2011 Vol. 29, No. 2
Display Method:
2011, 29(2): 207-216.
Abstract:
The genetic mechanism of secondary porosity is complex, there are a lot of factors that control the development of secondary porosity. Looking at the various theories, regardless of the theory of formation water, geothermal field, precipitation, or the theory of organic matter evolution. From the mechanism point of view, dissolution of sandstone and formation of secondary porosity can be attributed to the organic acid and inorganic acid. The burial effect of clastic rocks on reservoir properties has twoway effect, one is the negative effect, that is the deterioration of reservoir properties, mainly are mechanical compaction and diagenetic cementation; the other is a positive effect, that is making reservoir properties better, mainly are
the dissolution of acid fluid such as leaching of meteoric water, carbonate and bicarbonate, organic acids and so on.
Combining with the oil geological features of the western part of Daqing Placanticline, the controlling factors of secondary porosity are analysesed in this paper. The author believe that secondary porosity in the study area is mainly controled by mechanical compaction, diagenetic cementation, meteoric water filtering effects and dissolution of acid fluid such as organic acids. The three controlling factors such as compaction, organic acids and carbonate, confirmed their threshold value of controling development of secondary porosity and the corresponding quantitative assessment model were set up.
Porosity is greatly influenced by depth, Athy (1930) and Hedberg (1936) believed that porosity and depth are an exponential relationship under the normal effect of compaction.This paper established normal compaction curve formulas for three lithologies such as coarse sandstone, fine sandstone and siltstone in the study area, the formulas are (z)=44exp(-0.000 65·z),(z)=47.8exp(-0.000 68·z), (z)=48exp(-0.000 7·z), where Z represents the depth, (z) represents the porosity at present.
During the process of thermal evolution and degradation of organics, organic acid and CO2 are generated. They could form organic acid thermal fluid in water, which could corrode aluminosilicate mineral and carbonate cement in reservoir and thus form the secondary porosity（Surdam,1989). According to the crossplot analysis of the abnormal high porosity data and organic acid content in the formation water and the crossplot analysis of the abnormal high porosity data and the value of HCO3+ CO23 in the research area, data shows that they have a good correlation, and the four abnormal high porosity areas on vertical（500~850 m, 1 050~1 350 m, 1 500~2 050 m, 2 125~2 350 m；） correspond to high value areas of acid organic and HCO-3+ CO23 respectively.
Then how to give a plane quantitative prediction to the secondary porosity? It has been proved that when the value of organic acid is higher than 1 000 mg/l or the value of HCO-3+ CO23 is higher than 3 000 mg/l the secondary porosity is extremely developed. Then what is the threshold of the porosity? The threshold is a normal compaction curve formula and it varies with depth. The developed area of the secondary porosity can be drawn on plane by using the value higher than the thresholds. In order to predict the plane characteristics of the secondary porosity better, reservoir group can also be divided. The area where the value is higher than zero is the relative developed area of the secondary porosity, and the area where the value is lower than zero is the relative area of diagenesis cementation and zero area is the normal compaction area or cancellation area. The higher the value is, the more developed the secondary porosity is.
Sedimentary facies is a congenital factor that controls the original physical properties of the reservoir, while diagenetic facies, as an acquired factor, is the consequence of late reformation. Consequently, study of sedimentary facies and diagenetic facies should not be neglected when the secondary porosity is researched. Based on the predicted isoline map of abnormal high porosity, organic acid value and the concentration of HCO-3+ CO23, combined with the sedimentary facies and the diagenetic facies, secondary porosity can be predicted.In order to check applicability of this method, the predicted results and the reserve areas are superimposed in this paper. The reserve areas in each formation coincide well with the results predicted in the paper. Thin section analysis also gives convincing evidence that there is a lot of secondary porosity in this area. Therefore, this method is convincible to predict the secondary porosity. It also provides a direct indication method for exploration deployment and the submission of reserves.
The genetic mechanism of secondary porosity is complex, there are a lot of factors that control the development of secondary porosity. Looking at the various theories, regardless of the theory of formation water, geothermal field, precipitation, or the theory of organic matter evolution. From the mechanism point of view, dissolution of sandstone and formation of secondary porosity can be attributed to the organic acid and inorganic acid. The burial effect of clastic rocks on reservoir properties has twoway effect, one is the negative effect, that is the deterioration of reservoir properties, mainly are mechanical compaction and diagenetic cementation; the other is a positive effect, that is making reservoir properties better, mainly are
the dissolution of acid fluid such as leaching of meteoric water, carbonate and bicarbonate, organic acids and so on.
Combining with the oil geological features of the western part of Daqing Placanticline, the controlling factors of secondary porosity are analysesed in this paper. The author believe that secondary porosity in the study area is mainly controled by mechanical compaction, diagenetic cementation, meteoric water filtering effects and dissolution of acid fluid such as organic acids. The three controlling factors such as compaction, organic acids and carbonate, confirmed their threshold value of controling development of secondary porosity and the corresponding quantitative assessment model were set up.
Porosity is greatly influenced by depth, Athy (1930) and Hedberg (1936) believed that porosity and depth are an exponential relationship under the normal effect of compaction.This paper established normal compaction curve formulas for three lithologies such as coarse sandstone, fine sandstone and siltstone in the study area, the formulas are (z)=44exp(-0.000 65·z),(z)=47.8exp(-0.000 68·z), (z)=48exp(-0.000 7·z), where Z represents the depth, (z) represents the porosity at present.
During the process of thermal evolution and degradation of organics, organic acid and CO2 are generated. They could form organic acid thermal fluid in water, which could corrode aluminosilicate mineral and carbonate cement in reservoir and thus form the secondary porosity（Surdam,1989). According to the crossplot analysis of the abnormal high porosity data and organic acid content in the formation water and the crossplot analysis of the abnormal high porosity data and the value of HCO3+ CO23 in the research area, data shows that they have a good correlation, and the four abnormal high porosity areas on vertical（500~850 m, 1 050~1 350 m, 1 500~2 050 m, 2 125~2 350 m；） correspond to high value areas of acid organic and HCO-3+ CO23 respectively.
Then how to give a plane quantitative prediction to the secondary porosity? It has been proved that when the value of organic acid is higher than 1 000 mg/l or the value of HCO-3+ CO23 is higher than 3 000 mg/l the secondary porosity is extremely developed. Then what is the threshold of the porosity? The threshold is a normal compaction curve formula and it varies with depth. The developed area of the secondary porosity can be drawn on plane by using the value higher than the thresholds. In order to predict the plane characteristics of the secondary porosity better, reservoir group can also be divided. The area where the value is higher than zero is the relative developed area of the secondary porosity, and the area where the value is lower than zero is the relative area of diagenesis cementation and zero area is the normal compaction area or cancellation area. The higher the value is, the more developed the secondary porosity is.
Sedimentary facies is a congenital factor that controls the original physical properties of the reservoir, while diagenetic facies, as an acquired factor, is the consequence of late reformation. Consequently, study of sedimentary facies and diagenetic facies should not be neglected when the secondary porosity is researched. Based on the predicted isoline map of abnormal high porosity, organic acid value and the concentration of HCO-3+ CO23, combined with the sedimentary facies and the diagenetic facies, secondary porosity can be predicted.In order to check applicability of this method, the predicted results and the reserve areas are superimposed in this paper. The reserve areas in each formation coincide well with the results predicted in the paper. Thin section analysis also gives convincing evidence that there is a lot of secondary porosity in this area. Therefore, this method is convincible to predict the secondary porosity. It also provides a direct indication method for exploration deployment and the submission of reserves.
2011, 29(2): 226-234.
Abstract:
The study area is located in the southwest of the Ordos Basin, with the tectonic position in the intersection of Ordos Basin and QinlingQilian orogenic belt, and the area is adjacent to the western thrust fold belt in the west, and looking at Qinling orogenic belt across Weihe graben in the south. Since the Late Triassic, molasse formations were widely developed because of the neighbouring orogenic belts' multistage tectonic movements. Among them the Yijun conglomerate at the bottom of the Lower Cretaceous is the typical molasse formation. The study on it has great significance and value to discuss the original basin boundary and basinmountain evolution in the southwest of the Early Cretaceous basin.
Using fabric analysis method to study the composition, particle size, shape and preferred orientations of nine measuring points from west to east in Yijun conglomerate in the southern Early Cretaceous basin can clarify the composition, source and formation environment of these gravels. The composition analysis shows that the component gravels from the west to the east in the study area can be divided into three regions: Longxian Qianyang area's gravels are mainly carbonate rocks, LinyouYongping area's gravels are dominated by granite and metamorphic rocks, Binxian area's gravels are quartzite and metamorphic rocks. Analysis on particle size, shape and preferred orientations indicate that the gravels size show largesmalllargesmall rule from west to east, generally with poor sorting, subangular to subrounded and low degree weathering, and the material mainly come from the southern of the basin, which can be seen that the Yijun conglomerate is an alluvial fanfluvial sedimentary gravels with arid environment and rapid accumulation to deposit in the depression at the foot of mountain.
Combining with analysis of adjacent areas rock formations and regional geology, we consider that the provenance of Lower Cretaceous should not come from the old strata of southern Weibei uplift or the preCenozoic strata of Weihe garben. LinyouBinxian region's gravels mainly came from the North Qinling orogenic belt, and LongxianQianyang's gravels might come from the Paleozoic of Longshan area, which was the extrusion response of Qinling orogenic belt at southwestern basin in Late Mesozoic. It can be further explained by the evidence that the largescale uplift does not exist at Weibei area or it's southern to prevent the source material from Qinling orogenic belt. We can infer that the development time of Weibei largescale uplift may be in the late Early Cretaceous at least.
While the sedimentary facies of the southern basin in Lower Cretaceous are fluvial, delta and lake, therefore, it is considered that the southern boundary of the heyday sedimentary basin in the Cretaceous may be still extended to the south, which may be at least in Weihe region or even in the northern foot of Qinling mountain.
The study area is located in the southwest of the Ordos Basin, with the tectonic position in the intersection of Ordos Basin and QinlingQilian orogenic belt, and the area is adjacent to the western thrust fold belt in the west, and looking at Qinling orogenic belt across Weihe graben in the south. Since the Late Triassic, molasse formations were widely developed because of the neighbouring orogenic belts' multistage tectonic movements. Among them the Yijun conglomerate at the bottom of the Lower Cretaceous is the typical molasse formation. The study on it has great significance and value to discuss the original basin boundary and basinmountain evolution in the southwest of the Early Cretaceous basin.
Using fabric analysis method to study the composition, particle size, shape and preferred orientations of nine measuring points from west to east in Yijun conglomerate in the southern Early Cretaceous basin can clarify the composition, source and formation environment of these gravels. The composition analysis shows that the component gravels from the west to the east in the study area can be divided into three regions: Longxian Qianyang area's gravels are mainly carbonate rocks, LinyouYongping area's gravels are dominated by granite and metamorphic rocks, Binxian area's gravels are quartzite and metamorphic rocks. Analysis on particle size, shape and preferred orientations indicate that the gravels size show largesmalllargesmall rule from west to east, generally with poor sorting, subangular to subrounded and low degree weathering, and the material mainly come from the southern of the basin, which can be seen that the Yijun conglomerate is an alluvial fanfluvial sedimentary gravels with arid environment and rapid accumulation to deposit in the depression at the foot of mountain.
Combining with analysis of adjacent areas rock formations and regional geology, we consider that the provenance of Lower Cretaceous should not come from the old strata of southern Weibei uplift or the preCenozoic strata of Weihe garben. LinyouBinxian region's gravels mainly came from the North Qinling orogenic belt, and LongxianQianyang's gravels might come from the Paleozoic of Longshan area, which was the extrusion response of Qinling orogenic belt at southwestern basin in Late Mesozoic. It can be further explained by the evidence that the largescale uplift does not exist at Weibei area or it's southern to prevent the source material from Qinling orogenic belt. We can infer that the development time of Weibei largescale uplift may be in the late Early Cretaceous at least.
