Table of Content

28 February 2019, Volume 40 Issue 1

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Petroleum Geology

Dynamic characteristics and model of hydrocarbon accumulation in the large Chunxiao oil and gas field in the East China Sea Shelf Basin

Xu Hong, Zhang Weiwei, Ji Zhaopeng, Wang Li, Wang Qing, Su Dapeng, Lei Baohua, Yang Yanqiu

2019, 40(1):  1-11.  doi:10.11743/ogg20190101

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The research on the dynamics of hydrocarbon accumulation is a frontier of kinetics research.The study on the dynamics of hydrocarbon accumulation in the large Chunxiao oil and gas field is also a hot topic,but a bottleneck as well,in the study of hydrocarbon accumulation features in the East China Sea Shelf Basin for a long time.Through well-to-seismic tie interpretation and numerical simulation,we concluded three energy fields supporting the accumulation in the oil and gas field,including temperature field,pressure field and stress field, the dynamic elements of which were analyzed as well.It's pointed out that the study area is characterized by high geothermal gradient,up to 36~40℃/km,and 60 magmatic intrusive rocks are found therein,leading to the promotion of hydrocarbon accumulation by locally increasing temperature.The thermal maturity of source rocks is significantly inconsistent.The results of numerical simulation show that the maturity is high in degreein northeast and southeast of large Chunxiao area;the Tianwaitian and Chunxiao-Duanqiao structures come to the last in terms of the maturity of their source rocks,and the large Chunxiao structure is also late in this aspect;the NNE-trending tension stress field in the early and middle stages produces tensile normal faults,which are conducive to hydrocarbon migration and preservation.In the later period,the NWW-trending tension-shear stress field forms translational faults,which are also favorable for hydrocarbon preservation.Typical overpressure is developed and a large overpressure sac exists in the area,and an overpressure-pressure sac model is proposed to demonstrate the effect of overpressure and fracture systems on promoting hydrocarbon migration.In addition,the numerical simulation method is used to reveal the era,process and results of hydrocarbon migration.The key moment and events,and dynamic mechanism and process of hydrocarbon accumulation in the study area are integratedly analyzed,leading to the establishment of the dynamic model of hydrocarbon accumulation in large Chunxiao oil and gas field as well as its accumulation model.

Characteristics of tectonic-lithofacies paleogeography in the Lower Cambrian Xiaoerbulake Formation,Tarim Basin

Hu Mingyi, Sun Chunyan, Gao Da

2019, 40(1):  12-23.  doi:10.11743/ogg20190102

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The study aims to discover new prospective areas for deep oil and gas exploration based on the understanding of tectonic and sedimentary response characteristics and evolution patterns of the Xiaoerbulake Formationin,Tarim Basin.Under the guidance of marine carbonate rock sequence stratigraphy and reservoir sedimentology,it's made certain that the Lower Cambrian Xiaoerbulake Formation mainly develops restricted-semi-evaporitic platform,open platform,platform margin,slope-basin facies according to core,logging and seismic data and thin section observation,combined with the features of tectonic paleo-geomorphology.These sedimentary facies can in turn be subdivided into multiple subfacies and microfacies,and their distribution is characteristic of zonation and inheritance.The fine interpretation of the two-dimensional seismic lines across the basin helps to identify the features of seismic responses of the main Cambrian sedimentary facies in Tarim Basin,and recognize six types of seismic facies,namely the intra-platform microbial mound/shoal(SF1),intra-platform low(SF2),inter-mound/bank sea(SF3),gypsum-salt lake(SF4),platform margin(SF5)and slope-basin(SF6).Then the boundary of sedimentary facies for each facies zone was delineated and the relation of facies zones distribution with paleostructure and paleogeomorphology was revealed.According to the isopach map of the Xiaoerbulake Formation,it's made clear that there is an EW-trending paleo-uplift in the southern basin,and two depressions in the eastern and central basin.The First Terrace lies in the southwestern basin,which transits from the wide and gentle Level 1 slope-break towards the central depression,while the narrow and steep Level 2 slope-break in the central basin rapidly transfers to a deep depression towards the east.In addition,the seismic interpretation results,analysis of outcrop,observation of cores as well as microscopic observation of thin sections were used to generate the lithofacies paleogeographic map of the Lower Cambrian Xiaoerbulake Formation,providing favorable evidences for the exploration of deep carbonate oil and gas in the Lower Cambrian Xiaoerbulake Formation of Tarim Basin.

