Lithologic Reservoirs

Drilling logs, outcrop profiles, cores and seismic data were used to study the sequence-based lithofacies palaeogeography of Cambrian Xixiangchi Formation in Sichuan Basin and its periphery. The results show that:(1) Xixiangchi Formation was dominated by carbonate platform facies, in which mixed tidal flat, restricted platform, open platform, platform margin, and slope-basin units were developed from west to east. By applying the principles and methods of sequence stratigraphy, the Xixiangchi Formation was divided into five third-order sequences(SQ1-SQ5) from bottom to top.(2) The restricted platform was the dominant sedimentary facies type in Xixiangchi Formation, which can be subdivided into three sedimentary subfacies, namely platform, intra-platform shoal, and semi-restricted lagoon. The intra-platform shoals were distributed in points in the early stage, and later as the shoals increased in size, they were of continuous distribution.(3) There are two kinds of reservoirs of granular dolomite and crystal dolomite in Xixiangchi Formation, and the overall physical properties are characterized by low to extra-low porosity and permeability. The development of high-quality reservoirs was controlled by shoal facies deposition, dissolution reformation and fracturing action. The reservoir thickness is generally thin, but the vertical cumulative thickness mainly ranges from 5 to 40 m. The horizontal distribution of reservoirs is of poor continuity, and is concentrated in southern, eastern and northeastern Sichuan Basin.(4) There are five favorable exploration targets developed in Xixiangchi Formation in Sichuan Basin, which are located in the central Sichuan paleo-uplift, Weiyuan paleo-uplift, eastern to southeast Sichuan and northeastern Sichuan. The available exploration area exceeds 20 000 km2, which serves as a potential reserve exploration area in Sichuan Basin.

Sedimentary characteristics of Ordovician Yingshan Formation and Yijianfang Formation in Shunbei-1 area, Tarim Basin

PENG Jun, XIA Meng, CAO Fei, XIA Jingang, LI Feng

2022, Vol.34(2): 17–30 Abstract ( 386 ) PDF (10582 KB) ( 325 )

doi: https://doi.org/10.12108/yxyqc.20220202

In recent years, the continuous discovery of oil and gas reservoirs in Shunbei area of Tarim Basin reveals its great exploration potential.Through core observation, thin section identification, conventional logging curves and imaging logging image analysis and element geochemistry test, the sedimentary environment of the upper member of Yingshan Formation and Yijianfang Formation of Ordovician(O1-2y-O2yj) in Shunbei-1 area was analyzed. Combined with the identification marks of sedimentary facies, the sedimentary characteristics of the study area were described, and the high-quality reservoir types and favorable reservoir development areas were pointed out. The results show that:(1) The mass fraction of Mn of the upper member of Yingshan Formation and Yijianfang Formation of Ordovician in Shunbei-1 area 1 is greater than 200×10-6, the average value of Sr/BA is 2.025 4-3.260 2, and the average value of U/Th is 2.02, mostly greater than 0.75, the average value of δU is 1.29, mostly greater than 1, which shows that the whole study area was in a marine sedimentary environment with water depth changing from deep to shallow, normal salinity, warm and humid weak oxidation-weak reduction.(2) The upper member of Yingshan Formation and Yijianfang Formation of Ordovician in the study area is an open platform facies deposit, with two subfacies developed:platform flat and intra-platform shoal, and six microfacies, such as limestone flat, dolomite-limestone flat, dolomite flat, sandy debris shoal, bioclastic shoal and granular beach.(3) As a whole, the study area is mainly dominated by limestone flat microfacies.In the vertical direction, the sandy debris shoal and bioclastic shoal microfacies are more developed in Yijianfang Formation than in the upper member of Yingshan Formation.The development of intra-platform shoal subfacies is weakened from east to west in the plane.