While the sedimentary facies of the southern basin in Lower Cretaceous are fluvial, delta and lake, therefore, it is considered that the southern boundary of the heyday sedimentary basin in the Cretaceous may be still extended to the south, which may be at least in Weihe region or even in the northern foot of Qinling mountain.
2011, 29(2): 245-254.
Abstract:
The catastrophe and extinctions occurred in late Cretaceous is one of the most severe events during the geological history, a thin clay boundary worldwide deposited on the earth during this period was called "KT boundary". The nerth western Tarim Basin is an ideal experimental field preserving continuous Late CretaceousPaleogene marine stratum and excellent outcrops in China. Previous researchers have done a lot of work and made a series of important achievements, however, the research and discussion on the paleoenvrionment and paleoclimate from Late Cretaceous to Early Paleogene is seldom involved. In this study, Kuzigongsu section of Late CretaceousPaleogene strata were selected to investigate the lithology and sedimentary environment changes and geochemical characters of the Late CretaceousPaleogene marine stratum. The results showed that the sediment color, calcium carbonate, organic carbon content and carbon/oxygen stable isotope changed abnormally in the 5 cm thick clay layer distributed between the Yigeziya and the Tuyiluoke Formations.The three curves of L*(lightness), a*(reddness), b*(yellowness) representing the sediment color index changed slowly within Tuyiluoke Fm, but fluctuated greatly in the part of the Tuyiluoke Fm. The CaCO3 content was also divided into two parts: Lower Cretaceous Yigeziya Fm is higher (average content of 51.53%), paleocene Tuyiluoke Fm is lower (average 19.24%), while the content between the two Formations sharply decreased (4.92%). The organic carban content maintained stably in Yigeziya Fm. (average content 0.12%), the lowest value was detected in the graygreen clay between two Formations, the content recovered to >0.1% after entering the Yigeziya Fm. The curves of carbon and oxygen isotopes changed with the graygreen calcareous mudstone between the Yigeziya Fm and Tuyiluoke Fm as the turing point, δ13C suddenly from positive value to negative, and remained negative until 3.78 m deep at the bottom of Tuyiluoke Fm, then turn positive (varying from 0.8‰ to 3.5‰). Along with previous studies, we conformed that the 5 cm thick clay layer, which is the deposition production of the significant global abrupt events during Late Cretaceou, distributed between the Yigeziya and the Tuyiluoke Formations is the CretaceousPaleogene boundary (KT boundary). As a consequence, the marine deposit changed from magnificent transgression deposit of Yigeziya Group to dramatic regression deposit of Tuyiluoke Group, companied with the decrease of seawater temperature, increase of CO2 solubility, reduce of oxygen content and abrupt wane of oxidization, suggested fantastic environmental changes. Thus, this will surely enrich the events study from Cretaceous to Paleogene and profoundly influence the determination of the age sequence and regional paleoenvironment in Tarim Basin.
The catastrophe and extinctions occurred in late Cretaceous is one of the most severe events during the geological history, a thin clay boundary worldwide deposited on the earth during this period was called "KT boundary". The nerth western Tarim Basin is an ideal experimental field preserving continuous Late CretaceousPaleogene marine stratum and excellent outcrops in China. Previous researchers have done a lot of work and made a series of important achievements, however, the research and discussion on the paleoenvrionment and paleoclimate from Late Cretaceous to Early Paleogene is seldom involved. In this study, Kuzigongsu section of Late CretaceousPaleogene strata were selected to investigate the lithology and sedimentary environment changes and geochemical characters of the Late CretaceousPaleogene marine stratum. The results showed that the sediment color, calcium carbonate, organic carbon content and carbon/oxygen stable isotope changed abnormally in the 5 cm thick clay layer distributed between the Yigeziya and the Tuyiluoke Formations.The three curves of L*(lightness), a*(reddness), b*(yellowness) representing the sediment color index changed slowly within Tuyiluoke Fm, but fluctuated greatly in the part of the Tuyiluoke Fm. The CaCO3 content was also divided into two parts: Lower Cretaceous Yigeziya Fm is higher (average content of 51.53%), paleocene Tuyiluoke Fm is lower (average 19.24%), while the content between the two Formations sharply decreased (4.92%). The organic carban content maintained stably in Yigeziya Fm. (average content 0.12%), the lowest value was detected in the graygreen clay between two Formations, the content recovered to >0.1% after entering the Yigeziya Fm. The curves of carbon and oxygen isotopes changed with the graygreen calcareous mudstone between the Yigeziya Fm and Tuyiluoke Fm as the turing point, δ13C suddenly from positive value to negative, and remained negative until 3.78 m deep at the bottom of Tuyiluoke Fm, then turn positive (varying from 0.8‰ to 3.5‰). Along with previous studies, we conformed that the 5 cm thick clay layer, which is the deposition production of the significant global abrupt events during Late Cretaceou, distributed between the Yigeziya and the Tuyiluoke Formations is the CretaceousPaleogene boundary (KT boundary). As a consequence, the marine deposit changed from magnificent transgression deposit of Yigeziya Group to dramatic regression deposit of Tuyiluoke Group, companied with the decrease of seawater temperature, increase of CO2 solubility, reduce of oxygen content and abrupt wane of oxidization, suggested fantastic environmental changes. Thus, this will surely enrich the events study from Cretaceous to Paleogene and profoundly influence the determination of the age sequence and regional paleoenvironment in Tarim Basin.
2011, 29(2): 268-276.
Abstract:
Huhehu Depression is a key exploration area in the east grabens which is located in the southeast of Hailar Basin. The depression is a halfgraben rift subsidence structure, characterized by it being faulted in the east and overlapped in the west, and its area is 2 500 km2. The 2nd member of the Nantun Formation in Huhehu Depression has developed the coal measures buried deeply whose strata formed by a set of gray siltstone and dark thick mudstone which mixed with black coal, gray fine sandstone and grey conglomerate. And the rock layers developed a series of multilayer and thicker coal seam, its reflectance of vitrinite is between 0.8%～1.5%, and the coal rank between gas coalcoking coal stage, so the coal measures have a better hydrocarbon potential. Also, because of the weaker later tectonic movement, the coal measures contain rich oilgas (CBM) resources. This paper mainly analyzes the characteristic of the stratigraphic framework, coal accumulation environments and coal accumulation laws in the case of the coal measures of the 2nd member in the Nantun Formation to guide the petroleum exploration in Huhehu Depression in the future.
Based on the theory and method of sequence stratigraphy，according to the drilling, well logging and seismic data, the sequence interfaces (the top interface SB4, the bottom interface SB3) and system tract interfaces (defined by the initial and maximum flooding surfaces) of the 2nd member in the Nantun Formation are identified. Finally, the sequence stratigraphic framework is established by the comprehensive analysis of the tectonic evolution, paleontological features, paleoclimate changes and lake level variations. Also, the main controlling factors on sequence formation are analyzed briefly.
At present, there are 11 wildcat wells in the study area. Through the analysis of the single well facies and sections of correlation of the stratigraphic, combined with other single factor maps, such as the stratum isopach maps, sandstone isopach maps, sandstone content isoline maps, dark mudstone, carbonaceous shale and coal layers thickness isopach maps, with the system tracts as the units, the author has described the sedimentary system distribution and temporalspatial evolution characteristics within the sequence stratigraphic framework.
Through comprehensive analysis of sedimentary evolution and correlation of the sequence stratigraghy, a study of coal layers development features and the patterns of coal accumulation is carried out with the application of basic theory and method of the coalfield geology. The 2nd member in the Nantun Formation of Huhehu Depression which was in the faults extension period, characterized by the lowamplitude, weak and pulsative vibration of the tectonic subsidence, During this period, it experienced an infilling evolution process of water in withdrawal, and the lacustrinebraided river delta sedimentary systems developed dominantly. In the early stage of baselevel falling, the increasing rate of accommodation space was consistent with the supply rate of sedimentary source, and the lacustrine areas continuously existed in the center of the depression, so the peat swamp formed stably and accumulated coal in the lacustrine basin margin of the depression; With the base level rising up, the groundwater level rise regionally, the 2nd member of the Nantun Formation developed shoreshallow lacustrine swamp coalforming environments. Because of the peat beds were buried and preserved rapidly by the highstand system tract sediments, the coalforming conditions of transgressive system tract were favourable; Lastly, in the highstand system tract sedimentary period also developed the coal accumulation environment, as a result of progradation of braided river delta, a lot of peat layers were buried rapidly in the braided river delta plain. Accordingly, controlled by the lake level changing system and the relatively steady tectonic movement, the major coal accumulation environments are interdistributary estuary swamp of the delta plain and shoreshallow lacustrine swamp.
As mentioned above, the study area was controlled by the rates of the tectonic subsidence and the peat accumulation every system tract developed coalaccumulation sedimentary conditions. However, the degrees of development and distribution ranges of the coal layers are significantly different between the different system tracts. The results indicate that the coalaccumulation mainly developed in the later period of lowstand system tract and transgressive system tract. In vertical directions, the coal layers mainly formed in transgressive system tract where the water started to expand. During this period, a large area and long term lacustrinebraided river delta sedimentary systems were peaty and swampy largely in shoreshallow lacustrine zones where the water was shallow. Under such circumstances, because of the increasing rate of the accommodation space matched balanced with the sedimentary accumulation rate, the stratum was very conducive to form series thick and stable coal layers. But the coal accumulation was the weakest in the highstand system tract in which the distribution of formed coal layers was thin and more localized.
Controlled by the tectonic subsidence and sedimentary systems, the coal accumulation of each system tract shows obvious zonations and situations that Southeast of the Depression is strong, while Northeast is weak. The coalrich units are in accordance with the distribution of the lacustrinebraided river delta sedimentary facies, mainly distribute between lacustrine basin margin and the subsidence center, and near to the lacustrine basin margin. However, it was not conducive to develop peat swamp in the deeper lake water body, the coal layers became thin even pinchout from the braided river delta plain to deep lake.
Huhehu Depression is a key exploration area in the east grabens which is located in the southeast of Hailar Basin. The depression is a halfgraben rift subsidence structure, characterized by it being faulted in the east and overlapped in the west, and its area is 2 500 km2. The 2nd member of the Nantun Formation in Huhehu Depression has developed the coal measures buried deeply whose strata formed by a set of gray siltstone and dark thick mudstone which mixed with black coal, gray fine sandstone and grey conglomerate. And the rock layers developed a series of multilayer and thicker coal seam, its reflectance of vitrinite is between 0.8%～1.5%, and the coal rank between gas coalcoking coal stage, so the coal measures have a better hydrocarbon potential. Also, because of the weaker later tectonic movement, the coal measures contain rich oilgas (CBM) resources. This paper mainly analyzes the characteristic of the stratigraphic framework, coal accumulation environments and coal accumulation laws in the case of the coal measures of the 2nd member in the Nantun Formation to guide the petroleum exploration in Huhehu Depression in the future.
Based on the theory and method of sequence stratigraphy，according to the drilling, well logging and seismic data, the sequence interfaces (the top interface SB4, the bottom interface SB3) and system tract interfaces (defined by the initial and maximum flooding surfaces) of the 2nd member in the Nantun Formation are identified. Finally, the sequence stratigraphic framework is established by the comprehensive analysis of the tectonic evolution, paleontological features, paleoclimate changes and lake level variations. Also, the main controlling factors on sequence formation are analyzed briefly.
At present, there are 11 wildcat wells in the study area. Through the analysis of the single well facies and sections of correlation of the stratigraphic, combined with other single factor maps, such as the stratum isopach maps, sandstone isopach maps, sandstone content isoline maps, dark mudstone, carbonaceous shale and coal layers thickness isopach maps, with the system tracts as the units, the author has described the sedimentary system distribution and temporalspatial evolution characteristics within the sequence stratigraphic framework.
