Table of Content

28 October 2019, Volume 40 Issue 5

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Petroleum Geology

Effects of organic matter evolution on oil reservoir property during diagenesis of typical continental shale sequences

Hu Wenxuan, Yao Suping, Lu Xiancai, Wu Haiguang, Sun Funing, Jin Jun

2019, 40(5):  947-956,1047.  doi:10.11743/ogg20190501

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Following a review on the research progress of diagenesis and micro-and nano-scale pore development of shale sequences,we take the Paleogene Shahejie Formation in Bohai Bay Basin and the Permian Lucaogou Formation in Junggar Basin as examples to analyze the diagenetic characteristics and reservoir space forming mechanism of organic-rich continental shale sequences.The study focuses on the effect of diagenetic (hydrocarbon-generating) evolution on reservoir space development.The results show that the porosity of the shale decreases significantly along with compaction at the early diagenetic stage; this is especially shown in the transformation from macro-pores to micro-pores,resulting in the relative increase of mesoporosity-microporosity.As of the end of early and the middle diagenetic stages,the porosity may reaches a new peak value at a depth of 3000m to 3700m in Dongying Sag,under the impact of early hydrocarbon generation of organic matters.And then as compaction comes to dominate again,the porosity decreases sharply,even though secondary micro-porosity increases due to acid fluid dissolution.The dissolution of acid fluids occurs in the Shahejie Formation,but it turns to be dominant in the Lucaogou Formation,a major reason for the formation of reservoir space.Geochemical analyses on rock samples from the Lucaogou Formation show that the samples with higher TOC generally have a lower content of δ¹³C in carbonate minerals,and a higher secondary dissolution porosity.Therefore,the diagenesis in organic-rich shale sequences is significantly different from that in ordinary shales.The organic acids and acidic CO₂ accompanying the hydrocarbon generation are of great significance to the modification and improvement of reservoir quality by dissolution,and also major factors controlling the distribution of shale oil sweet spots.

Effects of differential diagenesis of deep carbonate rocks on hydrocarbon zonation and accumulation: A case study of Yingshan Formation on northern slope of Tazhong uplift,Tarim Basin

Lyu Xiuxiang, Chen Peipei, Chen Kun, Zhang Jie, Qian Wenwen

2019, 40(5):  957-971.  doi:10.11743/ogg20190502

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The carbonate rock reservoir on the northern slope of Yingshan Formation,Tazhong uplift,has undergone multistage diagenetic modifications of various kinds,featuring large thickness and deep burial depth.Based on the observation of core and thin section,the interpretation of well-logging data and the fracture identification by seismic attribute inversion,combined with the statistical analysis of logging and formation testing data,we found that the karst reservoir of the Yingshan Formation has gone through multistage dissolution and tectonic fracturing,and the reservoir space has experienced intensive,constructive rework,while the destructive cementation-filling is relatively weak.In comparison,the adjacent tight interval is mainly caused by intensive cementation and filling,and because the interval is remarkably weak in dissolution,its electric resistivity is significantly higher than that of adjacent reservoir intervals.So it is called "high-resistivity interval",a diagnostic character to identify tight layers.There are significant differences in diagenesis between the carbonate reservoirs and its adjacent tight caprocks.It is the differential diagenesis that results in the formation of multiple reservoir-caprock assemblages inside thick-bedded carbonate units.Vertically,multiple fracture-cavity sections (reservoir) alternate with tight sections (caprocks),having formed multiple superimposing reservoir-caprock assemblages.This has led to multiple hydrocarbon-bearing zones in the thick-bedded carbonate unit.Horizontally,due to the heterogeneity of carbonate rocks and the segmentation of strike-slip faults,the distribution of both karst reservoirs and tight high-resistivity caprock is restricted,leading to the hydrocarbon accumulation pattern in the Yingshan Formation characterized by vertical zonation and lateral compartmentalization.

Hydrocarbon charging history of the ultra-deep reservoir in Shun 1 strike-slip fault zone,Tarim Basin

Wang Yuwei, Chen Honghan, Guo Huifang, Zhu Zhihui, Wang Qianru, Yu Peng, Qi Lixin, Yun Lu