Characteristics and main controlling factors of Triassic Chang 6₃ reservoir in Fanjiachuan area, Ordos Basin

WEI Qinlian, WANG Chonghuan, LIU Junfeng, HU Rong, LIU Meirong, LYU Yujuan

2022, Vol.34(2): 31–44 Abstract ( 385 ) PDF (8853 KB) ( 350 )

doi: https://doi.org/10.12108/yxyqc.20220203

Chang 6 reservoir in Fanjiachuan area of Ordos Basin is a new development area, of which Chang 63 is the main reservoir.In order to study its tight reservoir characteristics and main controlling factors, casting thin sections, conventional physical properties and high-pressure mercury intrusion data were applied to study the petrological characteristics, physical properties and pore structure characteristics of Chang 63 reservoir in Fanjiachuan area of Ordos Basin, recognition functions for different pore structures and diagenetic parameters were established, and the main controlling factors of reservoir performance with different pore structures were analyzed.The results show that:(1) The Chang 63 reservoir in Fanjiachuan area consists of lithic feldspar sandstone with fine lithologies.Clay minerals mainly develop with hydromica, followed by chlorite, and carbonate cements mainly develop with iron dolomite, followed by iron calcite.(2) The Chang 63 reservoir in Fanjiachuan area mainly develops intergranular pores and feldspar dissolved pores, with poor physical properties, and mainly belongs to lowextra low porosity and ultra-low permeability reservoirs, with an average porosity of 9.05% and an average permeability of 0.17 mD.According to the classification standard of pore structure, the pore structures of Chang 6 3 reservoir can be divided into three types, and their throat types and distribution areas distinguished.(3) Sedimentary microfacies is a key factor controlling the physical properties and oil-bearing properties of Chang 63 reservoir in Fanjiachuan area.Compaction is the main diagenesis causing reservoir densification.Cementation and dissolution are important factors resulting in pore structure differences, and the influences of sedimentation and diagenesis are quite different in reservoirs with different pore structures.The research results have guiding significance for the comprehensive evaluation of tight oil of Changqing Oilfield.

Reservoir characteristics of Paleogene lacustrine carbonate rocks in western Qaidam Basin

CUI Jun, MAO Jianying, CHEN Dengqian, SHI Qi, LI Yanan, XIA Xiaomin

2022, Vol.34(2): 45–53 Abstract ( 340 ) PDF (6161 KB) ( 200 )

doi: https://doi.org/10.12108/yxyqc.20220204

Lacustrine carbonate rocks are widely developed in the upper member of Xiaganchaigou Formation (E32) and upper Youshashan Formation(N22) of Paleogene in the western Qaidam Basin. In order to study its distribution and reservoir development characteristics, core observation, thin section identification, scanning electron microscope analysis and X-ray diffraction analysis were carried out. The results show that:(1) The lacustrine carbonate rocks in the study area have the characteristics of many rock types, rapid phase transformation, terrigenous clastic development and general dolomitization. The rocks are mainly composed of massive carbonate rocks, laminar carbonate rocks, algal limestones and granular carbonate rocks, which are closely related to sedimen-tary microfacies and paleoenvironment.(2) The massive carbonate reservoir space in the study area is mainly composed of intergranular pores, and fractures are locally developed. The bedding microfractures of laminar carbonate rocks are developed, the algal limestone reservoir space is mainly composed of algal framework pores with strong heterogeneity, and the granular carbonate rocks are significantly affected by cementation.(3) The average porosity of massive carbonate rocks is 10.6%, the average matrix permeability is 0.03 mD, the displacement pressure is 10.0 MPa, and the average pore throat radius is 0.038 μm. The average porosity of laminar carbonate rocks is 7.4%, the average permeability is 0.76 mD, the displacement pressure is 11.5 MPa, and the average pore throat radius is 0.071 μm. The average porosity of algal limestones is 10.1%, the average permeability is 9.09 mD, the displacement pressure is 3.2 MPa, and the average pore throat radius is 0.117μm. The average porosity of granular carbonate rocks is 7.1%, the average permeability is 0.01 mD, the displacement pressure is 15.0 MPa, and the average pore throat radius is 0.026 μm.(4) The Paleogene high-quality reservoirs in Gasi area are mainly algal limestones, the carbonate rocks in Yingxi-Ganchaigou area develop fracture system, which is a high-yield "sweet spot" area, and Mangya Sag is an exploration potential area. The research results can be used as a reference for oil and gas exploration of lacustrine carbonate rocks in Qaidam Basin.