Characteristics of detritus development in the Permian lower Shihezi Formation in Hangjinqi area and its influence on reservoir physical properties

Qiu Longwei, Mu Xiangji, Li Hao, Zhang Jun, Ge Jun, Xu Shuang, Zhou Shibo

2019, 40(1):  24-33.  doi:10.11743/ogg20190103

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Hangjinqi area has great potential for tight gas exploration in Ordos Basin and the Lower Shihezi Formation is the major gas pay zone in the area.The reservoir rocks are mainly composed of lithic sandstones and lithic quartz sandstones.Based on the observation of cast thin section and the analyses of the SEM images,high pressure mercury injection,X-ray diffraction of clay minerals and cathodoluminescence,and combining with the relevant porosity and permeability data,we studied the petrological characteristics and physical properties of reservoirs in the Lower Shihezi Formation in Hangjinqi area of northern Ordos Basin and identified the developmental features of detrital grains and their influence on reservoir physical properties.The following results were obtained.The Lower Shihezi Formation reservoir is mainly composed of medium-coarse sandstones characterized by coarse grain size,with an average content of detritus as high as 40%.The reservoir of the study area is deeply buried,and experienced strong diagenesis and compaction later,leading to the filling of most primary pores and the secondary pores acting as the main reservoir space.The reservoir detritus is diversified in types.According to the influence of detritus on reservoir physical properties,the detritus as a whole can be divided into three types,namelypore reducing,pore increasing and pore retaining.The growth of pore reducing detritus leads to massive reduction of intergranular secondary solution pores and it is uneasy for these detritus to dissolve,resulting in the destruction of reservoir physical properties.Among other,a high content of phyllite detritus plays the most significant role in destroying the reservoir quality.While the growth of pore increasing detritus can function to reserve part of the intergranular pores and it is easy for the detritus to dissolve inside,so being of great help for improving the reservoir physical properties.A high content of metaquartzites is the most important factor for this improvement.Besides,the pore retaining detritus usually develops synchronously with the pore increasing one;it's strong in nature in resisting compaction and most of them is difficult to dissolve.Therefore,this detritus can protect the original pores,but could not improve the reservoir physical properties by self-dissolution,and is in indirect positive correlation with physical properties.

Geochemical characteristics and source of natural gas of the Upper Triassic Xujiahe Formation in Malubei area,northeastern Sichuan Basin

Du Hongquan, Wang Wei, Shi zhiqiang, Tan Jiankang, Cao Huanyu, Yin Xuefeng

2019, 40(1):  34-40.  doi:10.11743/ogg20190104

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Based on the analyses of geochemical data like gas composition and carbon and hydrogen isotopes,the study is made on the geochemical characteristics,origin and source of natural gas in the Upper Triassic Xujiahe Formation in Malubei area,northeastern Sichuan Basin.The results indicate that the natural gas in the Xujiahe Formation in Malubei area is mainly composed of methane,with the content ranging from 92.60% to 99.04%,averaging out at 97.59%,and its gas dry coefficient is generally higher than 0.990 0,with the average value of 0.992 2,indicating a higher degree of thermal evolution.Compared with the Xujiahe natural gas in adjacent areas,the values of gas δ¹³C₁ in Malubei area are bigger (ranging from -33.70‰ to -28.60‰,with an average of -30.88‰),but the values of δ¹³C₂ are smaller (ranging from -30.88‰ to -28.90‰,with an average of -33.11‰),and the carbon isotopes of methane and ethane in the gas of the study area are mainly in reversed distribution:the carbon isotope of methane is relatively heavier while the carbon isotope of ethane is lighter.The identification of gas origin and analysis of gas-source correlation indicate that the natural gas in the Xujiahe Formation in Malubei area is the mixed thermogenic gas of coal-type gas and oil-type gas generated by kerogen Type Ⅲ and Ⅱ,and mainly sourced from the coal-measure source rocks in the Upper Triassic Xujiahe Formation and the marine source rocks in the Upper Permian Wujiaping Formation.And the carbon isotopic reversal of methane and ethane for the Xujiahe gas just results from the mixing of the coal-type gas and oil-type gas.The accumulation and high productivity of the natural gas in the Xujiahe Formation in Malubei area is closely related to hydrocarbon generation by both marine and terrestrial source rocks and the connectivity of several quality marine and terrestrial source rocks to the Xujiahe reservoir by means of deep and large faults,which have played a significant role in natural gas accumulation and later adjustment in the Xujiahe Formation in Malubei area.