2019, 40(5):  972-989.  doi:10.11743/ogg20190503

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Major breakthrough in oil-gas exploration has been made in the ultra-deep strata of Shunbei area,Tarim Basin.Following the geochemistry comparison of crude oil and natural gas in the Ordovician Yijianfang Formation in Shunbei area,combined with the analysis of microscopic fluorescence spectrum parameters and fluorescence color,we put forward that the study area has undergone multi-stage hydrocarbon charging,the crude oil charged earlier has experienced biodegradation to a certain degree,while the gas is associated with the crude oil.In addition,through the comparison of crude oil in Shunbei and Shunnan areas in terms of isoprenoid parameters,biomarker compounds and light hydrocarbons in crude oils,we found that the hydrocarbon in Shunbei area is derived from the same source rocks as Shunnan area,that is,from the Cambrian Yurtusi Formation source rocks,and has migrated up to the Ordovician reservoirs along the faults cutting through the Cambrian.The samples collected in Shunbei area were analyzed systematically by fluid inclusion testing,as well as cold cathodoluminescence and fluorescent observations.The results show that there are three stages of oil charging in the late Caledonian,late Hercynian,and late Himalayan respectively,and one-stage gas charging in the late Himalayan.

Characteristics of the central segment of Shunbei 5 strike-slip fault zone in Tarim Basin and its geological significance

Deng Shang, Li Huili, Han Jun, Cui Deyu, Zou Rong

2019, 40(5):  990-998,1073.  doi:10.11743/ogg20190504

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The Shunbei 5 fault zone is known so far as the longest intracratonic strike-slip fault zone in Shunbei area and its surroundings,Tarim Basin.With a length of 270 km identified by both 2D and 3D seismic data,the Shunbei 5 fault zone is characterized by a long fault trace running through the Tabei uplift,Shuntuoguole low uplift and Tazhong uplift.Based on the overall change of fault strike,the Shunbei 5 fault zone can be divided into three segments:the northern segment (trending approximately NW20°),the central segment (trending nearly NS,i.e.NE10°) and the southern segment (trending approximately NE20°).The main faulting of the central segment (the part trending NE10°) of the Shunbei5 fault zone occurred in the 3rd episode of the middle stage of the Caledonian and the Late Caledonian,when the deep ridge-like structures and medium-deep grabens were formed under transpressional and extensional stress regimes,respectively.The shallow en echelon normal faults in the central segment of Shunbei 5 fault zone controlled the formation of reservoir space of fracture-cavity type in the Silurian clastic strata (namely sandstone and mudstone intervals).The fractured zones were developed in an asymmetric pattern in the hanging wall and footwall of the en echelon normal faults.The measured displacements of the northern and central segments of the Shunbei 5 fault zone are characterized by "longer at both ends and shorter in the center".Furthermore,the northern and central segments also remarkably differ in both geometries and kinematics,suggesting that Shunbei 5 fault zone was formed through the linking of various faults developed in different fault systems in early tectonic evolution.

Geochemical features and genesis of shale gas from the Lower Cambrian Shuijingtuo Formation shale in Yichang block,Middle Yangtze region

Luo Shengyuan, Chen Xiaohong, Liu An, Li Hai

2019, 40(5):  999-1010.  doi:10.11743/ogg20190505

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Strong gas shows are discovered in drilling the Lower Cambrian Shuijingtuo Formation shale in Yichang area,Middle Yangtze,and high commercial gas flow was tested following hydraulic fracturing of the Shuijingtuo shale in Well Yiye1 in the study area,which is a prospective area for shale gas exploration in the Lower Cambrian shale outside the Sichuan Basin.Nine shale gas samples were collected after hydraulic fracturing to analyze the geochemistry and origin of shale gas in the Shuijingtuo Formation,following the study of shale gas composition,as well as carbon,hydrogen,and He isotopic compositions.The results show that methane is the main constituent of the shale gas and varies in concentration from 87.17% to 92.75%,the content of ethane ranges from 0.83% to 0.94%,and the propane is in trace amount; the drying coefficient (C_l/C_l-5) is about 0.99,indicating typical dry gas.The content of nitrogen in non-hydrocarbon gases is slightly higher,with an average of 7.73%,the carbon dioxide content is less than 1%,and there is no H₂S.The δ¹³C methane in the Cambrian shale gas varies between -33.8‰ and -33.1‰ in content,the δ¹³C ethane value ranges from -39.2‰ to -36.0‰,δ¹³C propane value ranges from -39.4‰ to -38.5‰,and the δ¹³C value for CO₂ ranges from -16.8‰ to -14.6‰.The δD_CH4 methane in the shale gas ranges from -133.8‰ to -128.5‰ in content,while the δD_C2H6 ethane ranges from -168.0‰ to -146.1‰.The stable carbon isotopic distribution of the shale gas shows a totally reversed distribution pattern with δ¹³C₁ > δ¹³C₂ > δ¹³C₃,and δD_CH4 > δD_C2H6.The R/R_a ratio of helium in the Shuijingtuo shale gas ranges from 0.04 to 0.08,indicating the crustal origin of the helium.Considering the shale gas compositions and their ratios,as well as the reversed distribution of carbon and hydrogen isotopes at the high-over mature stage in thermal evolution,we believed that the shale gas of the Lower Cambrian Shuijingtuo Formation in Yichang area belongs to the oil-derived gas,featuring an origin of secondary cracking,which is similar to the Qiongzhusi shale gas in Sichuan Basin.