Through comprehensive analysis of sedimentary evolution and correlation of the sequence stratigraghy, a study of coal layers development features and the patterns of coal accumulation is carried out with the application of basic theory and method of the coalfield geology. The 2nd member in the Nantun Formation of Huhehu Depression which was in the faults extension period, characterized by the lowamplitude, weak and pulsative vibration of the tectonic subsidence, During this period, it experienced an infilling evolution process of water in withdrawal, and the lacustrinebraided river delta sedimentary systems developed dominantly. In the early stage of baselevel falling, the increasing rate of accommodation space was consistent with the supply rate of sedimentary source, and the lacustrine areas continuously existed in the center of the depression, so the peat swamp formed stably and accumulated coal in the lacustrine basin margin of the depression; With the base level rising up, the groundwater level rise regionally, the 2nd member of the Nantun Formation developed shoreshallow lacustrine swamp coalforming environments. Because of the peat beds were buried and preserved rapidly by the highstand system tract sediments, the coalforming conditions of transgressive system tract were favourable; Lastly, in the highstand system tract sedimentary period also developed the coal accumulation environment, as a result of progradation of braided river delta, a lot of peat layers were buried rapidly in the braided river delta plain. Accordingly, controlled by the lake level changing system and the relatively steady tectonic movement, the major coal accumulation environments are interdistributary estuary swamp of the delta plain and shoreshallow lacustrine swamp.
As mentioned above, the study area was controlled by the rates of the tectonic subsidence and the peat accumulation every system tract developed coalaccumulation sedimentary conditions. However, the degrees of development and distribution ranges of the coal layers are significantly different between the different system tracts. The results indicate that the coalaccumulation mainly developed in the later period of lowstand system tract and transgressive system tract. In vertical directions, the coal layers mainly formed in transgressive system tract where the water started to expand. During this period, a large area and long term lacustrinebraided river delta sedimentary systems were peaty and swampy largely in shoreshallow lacustrine zones where the water was shallow. Under such circumstances, because of the increasing rate of the accommodation space matched balanced with the sedimentary accumulation rate, the stratum was very conducive to form series thick and stable coal layers. But the coal accumulation was the weakest in the highstand system tract in which the distribution of formed coal layers was thin and more localized.
Controlled by the tectonic subsidence and sedimentary systems, the coal accumulation of each system tract shows obvious zonations and situations that Southeast of the Depression is strong, while Northeast is weak. The coalrich units are in accordance with the distribution of the lacustrinebraided river delta sedimentary facies, mainly distribute between lacustrine basin margin and the subsidence center, and near to the lacustrine basin margin. However, it was not conducive to develop peat swamp in the deeper lake water body, the coal layers became thin even pinchout from the braided river delta plain to deep lake.
2011, 29(2): 286-292.
Abstract:
Sequence stratigraphy of Ordovician strata in the south part of Ordos area, which appears continuous, clear, and its corresponding drilling,the south, is the ideal locality to analyze the sequence stratigraphy. Based on the measured outcrop sections, drilling and logging data, the structure and lithology interfaces are identified, structure interfaces includes the boundary of the Ordovician tops and bottoms, the boundary of Majiagou Formation bottom and the Pingliang Formation bottom; the boundary of lithology and lithofacies are located between Yeli Formation and Liangjiashan Formation, Majiagou Formation and Pingliang Formation interior, and the bottom interface Beiguoshan Formation.
Based on the 8 sequence interfaces, 7 thirdorder sequences are identified, which is SQ1 of Yeli Formation, SQ2 of Liangjiashan Formation, the average number of yearly hours is 8.25 Ma；SQ3 to SQ4 of Majiagou formation, the average number of yearly hours is 5.45 Ma；SQ5 to SQ6 of Pingliang Formation. SQ7 is the last one of Beiguoshan Formation. SQ1SQ6 was only consist by Transgressive system tract and Highstand system tract. SQ7 involves Lowstand system tract and Transgressive system tract since the Highstand system tract have been eroded due to tectonic uplift. SQ1 to SQ6 is a depositional system of open platform, SQ7 is the sedimentary in platform edge slope.
During EarlyMiddle Ordovician, it was turned to be slope facies as a whole in the study area, and deposited carbonate in general. From SQ1 to SQ4, the central part of the basin was terrestrial while the south of the basin accepted deposits from the QinlingQilian sea, resulting in a different rock association of the local Majiagou Formation contrasted with that of the easten part. In the SQ5 time, under the influence of the subduction, that of Qinlingqilian oceanic plate to the Huabei Plate, Erdos land margin turned from passive to positive, and deposited a series of terrigenous clastic sediment, while still the centraleastern of the south fringe deposited mainly with slope facies. Later at the SQ7 stage, pulled by the subduction of the Qinqi sea, south rim deposited widely a set of slope fan breccioid limestone.
Sequence stratigraphy of Ordovician strata in the south part of Ordos area, which appears continuous, clear, and its corresponding drilling,the south, is the ideal locality to analyze the sequence stratigraphy. Based on the measured outcrop sections, drilling and logging data, the structure and lithology interfaces are identified, structure interfaces includes the boundary of the Ordovician tops and bottoms, the boundary of Majiagou Formation bottom and the Pingliang Formation bottom; the boundary of lithology and lithofacies are located between Yeli Formation and Liangjiashan Formation, Majiagou Formation and Pingliang Formation interior, and the bottom interface Beiguoshan Formation.
Based on the 8 sequence interfaces, 7 thirdorder sequences are identified, which is SQ1 of Yeli Formation, SQ2 of Liangjiashan Formation, the average number of yearly hours is 8.25 Ma；SQ3 to SQ4 of Majiagou formation, the average number of yearly hours is 5.45 Ma；SQ5 to SQ6 of Pingliang Formation. SQ7 is the last one of Beiguoshan Formation. SQ1SQ6 was only consist by Transgressive system tract and Highstand system tract. SQ7 involves Lowstand system tract and Transgressive system tract since the Highstand system tract have been eroded due to tectonic uplift. SQ1 to SQ6 is a depositional system of open platform, SQ7 is the sedimentary in platform edge slope.
During EarlyMiddle Ordovician, it was turned to be slope facies as a whole in the study area, and deposited carbonate in general. From SQ1 to SQ4, the central part of the basin was terrestrial while the south of the basin accepted deposits from the QinlingQilian sea, resulting in a different rock association of the local Majiagou Formation contrasted with that of the easten part. In the SQ5 time, under the influence of the subduction, that of Qinlingqilian oceanic plate to the Huabei Plate, Erdos land margin turned from passive to positive, and deposited a series of terrigenous clastic sediment, while still the centraleastern of the south fringe deposited mainly with slope facies. Later at the SQ7 stage, pulled by the subduction of the Qinqi sea, south rim deposited widely a set of slope fan breccioid limestone.
2011, 29(2): 303-311.
Abstract:
The Matoushan Formation, one of the most important orebearing stratum, is located in the Redbed basin of central Yunnan Province, China. Its lithology is mainly clastic rocks, including mudstone, sandstone, as well as conglomerate. The geochemical compositions of sandstones and argillaceous rocks could discriminate the tectonic setting and provenance of source rock area. Based on the geochemical signatures of major elements, trace elements, and rare earth elements of the nonmineral rocks, which picked from Matoushan Formation of Mouding, reveal that the sandstones and argillaceous rocks of Matoushan Formation, which are characterized by: ①high contents of SiO2; ②Enrichment some chalcophile elements such as Ag,Pb,Bi and Ni, loss of some lithophile elements for example Be,Ga,Rb,Y,Nb,Ta,Hf,Sr and Sn; ③LREE enrichment, HREE depletion and slightly negative Eu anomaly. The differentiate diagrams refer that the provenance of Matoushan Formation belong to passive continental margin and active continental margin tectonic setting. The primary materials of sedimentary rocks should come from the upper continental crust, and are mainly composed by felsic rocks. Combined with the data of sedimentary facies and palaeogeography, we know that the principal source rock area of Matoushan Formation are Kangdian ancient land and Ailaoshan orogenic belt.
The Matoushan Formation, one of the most important orebearing stratum, is located in the Redbed basin of central Yunnan Province, China. Its lithology is mainly clastic rocks, including mudstone, sandstone, as well as conglomerate. The geochemical compositions of sandstones and argillaceous rocks could discriminate the tectonic setting and provenance of source rock area. Based on the geochemical signatures of major elements, trace elements, and rare earth elements of the nonmineral rocks, which picked from Matoushan Formation of Mouding, reveal that the sandstones and argillaceous rocks of Matoushan Formation, which are characterized by: ①high contents of SiO2; ②Enrichment some chalcophile elements such as Ag,Pb,Bi and Ni, loss of some lithophile elements for example Be,Ga,Rb,Y,Nb,Ta,Hf,Sr and Sn; ③LREE enrichment, HREE depletion and slightly negative Eu anomaly. The differentiate diagrams refer that the provenance of Matoushan Formation belong to passive continental margin and active continental margin tectonic setting. The primary materials of sedimentary rocks should come from the upper continental crust, and are mainly composed by felsic rocks. Combined with the data of sedimentary facies and palaeogeography, we know that the principal source rock area of Matoushan Formation are Kangdian ancient land and Ailaoshan orogenic belt.
2011, 29(2): 321-327.
Abstract:
Highresolution grain size, biogenic silica and its mass accumulation rate (MAR) records are gained by AMS14C dating and analyses of grain size and biogenic silica in Core MZ02 from the muddy area on the inner shelf of the East China Sea. The grain size results reflect a lowenergy shallow sea shelf depositional environment dominated by the coastal currents. The biogenic silica content in Core MZ02 is less than 1.5%, mainly controlled by the low supply of biogenic silica remains, high silica dissolution and terrigenous matter dilution. Comparing the biogenic silica content and its MAR with the climatic record inferred from the Eastern China mainland, the relatively high biogenic silica and its MAR generally concur with the warm and humid climatic intervals, and vice versa, suggesting the response of the biogenic silica to paleoclimatic variation of Eastern China. The high paleoproductivity indicated by biogenic silica and its MAR was induced by the intensified Asian summer monsoon, which could strengthen the upwelling and increase the supply of nutrition materials. In addition, the biogenic silica and its MAR correlated well with the δ18O records of GISP2 ice core and Hongyuan peat, which therefore revealed a regional response of biogenic silica to global climate change.
Highresolution grain size, biogenic silica and its mass accumulation rate (MAR) records are gained by AMS14C dating and analyses of grain size and biogenic silica in Core MZ02 from the muddy area on the inner shelf of the East China Sea. The grain size results reflect a lowenergy shallow sea shelf depositional environment dominated by the coastal currents. The biogenic silica content in Core MZ02 is less than 1.5%, mainly controlled by the low supply of biogenic silica remains, high silica dissolution and terrigenous matter dilution. Comparing the biogenic silica content and its MAR with the climatic record inferred from the Eastern China mainland, the relatively high biogenic silica and its MAR generally concur with the warm and humid climatic intervals, and vice versa, suggesting the response of the biogenic silica to paleoclimatic variation of Eastern China. The high paleoproductivity indicated by biogenic silica and its MAR was induced by the intensified Asian summer monsoon, which could strengthen the upwelling and increase the supply of nutrition materials. In addition, the biogenic silica and its MAR correlated well with the δ18O records of GISP2 ice core and Hongyuan peat, which therefore revealed a regional response of biogenic silica to global climate change.
2011, 29(2): 336-345.