Microfracture types of marine shale reservoir of Sichuan Basin and its influence on reservoir property

Wang Hu, He Zhiliang, Zhang Yonggui, Su Kun, Wang Ruyue, Zhao Conghui

2019, 40(1):  41-49.  doi:10.11743/ogg20190105

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The study analyzed the core samples from the Longmaxi Formation to investigate the types of fractures and their effects on reservoir properties by means of argon ion polishing-field emission scanning electron microscopy observation,experimental measurement of the porosity and permeability with overburden pressure,CT scanning and reconstruction,image mosaic,image grayscale recognition and threshold binarization,with a view to clarify the influence of microfractures on the physical properties of subsurface shale reservoir in Fuling shale gas field,Sichuan Basin.The results show that the microfractures for the shale samples can be divided into 6 types,including bedding fracture,particle edge fracture,dissolution fracture,diageneticcontraction fracture,abnormal pressure fracture and structural fracture.Microfractures can function to improve reservoir porosity and play an important role in increasing conductivity in shale reservoirs.Microfractures may form a net of fractures,connecting the various pores throughout the shale reservoir.Among them,particle edge fracturesare the main channels for vertical connection,while bedding fractures are mainly for lateral connection,both of which can be combined to form the best fracture network channels for shale reservoir.Effective fracture network can be composed by particle edge fractures in themselves,dissolution fractures can form fracture network in local area,improving the shale reservoir connectivity.The microfractures will exert a great influence on the permeability of shale reservoirs:the average permeability of microfracture samples is 62.9 times the mean permeability of microfracture-free shale samples.Under normal formation conditions,the microfractures in the shale reservoir should be open when they are shallow at a depth of 3,500 meters.Given the abnormal high pressure,sedimentation,tectonism and other rock mineral characteristics of shale reservoir fluid,the depth of the shale microfracture opening can be increased appropriately.

Petrologic characteristics and genesis analysis of the Ordovician Wufeng Formation-Silurian Longmaxi Formation in Laifeng-Xianfeng area of western Hubei

Wang Xiuping, Mou Chuanlong, Xiao Zhaohui, Chen Yao, Wang Qiyu

2019, 40(1):  50-66.  doi:10.11743/ogg20190106

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As part of western Hubei province,Laifeng-Xianfeng area is adjacent to the east of Sichuan Basin,with its Upper Ordovician Wufeng-Lower Silurian Longmaxi Formation dark rock series bearing lower organic abundance.The petrological and mineralogical characteristics of Upper Ordovician Wufeng-Lower Silurian Longmaxi Formations in Laifeng-Xianfeng area, were investigated according to detailed observation of the six outcrop sections and three cores, in combination with polarizing microscope,X-ray diffraction(XRD),field emission scanning electron microscopy(SEM)and organic carbon content(TOC)analysis.Large amount of clastic particles in silt size are developed in the study area whose mineral components are mainly quartz and clay minerals.Besides,siltstones and shales rich in organic matters are the main rock types in this area,which can be divided into four lithologic sections from the bottom-up including carbonaceous and siltyshales,carbonaceous and argillaceous siltstones,argillaceous siltstones and carbonaceous siltyshales.According to instructionsby petrological and mineralogical characteristics and geochemistry of element,and comparison of shale enrichment conditions with the corresponding formation of Sichuan Basin,it can be figured out that the shorter-term anoxic and reducing environment,rapid change of sea level and smaller rate of deposition,render the thickness and abundance of organic matters of organic-rich shales in the Wufeng-Longmaxi Formations in Laifeng-Xianfeng area with higher bioproductivity to be significantly worse than those of the corresponding formation in Sichuan Basin.

Microscopic characteristics of pores in Es³ shales and its significances for hydrocarbon retention in Dongpu Sag,Bohai Bay Basin

Shao Xinhe, Pang Xiongqi, Hu Tao, Xu Tianwu, Xu Yuan, Tang Ling, Li Hui, Li Longlong

2019, 40(1):  67-77.  doi:10.11743/ogg20190107

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The Es³ shales in Dongpu Sag were studied with XRD analysis,thin section and SEM observation,and low-temperature nitrogen adsorption experiment based on their basic geochemical characteristics.Microscopic characteristics of shale pores were analyzed,and thus controlling factors of shale pore development and its significance for hydrocarbon retention in shales have been identified.Results show:the Es³ shales in Dongpu Sag can be classified into silty shales,calcareous shales and gypsum shales according to their mineral compositions;the strong heterogeneity in shales'geochemical characteristics reflects a complex hydrocarbon retention pattern in Es³ shales;several pore types including interparticle,intraparticle and organic-matter pores and fractures,are identified in Es³ shales with different lithologies dominated by different pore types;the pore systems in Es³ shales can be dominated by micropores,macropores,or pores of multiple diameters based on low-pressure nitrogen adsorption experiment;pore development in Es³ shales is affected by both deposition and diagenesis,with deposition environment laying the foundation for pore development in various lithologies and diagenesis modifying pore structures during shale burial,respectively;oil and free gas mainly accumulate in large pores and microfractures,and micro-and meso-pores provide sites for adsorbed gas,while solution gas is of great significance to develop shale oil/gas resources.