Genesis and geological implication of illite coatings of Triassic Yanchang Formation in Ordos Basin

TIAN Jianfeng, GAO Yongli, ZHANG Pengbo, YU Jian, ZHANG Zhiguo

2022, Vol.34(2): 54–65 Abstract ( 345 ) PDF (6810 KB) ( 254 )

doi: https://doi.org/10.12108/yxyqc.20220205

Based on the anaysis of genesis and geological implication of illite coatings of Yanchang Formation, the occurrence, chemical composition, distribution characteristics of the illite coatings and its relationship with chlorite coatings were studied by thin section analysis, SEM observation and EDX. The results show that:(1) The illite coatings of Triassic Yanchang Formation in Ordos Basin that are of the same occurrence of authigenic smectite coatings, were formed by the mesogenetic replacement of authigenic smectite coatings.(2) The authigenic smecite and chlorite coatings grew vertically on detrital grain surfaces at the same time owing to the dissolution of volcanic materials whose composition controlled the type of coatings. The illitization of smectite coatings were often accompanied by the dissolution of K-feldspar.(3) The curved flake illite is much larger and thinner than chlorite, and the intercrystalline matrix porosity of illite coatings is much higher than that of chlorite coatings, resulting in that only chlorite and illite can be observed in the inner layer and outer layer of illite coatings in thin sections respectively, while only illite can be easily seen under the scanning electron microscope. Although illite coating reduces reservoir permeability, it is conducive to the preservation of pores and water wettability of sandstone, which is of great significance for oil and gas production. The research results have reference significance for the prediction and development of high-quality reservoirs of Triassic Yanchang Formation in Ordos Basin.

Thermochemical sulfate reduction(TSR) and reservoir reformation of the upper Paleogene Xiaganchaigou Formation in Yingxi area, Qaidam Basin

ZHAO Sisi, LI Jianming, LIU Jincheng, LI Jiyong, CUI Jun

2022, Vol.34(2): 66–74 Abstract ( 286 ) PDF (7601 KB) ( 190 )

doi: https://doi.org/10.12108/yxyqc.20220206

As an important mechanism of dissolution, thermochemical sulfate reduction(TSR) is of great significance in reservoir research. Through the analysis of core observation, thin section identification, scanning electron microscope and logging data, the TSR characteristics and its effect on reservoir reformation of the upper member of Xiaganchaigou Formation in Yingxi area of Qaidam Basin were studied. The results show that:(1) The reactants of TSR are mainly hydrocarbons and sulfate rocks, and the products are high-content H2S, CO, altered hydrocarbons, calcite, elemental sulfur, pyrite, sulfur-containing organic matter. The initial reaction temperature is 100-140℃, corresponding to the formation depth of 3 113-4 536 m in Yingxi area.(2) TSR of the upper member of Xiaganchaigou Formation in Yingxi area resulted that carbonate pores and fractures were filled with modified minerals such as granular and massive pyrite, elemental sulfur and asphalt. The δ34S value of sulfurbearing minerals is relatively heavy. The hydrocarbon components of natural gas are characterized by high methane content and low heavy hydrocarbon content, and the dryness coefficient is larger. The carbon isotope δ13 C1 and δ13 C2 values are larger, and the TSR products such as H2S and relatively high content of CO2 are common. The salinity of formation water is low.(3) TSR related fluids reformed carbonate reservoir, and burial dissolution developed, forming pore systems of different sizes, with an average pore increase rate of 3.5%, which optimized reservoir physical properties.

