Abstract:Polygonal faults are non-tectonic small stratabound extensional faults generally developed in the fine-grained sediments with numerous fault planes and small offset. The fault planes are arranged in irregular polygonal shape in plan view, and occur as extensional faults with similar or opposite inclinations in profile view. The geometry of the polygonal faults is considered to be affected by formation mechanisms, such as density inversion, syneresis and shear failure. The prominent indicator of density inversion is the wave-like surface between horizons. The prominent indicators of syneresis are listric fault planes and growth sequences, and the furrows on the seafloor and the micro-fractures in the sediment samples make up the evidence. The occurrence of polygonal faults improves the permeability of fine-grained sediments, which can provide vertical pathways for gas and fluid migration. The conductivity of listric faults induced by syneresis is inferred to be better than that of straight faults. As a product of shallow gas and fluid migration, the occurrence area of gas hydrate is possibly dominated by polygonal faults. Deep polygonal faults provide pathways of gas and fluid migration for gas hydrate, and shallow polygonal faults provide reservoir spaces for gas hydrate.

Abstract:Main marine shale formations in southern China, in addition to the Ordovician Wufeng Formation and the Silurian Longmaxi Formation which have become China's shale gas industrial production layers, include marine shale of the Cambrian Niutitang Formation and the Sinian Doushantuo Formation, and other marine shales. So far, the Sinian Doushantuo Formation is the oldest shale gas layer found in China and even in the whole world. Based on previous studies of regional geology and relying on drilling core data obtained in the middle Yangtze region, the authors used multi-scale methods for characterizing the reservoir properties and, combined with the organic matter abundance and evolution characteristics, carried out the comprehensive research. The reservoir and hydrocarbon potential of the world's oldest shale gas were analyzed for the first time. A comparison was made with the commercial shale gas reservoir in the United States and that in Sichuan basin of China. The results show that the Sinian Doushantuo Formation shale in the middle Yangtze area is developed in a relatively stable sedimentary environment, and the mud shale has large thickness and high organic matter content. The total organic carbon (TOC) content of most samples is more than 2%. The brittle mineral content of shale is high, and micro-nano pores and multi-scale fractures are developed. Compared with shale of Longmaxi Formation in Sichuan basin, the thermal evolution degree of organic matter in shale of Doushantuo Formation is moderate, the Raman maturity is between 2.0 and 3.0, the organic matter is in the stage of pyrolysis. The analysis indicates that Doushantuo Formation has the basic geological conditions for shale gas reservoirs and has a good resource and exploration prospect. The research results provide comprehensive basic data for the evaluation, exploration and development of new shale gas fields and new strata in the future.

Abstract:The double bottom simulating reflector (DBSR), which is important for the accumulation mechanism, distribution and enrichment characteristics as well as drilling and exploitation site optimization of gas hydrates, has become a new international research hotspot. The research on the DBSR abroad mainly focuses on the recognition, formation mechanism and the relationship with the accumulation of gas hydrates; nevertheless, the mechanism of DBSR is still ambiguous. Especially, the unification of DBSR with hydrates and free gases for carrying out the study of the dynamic accumulation of gas hydrates remains at the exploratory stage. Evidence of DBSR has been found in the South China Sea, but the research on them has been very insufficient and seriously lagged. Therefore, it is an urgent need to carry out the study of the recognition and depiction, the formation mechanism and controlling factors, the developing and evolution of DBSR and their relations with the dynamic accumulation of gas hydrates, as well as the effect of DBSR on the exploration and exploitation of multiple-type gas hydrates. The investigation of the relationship between DBSR and the dynamic accumulation of gas hydrates and establishment of the dynamic evolution models of the gas hydrate-free gas system will provide important scientific basis for the intensive research on the genetic mechanism, distribution regularities and geological evolution of the DBSR and gas hydrates. The research on reservioring theories as well as exploration and development of gas hydrates can also promote and develop the research.