Abstract:
Takermohur desert is located in Yili valley of west Xinjiang, which is also situated at semiarid desert geochemical environment in midlatitude westerlies of northern hemisphere. In the center of the desert, we observed a continuously sedimentary profile (with 3.6m outcropped thickness) which is formed with alternative aeolian layers and paleosol layers. On this profile, we collected 72 samples with 5cm equal interval and 3 samples for OSL dating. In the laboratory, we tested 18 trace elements content with Xray fluorescence and grainsize composition with laser particle sizer for all 72 samples. Also, we tested the content of CaCO3 and TOC of all samples. On this basis, we carried out correlation analysis and factor analysis for the content changes and assemblage of trace elements and the ratio of Sr/Ba. According to the different geochemical environment which is indicated by trace element migration with different geochemical properties and the ratio of characteristic elements, we discussed the paleoclimatic characteristics which are recorded by trace element shift and accumulation of different layer in the profile. Based on the OSL time scale, we reconstructed climatic sequence since 3.71 ka BP, which contrast the changes of grainsize composition, CaCO3 and TOC content and refer to the optimal cluster analysis of different parameters. On the whole, the climatic changes of late Holocene in research area could be divided into 5 stages: 3.71～3.06 ka BP, cold and humid; 3.06～2.78 ka BP, warm and arid; 2.78～2.10 ka BP, cool and humid; 2.10～0.50 ka BP ,cold and humid; 0.50 ka BP to present, changed from cold and humid to warm and arid. During the coldhumid climatic period, paleosol developed, and grainsizes of the layer are finer, and amount of CaCO3 and TOC increased. The accumulation of trace element such as Ti、P、Nb et.al which indicates the humid sediment environment increased, and the ratio of Sr/Ba is decreased. During the warmarid climatic period, aeolian layers developed, grain sizes are coarse, amount of CaCO3 and TOC at a relative low, and accumulation of Sr et al, which shows the arid environment increased, so the ratio of Sr/Ba is increased. In general, there has been a climatic aridification tendency in the region since late Holocene. And the paleoclimate characteristics alternating by arid and humid and climatic change stages were similar between the study area and other regions in northern Xinjiang. At centennial scales, the climatic change has teleconnection with that of north Atlantic, which is located in the upper wind of midhigh latitude westerlies. It may suggests that the climatic change of northern Atlantic regions as well as the global climate change is one of the most important driving factor for the climatic change of the study area since late Holocene. The climate change model of the study area indicated by assemblage and variation of trace elements is different from the monsoon zones in east China. It is reflected that there are regional difference refer to the process of trace element migration at supergene geochemical environment under the influence of global climate change.
Takermohur desert is located in Yili valley of west Xinjiang, which is also situated at semiarid desert geochemical environment in midlatitude westerlies of northern hemisphere. In the center of the desert, we observed a continuously sedimentary profile (with 3.6m outcropped thickness) which is formed with alternative aeolian layers and paleosol layers. On this profile, we collected 72 samples with 5cm equal interval and 3 samples for OSL dating. In the laboratory, we tested 18 trace elements content with Xray fluorescence and grainsize composition with laser particle sizer for all 72 samples. Also, we tested the content of CaCO3 and TOC of all samples. On this basis, we carried out correlation analysis and factor analysis for the content changes and assemblage of trace elements and the ratio of Sr/Ba. According to the different geochemical environment which is indicated by trace element migration with different geochemical properties and the ratio of characteristic elements, we discussed the paleoclimatic characteristics which are recorded by trace element shift and accumulation of different layer in the profile. Based on the OSL time scale, we reconstructed climatic sequence since 3.71 ka BP, which contrast the changes of grainsize composition, CaCO3 and TOC content and refer to the optimal cluster analysis of different parameters. On the whole, the climatic changes of late Holocene in research area could be divided into 5 stages: 3.71～3.06 ka BP, cold and humid; 3.06～2.78 ka BP, warm and arid; 2.78～2.10 ka BP, cool and humid; 2.10～0.50 ka BP ,cold and humid; 0.50 ka BP to present, changed from cold and humid to warm and arid. During the coldhumid climatic period, paleosol developed, and grainsizes of the layer are finer, and amount of CaCO3 and TOC increased. The accumulation of trace element such as Ti、P、Nb et.al which indicates the humid sediment environment increased, and the ratio of Sr/Ba is decreased. During the warmarid climatic period, aeolian layers developed, grain sizes are coarse, amount of CaCO3 and TOC at a relative low, and accumulation of Sr et al, which shows the arid environment increased, so the ratio of Sr/Ba is increased. In general, there has been a climatic aridification tendency in the region since late Holocene. And the paleoclimate characteristics alternating by arid and humid and climatic change stages were similar between the study area and other regions in northern Xinjiang. At centennial scales, the climatic change has teleconnection with that of north Atlantic, which is located in the upper wind of midhigh latitude westerlies. It may suggests that the climatic change of northern Atlantic regions as well as the global climate change is one of the most important driving factor for the climatic change of the study area since late Holocene. The climate change model of the study area indicated by assemblage and variation of trace elements is different from the monsoon zones in east China. It is reflected that there are regional difference refer to the process of trace element migration at supergene geochemical environment under the influence of global climate change.
2011, 29(2): 354-362.
Abstract:
A largescale hydrological and sedimentological investigation was carried out off the Huanghe (Yellow River) mouth and the nearby Bohai Sea during AugustSeptember 2007. Approximately 227 suspended sediments and 245 surface sediments samples were selected to determine organic carbon and nitrogen contents using an elemental analysis instrument. The analyzed data show that the TOC content in surface sediment is lower than POC content in bottom suspended sediments and especially lower than that in surface suspended sediments in the same sampling stations. The study area can be classified into five provinces based on TOC, TN and POC data. Province Ⅰlocated near the Qing8 outlet is characterized by higher POC content in suspended sediments, while lower TOC, TN contents and higher C/N values in surface sediments. In contrast, the POC contents in suspended sediments are lower, while TOC, TN contents and C/N values in surface sediments are higher in the province Ⅱ, which covers on both sides of the modern Huanghe mouth. Provinces Ⅲ, Ⅳ and Ⅴare distributed east of 119.5°E, where the POC contents are higher, while TOC, TN contents and C/N values are lower. The results demonstrate that the contents of POC, TOC, TN and their distribution are mainly controlled by sources and sedimentary dynamic environment.
A largescale hydrological and sedimentological investigation was carried out off the Huanghe (Yellow River) mouth and the nearby Bohai Sea during AugustSeptember 2007. Approximately 227 suspended sediments and 245 surface sediments samples were selected to determine organic carbon and nitrogen contents using an elemental analysis instrument. The analyzed data show that the TOC content in surface sediment is lower than POC content in bottom suspended sediments and especially lower than that in surface suspended sediments in the same sampling stations. The study area can be classified into five provinces based on TOC, TN and POC data. Province Ⅰlocated near the Qing8 outlet is characterized by higher POC content in suspended sediments, while lower TOC, TN contents and higher C/N values in surface sediments. In contrast, the POC contents in suspended sediments are lower, while TOC, TN contents and C/N values in surface sediments are higher in the province Ⅱ, which covers on both sides of the modern Huanghe mouth. Provinces Ⅲ, Ⅳ and Ⅴare distributed east of 119.5°E, where the POC contents are higher, while TOC, TN contents and C/N values are lower. The results demonstrate that the contents of POC, TOC, TN and their distribution are mainly controlled by sources and sedimentary dynamic environment.
2011, 29(2): 374-380.
Abstract:
One or two very low subpeaks appear in the coarse end of the grain size differential curves of the peaty sediments from the Yangtze Estuary with the conventional pretreatment method, which is difficult to interpret. The experimental results indicate that different pretreatment procedures and measuring time has obvious influence on the grainsize distribution. In order to obtain accurate measurement, three different methods are used in the present study. In Method 1, sample is immersed in a beaker successively by the distilled water and boiled with 10 ml 10% hydrogen peroxide (H2O2). The beaker is filled up with distilled water and supernate is then removed after settled for 24 hours. Repeat the settlement and removal to eliminate the surplus H2O2. The remained sample is then dispersed by (NaPO3)6 and ready for measurement. In Method 2, which is also called conventional method, samples are immersed by 10ml 10% hydrochloric acid (HCl) in addition to the H2O2. Other steps are the same as in Method 1. In Method 3, the difference from Method 2 is that concentrations of both H2O2 and HCl are increased from 10% to 30%, and other steps are the same as in Method 1. The results show that the Method 1 is preferred and grainsize differential distribution measured in the third minute since they are put in the lasergrain equipment should be selected. Compared with both in the first and the second minute, the grain size measured in the third minute is ideal, which possibly indicate that the flocculated particles need three minutes to disperse. The coarse composition demonstrated in the differential curves might be resulted from the insufficient pretreatment. Our experimental result of mineral grainsize measurement of peaty sediments suggest that clayey silt and silty clay, with mean grain size ranging between 9 μm and 25 μm, prevail and reflect the relative stable environment of still water.
One or two very low subpeaks appear in the coarse end of the grain size differential curves of the peaty sediments from the Yangtze Estuary with the conventional pretreatment method, which is difficult to interpret. The experimental results indicate that different pretreatment procedures and measuring time has obvious influence on the grainsize distribution. In order to obtain accurate measurement, three different methods are used in the present study. In Method 1, sample is immersed in a beaker successively by the distilled water and boiled with 10 ml 10% hydrogen peroxide (H2O2). The beaker is filled up with distilled water and supernate is then removed after settled for 24 hours. Repeat the settlement and removal to eliminate the surplus H2O2. The remained sample is then dispersed by (NaPO3)6 and ready for measurement. In Method 2, which is also called conventional method, samples are immersed by 10ml 10% hydrochloric acid (HCl) in addition to the H2O2. Other steps are the same as in Method 1. In Method 3, the difference from Method 2 is that concentrations of both H2O2 and HCl are increased from 10% to 30%, and other steps are the same as in Method 1. The results show that the Method 1 is preferred and grainsize differential distribution measured in the third minute since they are put in the lasergrain equipment should be selected. Compared with both in the first and the second minute, the grain size measured in the third minute is ideal, which possibly indicate that the flocculated particles need three minutes to disperse. The coarse composition demonstrated in the differential curves might be resulted from the insufficient pretreatment. Our experimental result of mineral grainsize measurement of peaty sediments suggest that clayey silt and silty clay, with mean grain size ranging between 9 μm and 25 μm, prevail and reflect the relative stable environment of still water.
2011, 29(2): 388-394.
Abstract:
Beier depression is the highly explored depression in Hailaer basin with complete and thick sedimentary sequence. The producing oil condition is very excellent, and it is the main oilgas depression in Hailaer basin.The main hydrocarbon source rocks is from the Nantun group, next is Damoguaihe group and Tongbomiao group.The average paleogeothermal gradient recovered by the vitrinite reflectance and homogenization temperatures of fluid inclusion and fission track method is about 3.7～5.8℃ /100m,it is obvious higher than present geothermal gradient 3.35℃/100m, the paleotemperature is higher than present geothermal, the paleotemperature research and thermal history simulation show the max paleogeothermal value were reached in late Cretaceous,paleogeothermal field controlled maturity of hydrocarbon source rocks of Lower Cretaceous.Results of the geothermal history on the basis of illite dating results and hydrocarbon generated and hydrocarbon expulsed process show that the forming time of oilgas reservoirs have two stages in Beier depression: late Cretaceous and early Tertiary. Late Creatceous is the main stage for Beier depression. The second stage is early Tertiary，it happened Lifting disintegration function in the stage,the goal stratum temperature started to reduce, the intensity of hydrocarbon production began to weaken, on the one hand,the oilgas reservoirs formed in the first stage was adjusted;one the other hand, it was the second formation process of oilgas reservoirs.The second forming stage of oilgas reservoirs is important for the formation of oil and gas reservoirs in Beier depression and also the important factor for oilgas accumulation in Beier depression.