Tight oil reservoir characteristics of lacustrine mixed marlstone in the Palaeogene Shahejie Formation of Shulu Sag,Bohai Bay Basin

Fu Xiaodong, Wu Jianping, Shou Jianfeng, Wang Xiaofang, Zhou Jingao, Zhang Tianfu, Guo Yongjun

2019, 40(1):  78-91.  doi:10.11743/ogg20190108

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Lacustrine marlstones of mixed silisiclastic-carbonate origin are widely developed in the lower third member of the Shahejie Formation(Es^3L)in the Shulu Sag,Bohai Bay Basin,and are the main target layers for tight oil exploration.Various data including core analysis,thin section observation,porosity and permeability measurement,mercury intrusion porosimetry,scanning electronic microscope(SEM),CT scanning and nuclear magnetic resonance(NMR) were used to clarify the geologic properties of the special marlstone reservoir of mixed silisiclastic-carbonate origin.Based on the comprehensive analyses in geology and geochemistry,the main controlling factors favorable for reservoirs development were revealed,the criteria for reservoir classification and evaluation were established,and furthermore the favorable plays for tight oil exploration were pointed out.The results indicate that the marlstones in Es^3L can be divided into two groups,namely lamellar marlstones of interlaminated mixing and blocky marlstones of structural mixing.The mineral compositions of both types are dominated by calcite,followed by dolomite,and are low in contents of clay,quartz,and feldspar.The re-servoir porosity is typically below 4.0%,and matrix permeability less than 10×10^-3 μm²,manifesting its nature of ultra-low porosity and extra-low permeability reservoir.There are diverse types of reservoir pores with complicated structures,which can be divided into two categories and eight subcategories.Most of the matrix pores are nanopores.Reservoir pro-perties and pore structures are distinctly different between marlstones with diverse structures:the pore distribution in the lamellar marlstones is highly heterogeneous,featuring double or multi-porosity media;the lamellar marlstones are better than blocky marlstones in terms of porosity and pore structure.The favorable marlstone reservoirs of mixed silisiclastic-carbonate origin are controlled by sedimentary subfacies,rock fabric,abundance and maturity of organic matters,and density of structural fractures.As for tight oil exploration,the lamellar marlstones with high TOC that are deposited in inner slope and semi-deep and deep lacustrine subfacies of trough areas are the favorable targets for tight oil exploration.

Petroleum geologic conditions and distributional features of large- and medium-sized oil and gas fields in Bohai Sea Basin

Zhong Kai, Zhu Weilin, Xue Yong'an, Zhou Xinhuai, Xu Changgui, Niu Chengmin

2019, 40(1):  92-100.  doi:10.11743/ogg20190109

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On the basis of reviewing the petroleum exploration history of the Bohai Sea Basin(the offshore part of the Bohai Bay Basin)and the related petroleum geologic theories,we summarized the Cenozoic tectonic-sedimentary evolution of the Bohai Sea Basin,and analyzed its petroleum geologic conditions,with focus on their significant differences from that of the onshore part of the Bohai Bay Basin.First of all,two sets of high-quality source rocks of the Shahejie Formation and Dongying Formation are developed as the results of two tectonic cycles of the Bohai Sea Basin.The distribution of source rocks of the Dongying Formation is limited to this basin,and the rapid subsidence in late stages contributes to its maturity,both of which manifest the more favorable hydrocarbon source conditions in the Bohai Sea Basin.On the other hand,shallow-extremely shallow-water deltas,a unique series of strata for exploration in the offshore part,are widely developed in the Neogene of the Bohai Sea Basin.In addition,the long-term sustained activities of the Tanlu fault zone in the offshore part eventually resulted in the play fairway with several large-and medium-sized oil and gas fields in the Bohai Sea Basin,which is an important field for oil and gas exploration in the Bohai Sea at present.It's of referential value for future Bohai Sea exploration and related studies to fully understand the particularity of evolution of the Bohai Sea Basin and its unique petroleum geologic conditions and oil and gas accumulation patterns.