Genesis of lacustrine dolomites of the fourth member of Paleogene Shahejie Formation in Leijia area, Western Liao Depression

LI Tian, DAI Zongyang, LI Yang, HUANG Lei, GONG Zhenchao, ZHAO Xiaoyang, ZHOU Xiaolong, HUANG Lan

2022, Vol.34(2): 75–85 Abstract ( 304 ) PDF (11162 KB) ( 199 )

doi: https://doi.org/10.12108/yxyqc.20220207

The petrological characteristics, major and trace elements, carbon and oxygen isotopes and other geochemical characteristics were used to analyz the genesis of lacustrine dolomites of the fourth member of Paleogene Shahejie Formation in Leijia area of Western Liaohe Depression. The results show that:(1) During the deposition of dolomites in the study area, the lake basin water bodies were relatively salty and in a strong reducing environment, dominated by micritic dolomites, with two types of occurrences:uniform dispersion and laminar enrichment.(2) The average values of δ18O and δ13C of the dolomites in the study area are -4.4‰ and 8.7‰ respectively, the carbon isotope is positive and the oxygen isotope is negative, reflecting the characteristics of overall salinized water body. The cathodoluminescence test results are no luminescence or dim light, dark red and orange light. The rare earth elements are relatively enriched in light rare earth elements, and weak Eu positive anomaly. The distribution model of rare earth elements shows that the analcite-bearing dolomites have a certain affinity with the alkaline basalts of the underlying Fangshenpao Formation and the basalts in the same layer.(3) There are two genetic types of dolomites in the study area:quasi-syngenetic dolomitization and hydrothermal sedimentary dolomitization. The quasi syngenetic dolomitization is dominated by argillaceous structure, with fine grains, low order of dolomite and rich-calcium and poor-magnesium, which is metasomatized by high salinity lake water. Deep hydrothermal fluids carrying a large number of cations spewed out of the surface and mixed with lake water to form hydrothermal sedimentary rocks.

Characteristics and geological significance of stromatolite dolomite of Ma 5₅ submember of Ordovician Majiagou Formation in Mizhi area, central-eastern Ordos Basin

LUO Zhenfeng, SU Zhongtang, LIAO Huihong, HUANG Wenming, MA Hui, SHE Wei

2022, Vol.34(2): 86–94 Abstract ( 398 ) PDF (6525 KB) ( 259 )

doi: https://doi.org/10.12108/yxyqc.20220208

A set of stromatolite dolomite developed at the bottom of Ma55 submember of Majiagou Formation in Mizhi area, central-eastern Ordos Basin, which is a good material for studying microbial rocks and microbial dolomitization. The stromatolite dolomite was studied through the analysis of petrology, mineralogy, cathodoluminescence, scanning electron microscope, carbon, oxygen and strontium isotopes, geochemistry, the sedimentary environment of Ma 55 submember was determined, and the genesis and geological significance of this set of stromatolite dolomite were discussed. The research results show that:(1) The stromatolites in the study area can be divided into columnar and dendritic according to the morphological characteristics, the dolomite crystal is microcrystalline, and it is dark red in the cathodoluminescence, and the dolomite is granular under the scanning electron microscope.(2) The δ13C value of stromatolite dolomite(-4.15‰--0.80‰) is generally within the range of seawater value in the same period, and the δ18O value(-13.19‰--6.62‰) is obviously negative compared with the seawater value in the same period. The 87Sr/86Sr value(0.708 9-0.709 8) is higher than that of seawater in the same period. The contents of Fe, Mn, Na of the stromatolite dolomite are relatively high, and the Sr content is relatively low. The dolomitization fluid of the stromatolite dolomite of Ma 55 submember in the study area is mainly marine-derived fluid, mixed with atmospheric fresh water, which is the product of shallow buried infiltration and reflux in the intertidal-subtidal zone.(3) The intertidal zone with seawater salinity conversion in the early stage of transgression is a favorable area for oil and gas exploration of stromatolite dolomite.