Beier depression is the highly explored depression in Hailaer basin with complete and thick sedimentary sequence. The producing oil condition is very excellent, and it is the main oilgas depression in Hailaer basin.The main hydrocarbon source rocks is from the Nantun group, next is Damoguaihe group and Tongbomiao group.The average paleogeothermal gradient recovered by the vitrinite reflectance and homogenization temperatures of fluid inclusion and fission track method is about 3.7～5.8℃ /100m,it is obvious higher than present geothermal gradient 3.35℃/100m, the paleotemperature is higher than present geothermal, the paleotemperature research and thermal history simulation show the max paleogeothermal value were reached in late Cretaceous,paleogeothermal field controlled maturity of hydrocarbon source rocks of Lower Cretaceous.Results of the geothermal history on the basis of illite dating results and hydrocarbon generated and hydrocarbon expulsed process show that the forming time of oilgas reservoirs have two stages in Beier depression: late Cretaceous and early Tertiary. Late Creatceous is the main stage for Beier depression. The second stage is early Tertiary，it happened Lifting disintegration function in the stage,the goal stratum temperature started to reduce, the intensity of hydrocarbon production began to weaken, on the one hand,the oilgas reservoirs formed in the first stage was adjusted;one the other hand, it was the second formation process of oilgas reservoirs.The second forming stage of oilgas reservoirs is important for the formation of oil and gas reservoirs in Beier depression and also the important factor for oilgas accumulation in Beier depression.
2011, 29(2): 402-409.
Abstract:
Orthogonal experimental on the system of noctylic acid and magnesium sulfate were carried out to investigate the relationship among temperature, reaction time and the amount of water in the system. The results show that temperature is the critical factor in the process of the formation of organic sulfides, which gives contributions of 59% to the experimental results. The second is the amount of water, 26 percent, and reaction time gives less effect. And also, the thermal simulation experiments and kinetics for systems of noctylic acid with magnesium sulfate and noctane with magnesium sulfate were carried out. It is shown that solid magnesium sulfate could react with noctylic and noctane gas to produce magnesium oxide, hydrogen sulfide, coke, carbon dioxide, and organic sulfides as the main products. With the increasing temperature, the conversion of inorganic sulfur to organic sulfur especially to thiophene sulfur increases. The values of apparent activation energy for noctylic acid with magnesium sulfate and noctane with magnesium sulfate are 45.23 kJ·mol1 and 48.6 kJ·mol1.
Orthogonal experimental on the system of noctylic acid and magnesium sulfate were carried out to investigate the relationship among temperature, reaction time and the amount of water in the system. The results show that temperature is the critical factor in the process of the formation of organic sulfides, which gives contributions of 59% to the experimental results. The second is the amount of water, 26 percent, and reaction time gives less effect. And also, the thermal simulation experiments and kinetics for systems of noctylic acid with magnesium sulfate and noctane with magnesium sulfate were carried out. It is shown that solid magnesium sulfate could react with noctylic and noctane gas to produce magnesium oxide, hydrogen sulfide, coke, carbon dioxide, and organic sulfides as the main products. With the increasing temperature, the conversion of inorganic sulfur to organic sulfur especially to thiophene sulfur increases. The values of apparent activation energy for noctylic acid with magnesium sulfate and noctane with magnesium sulfate are 45.23 kJ·mol1 and 48.6 kJ·mol1.
2011, 29(2): 217-225.
Abstract:
The trend of thirdorder sealevel changes and mechanism of dolomitization are studied, based on C, O isotope analysis of carbonate rocks, as well as microscopic research, CL analysis, and previous studies of Feixianguan Formation in Jichang of northeast Sichuan Basin. In convenience to analysis, the samples were grouped into two sets of micrite limestone and dolomite. The C, O isotope analysis of micrite limestone shows that the value of δ13C are all positive, and two stages are clear, that is, the first stage is a fast rising stage and followed a fallingstage with a wave shape. The result suggests that the sea level increases quickly in the early First Section of Feixianguan Formation, then, shows a slowly falling trend with sublevel fluctuation after maximum flooding surface. The δ13C evolution curve is rising totally, which is in accord with the rising trend of the Triassic global sea level. The implications of δ18O on the sealevel changes is obscure, due to the processes of late diagenesis. The C, O isotope analysis of dolomite shows that the orthogonal points of δ13C (0.91‰~2.09‰) and δ18O (3.45‰~9.29‰) fall in the second quadrant. The result, combining with microscope studies and cathodeluminescent analysis, suggests that dolomitization was formed in the buried condition.
The trend of thirdorder sealevel changes and mechanism of dolomitization are studied, based on C, O isotope analysis of carbonate rocks, as well as microscopic research, CL analysis, and previous studies of Feixianguan Formation in Jichang of northeast Sichuan Basin. In convenience to analysis, the samples were grouped into two sets of micrite limestone and dolomite. The C, O isotope analysis of micrite limestone shows that the value of δ13C are all positive, and two stages are clear, that is, the first stage is a fast rising stage and followed a fallingstage with a wave shape. The result suggests that the sea level increases quickly in the early First Section of Feixianguan Formation, then, shows a slowly falling trend with sublevel fluctuation after maximum flooding surface. The δ13C evolution curve is rising totally, which is in accord with the rising trend of the Triassic global sea level. The implications of δ18O on the sealevel changes is obscure, due to the processes of late diagenesis. The C, O isotope analysis of dolomite shows that the orthogonal points of δ13C (0.91‰~2.09‰) and δ18O (3.45‰~9.29‰) fall in the second quadrant. The result, combining with microscope studies and cathodeluminescent analysis, suggests that dolomitization was formed in the buried condition.
2011, 29(2): 235-244.
Abstract:
Upper Triassic Xujiahe Formation in Sichuan Basin is the principal producing formation. Six members of Xujiahe Formation form sandwichlike sedimentation structure with interbeded sandstone and mudstone. In order to discuss the contribution of paleoclimate on this kind of sedimentation structure, three commonlyused methods of restoring paleoclimate, namely sporopollen, elements and GRray logging were used to restore the paleoclimate of the time when each member of Xujiahe Formation was deposited. 78 genera of sporopollen were found in the samples of Xujiahe Formation, forming the sporopollen combination of DictyophylliditesConcavisporitesCycadopites. The combination indicates warm and humid climate in tropic and/or subtropic zone. The ratio of Ca/Mg has a positive correlation with paleotemperature while the ratio of Sr/Ba has a negative correlation with salinity(paleomoisture). The Gamma Ray can also illustrate the change of paleoclimate. The high GR data indicate humid climate, and vice versa. The proofs from sporopollen provided the climate background, and the other two methods described the details of paleoclimate. Combined all the three methods, we can find that the paleoclimate when Xujiahe Formation was sediment belongs to warmhumid tropicssubtropics zone. Paleoclimate when T3x1, T3x3 and T3x5 were formed showed hot and humid. In the meantime, when T3x2, T3x4 and T3x6 were formed it was warm and relatively dry. The climate of T3x5 was outstanding hot and humid, while the temperature and moisture of T3x2 was lowest. Controlled by the regular changes of paleoclimate, even numbered members were dominated by mudstone, and odd numbered members were dominated by sandstones. This kind of sandwichlike sedimentation structure formed by alternating layers of mudstone and sandstone provided favorite conditions for the formation of hydrocarbon reservoirs.
Upper Triassic Xujiahe Formation in Sichuan Basin is the principal producing formation. Six members of Xujiahe Formation form sandwichlike sedimentation structure with interbeded sandstone and mudstone. In order to discuss the contribution of paleoclimate on this kind of sedimentation structure, three commonlyused methods of restoring paleoclimate, namely sporopollen, elements and GRray logging were used to restore the paleoclimate of the time when each member of Xujiahe Formation was deposited. 78 genera of sporopollen were found in the samples of Xujiahe Formation, forming the sporopollen combination of DictyophylliditesConcavisporitesCycadopites. The combination indicates warm and humid climate in tropic and/or subtropic zone. The ratio of Ca/Mg has a positive correlation with paleotemperature while the ratio of Sr/Ba has a negative correlation with salinity(paleomoisture). The Gamma Ray can also illustrate the change of paleoclimate. The high GR data indicate humid climate, and vice versa. The proofs from sporopollen provided the climate background, and the other two methods described the details of paleoclimate. Combined all the three methods, we can find that the paleoclimate when Xujiahe Formation was sediment belongs to warmhumid tropicssubtropics zone. Paleoclimate when T3x1, T3x3 and T3x5 were formed showed hot and humid. In the meantime, when T3x2, T3x4 and T3x6 were formed it was warm and relatively dry. The climate of T3x5 was outstanding hot and humid, while the temperature and moisture of T3x2 was lowest. Controlled by the regular changes of paleoclimate, even numbered members were dominated by mudstone, and odd numbered members were dominated by sandstones. This kind of sandwichlike sedimentation structure formed by alternating layers of mudstone and sandstone provided favorite conditions for the formation of hydrocarbon reservoirs.
2011, 29(2): 255-267.
Abstract:
Minfeng subsag lies in the southeast of Jiyang Depression, and has super oilgenerative and depositional conditions. At present, the degree of lithologic hydrocarbon reservoir exploration is low, so the exploration potential is great. Delta, fan delta and near shore subaqueous fan depositional systems can be found in this area according to the research of seismic, logs and core. Distribution and interaction of depositional systems has been researched based on the identification and analysis of sedimentary feature in the middle of the third Member of Shahejie Formation, Paleogene System (Es3) in Minfeng subsag, using single factor analysis and multifactor comprehensive mapping method. Delta depositional system is the most important one, which can cover the most part of the subsag, including Yong'anzhen delta and a part of Dongying delta. It mainly contains delta front and foredelta subfacies, and the main rocks are in the fine sandstone, siltstone, and dark mudstone, etc. Underwater distributary channel facies visible trough cross bedding, wedgeshaped cross bedding, wavy bedding and washingfilling sedimentary structure. Lithology cycles generally show the upward fining cycles, logging curve shapes are the bellshaped and boxshaped. In distributary stream mouth bar, the sandstone are sorted well and tabular crossbedding, wavy bedding and bimodal crossbedding can be found. Lithology cycles generally show the backward fining cycles, logging curve shapes are the funnelshaped or dentelated funnelshaped. In distributary bay, horizontal beddings and lenticular beddings are the most popular sedimentary structure, also, wavegenerated ripples, shell deposition and phytoclasts can be found. The slump turbidity sandstone interlining in the thin mudstone can be found in foredelta subfacies. There are some typical deformation structure and liquefaction structure in the core and fingershaped and bayonetshaped in the log curves. The delta can be divided into 6 units which means 6 depositional phases according to the lithological changes in the single well and the forward characters of events in the seismic profiles. Three of them on the top partly omission in the east of the subsag. The fan delta and near shore subaqueous fan only deposited in the north of subsag closed to Chenjiazhuang uplift during earlier stage of Es3. Looking at the sedimentary evolution from building several profiles in different location, we can found a typical phenomenal that the forward and backward occurred in the same time sometimes. There are three different types: in the early stage, delta in the south continually forward while the near shore subaqueous fan in the north backward; in the middle stage, one lobe of Yong'anzhen delta's forward while another backward; in the latter stage, the Yong'anzhen delta forward to the south while the Dongying delta backward to the south. These can be summarized into three kinds of transformation of accommodation space according to the genesis: ① the transformation of accommodation space caused by tectonic movement between two different sedimentary systems in different structure layers; ② the transformation of accommodation space caused by deposition between two different sedimentary systems in same structure layer; ③ the transformation of accommodation space caused by deposition between two different parts of a sedimentary system in same structure layer.
