Table of Content

08 April 2014, Volume 35 Issue 2

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Distribution of large and giant coal-related gas fields in China and its significance

Wang Tingbin, Dong Li, Zhang Yaxiong

2014, 35(2):  167-182. 

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The majority of gas reserves in China are stored in large and giant coal-related gas fields coal-related gas.Despite the fact of numerous coal-bearing basins in China,there are only a few large-and medium-sized such basins having the right geological conditions for forming large or giant gas fieds.The tectonic geological environments after deposition of coal-bearing strata are thought to be more closely linked to coal-related gas reservoirs:the later the peak gas-generation and reservoiring periods were,the more favorable it was for coal-bearing basins transforming into gas(oil)-bearing basins and forming larege and giant coal-related gas fields.Coal-bearing/gas(oil)-bearing basins of para-foreland type,intracratonic depressions type and marginal rift type are considered to be most promising in terms of exploration for gas.Basins including Ordos basin,central,western and north-eastern Sichuan basin,northern Tarim basin(Kucha depression),Xihu sag in East China Sea basin,Yingqiong basin and northern Chaidan basin as well as those in Sanhu area,northern and southern Junggar basin,the deeper part of Songliao basin,basins in deep water area of northern South China Sea,will still be the major hope for near and middle term gas reserve growth in China for the century.

Heterogeneity of the Paleozoic tight sandstone reservoirs in Gaoqiao Area of Ordos Basin

Tian Jingchun, Liu Weiwei, Wang Feng, Chen Rong, Lin Xiaobing

2014, 35(2):  183-189. 

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The heterogeneity characteristics of the Paleozoic tight sandstone reservoirs in Gaoqiao area of Ordos Basin are studied in detail in this paper by using core data,logging data,and test data.The results show that there is a close relationship between heterogeneity and sedimentation of the Shanxi Formation and Xiashihezi Formation sandbodies.The sandbodies formed by vertical accretion show vertical positive rhythm or flanges positive rhythm with no obvious trend.In contrast,those sandbodies formed by lateral accretion show strong vertical positive rhythm.Sandbodies formed by progradation have apparent vertical anti-rhythm.Sandbodies of overland flow deposition display little change or a slight vertical counter-rhythm.He8 and Shanxi Fm reservoirs show strong intralayer heterogeneity.The heterogeneity of Shan2 segment is stronger than that of Shan1,while the heterogeneity of lower He8 segment is stronger than that of upper He8 segment.Among these four sub-layers,the upper He8 segment is most homogeneous.Interlayer heterogeneity significantly varies among different layers.He8 segment and Shanxi Formation also show strong lateral reservoir heterogeneity in Gaoqiao area.

Tight oil reservoir formation conditions and main controlling factors of Yanchang Formation in southeastern Ordos Basin

Ren Zhanli, Li Wenhou, Liang Yu, Wu Xiaoqing, Yu Qiang, Ren Long, Wang Wei

2014, 35(2):  190-198. 

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No breakthrough has been made in tight oil exploration of the Yanchang Formation in southeastern Ordos Basin because of the low level of research on tight oil accumulation conditions.In order to evaluate tight oil exploration prospect of the Yanchang Formation,we studied in-detail the tight oil accumulation conditions in the study area studying respects of sedimentary microfacies,reservoir rock,cap rock and hydrocarbon migration.The high quality source rocks of Chang 7(7^th Member of the Yanchang Fm)are widely developed in the southeast region of the basin.The reservoir of the Yanchang Formation is tight. The favorable source-reservoir combination provided good geological conditions for tight oil accumulation.The Chang 6, Chang 7 and Chang 8(the 6^th,7^th and 8^th Members of the Yanchang Fm)intervals are the main targets for exploration of tight oil in the Yanchang Formaiton in the study area.Tight oil reservoir formation and enrichment are controlled by sedimentary microfacies and quality reservoirs. Based on the study of inclusions temperature,thermal history,illite dating,saturation pressure and other methods,the tight oil accumulation period in Yanchang Formation is set at the Early Cretaceous and was controlled by the Late Mesozoic tectonic thermal event, thus tight oil accumulation occurred in late stage.