Abstract:Unconventional tight oil reservoirs, a lightspot in the present unconventional oil and gas exploration, have emerged as a significant source of oil supply in the world. The geological research, theoretical promoting, and present significant issues concerning tight oil will supply and prefect geological theory on unconventional hydrocarbon. Currently, the development of geological research on tight oil can be concluded as follows:① Three main formation conditions and six basic characteristics about tight oil as well as the differences between continental tight oil in China and marine tight oil in the USA have been summarized. ② Pore-throat-fracture network in the tight-oil reservoir has been characterized with advanced and high-precision analysis and testing technology, the formation of tight oil being the primary research object. ③ The accumulation mechanism of tight oil has been deeply studied, which includes charge dynamics, the threshold of charging, migration mechanism, accumulating model, etc. Based on the research status about tight oil, this paper puts forward several important questions about tight oil:① Holographic quantitative heterogeneity characterization on the tight oil reservoir at a micro-nano scale and its effect on tight oil migration and accumulation; ② the genesis of source-reservoir configuration and its control on tight oil accumulation and enrichment; ③ the influence factors and transformation conditions of the occurrence of tight oil, and the relative contribution of different kinds of occurrence status and space; ④ the internal relationship between the mixed sedimentary system in saline lacustrine basins and the formation of tight oil; ⑤ a comprehensive study of the tight oil accumulation factors, and the accumulation mechanism and its whole process. The answers of these questions will play a significant role in geological research on tight oil.

Abstract:The Qiangtang Basin is the largest Mesozoic marine sedimentary basin on the Tibetan Plateau. It is rich in source rocks and evaporites, and has become one of the most important places for petroleum exploration in China. The study of sedimentary facies of the Upper Triassic Bagong Formation in Tumen-Baqing area can not only provide evidence for the evolution of the Tethyan Ocean but also provide theoretical basis and practical guidance for the exploration of hydrocarbon and potassium resources in the Qiangtang Basin. An analysis of petrological characteristics, paleontology and sedimentary structures shows that the lagoon facies, tidal flat facies, delta facies and ten kinds of sub-facies are mainly developed in the study area. During the Bagong period of the Late Triassic, the study area was in a continental margin sea sedimentary environment under the extensional background from the north (Baqing-Suoxian) to the south (Ban-Nu suture zone).

Abstract:The study area is located in the central and western part of Weixin Sag in the northern Yunnan-Guizhou to the southern margin of the southern Sichuan Low-steep fold belt. This paper analyzed the basic geological characteristics of the Longmaxi black shale using drilling, field outcrops, regional geology and other data. And the shale gas resource of Longmaxi black shale were evaluated by using volume method. The research results show that the studied Longmaxi black shale is relatively high in TOC, maturity and brittle mineral content, relatively saprolitic in organic matter type, and medium in physical properties and total gas content. The above characteristics of Longmaxi black shale has certain comparability with the major gas-producing shale in Weiyuan, Changning and North America. The preliminary estimation of shale gas resource of the Longmaxi Formation in the study area is 24871.8×10⁸ m³, which shows a good prospect for shale gas exploration. But affected by characteristics of reservoir development and preservation conditions, the shale gas resources are severely unevenly distributed. Shale gas resources with good quality are mainly distributed in the first layer, and the southern Sichuan low-steep fold belt in the northern part of the study area is the main area equipped with good quality resources. The wide anticline and box anticline structural belts with good preservation conditions are favorable areas for the next exploration.

Abstract:The magnitude and direction of stress are important basic data for unconventional oil and gas exploration and analysis of regional tectonic stress field. In order to find out the present stress state in southern Guizhou area, the authors applied the Anelastic Strain Recovery (ASR) method to obtain the in-situ stress state in Carboniferous Dawuba Formation of southern Guizhou Area (Qianziye-1 well) ranging from 2877 to 2985 m in depth. Based on the features of regional tectonic development in Qiannan depression, the authors investigated the characteristics of regional tectonic stress field and the preservation conditions of oil and gas. The results show that the Carboniferous Dawuba Formation in southern Guizhou area gas retaining the normal fault stress regime, the orientation of maximum horizontal principal stress at the measuring point was in nearly SN direction, which was the same as the maximum principal stress direction of the paleotectonic stress field in the Yanshanian period. The late tectonic movements occurred mainly along faults, which suggests a relatively minor damage to the secondary oil and gas pools during the last phase of YanshanHimalayan movement. It is conducive to the preservation of oil and gas reservoirs in Qiannan depression.

