Table of Content

28 June 2020, Volume 41 Issue 3

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Petroleum Geology

Multi-scale fractures in tight sandstone reservoirs with low permeability and geological conditions of their development

Zeng Lianbo, Lyu Peng, Qu Xuefeng, Fan Jianming

2020, 41(3):  449-454.  doi:10.11743/ogg20200301

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Given the mechanical stratigraphic units and their interface properties constraining natural fracture propagation in tight sandstone reservoirs with low permeability,we propose the division criterion of multi-scale fractures,and discuss the geological conditions of multi-scale fracture formation.According to the scale of natural fracture and mechanical stratigraphy interfaces controlling their development,the natural fractures within reservoirs can be grouped into 4 levels,namely the large-scale,meso-scale,small-scale and micro-scale fractures.Large-scale fractures were developed within sandstone layer groups with a propagation of hundreds of meters,cutting through interlayers and constrained by barriers;meso-scale fractures were developed within composite sand bodies with a propagation of tens of meters,cutting through beddings and constrained by interlayers;small-scale fractures were developed within single sand bodies with a propagation ranging from several meters to over ten meters,and constrained by beddings;micro-scale fractures are to be observed and identified under microscopes,usually with a propagation of less than several centimeters.These fractures feature power-law distribution:the fracture size is in negative correlation with the number of fractures.Multi-scale fracture formation is mainly controlled by the differential rock mechanical properties,thickness of mechanical stratigraphic unit and tectonic stress.The greater tectonic stress is favorable for generating large-scale,cross-layer fractures,the propagation of which can be constrained only by relatively large differential rock mechanical properties and thick mechanical stratigraphic units;while the smaller tectonic stress is conducive to developing small-scale fractures,the propagation of which can be sufficiently constrained by relatively small differential rock mechanical properties and thin mechanical stratigraphic units.Fractures of multiple scales are of different contributions to the formation of tight,low-permeability reservoirs.Micro-scale fractures are the major reservoir space,improving the storage and permeability of tight,low-permeability reservoirs;thus they are key to the reservoir stable production of this kind;small-scale fractures act as both storage space and seepage channels,key to controlling the initial productivity of tight,low-permeability reservoirs;large- and meso-scale fractures mainly function as seepage channels,affecting the distribution pattern of remaining oil at the middle and late stages of reservoir development of the kind.

Analysis of conceptual models for the influence of rock mechanics on tensile zone in faulted anticlines

Sun Shuai, Hou Guiting

2020, 41(3):  455-462.  doi:10.11743/ogg20200302

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The study on the structural fracture development in the faulted anticline in Kelasu tectonic zone,Kuqa Depression,reveals a tensile fracture zone developed in the hinge zone of the faulted anticline.And the flow rate from single-well test in the tensile fracture zone is high,indicating a prolific zone.Therefore,it is of great significance to oil/gas exploration and development to make an in-depth study on the tension fracture zone and its controlling factors.Our previous study reveals that the tensile fracture zone is controlled by neutral surfaces,but the influence of rock mechanics on the zone remains unclear.Then,we build several 2D mechanical conceptual models with different rock mechanical parameters through finite numerical modeling,for analyzing the influence of Young's modulus,Poisson ratio and viscosity ratio on the tensile fracture zone.The results show that:(1) when the Young's modulus is greater than 70 GPa,the maximum strain value grows rapidly with the increasing Young's modulus;both the width and thickness of the zone feature an overall linear growth with the increasing Young's modulus;(2) when the Young's modulus is greater than 40 GPa,the maximum strain value and width of the zone grow progressively with the increasing Poisson ratio;(3) when the viscosity ratio is less than 25,the maximum strain value comes to zero,and the width or thickness of the tensile fracture zone can be neglected;while when the viscosity ratio is greater than 25,the maximum strain value grows greater than zero and increases gradually,and so do the width and thickness of the zone.

Geological factors contributing to high shale gas yield in the Wufeng-Longmaxi Fms of Sichuan Basin: A case study of Well JY6-2HF in Fuling shale gas field

Nie Haikuan, Zhang Baiqiao, Liu Guangxiang, Yan Caina, Li Donghui, Lu Zhiyuan, Zhang Guangrong

2020, 41(3):  463-473.  doi:10.11743/ogg20200303

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The great success of shale gas development in Fuling (the cumulative production of Well JY6-2HF has exceeded 300 million cubic meters) provides new enlightenment for the exploration and development of Wufeng-Longmaxi Formations' shale gas reservoir.The keys for the success are:(1) the Wufeng-Longmaxi Formations in Jiaoshiba area were deposited in deep water.The WF2-LM4 graptolitic zone shale is relatively thick,laying a better material basis for shale gas enrichment.(2) The lateral of Well JY6-2HF is mainly emplaced in WF2-LM4 graptolitic zone,which contains high-quality shale comparable to the "Lower Hot Shale" in Middle East-North Africa.Most of the laterals of shale gas wells in Weiyuan and Changning shale gas fields are also emplaced in this graptolitic zone,which is characterized by high total organic carbon content (TOC >3%) and high gas content (6.87-9.02 m³/t),and highly-developed organic pores (organic porosity >4%).Comprehensive analysis shows that under similar geological and engineering conditions,the wells with longer laterals emplaced in WF2-LM4 graptolitic zone generally have higher test production and estimated ultimate recovery (EUR).The research results are of referential value to the exploration and development of Wufeng-Longmaxi Formations' shale gas in Sichuan Basin and its periphery,and of the Silurian shale gas in other parts of the world.