Diagenesis and diagenetic facies of the Chang 4+5 tight sandstone reservoirs in Longdong area, Ordos Basin

Wang Feng, Chen Rong, Tian Jingchun, Li Mingrui, Zhang Huiruo, Liu Xin

2014, 35(2):  199-206. 

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Based on thin sections,SEM,X-ray diffraction,mercury injection,this paper analyzed the diagenesis and its impact on Chang4+5(the 4^th and 5^th Member of the Yanchang Fm)oil layers in Longdong area,Ordos Basin.Based on the analysis results,we studied the characteristics of diagenetic facies and the distribution of favorable facies.The Chang4+5 oil layers is at the mid-diagenetic A-B stage,and the early compaction and carbonate cementation resulted in loss of great numbers of primary pores,resulting in poor reservoir quality.However,the early chlorite thin film was favorable for the protection of primary pores and dissolution improved reservoir quality.According to the impact of the diagenesis on reservoir quality,five types of diagenetic facies were identified,namely chlorite thin film-residual intergranular pore facies,kaolinite cementation-feldspar dissolution facies,illite cementation-feldspar dissolution facies,carbonate cementation facies and siliceous-illite cementation facies.High quality reservoirs are closely related to the diagenetic facies,and the chlorite thin film-residual intergranular pore diagenetic facies is the best in reservoir physical property and petroliferous property,thus is the most favorable diagenetic facies for oil enrichment in this area.

Displacement characteristics and development effect of tight oil reservoir：a case from Chang 7 oil layer of the Yanchang Formation in Shangliyuan area, Ordos Basin

Han Yonglin, Liu Junfeng, Yu Yongjin, Wang Shenghua, Shen Qiang

2014, 35(2):  207-211. 

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In order to study displacement characteristics and development effect of tight oil reservoir,this paper analyzed the sensitivity,fluid percolation and water-driving displacement characteristics of Chang7 oil layer of the Triassic Yanchang Formation in Shangliyuan area,Ordos Basin.The results show that Chang7 oil layer has weak velocity sensitivity,weak acid sensitivity,no water sensitivity and no salt sensitivity.The starting pressure gradient of single-phase flow is up to 2.0 MPa/cm,and negatively correlates with permeability.The saturation of irreducible water and residual oil is high,the oil-water simultaneous vadose region is narrow,and the isoperm point is low,thus the increasing of water saturation can cause the rapidly decreasing of oil phase permeability.According to the changes of WC(water cut)and oil displacement efficiency in different displacement period,the water-driving in the Chang 7 oil layer can be divided into three different periods including no water period,WC-increasing-rapidly period,and high WC period.The oil displacement efficiency in no water period is 18.1%,and the final oil displacement efficiency is as low as 31.7%.In addition,the weak oil wettability of the oil layer will have a profound impact on later waterflooding.

Diagenetic characteristics and quantitative porosity estimation of tight sandstone reservoirs：a case from the 8^th Member of Permian Xiashihezi Formation in the Gaoqiao region, Ordos Basin

Zhang Xingliang, Tian Jingchun, Wang Feng, Chen Rong, Zhang Jinquan

2014, 35(2):  212-217. 

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Based on the thin section examination,SEM,X-ray diffraction,this paper studied the diagenetic types and their control on reservoirs in the 8^th Member of Xiashihezi Formation(He8)tight sandstone in Gaoqiao region.The He8 reservoir has experienced complex diagenesis after deposition.Compaction resulted in the loss of primary intergranular pores.And cementation reduces the primary intergranular pores while increasing the strength of the skeleton particles,which is conducive to the preservation of residual primary intergranular pore.Replacement in the study area features in feldspar replacing kaolinite and iron calcite replacing quartzes debris.Dissolution improves the reservoir physical properties by dissolving the unstable detrital grains and cements.Fracturing causes the formation of micro-fractures.Quantitative evaluation of the increase and decrease of the porosity caused by different diagenesis reveals that compaction is the most significant in porosity reduction,being about 22.40%-33.17%,with an average of 27.166%,followed by cementation and replacement,bing about 5.5%-17%,with an average of 10.01%.In contrast,dissolution increases porosity about 6.86%-8.41%,with an ave-rage of 8.586%.The porosity of the tight sandstone is the final result of the different diagenesis.