Minfeng subsag lies in the southeast of Jiyang Depression, and has super oilgenerative and depositional conditions. At present, the degree of lithologic hydrocarbon reservoir exploration is low, so the exploration potential is great. Delta, fan delta and near shore subaqueous fan depositional systems can be found in this area according to the research of seismic, logs and core. Distribution and interaction of depositional systems has been researched based on the identification and analysis of sedimentary feature in the middle of the third Member of Shahejie Formation, Paleogene System (Es3) in Minfeng subsag, using single factor analysis and multifactor comprehensive mapping method. Delta depositional system is the most important one, which can cover the most part of the subsag, including Yong'anzhen delta and a part of Dongying delta. It mainly contains delta front and foredelta subfacies, and the main rocks are in the fine sandstone, siltstone, and dark mudstone, etc. Underwater distributary channel facies visible trough cross bedding, wedgeshaped cross bedding, wavy bedding and washingfilling sedimentary structure. Lithology cycles generally show the upward fining cycles, logging curve shapes are the bellshaped and boxshaped. In distributary stream mouth bar, the sandstone are sorted well and tabular crossbedding, wavy bedding and bimodal crossbedding can be found. Lithology cycles generally show the backward fining cycles, logging curve shapes are the funnelshaped or dentelated funnelshaped. In distributary bay, horizontal beddings and lenticular beddings are the most popular sedimentary structure, also, wavegenerated ripples, shell deposition and phytoclasts can be found. The slump turbidity sandstone interlining in the thin mudstone can be found in foredelta subfacies. There are some typical deformation structure and liquefaction structure in the core and fingershaped and bayonetshaped in the log curves. The delta can be divided into 6 units which means 6 depositional phases according to the lithological changes in the single well and the forward characters of events in the seismic profiles. Three of them on the top partly omission in the east of the subsag. The fan delta and near shore subaqueous fan only deposited in the north of subsag closed to Chenjiazhuang uplift during earlier stage of Es3. Looking at the sedimentary evolution from building several profiles in different location, we can found a typical phenomenal that the forward and backward occurred in the same time sometimes. There are three different types: in the early stage, delta in the south continually forward while the near shore subaqueous fan in the north backward; in the middle stage, one lobe of Yong'anzhen delta's forward while another backward; in the latter stage, the Yong'anzhen delta forward to the south while the Dongying delta backward to the south. These can be summarized into three kinds of transformation of accommodation space according to the genesis: ① the transformation of accommodation space caused by tectonic movement between two different sedimentary systems in different structure layers; ② the transformation of accommodation space caused by deposition between two different sedimentary systems in same structure layer; ③ the transformation of accommodation space caused by deposition between two different parts of a sedimentary system in same structure layer.
2011, 29(2): 277-285.
Abstract:
By studying how base level and accommodation space change the strata between the forth member of Quantou Group and the first member of Nenjiang Group in Changling Depression Basin. The sequence stratigraphy has been devided into five third sequences and eight forth sequences, and high resolution sequence formation framework has been built up. Double delta sediments and lake sediments superimposed vertically because of twostage cycles of lake level. By analysising the relationship between elements of reservoir and development of sedimentary sequence in the sequence framework, the formation in depression contained three sets of allied reservoir plays. The lower play had good genetic conditions and the middle one's conditions were also good if migration pathway were unblocked. The rule of reservoir formed in this area has been concluded as follows. Firstly, sequence and cycle controlled where petrol accumulate and the main reservoirs lay near hydrocarbon rocks. Secondly, sedimentation zone controlled the distribution of reservoir, delta front sand bodies which contained more and better oil and gas were good reservoirs. In accordance with restriction of reservoir factors' combination and distribution, it is predicted that good reservoir of Changling area distributed
in delta front and shorenear crossover region.
By studying how base level and accommodation space change the strata between the forth member of Quantou Group and the first member of Nenjiang Group in Changling Depression Basin. The sequence stratigraphy has been devided into five third sequences and eight forth sequences, and high resolution sequence formation framework has been built up. Double delta sediments and lake sediments superimposed vertically because of twostage cycles of lake level. By analysising the relationship between elements of reservoir and development of sedimentary sequence in the sequence framework, the formation in depression contained three sets of allied reservoir plays. The lower play had good genetic conditions and the middle one's conditions were also good if migration pathway were unblocked. The rule of reservoir formed in this area has been concluded as follows. Firstly, sequence and cycle controlled where petrol accumulate and the main reservoirs lay near hydrocarbon rocks. Secondly, sedimentation zone controlled the distribution of reservoir, delta front sand bodies which contained more and better oil and gas were good reservoirs. In accordance with restriction of reservoir factors' combination and distribution, it is predicted that good reservoir of Changling area distributed
in delta front and shorenear crossover region.
2011, 29(2): 293-302.
Abstract:
This article mainly use the Hong143 and Hong146 well that rich in dawsonitebearing sandstones as reaearch objiects.According to the analysis of conventional thin sections, alizarin redstaining, SEM, INCA energy spectromete, microthermometry , the diagenetic paragenesis succession of dawsonitebearing sandstones, in particular, the homogenization tempreature of fluid inclusion have been studied in detail. The diagenetic paragenesis of the Honggang Oilfield is clay minerals→secondary quartz overgrowths→calcite→dawsonite→ankerite→quartz healing microfracture. On the basis of intensive microthermometry, the peak of the homogenization tempreature is 110～120℃ in the calcite; while 130～140℃ in the quartz healing microfracture. The formation temperature of dawsonite is between calcite and quartz healing microfracture, which is 120～130℃. Comparing with research results at home and abroad, the formation temperature of dawsonite that is transformed by magma is relatively low , which is 25～150℃. The formation temperature of dawsonite that appeared in the inclusion as daughter mineral is relatively high,which is 250～340℃. The dawsonite in the southern Songliao Basin belong to the first situation, the formation temperature is relatively low, which is 120～130℃. Scanning electron microscopy and energy spectrum analysis shows that the dawsonite of Hong gang oil filed (The formation of temperature between 120～130℃) is mainly needlelike, the chemical composition is rich in Si, poor in Na, Al and O; the dawsonite of Wuerxun Depression (The formation of temperature between 78.56~115.02℃) is mainly columnar pin, chemical composition is rich in Na, Al and O, poor in Si.
This article mainly use the Hong143 and Hong146 well that rich in dawsonitebearing sandstones as reaearch objiects.According to the analysis of conventional thin sections, alizarin redstaining, SEM, INCA energy spectromete, microthermometry , the diagenetic paragenesis succession of dawsonitebearing sandstones, in particular, the homogenization tempreature of fluid inclusion have been studied in detail. The diagenetic paragenesis of the Honggang Oilfield is clay minerals→secondary quartz overgrowths→calcite→dawsonite→ankerite→quartz healing microfracture. On the basis of intensive microthermometry, the peak of the homogenization tempreature is 110～120℃ in the calcite; while 130～140℃ in the quartz healing microfracture. The formation temperature of dawsonite is between calcite and quartz healing microfracture, which is 120～130℃. Comparing with research results at home and abroad, the formation temperature of dawsonite that is transformed by magma is relatively low , which is 25～150℃. The formation temperature of dawsonite that appeared in the inclusion as daughter mineral is relatively high,which is 250～340℃. The dawsonite in the southern Songliao Basin belong to the first situation, the formation temperature is relatively low, which is 120～130℃. Scanning electron microscopy and energy spectrum analysis shows that the dawsonite of Hong gang oil filed (The formation of temperature between 120～130℃) is mainly needlelike, the chemical composition is rich in Si, poor in Na, Al and O; the dawsonite of Wuerxun Depression (The formation of temperature between 78.56~115.02℃) is mainly columnar pin, chemical composition is rich in Na, Al and O, poor in Si.
2011, 29(2): 312-320.
Abstract:
The Daheishan area is located in the south of the Santanghu Basin, north to the BalikunKaerlikeshan, and is adjacent to the Tuha Basin, which is located in the Paleozoic Orogenic Belt of eastern edge of Junggar basin. A suite of thickbedded marine volcanicsedimentary rocks——consist mainly of tuffaceous sandstones and siltstones, tuffs, muddy siltstones, silty mudstones, mudstones, and conglomerates are distributed in the Daheishan region in east Junggar, of which the ages and stratigraphy are highly controversial. More importantly, as the study area is sandwiched between the ZhahebaAermantai and the KelamailiTakezhale ophiolitic melange belts, it is of importance to investigate the stratigraphy. Two crystal tuff horizons (Nos. DHS04N and DHS07N) are sampled from the Jiangbasitao Formation of Lower Carboniferous and are dated using zircon LAICPMS method. The analysis shows that the zircons are of typical magmatic origin with oscillatory zoning. Samples of DHS04N have a uranium content of 63~642 μg/g, thorium of 19~1037 μg/g, and a Th/U value range between 0.29 and 1.89. Whereas those of DHS07N have a uranium content of 40
~476 μg/g, thorium of 25~466 μg/g, and Th/U of 0.48~1.14. The dating results yield two weighted mean 206Pb/238U ages of (339.4±1.9) Ma (MSWD=0.39) and (336.3±2.6) Ma (MSWD=0.58), respectively. These two radiometric ages are well consistent and are interpreted to represent the crystallization age of the zircons from the tuff and also the age of the sedimentation of the tuff sequence, indicating that the tuff sequence should be ascribed to the Visean Stage (ca. 345～328 Ma) of the Early Carboniferous. Newly found fossil evidence—e.g., brachiopoda index fossils Stratifera magna, Gigantoproductus striatosulcatus, and Gigantoproductus edelburgensis—also indicates the Early Carboniferous age of the sequence. Combining the new paleobiological data, we assign these successions to the Early Carboniferous in age. Two zircon samples show concordia ages of 352 ~ 2 999 Ma and CL images of the zircons is characterized by well psephicity, relatively darker hue in luminescence, as well as ambiguously developed zonings, which together suggests a inherit nature of the zircons. In total, the radiometric data document not only a set of Paleozoic ages but also seven important 752 ~ 2 999 Ma Precambrian ages. With respect to the origin of the zircons, we suggest that they were derived from areas between the two ophiolite belts, because the study area was restricted by the paleooceanic basin (Zhaheba Aermantai ophiolite ophiolitic melange belt) and backarc basin (KelamailiTakezhale ophiolite ophiolitic melange belt). This would possibly indicate the existence of Precambrian blocks. Paleozoic strataformed mountains in northeast Junggar are surrounded by two ophiolite ophiolitic melange belts and form a “restricted area”. Our data may further indicate that this “restricted area” was presumably a Paleozoic hyperplasia belt that mixes ophiolite, ophiolitic melange block, island arc magmatic complex, and microlandmass.
The Daheishan area is located in the south of the Santanghu Basin, north to the BalikunKaerlikeshan, and is adjacent to the Tuha Basin, which is located in the Paleozoic Orogenic Belt of eastern edge of Junggar basin. A suite of thickbedded marine volcanicsedimentary rocks——consist mainly of tuffaceous sandstones and siltstones, tuffs, muddy siltstones, silty mudstones, mudstones, and conglomerates are distributed in the Daheishan region in east Junggar, of which the ages and stratigraphy are highly controversial. More importantly, as the study area is sandwiched between the ZhahebaAermantai and the KelamailiTakezhale ophiolitic melange belts, it is of importance to investigate the stratigraphy. Two crystal tuff horizons (Nos. DHS04N and DHS07N) are sampled from the Jiangbasitao Formation of Lower Carboniferous and are dated using zircon LAICPMS method. The analysis shows that the zircons are of typical magmatic origin with oscillatory zoning. Samples of DHS04N have a uranium content of 63~642 μg/g, thorium of 19~1037 μg/g, and a Th/U value range between 0.29 and 1.89. Whereas those of DHS07N have a uranium content of 40
~476 μg/g, thorium of 25~466 μg/g, and Th/U of 0.48~1.14. The dating results yield two weighted mean 206Pb/238U ages of (339.4±1.9) Ma (MSWD=0.39) and (336.3±2.6) Ma (MSWD=0.58), respectively. These two radiometric ages are well consistent and are interpreted to represent the crystallization age of the zircons from the tuff and also the age of the sedimentation of the tuff sequence, indicating that the tuff sequence should be ascribed to the Visean Stage (ca. 345～328 Ma) of the Early Carboniferous. Newly found fossil evidence—e.g., brachiopoda index fossils Stratifera magna, Gigantoproductus striatosulcatus, and Gigantoproductus edelburgensis—also indicates the Early Carboniferous age of the sequence. Combining the new paleobiological data, we assign these successions to the Early Carboniferous in age. Two zircon samples show concordia ages of 352 ~ 2 999 Ma and CL images of the zircons is characterized by well psephicity, relatively darker hue in luminescence, as well as ambiguously developed zonings, which together suggests a inherit nature of the zircons. In total, the radiometric data document not only a set of Paleozoic ages but also seven important 752 ~ 2 999 Ma Precambrian ages. With respect to the origin of the zircons, we suggest that they were derived from areas between the two ophiolite belts, because the study area was restricted by the paleooceanic basin (Zhaheba Aermantai ophiolite ophiolitic melange belt) and backarc basin (KelamailiTakezhale ophiolite ophiolitic melange belt). This would possibly indicate the existence of Precambrian blocks. Paleozoic strataformed mountains in northeast Junggar are surrounded by two ophiolite ophiolitic melange belts and form a “restricted area”. Our data may further indicate that this “restricted area” was presumably a Paleozoic hyperplasia belt that mixes ophiolite, ophiolitic melange block, island arc magmatic complex, and microlandmass.