Reservoir characteristics and controlling factors of Shahezi Formation in Xujiaweizi Fault Depression,Songliao Basin

Shao Zhaoyi, Wu Chaodong, Zhang Dazhi, Yang Buzeng

2019, 40(1):  101-108.  doi:10.11743/ogg20190110

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The Shahezi Formation in the Xujiaweizi Fault Depression is the main target for recent natural gas exploration in the northern Songliao Basin.It is of great significance to study the reservoir characteristics and controlling factors of such tight glutenite gas reservoir.The deposits in the fault depression formed in an early Cretaceous extensional tectonic environment,can be divided into four third-order sequences vertically,each of which develops depositions of fandelta and braided delta in the steep and gentle slope zones of the rifted lacustrine basin.The reservoir isdominated by coarse clastic rocks such as glutenites and pebbly coarse sandstones.The reservoir pores of the Shahezi Formation consist of primary intergranular pores and secondary pores,with porosity mainly in the range of 2% to 8% and with permeability usually less than 0.1×10^-3μm²,indicating a tight reservoir.The controlling factors for reservoir development in the Shahezi Formation include burial depth,sedimentary facies belt,rock type and epigenesis.The fan delta front(braided delta)with shallow depth and glutenite reservoir of plain faciesare characteristic of good reservoir quality,while the sandstones and glutenites with pyroclastic materials,tend to form dissolution pores and improve the reservoir performance.The favorable reservoirs of the Shahezi Formation in the Xujiaweizi Fault Depression are mainly located in the fan-delta front and the plain area with a burial depth less than 4 000 m at the east and west sides of Anda,as well as in the higher part of fan development zone of Xudong and Xuxi areas.The shore-shallow lake facies with shallow depth in central Anda and the Xudong and Xuxi slope fan development zones with deep depth are also favorable zones for reservoir development.

Sedimentary strata and organic matter features of the Cretaceous Laiyang Group in eastern Shandong Province

Zhou Yaoqi, Gu Yangjian, Zhou Tengfei, Yue Huiwen, Li Manjie, Yin Xingcheng, Liang Zhao, Cao Mengchun, Dong Shihui

2019, 40(1):  109-122.  doi:10.11743/ogg20190111

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The organic matter features of the Laiyang Group in Lingshan Island were systematically studied by means of sedimentology, sedimentary geochemistry and organic geochemistry and integrating with typical outcrop observation in Lingshan Island,Laoshan Yakou-Baxiandun area and Jiaolai Basin and the previous research on the elemental geochemistry of Lingshan Island and the organic geochemistry of Jiaolai Basin. In Jiaolai Basin,the terrestrial strata of fluvial and lacustrine facies and alluvial fan facies were developed in the Early Cretaceous Laiyang period.The organic geochemistry studies on the deposits show that the source rock samples from the Shuinan section are mainly of saprolite(Type Ⅰ)and humus and saprolite (Type Ⅱ₁),have entered the hydrocarbon generation threshold and are in the thermal mature stage;while in the Lingshan Island,LaoshanYakou-Baxiandun areas,marine strata dominated by deep sea gravity flow and deep water traction current deposits were developed in the Early Cretaceous Laiyang period.Meanwhile the elemental geochemical studies on siltstones and shales of Lingshan Island reveal that its sedimentary setting is a marine reducing environment,and the organic geochemical research shows that samples from Lingshan Island belong to the humic(Type Ⅲ)and humic and sapropel (Type Ⅱ₁)types in high to over mature stage,and the turbidites of Lingshan Island may have completed the process of hydrocarbon generation and expulsion.

Forebulge migration since the Mid-Miocene in the southern South China Sea: evidences from the Beikang Basin

Wang Longzhang, Yao Yongjian, Zhang Li, Zhou Jiangyu, XuXing, Xiao Jiaojing, Shen Ao, Xu Qiao

2019, 40(1):  123-132.  doi:10.11743/ogg20190112

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The 2D seismic data were used to discuss the migration of the Beikang forebulge in the southern South China Sea and its impact on trap formation.The strata overlying the Early Miocene Unconformity(EMU),a regional unconformity,could be simply categorized as draping and horizontal layer series in the Nansha sea area.The draping layer series overlie the EMU directly,show parallel seismic reflection characters of continuous thin layers and thicken southeastward,suggesting suspended sediments.While the horizontal layer series were deposited synchronously in structural lows and continuously onlapped and overlay the draping strata formed early.They are mixed thick sedimentary layers of turbidites and suspended sediments featuring discontinuous or weak seismic reflection.Judged by the migration of onlap reflection termination and thickening of the draping strata,the forebulge has migrated southeastward during its uplifting since the Mid-Miocene,indicating a process of intense flexure and equilibrium rebound occurring at the thinned continental crust in the Nansha sea area.The forebulge migration has caused the formation of a host of buried hills,with the draping and horizontal series of strata serving as favorable seals,which in turn will be conducive to the formation of traps.