Research progress in fault transformation zones as lateral or vertical hydrocarbon migration pathways

Sun Tongwen, Gao Xicheng, Lyu Yanfang, Fu Guang, Wang Haixue, Wang Haoran

2019, 40(5):  1011-1021.  doi:10.11743/ogg20190506

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Plenty of oil and gas exploration practices in faulted basins have shown that hydrocarbons are mainly distributed in fault transformation zones instead of the whole fault zone,including relay ramp,flexural turning point,intersection area and terminal,which show remarkable characteristics of migration pathways.In order to find out the advantageous conditions and mode of hydrocarbon migration in fault transformation zones,we made a systematic analysis of the characteristics of hydrocarbon migration based on the study of the formation and evolution of fault transformation zones.The results show that the fault transformation zones may serve as both lateral and vertical pathways for hydrocarbon migration at different evolution stages.When acting as lateral migration pathways,they are advantageous in terms of (1) the gradient of fluid potential,(2) well-developed sand bodies characterized by a higher probability of reservoir juxtaposition and better lateral connectivity,and (3) a relatively lower ratio of vertical to lateral permeability.There are two main modes of lateral migration:undamaged-laterally connected hydrocarbon migration and accumulation mode,and destroyed-vertically and laterally sealed hydrocarbon migration and accumulation mode,and hydrocarbon enrichment horizons are generally consistent with migration horizons.When acting as vertical migration pathways,they are advantageous in terms of (1) a higher activity intensity for the hard-linkage transformation zone,prone to be a migration pathway with episodic opening; (2) the fault plane of the hard-linkage transformation zone,usually a ridge-shaped low-potential area,favorable for hydrocarbon accumulation; (3) the stress concentration and fracture development,tending to destroy the caprock and resulting in vertical seepage.Similarly,there are also two main modes of vertical migration:incompletely destroyed-vertical hydrocarbon seepage through relay ramps,and destroyed-vertical hydrocarbon migration along fault planes,in which oil and gas tend to accumulate in shallower horizons.In addition,it can be proven that the fault transformation zone has once been the pathway of fluid migration in terms of mineral precipitation rate,diagenesis,groundwater seepage and water inrush in mine,etc.

Characteristics and main controlling factors of natural fractures in the Lower-to-Middle Ordovician carbonate reservoirs in Tahe area,Northern Tarim Basin

Hao Junmin, Wang Xiaoyao, Sun Jianfang, Sun Xiaotong, Shi Jinxiong, Cao Dongsheng, Zeng Lianbo

2019, 40(5):  1022-1030.  doi:10.11743/ogg20190507

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The characteristics of and major factors controlling the natural fracture development in the Middle-to-Lower Ordovician carbonate reservoirs in Tahe area are studied through observations of outcrops,cores and casting thin sections,as well as logging data analysis.The natural fractures in the study area come in three types:tectonic fractures,diagenetic fractures and composite fractures.Specifically,the tectonic fractures are mainly shearing fractures,followed by tensional fractures;the diagenetic fractures are mainly bedding-parallel fractures,followed by diagenetic stylolites;the composite fractures mainly include structural-diagenetic fractures,structural-epigenetic fractures and karst fractures.The tectonic shearing fractures are dominant in the study area,developed in four groups trending NE-SW,NW-SE,nearly EW and nearly NS,respectively.However,they differ remarkably in different horizons in terms of development degree,and occur preferentially along the NE-SW direction in these horizons.High-angle fractures are dominant with a dip angle of over 70°.The longitudinal propagation length of the fractures is controlled by the rock mechanical stratigraphy,within which the fractures are developed,and on the interface of which the fractures terminate.Fracture density shows obvious heterogeneity both vertically and horizontally,mainly controlled by geological factors including sedimentation,tectonics and karstification.Along with the increase of plastic mineral contents and rock layer thickness,the fracture density gradually decreases.Faults and folds play a vital role in fracture development,leading to evident variation in fracture density at various structural locations.The fracture density tends to gradually decrease along with the increasing of distance to fault plane and fold axial plane.Besides,karstification affects the degree of fracture development in the upper strata of karst cavens,where there are mainly three types of fracture systems,i.e.nearly vertical fractures,obliquely-crossing fractures and nearly horizontal fractures.