2011, 29(2): 328-335.
Abstract:
To explore the spatial and temporal evolution of sedimentation in the Pearl River Delta Area (PRDA) since Holocene, the PRDA is divided into 12 subregions, and the evolution history is distinguished into 4 stages. Sedimentation rates of 92 cores were calculated for representing different subregions and evolution stages. The obtained sedimentation rates are 1.27 mm/a, 2.34 mm/a, 1.94 mm/a, 2.27 mm/a in 10.0~7.5 ka BP, 7.5~5.0 ka BP, 5.0~2.5 ka BP, 2.5~0 ka BP, respectively. Maximum rate occurred in 7.5~5.0 ka BP and minimum rate occurred in 10.0~7.5 ka BP. The depocenters were found to be located in the middle area, rather than only at the mouth of the paleoestuary bay, which is quite different from the normal pattern established in many estuaries. Maximum and minimum rates during different statges were caused by differences in sealevel rise and accommodation space; Since 2 500 a B P, human activities have increased the sediment amount discharged into the bay, but the dispersal to the offshore was also increased by the formation of river network and limitation of the accommodation space relative to the supply. Therefore the largest sedimentation rate was not situated then, contrary to the preexisting understanding. The existence of multiply depocenters in the paleoestuary bay indicated a new evolution pattern of the delta that several sand bodies progradated multidirectionally and simultaneously. This is much different from several largescale delta systems in the world that propagate seaward unidirectionally.
To explore the spatial and temporal evolution of sedimentation in the Pearl River Delta Area (PRDA) since Holocene, the PRDA is divided into 12 subregions, and the evolution history is distinguished into 4 stages. Sedimentation rates of 92 cores were calculated for representing different subregions and evolution stages. The obtained sedimentation rates are 1.27 mm/a, 2.34 mm/a, 1.94 mm/a, 2.27 mm/a in 10.0~7.5 ka BP, 7.5~5.0 ka BP, 5.0~2.5 ka BP, 2.5~0 ka BP, respectively. Maximum rate occurred in 7.5~5.0 ka BP and minimum rate occurred in 10.0~7.5 ka BP. The depocenters were found to be located in the middle area, rather than only at the mouth of the paleoestuary bay, which is quite different from the normal pattern established in many estuaries. Maximum and minimum rates during different statges were caused by differences in sealevel rise and accommodation space; Since 2 500 a B P, human activities have increased the sediment amount discharged into the bay, but the dispersal to the offshore was also increased by the formation of river network and limitation of the accommodation space relative to the supply. Therefore the largest sedimentation rate was not situated then, contrary to the preexisting understanding. The existence of multiply depocenters in the paleoestuary bay indicated a new evolution pattern of the delta that several sand bodies progradated multidirectionally and simultaneously. This is much different from several largescale delta systems in the world that propagate seaward unidirectionally.
2011, 29(2): 346-353.
Abstract:
Estuarine and coastal eutrophication has been one of the prime environmental problems worrying the coastal countries, because of human frequent activities in the recent years. Sanmen Bay is a typical bay under the influence of human activities. On the basis of the present eutrophication analysis and combined 210Pb age data, sedimentary age records is constructed. Through the organic tight combination of more chemical indexes(The vertical distribution of TOC ,TN ,TP and BSi ; the ratios and burial fluxes of biogenic elements.) and age records from sedimentary pole and historical data, sedimentary records are extracted from the sedimentary pole, the sedimentary history of Sanmen Bay eutrophication in the past fifty years are presented. From varied trends of TOC and TN, the cores of SM17and SM3 also experience approximately three stages and show an increase from bottom to top, but the former relatively more obvious than the latter. The relationship between TOC, TN and BSi in the two cores show positive correlation. Organic matter within the two cores is of aquatic and terrestrial sources deduced from the ratio of C/N. Although a few fluctuation burial flux of TOC for core SM17 basically vary little from the early 1950s to middle 1980s. The fluxes of various indexes have also an increasing trend from the middle 1980s to early 20th century. The fluxes of TOC and TN do not increase. "three stages theory" on sedimentary history of Sanmen Bay eutrophication is put forward from comprehensive analyses of environmental indexes, namely natural evolution stage under the interference of human activities（from the establishment of the People’s Republic of China to the beginning of reform and opening up）; increasing stage under the interference of human activities（from the beginning of reform and opening up to early 20th century）; slow coordination stage under the interference of human activities（since early 20th century）.
Estuarine and coastal eutrophication has been one of the prime environmental problems worrying the coastal countries, because of human frequent activities in the recent years. Sanmen Bay is a typical bay under the influence of human activities. On the basis of the present eutrophication analysis and combined 210Pb age data, sedimentary age records is constructed. Through the organic tight combination of more chemical indexes(The vertical distribution of TOC ,TN ,TP and BSi ; the ratios and burial fluxes of biogenic elements.) and age records from sedimentary pole and historical data, sedimentary records are extracted from the sedimentary pole, the sedimentary history of Sanmen Bay eutrophication in the past fifty years are presented. From varied trends of TOC and TN, the cores of SM17and SM3 also experience approximately three stages and show an increase from bottom to top, but the former relatively more obvious than the latter. The relationship between TOC, TN and BSi in the two cores show positive correlation. Organic matter within the two cores is of aquatic and terrestrial sources deduced from the ratio of C/N. Although a few fluctuation burial flux of TOC for core SM17 basically vary little from the early 1950s to middle 1980s. The fluxes of various indexes have also an increasing trend from the middle 1980s to early 20th century. The fluxes of TOC and TN do not increase. "three stages theory" on sedimentary history of Sanmen Bay eutrophication is put forward from comprehensive analyses of environmental indexes, namely natural evolution stage under the interference of human activities（from the establishment of the People’s Republic of China to the beginning of reform and opening up）; increasing stage under the interference of human activities（from the beginning of reform and opening up to early 20th century）; slow coordination stage under the interference of human activities（since early 20th century）.
2011, 29(2): 363-373.
Abstract:
This paper studied contemporary nontectonic cracks in mud sediment of delta plain at Yellow River Delta. Subaqueous shrinkage cracks, desiccation cracks and mixed origin cracks are studied in detail at the aspects of quantitative classification, fractal features, planar and sectional growth pattern and development models. This paper has established the quantitative characterization and comparison standards of different nontectonic cracks in modern mud sediment.
1. Description of crack types
Nontectonic cracks at Yellow River Delta can be divided into three types: Subaqueous shrinkage cracks, desiccation cracks and mixed origin cracks
Characteristics of subaqueous shrinkage cracks are: deep overlying water body; most with just one grade of cracks; smooth crack walls; large base crack unit area; cracks are more developed in sectors of shallow water than those in deep water in a long and narrow water body.
Characteristics of desiccation cracks are: usually multiple crack grades; Vshaped in vertical section; upward curling of crack unit margins due to desiccation.
Characteristics of mixed origin cracks are: the first type observed has secondary crack grades developed on the basis of previous subaqueous shrinkage crack due to subaerial exposure; the second type observed has two independent crack systems desiccation cracks overlying filled subaqueous shrinkage cracks.
The occurrence of many different crack types in this study area without significant microfacies changes can be attributed to the various volume and depth of overlying water body: for areas with deepest water and largest volume, subaqueous shrinkage cracks easily form because of more mud containing water; for areas with least water depth, only desiccation cracks develop; and for areas with water parameters between those of the first two, different kinds of mixed origin cracks develop highly possibly.
2. Statistical analyses of cracks
Quantitative classification of firstgrade crack unit area is applied to all study areas under the same criteria. The distribution of areas of subaqueous shrinkage cracks is the most centralized, which are mostly more than 500cm2; desiccation cracks unit areas have the largest area span; while the mixed origin cracks fall in between the first two, lacking areas of small figures.
Fractal dimensions are calculated for each crack area by the overlapping method. It is found that the fractal dimension rises from subaqueous shrinkage cracks to desiccation. Fractal theory can be well applied to analyze the planar density, complexity and connectivity of cracks, thus a good method in characterizing level of crack development.
3. Analyses of crack growth patterns
Similar planar growth patterns of all kinds of cracks are observed: New cracks develop at inflection points on the side of convex curvature. The intersection angle of new crack to host crack is greater than 70 , while the angle of bending crack to its original direction is less than 70 . A theory of shrinkage circle is adopted to demonstrate crack growth pattern. In vertical section, shrinkage sphere is used to illustrate the structure of bottomup subaqueous shrinkage cracks which are similar to the "flower structure" used to describe faults, whereas there is a distinct difference from the Vshaped topdown desiccation cracks.
4. Development models
Development models are established respectively for all types of cracks, in which relationships of mutual transformation of certain crack types are shown. Generally, from deep to shallow areas, subaqueous shrinkage cracks, mixed origin cracks and desiccation cracks develop in succession, and some of them can transform if affected by weather. It is possible that more types of mixed origin cracks appear under more complicated conditions.
5. Discussion
Although there seem to be numerous studies on spatial distribution of nontectonic mud shrinkage cracks, convincing theories of factors affecting crack development is still yet to be discovered. It is recommended more studies of crack simulation experiment on certain crack development factor(s) to be done in order to gain a better understanding of nontectonic shrinkage cracks.
This paper studied contemporary nontectonic cracks in mud sediment of delta plain at Yellow River Delta. Subaqueous shrinkage cracks, desiccation cracks and mixed origin cracks are studied in detail at the aspects of quantitative classification, fractal features, planar and sectional growth pattern and development models. This paper has established the quantitative characterization and comparison standards of different nontectonic cracks in modern mud sediment.
1. Description of crack types
Nontectonic cracks at Yellow River Delta can be divided into three types: Subaqueous shrinkage cracks, desiccation cracks and mixed origin cracks
Characteristics of subaqueous shrinkage cracks are: deep overlying water body; most with just one grade of cracks; smooth crack walls; large base crack unit area; cracks are more developed in sectors of shallow water than those in deep water in a long and narrow water body.
Characteristics of desiccation cracks are: usually multiple crack grades; Vshaped in vertical section; upward curling of crack unit margins due to desiccation.
Characteristics of mixed origin cracks are: the first type observed has secondary crack grades developed on the basis of previous subaqueous shrinkage crack due to subaerial exposure; the second type observed has two independent crack systems desiccation cracks overlying filled subaqueous shrinkage cracks.
The occurrence of many different crack types in this study area without significant microfacies changes can be attributed to the various volume and depth of overlying water body: for areas with deepest water and largest volume, subaqueous shrinkage cracks easily form because of more mud containing water; for areas with least water depth, only desiccation cracks develop; and for areas with water parameters between those of the first two, different kinds of mixed origin cracks develop highly possibly.