Abstract:Sichuan Basin, as one of the six atmospheric regions in China, has made outstanding contribution to the rapid development of natural gas in China. The thickness of marine craton deposits is 4000-7000 m, and marine carbonate rocks are the main gas enrichment strata. In order to further guide the direction and target of natural gas exploration for marine carbonate rocks in the basin, the authors, based on previous research results and many years'exploration experience and starting with key geological factors controlling hydrocarbon accumulation, have systematically summarized the regularity of natural gas enrichment in marine carbonate rocks in Sichuan Basin, and reached some conclusion:"source control" on the whole controls complementary distribution of source rocks in space, and vertical and horizontal distribution of natural gas shows that the gas-bearing property of reservoirs which are close to source rocks in vertical direction is generally better, and the horizontal distribution of oil and gas reservoirs around high-quality source rocks (gas or oil center) is circular; the "facies-controlled" favorable sedimentary facies zone has obviously controlled reservoir development and natural gas enrichment, that is, there exist "facies-controlled reservoir and reservoir-controlled reservoir"; the karstification improves carbonate rocks, i.e., improves reservoir permeability conditions of gas reservoirs; inheritance development of paleo-uplift controls the early migration and accumulation of oil and gas reservoirs; and present structure plays an important role in controlling the readjustment and redistribution of gas reservoirs. By comprehensively applying the regularity of natural gas enrichment, Sichuan Basin has made several major discoveries and breakthroughs and submitted proven reserves exceeding trillion square meters, which has greatly promoted the rapid development of natural gas in China.

Abstract:To identify the impact of deposition facies on Longmaxi shale gas enrichment in southern Sichuan Basin, the authors studied sedimentary characteristics, stratigraphic distribution, lithology, sedimentary structures, organic carbon content, mineral composition, reservoir space type and other features as well as the impact of sedimentation on favorable shale distribution with the purpose of providing a basic evidence for the evaluation of shale gas enrichment conditions. By the observation and description of drilling cores from six shale gas wells in the study area and according to log response, experimental analysis, Longmaxi Formation was divided into three subfacies, which are shallow water muddy shelf, half-deep water shelf and deep water shelf. It is shown that shale gas accumulation is affected by organic carbon content, the development and distribution of shale, mineral composition, and the development of pores, which are all controlled by sedimentary facies. The deeper the water, the closer to the deposition center, TOC content is higher; Longmaxi Formation is thick but the effective section is much thinner, and it becomes thicker where it is closer to the deposition center. Organisms deposited in deepwater make the effective shale have higher organic matter content and more brittleness; Main pore types in Longmaxi shale are clay mineral layer pores and organic pores, and organic pores are more developed in the section with higher TOC content. Deep-water shelf is the most favorable facies for shale gas enrichment with abundant organic matter content, high brittleness and abundant organic pores.

Abstract:The development of oil field has entered the middle or late stage, and the dependence of oil and gas resources on imports is gradually increasing; therefore, the search for new oil and gas areas is an urgent task. Sanjiang Basin, which is located in the east of Heilongjiang Province, is the largest basin in the eastern periphery of Songliao Basin, but there has been no breakthrough in oil and gas exploration there. Through field geological survey and drilling exploration in Sanjiang Basin, the authors summarized the lithological and distribution characteristics of siliceous rock series of Dajiashan Formation deposited in early Mesozoic in this area. The siliceous rocks and dark mudstone strata are widely developed in the eastern part of the basin and has a large thickness, with total organic carbon abundance up to 0.95%; Ⅱ1 is the main organic matter type and is developed at the high maturation stage, and hence it is expected to become a new layer of oil and gas exploration in the Sanjiang Basin.

Abstract:Being a new layer of shale gas exploration in China, the Sinian Doushantuo Formation located in western Hubei Province, which shale reservoir mineral composition is dominated by dolomite and therefore its log response characteristics is obviously different with the siliceous shale of Ordovician Wufeng Formation, Silurian Longmaxi Formation and the Cambrian Niutitang Formation. The previous "seven parameter relationship" study on Longmaxi and Niutitang Formation is not support effectively for the fine characterization and fracturing construction of the Doushantuo Formation shale reservoir. In this paper, after analyzed the conventional logging, special logging and sample data of Eyangye 1 well, we suggested that the logging response characteristics of second member of Sinian Doushantuo Formation with low gamma, low uranium, low acoustic time difference, low neutron, high resistivity, and high density on the conventional logging curve. Furthermore, we proposed that the elemental logging is the most direct and effective logging method for evaluating the TOC of the second member of Doushantuo Formation shale, and analyzed that MRIL-P type nuclear magnetic survey is suitable for the evaluation of porosity parameters. At last, we evaluated the shale gas content via the new fitting method, and obtained favorable evaluation parameters of shale reservoirs. The research results above mentioned will provide parameter basis for the logging evaluation of the same type shale reservoir.