A model of hydrocarbon migration and accumulation in non-hydrocarbon-generating subsag at basin margins and its exploration significance: A case study on Sanhecun subsag of Zhanhua Sag,Bohai Bay Basin

Peng Cuncang

2019, 40(5):  1031-1037.  doi:10.11743/ogg20190508

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The oil and gas exploration in the non-hydrocarbon-generating subsags at basin margins is usually restricted by relatively poor pathway systems,and often regarded as the "forbidden area" for exploration.In order to examine various hydrocarbon migration and accumulation patterns in non-hydrocarbon-generating subsags at basin margins and expand the realm of hydrocarbon exploration,we have taken the Sanhecun subsag of Zhanhua Sag,Bohai Bay Basin as an example to analyze its hydrocarbon migration and accumulation pattern based on a systematic review of previous knowledge and data including drilling,logging,seismic analysis of Sanhecun subsag of Zhanhua Sag.The results show that Sanhecun subsag has undergone two hydrocarbon charging periods.These are the early-stage hydrocarbon accumulation during the sedimentation of Dongying Formation and the middle-late-stage hydrocarbon accumulation during the sedimentation of Guantao-Minghuazhen Formation.In the early-stage accumulating period,the hydrocarbon-generating Bonan sag was connected to Sanhecun subsag,and hydrocarbon from Bonan sag accumulated in the Shahejie Formation of Sanhecun subsag through lateral migration.In the middle-late-stage accumulating period,through combined migration and accumulation mechanisms of "meshwork-carpet" and "unconformity",hydrocarbon generated in Bonan sag was charged to the Dongying Formation and shallower strata in the Sanhecun sag.The speculation of hydrocarbon migration and accumulation mechanisms in a non-hydrocarbon-generating subsag at basin margins has a theoretical significance by extending hydrocarbon exploration realm,and has led to the discovery of Sanhecun oilfield in practice.

Characteristics and major controlling factors of gas-water distribution in tight volcanic gas reservoir in Xushen gas field,Songliao Basin

Zhou Xiang, Yu Shiquan, Zhang Dazhi, Yu Haisheng, Chen Xi

2019, 40(5):  1038-1047.  doi:10.11743/ogg20190509

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Xushen gas field is a typical tight volcanic gas reservoir located in Songliao Basin.With the development of gas reservoir in depth,water breakthrough in wells appears,and the intensive water invasion and rising gas-water ratio have severely restricted the efficient development of volcanic gas reservoir.Therefore,the understanding of inner gas-water distribution,along with its major controlling factors,has becoming a key in enhancing gas well productivity.In the study area,the gas-water distribution is complex in the volcanic gas reservoir,and characterized by "gas on top and water at bottom" overall.Besides,volcanic edifice is the elementary cell of gas-water distribution; there is no unified gas-water interface throughout the study area,while more than one gas-water systems may exist in the same volcanic edifice; and different gas-water systems may share a gas-water interface.Based on the description of volcanic edifices in the Yingcheng Formation,we systematically analyzed the gas-water distribution characteristics of,and clarified the major factors controlling the gas-water distribution in the tight volcanic gas reservoir in Xushen gas field,using production test,as well as some static data in terms of physical property,mercury intrusion porosimetry (MIP) and thin section.The results show that the source rock distribution controls the macroscopic gas-water distribution in the volcanic gas reservoir in the Yingcheng Formation,while the volcanic edifice and structure have jointly had an influence on the accumulation and adjustment of gas,and the complicated microstructure of volcanic rocks is a key to gas-water fractionation in the reservoir.

Improvement and application of the method for quantifying lateral transport capacity of fault-sandstone configuration

Fu Hongjun, Jiang Hongfu, Wang Yunzeng, Li Ang, Fu Guang, Hu Huiting, Fan Zili

2019, 40(5):  1048-1055.  doi:10.11743/ogg20190510

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In order to more precisely reveal the hydrocarbon distribution of fault zones in the upper source rock-lower reservoir assemblage,we present an improved method of quantifying the lateral transport capacity of the fault-sandstone configuration,considering both the effect of fault-sandstone contact length and physical property of sand bodies,and the effect of dip angles of sand bodies.Twelve faults in the middle-shallow layers were selected from five typical blocks in Nanpu Sag,Bohai Bay Basin,to quantitatively estimate and compare the lateral transport capacity of the configurations of the 12 faults and 67 sandstone layers in 16 wells by using the original and improved methods,respectively.The results show that:the parameter values obtained by using the improved method correlate positively with the height of oil-gas column in sand bodies,that is,the bigger the values are,the higher the oil-gas column becomes; and vice versa.When the parameter value is over 0.4,the sandstone reservoir contains oil or both oil and gas,while when its value is less than 0.4,the sandstone reservoir contains water or is dry,indicating the applicability of the improved method.The parameter values obtained with the improved method are remarkably lower than that worked out by the original method,indicating an overestimation by the original method and thus resulting in risks to oil-gas exploration.

