic terpanes and other biomarkers in different crude oils in the study area, they could be classified into two types. One of them from well Shi 62 and Shixi 58-1 has unusually abundant C19-49 tricyclic terpanes, the C19-30T/C27-35H and C23T/C30H ratios are more than 1.60 and 1.40, respectively. In addition, they have higher sterane/hopane （>4.50） and β-carotane/nC37 ratios （>3.20） and lower nC37/nC36 ratios （<1.0）, suggesting that algae are their main biological input. The other one from well Shi 57 and Shi 61 has the moderate content of extended C19-30 tricyclic terpanes, the C19-30T/C27-35H and C23T/C30H ratios are less than 0.50 and 0.55, respectively. In this king of crude oils, the sterane/hopane and β-carotane/nC37 ratios are less than 2.0 and 1.50, respectively, but the nC37/nC36 ratios are more than 1.0, demonstrating that the algae biological input is relatively low compared to the first one. In the study crude oils, the sterane maturity parameters such as C29 20S/（20S+20R） and ββ/（αα+ββ） have reached their equilibrium values and can not be used to eva-luate their maturity. Furthermore, aromatic maturity parameters related to methyl phenanthrene isomers such as MPI1 and MPR are usually used as maturity indicators, but their distribution and composition in the Paleogene saline source rocks in the study area does not change with increasing the maturity of organic matter, suggesting that they may not be effective maturity parameters in saline environment and can not be used to evaluate the maturity of crude oils. However, the distribution and composition of diamondoid hydrocarbons in source rocks and crude oils are mainly controlled by thermal evolution of organic matter and widely used to evaluate the maturity of organic matter and the cracking extent of crude oils. Based on the maturity parameters MAI and MDI related to diamondoid hydrocarbons, the maturity of these oils from the Yingxi area is very similar, and their Ro values are approximately 1.1%⁃1.3%, suggesting that they were generated after the hydrocarbon generation peak, but before the condensate stage. Conclusions In other words, biological input, not maturity, is the main factor affecting the distribution and abundance of extended tricyclic terpanes in different oils from the Yingxi area. Therefore, the two types of crude oils in the study area should have their own sources, and more attention should be given to them in the future.
2024, 42(5): 1784-1798.
doi: 10.14027/j.issn.1000-0550.2022.101
Abstract:
Objective The Ediacaran-Cambrian, as a key transition period in geological history, saw a dramatic transformation of the Earth system in terms of tectonic evolution, climate fluctuations, marine environment and biological community evolution. It is of great significance to study in depth the sediment sources, paleo-water depth, paleosalinity and redox conditions during this period. Methods Samples from the Maidiping Formation and the Qiongzhusi Formation of the well JS1 in the southwestern part of the Sichuan Basin were selected and analyzed in detail for total organic carbon （TOC）, major elements, trace elements and rare earth elements （REE）. [Results and Conclusions] The material sources of the Maidiping and Qiongzhusi Formations were found to be relatively consistent. The parent rock primarily consists of sedimentary rock and granite, and the material source area was created in the environment found at the continental margin. The Maidiping Formation contains more light rare earth elements （LREE） than heavy rare earth elements （HREE） （（La/Yb）N=6.05）, slightly to the right, and the average δEu is slightly <1, indicating normal seawater deposition in a shallow water body that was not significantly affected by hydrothermal action. The Qiongzhusi Formation has obvious LREE differentiation （（La/Yb）N=7.69）, LREE enrichment, HREE depletion and an obvious right dip, with weak negative δCe and δEu anomalies, possibly related to intermittent hydrothermal activity. In addition, comparative analyses for different wells have shown that the water body during the sedimentary period of the Maidiping Formation was in an oxidizing environment, and the sedimentary system in the region ranges from tidal-flat to shallow-shelf facies. The Qiongzhusi Formation as a whole belongs to an anaerobic environment, with a reducing environment its base and the degree of anoxia decreasing upwards. The regional develop-ment was a shallow- to deepwater shelf sedimentary system.