2. Statistical analyses of cracks
Quantitative classification of firstgrade crack unit area is applied to all study areas under the same criteria. The distribution of areas of subaqueous shrinkage cracks is the most centralized, which are mostly more than 500cm2; desiccation cracks unit areas have the largest area span; while the mixed origin cracks fall in between the first two, lacking areas of small figures.
Fractal dimensions are calculated for each crack area by the overlapping method. It is found that the fractal dimension rises from subaqueous shrinkage cracks to desiccation. Fractal theory can be well applied to analyze the planar density, complexity and connectivity of cracks, thus a good method in characterizing level of crack development.
3. Analyses of crack growth patterns
Similar planar growth patterns of all kinds of cracks are observed: New cracks develop at inflection points on the side of convex curvature. The intersection angle of new crack to host crack is greater than 70 , while the angle of bending crack to its original direction is less than 70 . A theory of shrinkage circle is adopted to demonstrate crack growth pattern. In vertical section, shrinkage sphere is used to illustrate the structure of bottomup subaqueous shrinkage cracks which are similar to the "flower structure" used to describe faults, whereas there is a distinct difference from the Vshaped topdown desiccation cracks.
4. Development models
Development models are established respectively for all types of cracks, in which relationships of mutual transformation of certain crack types are shown. Generally, from deep to shallow areas, subaqueous shrinkage cracks, mixed origin cracks and desiccation cracks develop in succession, and some of them can transform if affected by weather. It is possible that more types of mixed origin cracks appear under more complicated conditions.
5. Discussion
Although there seem to be numerous studies on spatial distribution of nontectonic mud shrinkage cracks, convincing theories of factors affecting crack development is still yet to be discovered. It is recommended more studies of crack simulation experiment on certain crack development factor(s) to be done in order to gain a better understanding of nontectonic shrinkage cracks.
2011, 29(2): 381-387.
Abstract:
The terminal Huangqihai Lake is located at the northern margin of the modern East Asian summer monsoon, and its evolution is sensitive to climate changes. Profile H3 is a few kilometers away to the southeast of the lake which was consisted of fine sand and silt deposited during ca 8.5~2.0 ka BP. During this period, Huangqihai Lake experienced its highest lake level in Holocene as well as largest summer monsoon precipitation. Analysis of Rb, Sr, Ba, Mn, Ti, Zr elements in entire sample, the fraction of grain size of 20~77 μm and <20 μm, and Carbonate Minerals content in <20 μm fraction indicated that element abundance in different grain size exhibited different features. Ti and Zr has enrichment tendency in the fraction of grain size of 20~77 μm; Mn, Sr and Rb was apt to enrich in the fraction of grain size of <20 μm, and the enrichment of Ba was not distinct. The enrichment of Ti, Zr, Mn and Rb probably were the fragmentating characteristics of their silicate minerals, and that of Sr was caused by lacstrine autogenic carbonate. Zrc/（Rbc+Bac）ratio, Zrc/Rbc ratio, and the grain size effect of Zr and Ti(or Tic/Tif, Zrc/Zrf) showed the relative content of heavy and light minerals, which indicated the intensity of erosion in basin and transportation from basin to lake, and lake enlarging and reducing information. The information which indicated by Baf/Srf ratio, Rbf/Srf ratio in <20 μm fraction was complicated and has no directly environmental implication.
The terminal Huangqihai Lake is located at the northern margin of the modern East Asian summer monsoon, and its evolution is sensitive to climate changes. Profile H3 is a few kilometers away to the southeast of the lake which was consisted of fine sand and silt deposited during ca 8.5~2.0 ka BP. During this period, Huangqihai Lake experienced its highest lake level in Holocene as well as largest summer monsoon precipitation. Analysis of Rb, Sr, Ba, Mn, Ti, Zr elements in entire sample, the fraction of grain size of 20~77 μm and <20 μm, and Carbonate Minerals content in <20 μm fraction indicated that element abundance in different grain size exhibited different features. Ti and Zr has enrichment tendency in the fraction of grain size of 20~77 μm; Mn, Sr and Rb was apt to enrich in the fraction of grain size of <20 μm, and the enrichment of Ba was not distinct. The enrichment of Ti, Zr, Mn and Rb probably were the fragmentating characteristics of their silicate minerals, and that of Sr was caused by lacstrine autogenic carbonate. Zrc/（Rbc+Bac）ratio, Zrc/Rbc ratio, and the grain size effect of Zr and Ti(or Tic/Tif, Zrc/Zrf) showed the relative content of heavy and light minerals, which indicated the intensity of erosion in basin and transportation from basin to lake, and lake enlarging and reducing information. The information which indicated by Baf/Srf ratio, Rbf/Srf ratio in <20 μm fraction was complicated and has no directly environmental implication.
2011, 29(2): 395-401.
Abstract:
Based on the analysis of the vitrinite reflectance and apatite fission track inclusions system testing, we carried out the calculation of paleotemperature gradient and reconstruction of thermal history , and then identified the paleotemperature gradient of west slope and Bianminyang tectonic zone of Jinhu depression. According to the vitrinite reflectance, we calculated that the range of the paleotemperature being between 45.6~128.4℃ and the paleotemperature gradient was
45.5 ℃/km in the west slope, the paleotemperature in Bianminyang tectonic zone was 26.4~120.3℃ and the paleotemperature gradient was 42.7℃/km. According to the apatite fission track, we calculated that the paleotemperature gradient in the west slope was 40.7℃/km, and in Bianminyang tectonic zone was 45.8℃/km. From the comparative analysis with different tectonic zones of Jinhu depression, we concluded a law that the paleo temperature gradient was higher than presentday geothermal gradient, specifically as follows: in the west slope, paleotemperature was 10.4~15.2℃/km higher than the current, and in Bianminyang tectonic zone paleotemperature was 12.4~15.3℃/km higher than the present.By the thermal history modeling of typical wells in west slope and Bianminyang tectonic zone, it could be seen that the paleogeothermal gradient became lower with the stratigraphical time changed for the new. It shows that the geothermal gradient of K2t~E1f was higher than E2d~Ny. Before the uplift and erosion caused by the Sanduo tectonic events, the paleotemperature of depression had reached the maximum. The maturity history of depression reflected that the Ro was 0.4% in the depth of 1 000 m of Jinhu depression. The source rock was at the lowmature stage.The Ro was 0.65% in the depth of 1 900 m and the temperature reached 90℃, the hydrocarbon source rocks entered the peak phase. The homogenization temperature of fluid inclusion samples was between 62~93℃ of Well Cui 2 in the west slope. Through the comprehensive analysis of the burial historythermal history of typical singlewell and the homogenization temperature of fluid inclusions, it can be identified that the accumulation period of Jinhu depression was between 47~41.5 Ma.Then it could be judged that the Sanduo period(E2s)was the main hydrocarbon accumulation period of Jinhu depression.
Based on the analysis of the vitrinite reflectance and apatite fission track inclusions system testing, we carried out the calculation of paleotemperature gradient and reconstruction of thermal history , and then identified the paleotemperature gradient of west slope and Bianminyang tectonic zone of Jinhu depression. According to the vitrinite reflectance, we calculated that the range of the paleotemperature being between 45.6~128.4℃ and the paleotemperature gradient was
45.5 ℃/km in the west slope, the paleotemperature in Bianminyang tectonic zone was 26.4~120.3℃ and the paleotemperature gradient was 42.7℃/km. According to the apatite fission track, we calculated that the paleotemperature gradient in the west slope was 40.7℃/km, and in Bianminyang tectonic zone was 45.8℃/km. From the comparative analysis with different tectonic zones of Jinhu depression, we concluded a law that the paleo temperature gradient was higher than presentday geothermal gradient, specifically as follows: in the west slope, paleotemperature was 10.4~15.2℃/km higher than the current, and in Bianminyang tectonic zone paleotemperature was 12.4~15.3℃/km higher than the present.By the thermal history modeling of typical wells in west slope and Bianminyang tectonic zone, it could be seen that the paleogeothermal gradient became lower with the stratigraphical time changed for the new. It shows that the geothermal gradient of K2t~E1f was higher than E2d~Ny. Before the uplift and erosion caused by the Sanduo tectonic events, the paleotemperature of depression had reached the maximum. The maturity history of depression reflected that the Ro was 0.4% in the depth of 1 000 m of Jinhu depression. The source rock was at the lowmature stage.The Ro was 0.65% in the depth of 1 900 m and the temperature reached 90℃, the hydrocarbon source rocks entered the peak phase. The homogenization temperature of fluid inclusion samples was between 62~93℃ of Well Cui 2 in the west slope. Through the comprehensive analysis of the burial historythermal history of typical singlewell and the homogenization temperature of fluid inclusions, it can be identified that the accumulation period of Jinhu depression was between 47~41.5 Ma.Then it could be judged that the Sanduo period(E2s)was the main hydrocarbon accumulation period of Jinhu depression.
2011, 29(2): 410-416.
Abstract:
Ordos basin is a large inner continental sedimentary basin in China. Mesozoic crude oils in the basin are derived from nonmarine source rocks, so that it has also been used as a typical model of a nonmarine sedimentary basin generating oil. Maling oilfield is located in the southwestern Yishan slope of Ordos Basin. It was discovered in Lower Jurassic Yanan Formation in 1970 and is one of the earliestdiscovered oilfields in Ordos Basin. It also is typical representative of oil reservoir in land fluvial deposits. However, previous geochemical studies of oils from Maling oilfield are limited and the mechanism of hydrocarbons accumulation for this oilfield has not been well known. Understanding the formation law of this kind of oil reservoir is very important for petroleum exploration and production in fluvial deposits. We collected the crude oil samples from different regions of Maling oilfield and source rocks in the Ordos Basin, and analyzed biomarker compounds in the oils and source rocks. In this paper, the formation conditions of oil reservoir in Maling oilfield, such as source rock, reservoir rock, oil migration, traps and accumulation, were systematically studied, and its forming mechanism was discussed dynamically. The results show that crude oil of Maling oilfield was derived from the Yanchang Formation source rocks. Reservoir rocks are mainly the Low Jurassic Yanan Formation fluvial channel sandstones, while coal seam, carbonaceous mudstone and mudstone constituted the cap rock. Hydrocarbon vertical migration was controlled by charging position and migration window. The lateral migration pathway of crude oil is sandbodies of river channels. The force for the vertical migration was likely the capillary pressure and the abnormally high strata pressure. There were five types of traps.
Ordos basin is a large inner continental sedimentary basin in China. Mesozoic crude oils in the basin are derived from nonmarine source rocks, so that it has also been used as a typical model of a nonmarine sedimentary basin generating oil. Maling oilfield is located in the southwestern Yishan slope of Ordos Basin. It was discovered in Lower Jurassic Yanan Formation in 1970 and is one of the earliestdiscovered oilfields in Ordos Basin. It also is typical representative of oil reservoir in land fluvial deposits. However, previous geochemical studies of oils from Maling oilfield are limited and the mechanism of hydrocarbons accumulation for this oilfield has not been well known. Understanding the formation law of this kind of oil reservoir is very important for petroleum exploration and production in fluvial deposits. We collected the crude oil samples from different regions of Maling oilfield and source rocks in the Ordos Basin, and analyzed biomarker compounds in the oils and source rocks. In this paper, the formation conditions of oil reservoir in Maling oilfield, such as source rock, reservoir rock, oil migration, traps and accumulation, were systematically studied, and its forming mechanism was discussed dynamically. The results show that crude oil of Maling oilfield was derived from the Yanchang Formation source rocks. Reservoir rocks are mainly the Low Jurassic Yanan Formation fluvial channel sandstones, while coal seam, carbonaceous mudstone and mudstone constituted the cap rock. Hydrocarbon vertical migration was controlled by charging position and migration window. The lateral migration pathway of crude oil is sandbodies of river channels. The force for the vertical migration was likely the capillary pressure and the abnormally high strata pressure. There were five types of traps.