Mechanism of “strike-slip-extensional” composite faults-controlled hydrocarbon accumulation in the northeastern Laizhouwan Sag, and its enlightenment for exploration

Shi Wenlong, Niu Chengmin, Yang Bo, Wang Liliang, Deng Hui, Wang Hang, Zhang Delong

2019, 40(5):  1056-1064.  doi:10.11743/ogg20190511

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The study area,jointly controlled by the NE-trending strike-slip stress and the NS-trending extensional tectonic stress,is located in the northeastern Laizhou Bay Sag,and the southern eastern branch of the Tanlu strike-slip fault zone.Analysis of the fault distribution and structural evolution of the composite stress zone reveals that KL6A block is mainly controlled by extensional stress;that the most favorable trap is faulted anticline formed jointly by the inherited extensional fault controlling traps and the strike-slip transpressional stress;and that the anticline and faulted anticline trap,developed at the strike-slip pressure buildup section of KL6B and KL6C blocks situated in the major strike-slip stress zone,are the most favorable areas for hydrocarbon preservation.Thereafter,we put up an innovative concept of differential reservoir-controlling effect of strike-slip faults,featuring "transpression press-controlled trap,transtension stress-controlled hydrocarbon migration".The transtentional segment of the strike-slip faults serves as the main path for oil-gas migration,making a breakthrough in the exploration of slope areas.In addition,a ternary reservoir-controlling model of "Source-Fault-Trap" is established,and it is believed that the discovered oil-gas reservoirs can act as transfer stations for further migration up to traps of even higher horizons,facilitating the exploration of slope zones.Blocks of hydrocarbon enrichment are discovered in the top traps of the Dongying Formation,which was originally considered to be poor in migration conditions,marking a major breakthrough in oil-gas exploration.

Cement characteristics of overgrown authigenic ferron dolomite and its impact on tight sandstone reservoirs qualities: Case study of the Chang 4 & 5 members of Triassic Yanchang Formation in Longdong area,Ordos Basin

Gao Yang, Liu Chun, Bai Xiaojia, Wang Zhizhang, Yang Zhen, Chen Jiahao, Cui Hang

2019, 40(5):  1065-1073.  doi:10.11743/ogg20190512

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The tight sandstone reservoirs have drawn more and more attention along with increasing hydrocarbon exploration activity.This is particularly true with regard to the mechanism of tightening and its relation with the timing of hydrocarbon charging.We analyzed the occurrence and growth mechanism of ferron dolomite cements in the Chang 4&5 members and their impact on reservoir properties in the Triassic Yanchang Formation in Longdong area,Ordos Basin,combined with a study on lithology,petrogeochemistry and physical properties of reservoirs.The results show that the authigenic ferron dolomite mainly occurs as "secondary overgrowth" of detrital dolomite,the generation of which is caused by the pressure solution of detrital dolomite and the ferron metasomasis at the late stage of early diagenesis.That is a small scale metasomasis of feldspar particles by ferron dolomite.The size of grains,the content of biotite debris as well as the sedimentary microfacies function to control the content and distribution of ferron dolomite cements,which in turn will control the reservoir property.The content of ferron dolomite cements in Zhenyuan and Huanxian areas is higher than that in Huachi and Baibao areas.The amount of ferron dolomite cements tends to increase at the middle-to-upper section of submerged distributary channels and the middle mouth bars.The ferron is mainly originated from the dissolution of biotite debris.The sand bodies of the bottom of submerged distributary channels facies usually have an overall good porosity and permeability only if the content of cements is less than 3%.However,the cement content tends to exceed 3% in the sand bodies on top of submerged distributary channels facies,resulting in lower reservoir quality.The results obtained in the study can provide indications to the analysis of tight reservoir formation mechanism in similar geological conditions.