Origin of reservoir bitumen and its implications for adjustment and reformation of hydrocarbon-accumulation in Baiyuan Sag, Pearl River Mouth Basin

Li Meijun, Zhang Zhongtao, Chen Cong, Yang Chengyu, Lu Xiaolin, Fu Jian, Dai Jinhui, Tang Youjun

2019, 40(1):  133-141.  doi:10.11743/ogg20190113

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Reservoir bitumen has been discovered in multiple Paleogene series of strata in the Baiyun Sag,Pearl River Mouth Basin.The abundance of reservoir bitumen gradually increases from the shallower Zhuhai and Zhujiang formations to deeply buried Enping and Wenchang Formations.The reservoir bitumen only sporadically occurs in coarse-grained sandstones with better permeability and porosity, while occurs in siltstones and mudstones in the form of veins.Almost all reservoir bitumen samples do not show apparent anisotropy with bitumen reflectance lower than 2.0%,suggesting that the reservoir bitumen in the Baiyun Sag doesn't mainly result from high-temperature cracking.All bitumen extracts lack 25-norhopane series according to the analysis of molecular geochemical composition.No gas chromatogram base-line "hump" was discovered in all saturated hydrocarbon of reservoir bitumen.The reconstructed burial history of representative individual wells shows that the reservoir strata have not experienced large-scale uplifting and denudation.Therefore,the reservoir bitumen in the Baiyun Sag is not of biodegradation origin.Based on the analyses of the geological setting and characteristics of oil and gas reservoirs,the origin of reservoir bitumen could be attributed to later "gas washing" for deeply buried hydrocarbon-accumulation generated by source rocks of Enping and Wenchang Formations during the early thermal maturity stage.The time differences of thermal evolution for source rocks and reservoirs of various stratigraphic horizons leads to the adjustment and altering of oil and gas reservoirs in the Baiyun Sag,subsequently resulting in the formation of light and volatile oil reservoirs.

Types and characteristics of Paleogene coal-forming sedimentary systems in Qiongdongnan Basin

Liu Ying, Liu Haiyan, Yang Haizhang, Wang Dongdong, Song Guangzeng, Lyu Dawei, Chen Ying, Li Zengxue

2019, 40(1):  142-151.  doi:10.11743/ogg20190114

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The practice for exploring natural gas in Qiongdongnan Basin proves that coal measures and coal seams are the most important hydrocarbon source rocks.Based on the characteristic analysis of coal-bearing deposits in the Oligocene Paleogene by the seismic data,drilling core and logging data in Qiongdongnan Basin,the corresponding relationships between sedimentary facies and seismic facies are established,and 14 kinds of sedimentary assemblages are identified in the Oligocene of Qiongdongnan Basin,including the alluvial fan-tidal flat-lagoon,alluvial fan-tidal flat-coast,alluvial fan-sea bog sand ridge plain-coast assemblages.According to the micro-and macro-physicochemical properties of cored coal intervals,it is made certain that the Yacheng Formation coal seam was affected by seawater during its formative process and its sedimentary environment tended to be strong in reduction,indicating that the coal seam is developed in the offshore marsh environment.In view of the characteristic and environmental analysis for coal forming,four types of coal-forming sedimentary systems were classified,namely fan delta,braided river delta,tidal flat-lagoon and barrier-coast-sea marsh sand ridge plain systems.Among them,the thin coal seam in the fan delta system,prone to bifurcation,is mainly developed in the fan delta plains,abandoned lobes,and interlobe depressions;the coal seam in the braided river delta system,featuring thinness and instability,mainly grows in the braided river delta plains,diversion bays,and floodplains.The coal seam in the tidal-lagoon system is mostly developed in the swamps of the supratidal flats and the inter-tidal-channel bays of the upper intertidal flats,the tidal delta plains and the silted lagoons,and in areas of multiple tidal channels,the coal seam layers are relatively large in number,and characterized by thinness and larger spacing, while in areas of single tidal channels, the number of coal-bearing seams,complex in structure,is relatively small,but their thickness is relatively large,and their spacing is small.At last, the coal seam in the barrier coast-sea marsh sand plain system is mainly developed in coastal plain marsh areas,characterized by multiple layers and faster change in lateral,but thinness for each layer.

Sedimentary features and geological conditions for hydrocarbon generation and accumulation in Taixi Basin

Zhang Li, Lei Zhenyu, Xu Hong, Shuai Qingwei, Luo Shuaibing, Qian Xing

2019, 40(1):  152-161.  doi:10.11743/ogg20190115

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The Taixi Basin is a superimposed Mesozoic-Cenozoic basin located in the Taiwan Strait,with its main body lying between Taiwan Island and onshore Fujian.Large volume of hydrocarbon has been discovered in the eastern basin and Taiwan Island.Based on the previous studies,the study focused on stratigraphic distribution,sedimentary features and hydrocarbon generation potentials of the basin through fine interpretation of seismic data and marine-continental comparative analysis.The results show that evolution of the Mesozoic-Cenozoic basin could be subdivided into three stages,namely the Cretaceous-Eocene rift depression lacustrine stage,the Oligocene uplift denudation stageand the Miocene-present sedimentation and subsidence stage.Correspondingly,three sets of source rocks are developed in this basin including the Lower Cretaceous,Paleocene and Eocene source rocks.The reservoir types are mainly clastic rocks,such as lacustrine facies,delta,subaqueous fan and channel sands.Medium-sized oil-gas fields may develop in the study area due to its favorable geological conditions for hydrocarbon generation and accumulation,thus its hydrocarbon exploration potential is believed to be relatively high.