Abstract:The lithofacies differentiation of the Ordovician-Silurian deposits in the Lower Yangtze region is obvious, with the development of many types of lithostratigraphic units. There are significant differences in color, lithology, thickness and occurrence of fossil assemblages of the sedimentary rocks. Based on the investigation of outcrops and drilling profiles in the study area, the authors summarized the achievements of previous researches and used the methods of petrology, paleontology, ecology and indoor analysis to divide the lithofacies into six sedimentary facies, i.e., coastal facies, isolated platform facies, shallow continental shelf facies, shelf break facies, under-compensated basin facies and turbidite basin facies. The lithofacies palaeogeography research shows that, in the Late Ordovician Kaidi period, due to the Caledonian tectonic movement, the collision and extrusion of the South China Plate were significant. The "Southern Yangtze Plateau" continued to rise and expand, which led directly to the disappearance of the old "platform-slope-basin" pattern and the formation of a new paleogeographic pattern, i.e., the paleogeographical pattern in which the water body which had been developed from the northwest to the southeast was completely transformed into the pattern that the all three sides of northwest, southwest and southeast directions were surrounded by the ancient land and got deepened toward the northeast in paleogeography. During the Late Ordovician Hernannian period, due to the declining of the global sea level and the climatic cooling caused by the Antarctic ice sheet condensation, the sea surface of the Yangtze platform got shallower so that the submarine basin rose above the lowest oxygen-containing depth in the oxygenated water layer, and the oxygen-rich environment replaced the anoxic environment. The Changwu-Yuqian Formation dolomitic sedimentary assemblage in the Jiangnan sedimentary area was transformed into shallower Wenchang Formation-Yankou Formation close to molasse combination. In the early Silurian period, with the rising of global sea level, the uplifted area continued to expand, the tectonic confinement effect was intensified and the black shale reappeared. Especially along the Anqing-Tongling-Chaohu line in the south of Anhui Province, the Wufeng Formation of Nanjing-Jurongin area in the middle part of Jiangsu and the black siliceous shale and carbonaceous shale at the bottom of Gaojiabian Formation, the Longmaxi Formation has good consistency with thickness of 40-60 m and TOC content of 1.2%-4%. These areas have good potential for hydrocarbon generation and should be regarded as the key research areas for the exploration of source rock and shale gas in further exploration.

Abstract:Significant breakthrough of shale gas exploration has been made in Lower Cambrian and Sinian shale on the north margin of the Yangtze block, South China. In this paper, the authors focused on the preservation condition of shale gas, with the purpose of investigating the tectonic evolution history of the Huangling uplift by means of geological survey and 2D seismic, geochronological and drilling data. The results are as follows:at~800Ma, granite intrusion took place in the Huangling dome basement, primarily shown by granites replacing metamorphism rocks; during 800~200Ma, no significant tectonic movement occurred, with no geochorologic data; the Sinian shale was buried at the depth of about 8km during Middle Jurassic. This study indicates that the rigid basement of the Huangling uplift was mainly composed of homogeneous granite, without intensive deformation. As the main challenges of the widely distributed Lower Cambrians and Sinian shale are high-maturity and intensive deformation, a geological unit with a dome probably is a favorable zone for the old age shale gas. Therefore, it is shown that the adjacent zones of the the Xuefengshan, Shennongjia and Hannan are geological units with a dome and probably have potentials for the exploration of shale in the Lower Cambrian strata.