Miocene river-wave dominated sedimentary system in south belt of Enping Sag and its significance

ZHANG Xiaozhao, WU Jing, PENG Guangrong, XU Xinming, ZHENG Xiaobo

2022, Vol.34(2): 95–104 Abstract ( 346 ) PDF (9758 KB) ( 170 )

doi: https://doi.org/10.12108/yxyqc.20220209

The south belt of Enping Sag is a favorable oil and gas enrichment area with a sedimentary background of developing lithologic traps. However, the exploration of lithologic reservoirs in this area stared relatively late, the understanding of lithologic traps is not enough, which restricts the progress of lithologic trap exploration. In Sag, based on high-precision sequence framework and sedimentary filling sequence, the sedimentary characteristics, sedimentary facies distribution and evolution law of river-wave dominated system were analyzed by using logging, core and seismic data, and then the identification marks, distribution law and development characteristics of lithologic traps of sand bodies under river-wave dominated system were dissected. The results show that river-wave dominated delta sedimentary system is developed in the Miocene in the south belt of Enping Sag, EP1, EP2 and EP3 oil areas are favorable areas for lithologic trap development, the fifth and sixth members of Hanjiang Formation and the first member of Zhujang Formation are favorable zones for lithologic trap development, and wave-reformed sandstone is the main reservoir of lithologic traps. The research results play a guiding role in lithologic exploration in south belt of Enping Sag.

Fault development characteristics and reservoir control in Chengbei fault step zone, Bohai Bay Basin

MIAO Huan, WANG Yanbin, HE Chuan, LI Jianhong, ZHANG Wei, ZHANG Yujian, GONG Xun

2022, Vol.34(2): 105–115 Abstract ( 327 ) PDF (5285 KB) ( 308 )

doi: https://doi.org/10.12108/yxyqc.20220210

In order to study the development characteristics of the main faults in Chengbei fault step zone of Bohai Bay Basin and its relationship with the secondary hydrocarbon generation and accumulation of Upper Paleozoic source rocks, seismic data interpretation, drilling data analysis and bottom structural characteristics of each era were carried out, and the geometric shape, activity and sealing characteristics of four main faults in Zhangbei, Zhangdong, Zhaobei and Yang'erzhuang were characterized. The results show that:(1) The main fault section in the study area is shovel type or slope flat type, and the secondary faults on both sides are in "Y" shape and flower shape in the section, and in feather and en echelon shape in horizontal plane. The activity of each main fault is the strongest in Es stage and the weakest in Ng stage. The sealing properties of Zhangdong fault and Zhaobei fault were poor in Ed stage but good in Ng stage, the sealing properties of Zhangbei fault were good in Ed stage but poor in Ng stage, and the sealing properties of Yang'erzhuang fault were good in both Ed stage and Nm stage.(2) The Upper Paleozoic source rocks in Qikou main depression reached the peak of hydrocarbon generation in Es3 stage, and now it is an over mature stage. The Upper Paleozoic source rocks in Chengbei fault step zone reached the peak of hydrocarbon generation in Neogene, and now are still in the peak of hydrocarbon generation.(3) The reservoir accumulation elements in Chengbei fault step zone are well matched from Ed stage to present which are the key stage for the formation of Upper Paleozoic reservoirs. The primary reservoirs of Upper Paleozoic are mainly self-source and self-reservoir anticline reservoirs, fault block and fault nose reservoirs, and buried hill reservoirs with young source in old reservoir, among which the former two are distributed in the whole area, and the last are distributed in Zhangdong buried hill. The research results can provide a basis for Upper Paleozoic oil and gas exploration in Chengbei fault step zone.