Objective The Ediacaran-Cambrian, as a key transition period in geological history, saw a dramatic transformation of the Earth system in terms of tectonic evolution, climate fluctuations, marine environment and biological community evolution. It is of great significance to study in depth the sediment sources, paleo-water depth, paleosalinity and redox conditions during this period. Methods Samples from the Maidiping Formation and the Qiongzhusi Formation of the well JS1 in the southwestern part of the Sichuan Basin were selected and analyzed in detail for total organic carbon （TOC）, major elements, trace elements and rare earth elements （REE）. [Results and Conclusions] The material sources of the Maidiping and Qiongzhusi Formations were found to be relatively consistent. The parent rock primarily consists of sedimentary rock and granite, and the material source area was created in the environment found at the continental margin. The Maidiping Formation contains more light rare earth elements （LREE） than heavy rare earth elements （HREE） （（La/Yb）N=6.05）, slightly to the right, and the average δEu is slightly <1, indicating normal seawater deposition in a shallow water body that was not significantly affected by hydrothermal action. The Qiongzhusi Formation has obvious LREE differentiation （（La/Yb）N=7.69）, LREE enrichment, HREE depletion and an obvious right dip, with weak negative δCe and δEu anomalies, possibly related to intermittent hydrothermal activity. In addition, comparative analyses for different wells have shown that the water body during the sedimentary period of the Maidiping Formation was in an oxidizing environment, and the sedimentary system in the region ranges from tidal-flat to shallow-shelf facies. The Qiongzhusi Formation as a whole belongs to an anaerobic environment, with a reducing environment its base and the degree of anoxia decreasing upwards. The regional develop-ment was a shallow- to deepwater shelf sedimentary system.
2024, 42(5): 1799-1816.
doi: 10.14027/j.issn.1000-0550.2022.128
Abstract:
Objective The paleosedimentary environment of the Gulong shale is the basis for the prediction of shale oil desserts. Methods Based on the salinity of modern lakes in Songliao Basin, the common and trace elements, rare earth elements, and biomarkers of the Gulong shale, the geochemical characteristics of the sedimentary environment and the enrichment mechanism of the organic matter were studied. Results The results show that during the formation of the Gulong shale, the climate was warm and humid, a freshwater brackish water reduction environment was developed, paleoproductivity was high, the sedimentational rate was low, and the water body was deep, which provided a geological basis for the formation, preservation, and enrichment of organic matter. The paleoclimate index, chemical index of alteration （CIA） during the shale deposition period was 63-74 and was dominated by a warm and humid climate. During the shale deposition period, the paleosalinity w（Sr）/w（Ba） was between 0.23-1.00, which was a fresh - brackish water environment, and w（V）/ w（V+Ni） was between 0.6-0.9, a dysoxic-anoxic environment. The paleowater depth was 25-117 m, indicating a semi deep-to-deep lake deposition. The （La/Yb）N value fluctuated between 0.90-1.41, representing a low sedimentational rate. Conclusions The enrichment of organic matter in the Gulong shale was caused by the favorable coupling of paleoclimate, paleosalinity, paleowater depth, sedimentation rate, and paleoproductivity. The research results can effectively guide the prediction, exploration, and development of Gulong shale oil desserts in Songliao Basin.
Objective The paleosedimentary environment of the Gulong shale is the basis for the prediction of shale oil desserts. Methods Based on the salinity of modern lakes in Songliao Basin, the common and trace elements, rare earth elements, and biomarkers of the Gulong shale, the geochemical characteristics of the sedimentary environment and the enrichment mechanism of the organic matter were studied. Results The results show that during the formation of the Gulong shale, the climate was warm and humid, a freshwater brackish water reduction environment was developed, paleoproductivity was high, the sedimentational rate was low, and the water body was deep, which provided a geological basis for the formation, preservation, and enrichment of organic matter. The paleoclimate index, chemical index of alteration （CIA） during the shale deposition period was 63-74 and was dominated by a warm and humid climate. During the shale deposition period, the paleosalinity w（Sr）/w（Ba） was between 0.23-1.00, which was a fresh - brackish water environment, and w（V）/ w（V+Ni） was between 0.6-0.9, a dysoxic-anoxic environment. The paleowater depth was 25-117 m, indicating a semi deep-to-deep lake deposition. The （La/Yb）N value fluctuated between 0.90-1.41, representing a low sedimentational rate. Conclusions The enrichment of organic matter in the Gulong shale was caused by the favorable coupling of paleoclimate, paleosalinity, paleowater depth, sedimentation rate, and paleoproductivity. The research results can effectively guide the prediction, exploration, and development of Gulong shale oil desserts in Songliao Basin.