Key Cenozoic tectonic transformation in Jiulongjiang Sag,Taiwan Strait Basin and its petroleum geologic significance

Qian Xing, Zhang Li, Yi Hai, Wei Zhenquan, Lei Zhenyu, Luo Shuaibing

2019, 40(1):  162-169.  doi:10.11743/ogg20190116

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Hydrocarbon accumulation is closely related with regional tectonic setting and basin formation and evolution.It's pointed out that four stages of critical tectonic transformation occurred in Jiulongjiang Sag through the analysis of two-dimensional multichannel seismic data and regional geological data.As a result, four unconformities formed during the Taiping,Oujiang,Puli and Hai'anshan movements.Controlled by different geodynamic backgrounds and tectonic stress fields at different stages,three styles of extensional structures,namely the stepwise,horst-graben and sliding fault terrace,and two styles of inverted structures,namely fault-restricted and flexural fold structures,are developed in the sag.Through the analysis of tectonic stress backgrounds and hydrocarbon accumulation conditions at different stages,it is believed that the Taiping and Oujiang movements laid the foundation for the formation of traps in the Jiulongjiang Sag,and was a major factor for the development of extensional structural traps;while the uplifting caused by the compressive force from the late Oujiang movement prompted the formation of inverted structures,providing favorable trap conditions for hydrocarbon accumulation.

Stratigraphic framework and temporal-spatial distribution of Oligocene deepwater sedimentary sequence in Ruvuma Basin,East Africa

Sun Hui, Liu Shaozhi, Lyv Fuliang, Fan Guozhang, Ma Hongxia

2019, 40(1):  170-181.  doi:10.11743/ogg20190117

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It is difficult to divide deep water sequences in sequence stratigraphy study due to a lack of direct connection between the sequence and the shelf margin deposition.The Oligocene gas-bearing sandstones in Ruvuma Basin,East Africa,were deposited by gravity flow in deep water environment.A reasonable sequence stratigraphic division is helpful to the interpretation of favorable targets and regional oil and gas exploration.Under the guidance of the mixed sequence stratigraphic model,we put 3D seismic data into application and comprehensively analyzed multiple data to study the sequence stratigraphy,disclosing the genetic relationship between sedimentary processes and sedimentary systems in deep water environment.Sequence boundaries were identified by the strata termination, condensation section and MTD.Consequently,the corresponding relationship between sedimentary architecture element and system tract was established by analyzing the nature and scale of sediments and density difference of turbidity currents leading to deep water deposition,the Oligocene isochronous stratigraphy framework was established and the rules of deep water sedimentary evolution was analyzed.The Oligocene in Ruvuma Basin can be divided into two third-order sequences,for both of which large-scale MTDs accumulated locally on continental slopes near shelf break in early forced regression.But,the sedimentary characteristics of the two sequences show differences.The large lobes in the Lower Oligocene third-order sequence that are dominated by coarse-medium-coarse-grained sandstones and have large sand/shale ratio and high density are products of late forced regressions.The sediments of the leveed channel system were located in the lowstand system tract(LST)and early transgression, with the condensation section overlying the leveed channel system, all of which contribute to the complete system tract of the Lower Oligocene sequence.Due to the limited data obtained within the study area,large lobes are absent in the Upper Oligocene third-order sequence,within which there is a lack of late deposition in falling stage system tract(FSST).Besides,the sedimentary architecture elements in other system tracts are quite similar to those of the Lower Oligocene sequences.

Petroleum Development

Waterflooding and optimum displacement pattern for ultra-low permeability reservoirs