Sedimentary characteristics of oolitic beach of Triassic Feixianguan Formation in eastern Kaijiang-Liangping trough, Sichuan Basin

LI Yang, WANG Xingzhi, PU Baiyu, XU Changhai, YANG Xiyan, ZHU Yiqing, HUANG Zisang, KANG Jiahao

2022, Vol.34(2): 116–130 Abstract ( 452 ) PDF (15777 KB) ( 232 )

doi: https://doi.org/10.12108/yxyqc.20220211

In order to clarify the temporal and spatial distribution characteristics of oolitic beach under the sequence stratigraphic framework of Triassic Feixianguan Formation in eastern Kaijiang-Liangping trough in Sichuan Basin, and the control mechanism of sea-level change on oolitic beach migration, lithologic characteristics analysis, identification of sedimentary structural markers and division of sequence stratigraphic framework of sequence boundary were carried out.The results show that:(1) Two third-order sequences(SQ1 and SQ2) are developed in Feixianguan Formation in eastern Kaijiang-Liangping trough under the background of continuous regression, which can be further subdivided into five fourth-order sequences(Ⅰ to Ⅴ) (. 2) The oolitic beach of Feixianguan Formation is mainly developed in sequenceⅠ to Ⅳ, and the oolitic beach in sequence Ⅱ is the most developed, with large single thickness and good continuity, while the sedimentary thickness of oolitic beach in sequence Ⅲ, Ⅰ and Ⅳ decreases in turn, and the least developed oolitic beach is with discontinuous and sporadic distribution.(3) Besides vertical aggradation, oolitic beach has obvious lateral migration characteristics, migrating from the interior of the platform to the slope as a whole, and the lateral migration of oolitic beach in sequence I to is the most obvious. (4) The high-frequency cycles in local highlands and the production rate of carbonate rocks change synergistically, resulting in vertical aggradation and multi-stage superposition of oolitic beach of Feixianguan Formation in the study area. When the sea level drops below the base level, the hydrodynamic conditions suitable for beach growth change from local highlands to local depressions, resulting in the change of oolitic beaches from vertical growth to lateral progradation.(5) On the plane, the oolitic beach of Feixianguan Formation in the study area is mainly distributed along NW-SE direction(platform edge), most developed in Tieshanpo and Puguang areas, followed by Dukouhe, Qilibei and Luojiazhai areas. The research results can provide a basis for the exploration of highquality beach facies reservoirs of Feixianguan Formation in Kaijiang-Liangping trough.

Sedimentary characteristics and controlling factors of sublacustrine fans of the third member of Paleogene Dongying Formation in Liaozhong Sag

MA Zhengwu, GUAN Dayong, WANG Qiming, LIU Yaojun, LI Xiaohui

2022, Vol.34(2): 131–140 Abstract ( 298 ) PDF (6935 KB) ( 235 )

doi: https://doi.org/10.12108/yxyqc.20220212

Sublacustrine fans are developed in the third member of Dongying Formation of Paleogene(Ed3) in Liaozhong Sag. In order to clarify the sedimentary characteristics, controlling factors and sedimentary distribution characteristics of the sublacustrine fans in the study area, the characteristics of cores, petrology, logging response and seismic response were comprehensively analyzed in combination with regional structural development characteristics and paleogeomorphic analysis. The results show that sublacustrine fans are mainly developed in the Ed3 of the study area. The sublacustrine fans can be divided into three subfacies such as inner fans, middle fans and outer fans, and several microfacies including main channels and braided channels. The sedimentary characteristics of sublacustrine fans are controlled by provenance supply, paleogeomorphology and the distribution of slope break zone. The distal delta is the prerequisite for the development of sublacustrine fans. The paleogeomorphology provides accommodation space and controls the location and scale of sublacustrine fans. The slope break zone controls the deposition rate of sediments, and the regulation and redistribution of sublacustrine fans. In the early stage of Ed3, three stages of sublacustrine fans were only developed in structural areas B and C. In the middle stage, braided river delta system developed, and large-scale sublacustrine fans were developed. In the late stage, sublacustrine fans were only developed in the central and structural area B. The structural area B is a favorable exploration target with large sublacustrine fan distribution, large sand body thickness, good reservoir physical properties and good vertical superposition, and it is close to hydrocarbon generation center and favorable for hydrocarbon migration and accumulation.