Mechanisms of gas-water distribution in tight sandstone gas reservoirs of Xujiahe Formation,central Sichuan Basin

Chen Taotao, Jia Ailin, He Dongbo, Shao Hui, Ji Lidan, Yan Haijun

2014, 35(2):  218-223. 

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Tight sandstone gas reservoirs of the Xujiahe Formation in central Sichuan Basin were observed to have many features.They contain gas-water transition zones that may be due to inadequate supply of gas from source rocks,pervasive gas distribution possibly as a result of early gas charging during gas accumulation,mixed gas/water distribution because of the tightness of sandstone reservoirs,control of local structural amplitude on gas-water differentiation and fractures acting as seepage channels for gas and water.The gas-water distribution pattern in the reservoirs may be explained as follows:immature-low mature gas that were generated during the Late Triassic to Late Jurassic accumulated and formed the pure gas cap on structural high under the control of local structure;the large amount of gas generated during the Early Cretaceous to Paleogene could not completely displace pore water due to the tightness of sandstone reservoir,resulting in mixed gas/water distribution;the physical properties of the reservoirs werer greatly improved by numerous fracutures formed during the Himalayan movement since the Paleogene.All these conditions worked together to form the present tight gas reservoirs in the formations of the basin.

Characteristics and controlling factors of tight sandstone reservoirs in the 5th member of Xujiahe Formation in the central of western Sichuan Depression

Lin Xiaobing, Liu Liping, Tian Jingchun, Peng Shunfeng, Yang Chenyu, Su Lin

2014, 35(2):  224-230. 

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Characteristics and dominant controlling factors of tight sandstone reservoirs in the 5th member of Xujiahe Formation in the central of western Sichuan Depression was researched through core observation,logging interpretation,phy-sical property analysis,thin section examination and scanning electron microscope,in order to provide the basis for deployment of exploration and development of new sequences.The tight sandstone reservoirs occur in the 5th member source rocks and consist of delta front underwater distributary channels,channel bars,sheet sand and shallow lake sand bars,with single layer thickness ranging from 2 to 25 m.They belong to super-low porosity and low permeability reservoirs,with an average porosity of 2.06% and permeability of 0.162×10^-3 μm² respectively.High contents of plastic particles,clay matrix and carbonate cements are the principal factors that led to the densification of the 5th member reservoirs.The medium-fine grain calcarenaceous sandstones and carbonate lithic sandstone deposited in relatively stronger hydrodynamic environments are favorable for the preservation of primary pores and exchange of fluids,thus are reservoirs with relatively high quality.Dissolution of feldspar resulted in the low content of feldspar and the formation of some dissolution pores.And the unique cracks caused by hydrocarbon generation pressurization in source rocks were helpful for the organic acid to enter into the residual inter-granular pores,enlarging pre-existing dissolution pores.Therefore,the delta front underwater distributary channels and channel bars sandbodies adjacent to the source rock with a certain range of thickness are the most favorable targets for tight sandstone reservoirs exploration.

Matching between sandstone tightening and hydrocarbon accumulation of the Xujiahe Formation,Guanyinchang area in southern Sichuan Basin

Zhang Xiang, Tian Jingchun, Du Benqiang, Xia Jiwen, Gu Na, Lin Xiaobing, Deng Feiyong, Liang Yuchen

2014, 35(2):  231-237. 

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Tight sandstone reservoir of the Xujiahe Formation in Guanyinchang area mainly consists of delta plain distribu-tary channels sandbodies.Based on analyses of reservoir diagenesis and tightening,detailed study shows that the hydrodynamic conditions during sedimentation determine the storage capacity of reservoirs.Strong compaction is the major factor causing the decrease of inter-particle primary porosity,and quartz overgrowth in diagenetic stage is the key factor causing the tightening of reservoir.The temperatures of fluid inclusions in quartz overgrowth is between 82.5-125.1 ℃,accor-ding to which the tightening depth is estimated to be between 1 725-2 864 m.This depth interval corresponds to the period of Yanshan movement.According to analysis of the tightening process and hydrocarbon accumulation stages in the Xujiahe Formation sandstone reservoir,the following understandings were obtained.The sandstone reservoir was not tightened at the initial phase of gas accumulation,so gas could migrate long distance laterally.Before the peak gas accumulation,the sandstone reservoir had become tight,thus the gas could not migrate laterally for a long distance,instead,they migrated along the unconformity of channel and microfractures opened by high pressure.of the composite structural and lithologic gas reservoir got finalized during Himalayan epoch,and the gas reservoir was controlled by both lithology and structures,so their heterogeneity was strong.