2024, 42(5): 1817-1831.
doi: 10.14027/j.issn.1000-0550.2023.088
Abstract:
Objective The Middle Ordovician Yanxi Formation is a recently identified set of marine organic rich black fine-grained rock series in central Hunan and its surrounding areas, with good shale gas exploration potential. The research on the sedimentary characteristics of this black fine-grained rock series urgently needs to be strengthened. Methods The lithology and lithofacies combination types of the Yanxi Formation were identified through field exploration and slice identification, and the sedimentary environment was determined through element testing. Basin analysis technology, combined with regional tectonic movement history, was used to identify the nature and morphology of the prototype basin, determining favorable sedimentary facies zones, spatiotemporal distribution, and sedimentary patterns of the black fine-grained rock series. Results The lithology of the Yanxi Formation is primarily siliceous rock and carbonaceous shale, followed by silty shale and siltstone. Siliceous rocks are mainly biogenic in shallow marine environments, and carbonaceous shale is formed in a quiet and anoxic deep water detention environment. Four types of lithofacies combinations can be identified within the Yanxi Formation： thick pure carbon shale, carbon shale and siliceous rock combination, silty shale and siliceous rock combination, and silty fine sandstone with thin layer shale combination. Based on different rock facies combinations, the Yanxi Formation can be vertically divided into three sections： upper, middle, and lower； the distribution thickness of the three sections in the region is relatively stable and has good comparability. The thick layer of carbonaceous shale in the middle section is the most important source rock series. The Yanxi Formation is characterized by a thick southeast and thin northwest distribution in the study area, and the types of sedimentary facies include deep water basins, deep water continental shelves, shallow water continental shelves, and turbidite fans. The distribution positions of each sedimentary facies zone varied in different periods of the Ordovician, with clear zoning and continuous changes. Conclusions The central and surrounding areas of Hunan were in a stagnant hypoxic water environment during the Middle Ordovician and were a stable depression basin within the block. The deep-water continental shelf facies and deep-water basin facies are favorable facies belts for the development of organic shale in the Yanxi Formation.
Objective The Middle Ordovician Yanxi Formation is a recently identified set of marine organic rich black fine-grained rock series in central Hunan and its surrounding areas, with good shale gas exploration potential. The research on the sedimentary characteristics of this black fine-grained rock series urgently needs to be strengthened. Methods The lithology and lithofacies combination types of the Yanxi Formation were identified through field exploration and slice identification, and the sedimentary environment was determined through element testing. Basin analysis technology, combined with regional tectonic movement history, was used to identify the nature and morphology of the prototype basin, determining favorable sedimentary facies zones, spatiotemporal distribution, and sedimentary patterns of the black fine-grained rock series. Results The lithology of the Yanxi Formation is primarily siliceous rock and carbonaceous shale, followed by silty shale and siltstone. Siliceous rocks are mainly biogenic in shallow marine environments, and carbonaceous shale is formed in a quiet and anoxic deep water detention environment. Four types of lithofacies combinations can be identified within the Yanxi Formation： thick pure carbon shale, carbon shale and siliceous rock combination, silty shale and siliceous rock combination, and silty fine sandstone with thin layer shale combination. Based on different rock facies combinations, the Yanxi Formation can be vertically divided into three sections： upper, middle, and lower； the distribution thickness of the three sections in the region is relatively stable and has good comparability. The thick layer of carbonaceous shale in the middle section is the most important source rock series. The Yanxi Formation is characterized by a thick southeast and thin northwest distribution in the study area, and the types of sedimentary facies include deep water basins, deep water continental shelves, shallow water continental shelves, and turbidite fans. The distribution positions of each sedimentary facies zone varied in different periods of the Ordovician, with clear zoning and continuous changes. Conclusions The central and surrounding areas of Hunan were in a stagnant hypoxic water environment during the Middle Ordovician and were a stable depression basin within the block. The deep-water continental shelf facies and deep-water basin facies are favorable facies belts for the development of organic shale in the Yanxi Formation.
2024, 42(5): 1832-1848.