Differential structural deformation of the Dajiaoshiba area,East Chongqing

Tuo Xiusong, Chen Kongquan, Luo Shunshe, Tang Jiguang, Zhang Shuishan, Liu Qijun

2019, 40(5):  1074-1083.  doi:10.11743/ogg20190513

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The structural deformation characteristics of different layers,zones and stages at the Dajiaoshiba area were analyzed based on seismic and regional geological data.Thereafter,we summarized the differential deformation patterns of the north and south sections,and discussed the mechanism for the formation of the S-N oriented Wujiang and Daershan structural fault deformation zones.The results show that four sets of structural deformation layers occur vertically in the Dajiaoshiba area,while horizontally it can be divided into thrust-fold belt,thrust-decollement fold belt and decollement-fold belt from east to west,which have undergone SE-NW-trending,nearly WE-trending and NE-SW-trending compressive stress since the Yanshan Movement.The studied area represents the front belt of the thrust nappe structure,and the major decollement formation shifts from the Precambrian to the Cambrian,which has gone through the piggy-back progressive deformation from east to west.Due to the segmentation of the Qiyueshan fault and the structural difference of the Precambrian basement decollement fault,the composition of east to west deformation zones changes along the trend of the nearly SN-trending Daershan and Wujiang structures,with a deformation pattern of thrusting-compresso-shearing-fault-detaching-decollement in the northern section,thrusting-compresso-shearing-fault-propagation-decollement in the middle section and thrusting-fault-bending-fault-propagation-decollement in the southern section,respectively.

An evaluation on logging data of horizontal wells in tight sand gas reservoir -A case study of drilling sector X in Daniudi gas field,Ordos Basin

Wei Xiuping, Hu Xiangyang, Li Hao, Zhao Ji, Zhao Lianshui

2019, 40(5):  1084-1094.  doi:10.11743/ogg20190514

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There are quite a few horizontal wells featuring insufficient logging suite,and thus are difficult to evaluate.However,the way to effective evaluation on horizontal wells with limited logging data is of great significance to successful development of gas reservoirs.We propose a productivity evaluation method based on two parameters,which are the peak value of Gamma ray logging and the effective total hydrocarbon indicator,because for many horizontal wells in the drill sector X of Daniudi gas field in Ordos Basin,only Gamma LWD and gas logging data are available.In addition,the method proposed also employs some of the dynamic geological and production data.It effectively improves the accuracy of productivity evaluation on horizontal wells,and may be applied to logging evaluation on horizontal wells in gas reservoirs of similar type.The study shows that the Gamma peak value is an effective indicator of reservoir quality:for the reservoirs in Shanxi and Lower Shihezi Formations of the Daniudi drilling sector X,the LWD Gamma data of high production wells are characteristic of single peak statistical distribution,while that of low production wells shows double peaks or no obvious peak,which is largely related to the stability of sand bodies encountered in drilling wells.Furthermore,using the gas logging data in the drilling sector X,we evaluated the single-well productivity based on the effective total hydrocarbon indicator,and propose a gas logging indicator for commercial productivity of horizontal wells in the Shanxi and Lower Shihezi Formations.By integrating these two indicators,we established the dual-parameter evaluation method for horizontal well productivity,and made an accurate prediction on the single-well productivity of the horizontal wells in the drilling sector X.

Accumulation conditions and favorable plays for deep gas exploration at the northern margin of Qaidam Basin

Tian Jixian, Li Jian, Zeng Xu, Kong Hua, Wang Peng, Sha Wei, Wang Mu, Shi Zhenghao, Song Dekang

2019, 40(5):  1095-1105.  doi:10.11743/ogg20190515

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The potential of deep natural gas resources is great at the northern margin of Qaidam Basin,but it is lower in exploration maturity.The gas accumulation conditions and plays for exploration are still to be revealed,restricting the exploration progress of natural gas.Given studies on seismic,geological,geochemical and drilling data,we analyzed the conditions for gas accumulation in deep strata at the northern margin of Qaidam Basin comprehensively in terms of source rock,reservoir,structure,and carrier system,and pointed out the favorable plays for deep gas exploration based on the existing exploration results and geological conditions.The results show that due to the widespread distribution of the Jurassic,a new Jurassic hydrocarbon generation depression is discovered in the western segment of the piedmont of Altun Mountain,featuring high potential of hydrocarbon generation in the deep high-to-over-mature source rocks.Both the weathering crust of bedrocks and clastic rocks with well-developed secondary pores may serve as the effective reservoirs in deep horizons:the physical properties of the former are not controlled by depth,and the latter were well developed characterized by high quality in deep horizons.The fracture-dominated carrier system of wide distribution is conducive to the vertical and horizontal migration of natural gas generated from ultra-deep source rocks.Multi-stage tectonic activities have led to the formation of multiple types of traps in deep layers at the northern margin of Qaidam Basin; meanwhile the continuous hydrocarbon generation of source rocks have facilitated the earlier,multi-stage hydrocarbon accumulation in these deep traps.In view of the above results,it is concluded that the traps around the hydrocarbon generation sag with well-developed deep large faults are favorable for deep gas accumulation,and the paleo-uplift at the basin margin and the large-scale anticlinal traps in the late tectonic zone are prospective areas for hydrocarbon exploration,with the deep layers in the piedmont of the Altun Mountain being the most prospective one.