Abstract:Both of the Hala Lake Depression in Qilian Mountain and the Muli Depression in which natural gas hydrate was found are depressions of the South Qilian Basin and have similar hydrate accumulation conditions. However, due to the low degree of geological work in the Hala Lake Depression, the geological structure information in the depression under the Quaternary overburden is unclear, and the gas hydrate accumulation conditions in this area are not well understood. In order to find out the geological structure characteristics and gas hydrate accumulation conditions of the Hala Lake Depression, the authors carried out a comprehensive study of geophysical and geochemical exploration in the depression. Some conclusions have been reached:(1) The investigation area can be divided into four sags, two salients and twenty-six faults; (2) As for the distribution characteristics of permafrost layers in the investigation area, the Cenozoic sedimentary areas are mainly in sheet distribution in large area and the outcrops of bedrock are mainly in island-shaped distribution; the development of the permafrost layer is closely related to factors such as elevation, topography, land cover and surface soil moisture; (3) Two geochemical anomalies were found in the investigation area, and the indicators combination of heavy hydrocarbon in acidolysis hydrocarbons, aridity coefficient of acidolysis hydrocarbons, methane in headspace gases and fluorescence spectra characteristics indicates that there are two different types of anomalous features characterized by an obvious anomalous concentration center with relatively high intensity; (4) The source rocks in the Hala Lake Depression are relatively poor, and the preservation integrity of source rocks and the Indosinian-Yanshanian trap structure are the key factors to hydrate accumulation. The northern part of the Hala Lake Depression and the No. I geochemical anomaly zone can be the prospective areas for hydrate accumulation, and these findings provide a basis for future investigation and evaluation of the gas hydrate resources in the South Qilian Basin.

Abstract:Characterizing the internal architecture of the reservoir of the Lower Xiaganchaigou Formation in the slope area of Western Qaidam Basin can provide some reference for the exploration and development of lithologic reservoirs in this area. Guided by high-precision sequence stratigraphy, sandbody architecture theory and method, the authors studied the characteristics of sequence stratigraphy and sandbody architecture in the Lower Xiaganchaigou Formation on the basis of well-seismic combined calibration and comprehensive analysis of seismic, well logging, drilling and core data. The results show that the Lower Xiaganchaigou Formation in the slope area of Western Qaidam Basin is a large regional lacustrine invasion sequence, which can be divided into 2 third-order sequences and 4 fourth-order sequences (EMSQ1~4). From early to late, A/S (the ratio of accommodating space and sediment supply) continues to increase, whereas the sequence structure of the fourth sequence gradually changes from deepening-upward semi-cycle to non-symmetric deepening-upward full-cycle. Meanwhile, the sedimentary facies changes from braided river delta plain to delta front, and finally changes to shore-shallow lacustrine facies. The stacking patterns and distribution regularity of different types of sandbodies are closely related to the fourth-order sequence structure. The distributary channel sandbodies in delta plain facies are mainly developed in EMSQ1, and cut each other and overlap vertically so that their connectivity is very good. The subaqueous distributary channel sandbodies in delta front are mostly developed in EMSQ2~3, and are isolated or overlapped occasionally so that their connectivity is not very good. The beach-bar sandbodies of the shoreshallow lacustrine facies are chiefly developed in EMSQ4, and are isolated from each other so that their connectivity is bad. It is concluded that the further exploration of lithologic reservoirs should focus on shore-shallow lacustrine beach-bar sandbodies in EMSQ4 sequence.

Abstract:Clay shale samples from Cretaceous Madongshan Formation in Well Guye 1 in Liupanshan Basin were collected for determining shale gas exploration prospect through outcrops and core description, rock pyrolysis, X-ray diffraction, and element analysis of kerogen and isothermal adsorption. The following conclusions have been reached:(1) Cretaceous Madongshan Formation of Liupanshan Basin has abundant oil gas shows consisting of fluorescence, oil stains, bitumen and dark shale or oil shale, with a thickness nearly 200 meters in core Guye1. (2) The organic matter type of Madongshan Formation is Ⅱand Ⅲ, the 65% shale samples of Madongshan Formation in core Guye1 have a TOC content higher than 1.0%, with 37.2% shale samples having TOC content higher than 2.0%. The vitrinite reflectance of nearly half samples is higher than 0.8%, and the Tmax is 435~445℃, suggesting that Madongshan Formation is at the low mature to mature stage, and gas reservoir is mixture of biogenic gas and thermal genetic gase. (3) The shale of Madongshan Formation has characteristics of low clay content and higher feldspar and carbonates. The average content of carbonate, feldspar, quartz and clay is 38.2%, 24.4%, 17.2% and 13.2%, respectively, which can meet the demand of shale gas exploration standard of clay content less than 30% and brittle mineral content higher than 40%. The components of clay are mainly illite and illite-montmorillonite, with the average content being 69% and 26.1%. The porosity is between 0.117% and 1.268%, the permeability is between 0.0009 mD and 0.0042 mD, suggesting a low porosity and low permeability reservoir. The isothermal adsorption test shows that Madongshan shale has a strong gas storage capacity, with the methane content being 0.96-4.6 m³/t and the average content being 2.26 m³/t. (4) According to a comparison of geological and geochemical characteristics with shale of Yanan Formation, the authors hold that Madongshan Formation of Liupanshan Basin has a huge shale gas exploration potential and could be regarded as a high quality shale gas reservoir for exploration and development, and the achievements obtained by the authors provide experience for exploration of the unconventional oil and gas reservoirs in the Cretaceous strata of the Hexi Corridor.