Wang Wenhuan, Peng Huanhuan, Li Guangquan, Yu Tao, Wei Chenji

2019, 40(1):  182-189.  doi:10.11743/ogg20190118

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This study reveals the waterflooding pattern of ultra-low permeability reservoir after the analysis of waterflooding recovery characters of plenty ultra-low permeability oil reservoirs,with a view to further bettering the development of ultra-low permeability oil reservoirs and enhancing oil recovery.It can be concluded that fractures induced by water injection will open along the injection-production line in the direction of in-situ maximum horizontal principal stress.In addition,as the angle between the in-situ maximum principle stress and the injection-production line becomes smaller with the increase of water injection pressure,the induced fracture is more likely to open,resulting in an invalid circulation of injected water to injection-production wells and poor waterflooding effect.Moreover,the seepage resistance of linear lateral displacement of the fracture is less than that of the radial displacement calculated by equivalent percolation resistance method.In turn a well pattern conversion mode of "linear injection into dynamic fractures along the direction of in-situ maximum horizontal principal stress with matrix displacement of lateral side" is proposed.The optimized well pattern can improve water sweep efficiency by mitigating the effect of dynamic fractures on reservoir heterogeneity and reducing seepage resistance.In application,the productivity of single wells has doubled,the plane sweep coefficient has increased by 43.8%,and the waterflooding recovery ratio has increased by 19.3%.

Methods and Technologies

Correlation between relative permeability, capillary pressure and resistivity index

Ma Dong, Zhang Kun, Hu Juncheng, Li Hehui, Ma Long, Tang Tang

2019, 40(1):  190-195.  doi:10.11743/ogg20190119

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Parameters such as capillary pressure,relative permeability and resistivity index are important to describe reservoir properties and multiphase fluid flow characteristics in reservoirs.However,the relative permeability and capillary pressure could not be easily measured through experiments which are often time-consuming,demanding in instrument precision,and restricted by subjective conditions.The study discussed the correlation amongcapillary pressure,relative permeability and resistivity index based on the capillary tube model and Archie equation,and in turn built a transformation model among the three parameters.Besides,the accuracy of the transformation model was verified by rock-electrical property experiment.The results show that the relative permeability and the capillary pressure calculated with the transformation model are very close to that measured through in-situ experiments.The method proposed in this study can effectively calculate the relative permeability and capillary pressure with resistivity data,making it easier to figure out the two parameters.

Pre-stack fluid identification for fractured-vuggy carbonate reservoir in Tazhong area

Xian Qiang, Feng Xukui, Liu Yonglei, Lyv Dong, An Haiting, Cai Zhidong, Wang Zujun, Yuan Hengchao

2019, 40(1):  196-204.  doi:10.11743/ogg20190120

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The reservoir space of the Ordovician carbonates in Tazhong area,Tarim Basin,is mainly composed of fractures and secondary dissolved pores and cavities,and its seismic responses are bead-like reflections.The fact that the reservoir is covered by surface dunes and buried in depth leads to a relatively low signal-to-noise ratio of seismic data and the poor imaging precision for the reservoir.The oil-water contact is complex in the reservoir,making it hard to effectively identify its features by using post-stack hydrocarbon detection method.These problems have seriously restricted the progress of exploration and development of carbonate reservoirs in this area.In order to address these problems,we conducted research based on high-density seismic data in Tazhong area.As the high-density seismic data have higher signal-to-noise ratio and reservoir imaging precision,they are beneficial to study pre-stack hydrocarbon detection method for fractured-vuggy reservoirs.Firstly,more accurate P and S wave velocity can be worked out by Rock Physics Model.Secondly,AVO forward modeling of oil/gas and water wells is conducted,showing discrepant responses from different wells as a result.Therefore,the AVO attributive analysis technique can be used to identify liquids in carbonate reservoirs.Finally,pre-stack simultaneous inversion technique is used to quantitatively identify fluid types in reservoirs in the research area; The predictions are in turn compared with the results of formation testing,revealing an accuracy of liquid predictionsof 83.3%,which manifests that this method provides effective technical support for improving the success rate of drilling and reducing risks of hydrocarbon exploration.

The stress concentration effect at fault end under various boundary conditions and its implications for fracture development

Qiu Dengfeng, Yun Jinbiao, Liu Quanyou, Zhou Yan, Ning Fei, Song Haiming

2019, 40(1):  205-214.  doi:10.11743/ogg20190121

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A fracture development play related to stress concentration of fault zone has high potentials for tight reservoirs and shale oil and gas.This study combined the methods of photo elastic physical modeling and finite element simulation to discuss the planar distribution patterns of stress field at fault's end,and took advantage of the elasticity mechanical theory to analyze the mechanism of stress field distribution.The results indicate that the fault's end displays the concentration of compression stress under the compression background.The stress concentration district extends outward in the shape of numeral "8" following a hyperbolic path centered on the most salient point of the fault's arc end,and its influence range increases with the growth of boundary stress intensity.The stress concentration degree tends to get stronger with the angle between boundary stress direction and fault strike increasing when the angle in the range of 0° to 90°,and reaches its summit at 75°.Besides,both the shape and the modeling method of fault have a significant impact on fault's end stress field.All in all,the stress concentration district at the fault's end has great potentials for the exploration and development of fracture reservoirs.