Lithofacies and causal mechanism of organic matter enrichment in the lower submember of the 3rd member of Shahejie Formation, Bonan Sag,Bohai Bay Basin

Wang Yuhan, Ding Weiming, Liu Xuan, Wei Ren, Dong Lin

2019, 40(5):  1106-1114.  doi:10.11743/ogg20190516

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Thick carbonate rocks bearing detrital components are widespread in the 3rd member of Shahejie Formation in Bonan Sag,Bohai Bay Basin,where shale oil resource potential is huge.The causal mechanism of organic matter enrichment in Bonan Sag was investigated drawing on lithofacies description,as well as analyses of terrigenous debris sources,primary productivity and redox environment.The core of Well L69 was systematically sampled,with the samples being mainly classified as massive micrite,lenticular micrite,laminar micrite and laminar sparite in terms of lithofacies features and sedimentary structures.In addition,the terrigenous debris sources were discussed based on analysis of major and trace elements,organic carbon and nitrogen isotopes,and inorganic carbon and oxygen isotopes,and the Al/K and Al/Mg were shown to be stable,indicating that the clastic components of shale in Well L69 are homogeneous.Under the comparison of organic nitrogen isotopes between bulk sample and kerogen,the redox environment of water and sediments was restored; the Ba content,organic carbon isotopes of kerogen,and their coupling relationships with organic nitrogen isotopes were employed to demonstrate the primary productivity variation.On this basis,the generating and preserving mechanisms of hydrocarbon in the study area were investigated.The terrigenous clastic input is supposed to bring nutrients to the study area,promoting the growth of algae,and producing a large amount of organic matter as a result.The sedimentation of organic matter consumed oxygen and resulted in a reducing environment in water,which has in turn facilitated the accumulation and preservation of organic matter.

Pore types and geological significance of calcarenaceous sandstones in the 3rd member of Xujiahe Formation in Yuanba area,Northeastern Sichuan Basin

Tang Zicheng, Zhong Dakang, Wang Wei, Shi Wenbin, Du Hongquan, Sun Haitao, Wang Ai, Zhou Zhiheng, Jia Xiaolan

2019, 40(5):  1115-1125.  doi:10.11743/ogg20190517

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Calcarenaceous sandstone is a type of sandstone in terrigenous clastic rocks in terms of genesis.The diagenesis in the burial process,reservoir space and hydrocarbon production capacity of calcarenaceous sandstones are determined to be quite special due to its uncommon compositions.Given the observation of the core,common thin sections,casting thin sections,cathode luminescent thin sections,scanning electron photomicrograph,as well as X-ray diffraction data,mercury intrusion data,and physical properties,we made a systematic study on the pore types,as well as quantitative evolution and validity of pores in the calcarenaceous sandstones of the 3rd member of Xujiahe Formation (T₃x₃) in western Yuanba area,Northeastern Sichuan Basin.The results show that there are fewer primary pores in these sandstones,and the secon-dary pores are mainly categorized into three types:intercrystalline micropores,dissolved pores and fractured pores.They are different in terms of genetic mechanism-the formation of intercrystalline micropores are mainly related to the recrystallization of calcareous debris and cements,and some intercrystalline micropores of calcareous debris are a result of the dolomitization of host rocks; while there are two causes for fractures:the intergranular fractures were developed out of unsynchronized recrystallization of calcareous debris and cements,and the structural fractures were formed due to the intense tectonic movements during the deposition of the T₃x₃.In addition,the dissolved pores were less developed and just formed partial "sweet spots",unable to improve the reservoir quality as a whole; while the well-developed intercrystalline pores,most of which are effective,serve as major contributors to the reservoir space of gases in calcarenaceous sandstones,combined with the communication provided by microfractures.Areas with high contents of calcareous debris and calcareous cements (such as high-energy distributary channels) are the high-quality reservoir zones in the T₃x₃,and could be the key areas for exploration and development,since strong recrystallization functions to produce a large number of intercrystalline micropores and the connection provided by fractures is also effective.