Abstract:Taking the Chang 7 shale system of Yanchang Formation in Ordos Basin as the research target and based on the statistics of a large number of geological samples, the authors established the oil generation and expulsion model. The movable threshold (hydrocarbon expulsion threshold) of the Chang 7 shale system was considered to be 70 mg/g TOC, and the evaluation template of shale oil in Chang 7 shale system was established, which included oil-bearing potential and mobility. In the evaluation template, the shale oil in this area was divided into three categories:effective resources, potential resources and ineffective resources. Based on the actual geological application examples of well H317, the different lithologies were systematically classified using the evaluation template. Sandstone, silty mudstone and some massive mudstone in the Chang 7 shale system of well H317 are probably the effective resources and favorable targets for shale oil exploration and development. Although the oil-bearing capacity of black shale is good, the composition is heavy and that will cause the fluidity to be not as good as the other kinds of lithologies mentioned previously. However, by using engineering techniques such as situ mining, it may become a potential target for shale oil exploration in this area.

Abstract:The first member of Upper Cretaceous Qingshankou Formation (K₂qn₁) in Songliao Basin has considerable shale oil resource potential because of the wide development of dark mudstone and high residual hydrocarbons. The paleo-sedimentary environment directly controls the dark mudstone distribution, organic matter development, and residual hydrocarbon content. Therefore, analyzing the sedimentary features and restoring the paleo-environment of K₂qn₁ are of great significance for revealing the regularity of shale oil enrichment. Based on the core observation and logging data of well Jishanyou 1 in Qian'an area of Changling Sag, the authors analyzed the sedimentary characteristics of K₂qn₁. According to the element data obtained by natural gamma ray spectrometry logging and lithologic scanning logging, the evolution of the paleo-environment of K₂qn₁ was restored by using suitable parameters. Furthermore, the correlation of paleo-environment parameters and residual hydrocarbon content in mudstone shale was also studied. The results show that the dark mudstone of K₂qn₁ in the study area was developed in the outer Delta front to shallow lake-semi-deep lake sedimentary system. The palaeo-environment of K₂qn₁ underwent a change of water depth from shallow to deep, climate from semi-humid to humid, salinity from fresh to brackish, reduction conditions from weak to strong, and sedimentary hydrodynamics from moderate to weak. There is an obvious interface of sedimentary environment within K₂qn₁. The content of residual hydrocarbon in the mudstone is mainly controlled by paleo-climate and paleo-reduction conditions. The oil-bearing organic matter was formed in a wet and reducing environment, with strong oil-generating capability and high content of residual hydrocarbon.

Abstract:The formation mechanism of the oil shale developed in the 1st and 2nd members of Nenjiang Formation in Songliao Basin was studied by combining theoretical model with geological data such as total carbon content, sulfur isotope of pyrite, and whole rock element from the Songke-1 Well (south hole). Some conclusions have been reached:(1) It may not be fully correct to attribute the formation of oil shale to seawater intrusion into the lake basin. Although seawater invasion can lead to salinization of the lake and subsequent algae blooming, the change of bottom water redox condition caused by the invasion of oxygen-rich seawater into the bottom lake can inhibit the accumulation and preservation of organic matter in sediments; (2) The impact of seawater transgression on oil shale formation was mainly related to algae blooming caused by the continuous salinization of the lake water after transgression; (3) Volcanic tuff deposits are widely distributed in the Songliao basin and usually associated with oil shale deposits. The increase of productivity in ancient Songliao lake basin was directly related to nutrients such as phosphorus and iron brought by volcanic ash input; (4) The formation of oil shale in the Nenjiang Formation was controlled by a combination of mechanisms such as water eutrophication events induced by instantaneous mass input of volcanic ash and seawater invasion. It is concluded that organic matter enrichment in the oil shale is controlled by both water depth and the influence of sediment input from rivers.

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