Upper limit and distribution of physical property of tight gas sandstones

Wang Pengyan, Liu Fengxuan, Ma Feng, Yang Mian, Lin Yikang, Lu Chong

2014, 35(2):  238-243. 

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The formation of tight gas is controlled by the low porosity and permeability of sandstone reservoirs.It has been recognized that the accumulation of pervasive low abundance tight gas is possible only when the porosity and permeability of reservoir is above a certain limit value.However,there are no widely accepted criteria for defining this limit value.Based on collection of the measured reservoir property of both domestic and international typical tight gas sands,we stu-died the statistical distribution patterns of permeability and porosity of tight reservoirs,and analyzed the causes for reservoir tightening and the characteristics of tight gas accumulation.Data show that over 80% routine permeability data points have a permeability less than 0.1×10^-3 μm².The corrected in-situ permeability is about one order of magnitude lower than the routine permeability.Taking the routine permeability value of 0.1×10^-3 μm² as the upper limit of tight gas sands is reasonable and practical.Statistics of tight gas sands over the world shows that tight sandstone reservoirs have diversity of sedimentary environments,large depth span,and long time range in geological age.Tectonic activity,sedimentary environment and diagenesis are the main factors for the formation of tight reservoir,but they play a different role in different basins.The distribution of tight gas sands can be delineated by analyzing source rock,tectonic movements and rese-rvoir tightness.

Controlling factors on physical property of tight volcanic rocks and reservoir quality prediction：a case study of the Tiaohu Formation in Marlang-Tiaohu Sag

Meng Yuanlin, Hu Yue, Li Xinning, Hu Anwen, Wu Chenliang, Zhao Zitong, Zhang Lei, Xu Cheng

2014, 35(2):  244-252. 

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Commercial oil flows were tested in the tight vitric and crystal sedimentary tuff reservoirs of the 2nd Member of Tiaohu Formation(P₂t²)in Malang-Tiaohu Sag,Santanghu Basin.In order to reveal the distribution pattern of tight oil,we studied the influences of lithofacies and diagenesis on physical properties of volcanic reservoirs.And plane distribution of four types tight reservoir of P₂t² were predicted through numerical modeling of diagenesis and in combination with lithofacies map.The results show that the quality volcanic reservoir decreases with increasing levels of diagenesis;Pyroclastic rock and tuff are optimal reservoirs,followed by vitric and crystal sedimentary tuffs and pyroclastic rocks,and lavas show the worst reservoir property.The conventional reservoirs(porosity≥9%)distribute in the southern Malang-Tiaohu Sag,and slight tight reservoirs(7%≤porosity<9%)and strong tight reservoirs((4%≤porosity<7%)are located in the central-northern of Malang-Tiaohu Sag.Effective traps are the main exploration targets in area with conventional reservoirs,while the reservoirs and hydrocarbon source should be the focuses of study in area with tight reservoirs.

Fracture prediction of bedrock buried hill tight reservoirs in Beier Depression

Yang Mian, Xu Ziyang, Yang Baisong, Wei Wei, Li Qiju

2014, 35(2):  253-257. 

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Bedrock buried hill reservoirs in Beier Depression belong to tight reservoirs with low to ultralow porosity and low to ultralow permeability,and have structural fractures as the main reservoir spaces.Mapping of these structural fractures is the premise and guarantee for looking for high quality reservoirs and improve exploration success rate.According to the top structural form of bedrock and distribution of basement faults in Beier Depression,we mapped two types of structural fractures by using curvature method and finite element numerical simulation method.One is fracture related to folds and the other is fracture related to faults.The fold-related fracture on top of the bedrock in Beier Depression are distributed mainly along strike,and are located in east of well Bei302-Bei13 and BeiD4 of west Beixi sub-depression,periphery of well Huo10 and Bei42 to Bei16 of Beizhong fault-uplift zone,and periphery of well Xi13-Xi5-De8 of Beizhong sub-depression.The fault-related fractures are mainly distributed in the southwest and central of Beixi sub-depression,periphery of well Bei30 of Beizhong fault-uplift zone,and periphery of well De8-Xi5.Cores observation shows that curvature-controlled dissolution fractures are common on top of the bedrock in well Bei16,which coincides with fracture prediction results.