Main controlling factors of Ordovician Wufeng-Silurian Longmaxi shale gas enrichment in Tiangongtang structure, southwestern margin of Sichuan Basin

ZHANG Menglin, LI Guoqin, HE Jia, HENG De

2022, Vol.34(2): 141–151 Abstract ( 369 ) PDF (5894 KB) ( 308 )

doi: https://doi.org/10.12108/yxyqc.20220213

The data of seismic, drilling, logging and core analysis and test were comprehensively used to study the structure, deposition and basic reservoir geological characteristics of Ordovician Wufeng-Silurian Longmaxi Formation in Tiangongtang structure in southwestern margin of Sichuan Basin, and the main controlling factors of shale gas enrichment and accumulation model were discussed. The results show that:(1) The high-quality shale in Tiangongtang structure is mainly developed in O3w-S1l1-1. Type Ⅰ reservoir is mainly developed in S1l1-1-1-S1l1-1-3 layers, with a thickness of 0.8-17.0 m, and characterized by high TOC content, good gas-bearing property and highly brittle minerals.(2) Tiangongtang structure has experienced rapid subsidence, deep burial and a large amount of hydrocarbon expulsion before the Late Cretaceous, forming an early enriched overpressure shale gas reservoir. After the Late Cretaceous, it was uplifted and reformed in two phases, and the early enriched overpressure shale gas reservoir gradually transformed to high pressure-normal pressure shale gas reservoir.(3) The preservation conditions of Tiangongtang structure are affected by stratigraphic dip angle and Gong-1 fault, resulting in differential enrichment of shale gas in different structural zones.(4) Tiangongtang structure is an anticline structure-type accumulation model of "early deep burial promotes enrichment, later uplift adjusts enrichment, and preservation conditions control enrichment". The southwest tip of the structure is a potential favorable area for next shale gas exploration and development.

Lithofacies and sedimentary environment of shale of Permian Longtan Formation in eastern Sichuan Basin

XIAO Wei, ZHANG Bing, YAO Yongjun, WANG Yan, YANG Hongyu, YANG Kai

2022, Vol.34(2): 152–162 Abstract ( 419 ) PDF (7564 KB) ( 281 )

doi: https://doi.org/10.12108/yxyqc.20220214

Permian Longtan Formation in eastern Sichuan Basin belongs to transitional-marine strata, with a broad shale gas exploration prospect. According to the mineralogical and petrological characteristics of shale in the study area, the lithofacies was divided, the paleosedimentary environment was reconstructed by using geochemical data, and the controlling factors of organic matter enrichment were discussed. On this basis, the coupling relationship among lithofacies, organic matter enrichment and sedimentary environment was established. The results show that:(1) The shale of Longtan Formation in eastern Sichuan Basin mainly develops four lithofacies:siliceous shale, clay siliceous mixed shale, clay calcareous mixed shale and calcareous siliceous mixed shale. The vertical distribution of lithofacies can be divided into early relatively clay rich stage and late relatively high siliceous content stage.(2) During the early stage of Longtan Formation, the sea level was low, it was mainly in oxidation-suboxidation environment. The average U/Th is 0.42, the average EFMo is 6.66, and the average V(/V+Ni) is 0.77. The paleoproductivity is low and the terrigenous input is large, and the siliceous source was terrigenous transport. During the late stage, the sea level continued to rise, it was mainly in reduction environment. The average U/Th is 5.96, the average EFMo is 148.01, and the average V(/V+Ni) is 0.79, with high paleoproductivity and little terrigenous input, and a large amount of biogenic silicon was enriched.(3) The organic matter enrichment in the sedimentary stage of Longtan Formation have obvious differences, it was jointly affected by terrigenous input and paleoproductivity in the early stage, and a large number of terrigenous higher plant debris were brought in, resulting in a high content of organic carbon. It was controlled by paleoproductivity in the late stage, and the supply of terrigenous debris was weaker than that in the early stage, with organic matter mostly from marine algae.