doi: 10.14027/j.issn.1000-0550.2022.153
Abstract:
Results Six lithofacies are identified from the shales of the Wufeng Formation-Longmaxi Formation： biological siliceous, clayey, （felsic-calcareous） silty, calcareous silty, calcareous, and felsic silty shale. The shale sedimentary environment of the Wufeng Formation-Longmaxi Formation underwent five stages of bottom-up changes. The paleo-climate shifted from warm and humid at the bottom of the Wufeng Formation to dry and hot at the middle to dry and cold at the top, and then from dry and hot at the lower part of the Longmaxi Formation to warm and humid at the middle and upper sections. In the lower parts of the Wufeng and Longmaxi formations, the sea level had risen owing to two large-scale transgressions. When the sea level is high, the extent of basin retention is weak, strong in reduction, low in salinity, and high in paleo-productivity. The glacier event at the top of Wufeng Formation caused the sea level to drop. When the sea level is low, the extent of basin retention is strong, weak in reduction, high in salinity, and low in paleo-productivity. The shale of the Wufeng Formation-Longmaxi Formation was mainly formed by suspended sedimentation under low energy conditions. Upwelling sedimentation developed in the middle Wufeng Formation and the lower Longmaxi Formation. A small amount of storm surge sedimentation developed at the top of the Wufeng Formation. In the upper Longmaxi Formation, gravity slumping, debris flows, and turbidity currents developed because of the shallowing of water and the increase of terrige-nous input. Therefore, the sedimentary model of the Wufeng Formation-Longmaxi Formation shale was established by integrating the lithofacies, depositional processes type, tectonic evolution, and sedimentary environment evolution. Conclusions The frequent changes of sedimentary environment and the diversity of depositional processes control the types and characteristics of lithofacies, further leading to significant differences in organic matter content, porosity, gas content, and other reservoir quality factors of different lithofacies. Further research on shale lithofacies and reservoir quality is required to provide theoretical basis for shale gas exploration and development. [Objective and Methods] The black shales of the Upper Ordovician and Lower Silurian in the southern Sichuan Basin are the main target of shale gas exploration, having attracted extensive attention among domestic and foreign scholars. To provide the theoretical basis for shale exploration and development in the future, the lithofacies division, sedimentary environment, and depositional processes of shale are explored. Through core observation and analysis using optical and scanning electron microscopes, the lithofacies are divided based on mineral composition and content, sedimentary structural characteristics, and comprehensive consideration of biological process and diagenesis. The paleoenvironment is analyzed by oxidation-reduction （U/Th,V/Cr,V/（V+Ni）,Ni/Co）, sea level change （Ce*）, hydrographic restriction （Mo/TOC）, paleo-climate （Sr/Cu）, paleo-salinity （Sr/Ba）, and paleo-productivity （Cu, biogenic Ba） indices. Depositional processes are identified by core and thin section observation, X-ray fluorescence scanning, and mineral X-ray diffraction whole rock analysis.
Results Six lithofacies are identified from the shales of the Wufeng Formation-Longmaxi Formation： biological siliceous, clayey, （felsic-calcareous） silty, calcareous silty, calcareous, and felsic silty shale. The shale sedimentary environment of the Wufeng Formation-Longmaxi Formation underwent five stages of bottom-up changes. The paleo-climate shifted from warm and humid at the bottom of the Wufeng Formation to dry and hot at the middle to dry and cold at the top, and then from dry and hot at the lower part of the Longmaxi Formation to warm and humid at the middle and upper sections. In the lower parts of the Wufeng and Longmaxi formations, the sea level had risen owing to two large-scale transgressions. When the sea level is high, the extent of basin retention is weak, strong in reduction, low in salinity, and high in paleo-productivity. The glacier event at the top of Wufeng Formation caused the sea level to drop. When the sea level is low, the extent of basin retention is strong, weak in reduction, high in salinity, and low in paleo-productivity. The shale of the Wufeng Formation-Longmaxi Formation was mainly formed by suspended sedimentation under low energy conditions. Upwelling sedimentation developed in the middle Wufeng Formation and the lower Longmaxi Formation. A small amount of storm surge sedimentation developed at the top of the Wufeng Formation. In the upper Longmaxi Formation, gravity slumping, debris flows, and turbidity currents developed because of the shallowing of water and the increase of terrige-nous input. Therefore, the sedimentary model of the Wufeng Formation-Longmaxi Formation shale was established by integrating the lithofacies, depositional processes type, tectonic evolution, and sedimentary environment evolution. Conclusions The frequent changes of sedimentary environment and the diversity of depositional processes control the types and characteristics of lithofacies, further leading to significant differences in organic matter content, porosity, gas content, and other reservoir quality factors of different lithofacies. Further research on shale lithofacies and reservoir quality is required to provide theoretical basis for shale gas exploration and development. [Objective and Methods] The black shales of the Upper Ordovician and Lower Silurian in the southern Sichuan Basin are the main target of shale gas exploration, having attracted extensive attention among domestic and foreign scholars. To provide the theoretical basis for shale exploration and development in the future, the lithofacies division, sedimentary environment, and depositional processes of shale are explored. Through core observation and analysis using optical and scanning electron microscopes, the lithofacies are divided based on mineral composition and content, sedimentary structural characteristics, and comprehensive consideration of biological process and diagenesis. The paleoenvironment is analyzed by oxidation-reduction （U/Th,V/Cr,V/（V+Ni）,Ni/Co）, sea level change （Ce*）, hydrographic restriction （Mo/TOC）, paleo-climate （Sr/Cu）, paleo-salinity （Sr/Ba）, and paleo-productivity （Cu, biogenic Ba） indices. Depositional processes are identified by core and thin section observation, X-ray fluorescence scanning, and mineral X-ray diffraction whole rock analysis.