Key parameter selection for tight gas resource potential：a case from Turfan-Hami Basin

Wang Weiming, Zhao Xu, Sun Jiwen, Duan Shengqiang, Wang Yang, Wang Guilei, Yan Xu, Tian Weichao

2014, 35(2):  258-265. 

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Selection of methods and relevant parameters is crucial for estimation of tight gas resources.The volumetric method was selected in this paper based on analysis of the advantages and disadvantages of different evaluation methods and relevant two key parameters were defined,namely tightness limit and gas saturation.The results show that volumetric method is the most reliable method to evaluate tight gas resource potential.However,the accurate determination of relevant parameters should be based on the features of tight sandstone reservoir.Tight sandstone reservoirs are generally cha-racterized by small throat,strong hydrophilicity and high bound water saturation.The size of throat controls the permeabi-lity.Hydrophilicity is controlled by clay mineral components and their contents rather than the total amount of clay mine-rals.The higher content of Illite is,the smaller the wetting angle is and the stronger the hydrophilicity is.The contrary is true for chloride.The balance relationship between gas buoyancy and capillary force determines the limits of reservoir properties,which is significantly influenced by wetting angle.The critical tightening limit progressively decreases from central to edge of the basin.Electric double layer formed by the strong adsorption water on Illite surface has strong conductivivity,which makes the Archie formula not suitable for the interpretation of tight gas saturation.While Indonesia's formula fully takes the influence of shale content into consideration,thus is more suitable for determining gas saturation of tight sandstone reservoir.

‘Four-pore’modeling and its quantitative logging description of shale gas reservoir

Li Jun, Lu Jing, Li Zheng, Wu Qingzhao, Nan Zeyu

2014, 35(2):  266-271. 

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The micropores of the shale gas reservoir consist of organic pore,clastic pore,clay pore and micro-fracture.After a comprehensive analysis of core properties and the logging response characteristics,several new methods are proposed to evaluate total porosity and porosity of each micropore type of shale gas reservoir quantitatively.1)Based on the volume model,the DEN or DT is used to determine the total porosity of shale gas reservoir.2)Conventional logs and SGR(Natural Gamma Ray Spectroscopy)logs are utilized to determine the volume-percent of organic matter content of shale gas reservoir,a scale parameter is determined by SEM technique,and finally a method for calculating the organic porosity is set up.3)Considering the clay pore is the main storage space for bound water,clay content is obtained from logs and correlated with the adjoining shale to get the evaluation method of clay porosity.4)Based on the response characteristics of dual laterolog for micro-fracture,an evaluation method of micro-fracture porosity is proposed by forward and inverse numerical simulation computations.5)The clastic porosity can be determined by subtracting organic porosity,clay porosity and micro-fracture porosity from the total porosity.A good agreement is found between the core analysis and the micropore porosity computed by using the above methods,which indicates the validity of the methods proposed in this paper.

Shale gas accumulation conditions of the Lower Cambrian Niutitang Formation in Upper Yangtze region

Hu Mingyi, Deng Qingjie, Hu Zhonggui

2014, 35(2):  272-279. 