Methods and Technologies

Establishment of training images of turbidity channels in deep waters and application of multi-point geostatistical modeling

Hu Xun, Yin Yanshu, Feng Wenjie, Wang Lixin, Duan Taizhong, Zhao Lei, Zhang Wenbiao

2019, 40(5):  1126-1134.  doi:10.11743/ogg20190518

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Deep-water turbidity channels are rich in oil and gas,and a hot area of hydrocarbon exploration and development.The deep-water turbidity channels in Angola are taken for reservoir modeling based on multi-point geostatistics.Alluvsim algorithm based on sedimentary process was improved to capture deep-water channels' sedimentation and migration characteristics.First,the integral migration and deposition processes of turbidity channels were simulated by adding integral migration algorithm to Alluvsim algorithm.Second,the curvature index constraint was also added to represent the original curvature characteristics of turbidity channels,to simulate the morphology of fewer crevasses and higher sinuosity for the channels of Angola deep waters.In addition,we obtained the morphology and statistical characteristic parameters of turbidity channels in the study area through literature surveys on the geological knowledge database of deep-water turbidity channels in Angola.The so acquired training images of turbidity channels in the study area improved Alluvsim algorithm.Thereafter,a three-dimensional geological model of the deep-water channels in Angola was established based on Snesim algorithm of multi-point geostatistics,to reproduce the spatial distribution of these channels,in which the training image was the model,logging data were the conditions,and seismic data were the trend constraint.The inspection of the model result shows that the variogram calculated by the geological model is in good agreement with that obtained by seismic attributes,and the geological model set up in this study may be used for subsequent reservoir engineering and numerical simulation research to guide oilfield exploration and development.Apart from providing a complete modeling workflow for the application of multi-point geostatistics to actual reservoir modeling,the study also demonstrates the technical feasibility for the automatic generation of turbidity channel training images.

Spatial analysis of the sedimentary stages for composite channels based on sand-stratum ratio: A case study of the 2rd submember of the lower 8th member of Shihezi Formation in the infilled pattern test area of Block ST,Sulige gas field,Ordos Basin

Wang Wensheng, Lan Yifei, Shi Hongran, Tian Qinghua, Zhang Zhigang, Xue Wen

2019, 40(5):  1135-1140.  doi:10.11743/ogg20190519

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Problems including the difficulty to determine the pinch-out location of cross-well sand bodies in the fine description of reservoir in Sulige gas field,Ordos Basin,usually bring great challenges for the quantitative characterization of sand body scale.To solve these problems,a method which may be applied to determine the pinch-out position of the single period channel in a rapid and quantitative manner,has been established following the fine,comprehensive geological research.That is,the sand-stratum ratio is applied to demarcate the scope of superimposing channels of different stages; under the planar constraints,we analyzed the multistage channel assemblage on the section position,and then described the distribution characteristics of composite channels in the three-dimensional space.A case study of the 2rd submember of the lower 8th member of Shihezi Formation in the infilled pattern test area of Block ST,Sulige gas field,has shown that:① a small-scale layer may be divided into n monolayers of almost equal thickness,and the sand-stratum ratio at the stage i ranges between (i-1)/n and i/n in the superimposing area;② due to the down-cutting of channels and the thinning of sand bodies on river edge,there may be partial deviations in the interpretation of superimposing area,but this does not affect the overall applicability of the methods,and appropriate adjustments to the area are necessary.It is concluded that the method proposed in the study can be applied to immediately identify the spatial distribution range of monolayer channels,and provide technical support for the quantitative characterization of reservoir and prediction of play fairway.

Effect of SEM parameters on quantitative evaluation of shale micropores

Zhao Rixin, Lu Shuangfang, Xue Haitao, Tian Shansi

2019, 40(5):  1141-1154.  doi:10.11743/ogg20190520

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We studied the microscopic pore structure of the organic-rich Longmaxi Formation shale in Sichuan Basin through Argon Ion milling-Field Emission Scanning Electron Microscopy characterization with different SEM parameters,and investigated the effect of electron microscopic parameter variation on the quantitative evaluation of the microscopic pores of shale based on the pore data extracted by the "optimal threshold method" within the same field of view.The results show that the probe current and accelerating voltage among these electron microscopic parameters,have greater influence on the accuracy of quantitative pore data extraction.When the beam exceeds 170PA,the higher the voltage is adjusted to,the smaller the large pore area is extracted.When the voltage exceeds 15KV,the beam current is lower,and the pores extracted are relatively more complete; while as the beam current is lower than 86PA,the longer the scanning time is,the more completely the pore is extracted,but as it exceeds 86PA,the scanning time has little effect on pore extraction.In addition,luminance and contrast values of excessive lowness or highness are detrimental to pore extraction,and the pore extraction comes to the most complete when their values are 47.5 and 85 respectively.In conclusion,the parameters of electron microscopy have a remarkable influence on the extraction of organic pores,and the proper selection of parameters can significantly improve the accuracy of quantitative data extraction for shale micropores,in a bid to obtain more realistic data of shale micropore structure.