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Based on outcrops,drilling and core data,combining with analysis of thin section,SEM,X-ray diffraction,mercury porosimetry and isothermal adsorption,this paper studied the accumulation conditions and characteristics of shale gas of the Lower Cambrian Niutitang Formation in Upper Yangtze Region,with focus on the stratigraphic,petrologic,geoche-mical and geophysical characteristics as well as gas storage capacity.The results show that the distribution of the organic-rich shale of the Lower Cambrian Niutitang Formation in Upper Yangtze Region is controlled by the shallow to deep water shelf sedimentary environment,where shale thickness generally ranges from 50 to 300 m.The shale has a total organic carbon content over 1.5% and is highly mature to over mature.The organic-rich shale has abundant brittle minerals and the clay minerals with high evolution are dominated by stable minerals such as illite but without any montmorillonite.The various types of pores and fractures provide effective reservoir space for shale gas,and the gas content reaches the lowest commercial development value(2.0 m³/t).Comprehensive analysis of these conditions reveals that the Ziyang-Zigong area,the northern Guizhou,eastsouthern Chongqing and eastern Hubei and Hunan Province are favorable exploration areas.

Geological conditions and exploration potential of shale oil in Qingshankou Formation, Northern Songliao Basin

Liu Bo, Lü Yanfang, Ran Qingchang, Dai Chunlei, Li Mei, Wang Meng

2014, 35(2):  280-285. 

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Shale oil and gas is one of the most important resources to replace the conventional oil and gas.According to regional geological surveying,the geological conditions of the development of shale oil in northern Songliao Basin were studied in this paper.Also,the favorable target intervals were selected and their potentials of exploration were assessed.The Qingshankou Formation has obvious cyclicity due to the control of lake level change,with mudstone rich in organic matter and gray matter and silty mudstone appearing alternately.The dark mudstone in the lower subcycle is characterized by large single layer thickness,broad distribution area and high content of organic matter,and is still at the stage of hydrocarbon generation.The upper subcycle is characterized by enrichment of quartz-feldspathic minerals and ultra-low porosity and permeability.Many micro pores and cracks are observed in scanning electron microscope,and they offer reservoir space for oil and gas.In addition,the shale reservoirs in K₁qn¹ are heterogeneous and mostly have abnormal pressure,thus are favorable for the shale oil accumulation.Geochemical analysis data and isothermal adsorption test of shale interval show that the 1st member of Qingshankou Formation is richest in oil,and contains some gas.So it has the larget exploration potential for shale oil exploration.Based on the above-mentioned geological conditions and oil-gas-bearing possibility study,the space distribution of favorable reservoir in K₁qn¹ is determined,and the OOIP of shale oil is calculated to be 10.56 billion tons by using probability volume method.

Microscopic pore characteristics of Sha-3 and Sha-4 shale and their accumulation significance in Liaohe Depression

Yang Chao, Zhang Jinchuan, Li wanjun, Jing Tieya, Sun Rui, Wang Zhongpeng, He Wei, Lu Yaya

2014, 35(2):  286-294. 

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In order to further study the pore characteristics of shale reservoir oin Sha-3 and Sha-4(the 3^rd and 4^th Member of Shahejie Formation respectively)and their accumulation significance in Liaohe depression,samples from 5 wells were chosen to conduct a series of experiments including focused ion beam-scanning electron microscope(FIB-SEM),helium adsorption and desorption,whole rock analysis by XRD,microscopic examination of kerogen and related organic geochemistry experiments.The results show that there are 6 types of pore including intergranular pore,intragranular pore,inter-crystal pore,dissolution pore,organic pore and micro-cracks.Furthermore,development of these pores are not only related to sedimentary compaction,cementation and dissolution,but also closely related to the lithology,mineral fabric,type of organic matter and organic geochemistry.The intergranular pores and interlayer micropores and cracks of clay mineral aggregates make the greatest contribution to the storage and migration of shale oil and gas,and they are also the major controlling factor of the anisotropic(permeability)of the shale reservoir,followed by the dissolution pores and inter-crystal pores.The organic pores contribute the least due to the relatively poor development,but they still plays an important role in storage of absorbed shale oil and gas.Meanwhile, micro-cracks are also negligible in connecting macro-pores and meso-pores.In addition,large amount of organic pores are found in adhesive-combining organic matter with low maturity,indicating that clay minerals play an important role in the early period of hydrocarbon generation.The feature of close symbiosis between organic matter and minerals(mainly clay minerals)also reflects the close relationship between the preservation pattern of organic matter and hydrocarbon generation.The experiments also demonstrate that the amorphous solid is the major contributor to the generation of shale oil and gas in Sha-3 and Sha-4 shale.