Characteristics of natural fractures in marine shale in Fuling area,Sichuan Basin, and their influence on shale gas

Tian He, Zeng Lianbo, Xu Xiang, Shu Zhiguo, Peng Yongmin, Mao Zhe, Luo Bing

2020, 41(3):  474-483.  doi:10.11743/ogg20200304

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Fractures of various types show different impacts on shale gas enrichment and preservation.Therefore,categorized evaluation and prediction are quite necessary in studying the natural fractures in shale gas reservoirs.A wealth of outcrop,coring and thin section observation,as well as FMI logging interpretation is applied to the discussion of the genetic types,development characteristics,and main controlling factors of natural fractures in marine shale,and the impact of various types of natural fractures on shale gas enrichment and preservation in the Wufeng-Longmaxi Formations,Fuling area,Sichuan Basin.The natural fractures in the study area can be divided into tectonic,diagenetic and abnormally high-pressure fractures in terms of geological origin.In turn,the tectonic fractures can be subdivided into intra-layer aperture fractures,cross-layer shear fractures,layer-parallel shear fractures,whereas diagenetic fractures mainly include lamellar fractures and constricted fractures.The shale formation of the study area is dominated by tectonic and lamellar fractures.The development of tectonic fractures is mainly controlled by structural position,rock mechanical stratigraphy thickness and brittle mineral content.While the development of the lamellar fractures is determined by laminae development,brittle mineral content,abnormal high pressure in fluids,and content and thermal evolution stage of organic matter.Natural fractures of various origins have different influence on shale gas.The aperture fractures and lamellar fractures developed within the shale formation are small in scale,providing better reservoir space for shale gas accumulation.The larger-scale cross-layer shear fractures and layer-parallel shear fractures mainly impact the preservation conditions of shale gas.The underdevelopment of constricted and abnormally high-pressure fractures provides poor effective porosity and thus has little influence on shale gas enrichment and preservation.

Fracture characteristics and their influence on natural gas production: A case study of the tight conglomerate reservoir in the Upper Triassic Xujiahe Formation in Jian'ge area,Sichuan Basin

Lyu Wenya, Miao Fengbin, Zhang Benjian, Zeng Qi, Xu Xiang, Ji Min

2020, 41(3):  484-491,557.  doi:10.11743/ogg20200305

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Natural fractures are important storage space and main seepage channels in tight conglomerate reservoirs,which function to influence the accumulation,enrichment and single-well productivity of tight gas.An integration of analogous outcrop,core,thin section,well log and production data,is applied to the study on the types and characteristics of natural fractures in the tight conglomerate reservoir of the Upper Triassic Xujiahe Formation in Jian'ge area,Sichuan Basin,and then the influence analysis of natural fractures on tight gas productivity.Trans-gravel,intra-gravel and gravel-edge fractures are developed in the tight conglomerate reservoirs.In terms of geological origin,trans-gravel fractures are tectonic fractures; intra-gravel fractures include tectonic,tectonic-diagenetic and parent-rock fractures; and gravel-edge fractures are tectonic-diagenetic fractures.Among others,trans-gravel fractures are the largest in areal density,porosity and permeability,followed by intra-gravel fractures and gravel-edge ones.Thus,the trans-gravel fractures are more contributive to the formation of the tight conglomerate reservoirs,and the most conducive to natural gas enrichment with high yield.The degree of fracture development is positively correlated with gas production:the greater the fracture density is,the greater the tight gas productivity could be.Under the present-day in-situ stress trending towards the nearly east-west (EW) to NWW-SEE direction,the contributions made by fractures of diverse directions are differential on the tight gas productivity.The nearly EW-trending fractures feature bigger apertures and good connectivity,being the most favorable area for tight gas production,followed by nearly NS- and NW-SE-trending fractures,and then the NE-SW-trending ones.

Timing of hydrocarbon accumulation for paleo-oil reservoirs in Anyue gas field in Chuanzhong Uplift

Yang Chengyu, Wen Long, Wang Tieguan, Luo Bing, Li Meijun, Tian Xingwang, Ni Zhiyong

2020, 41(3):  492-502.  doi:10.11743/ogg20200306

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Anyue gas field,resulted from the in-situ pyrolysis of paleo-oil reservoirs in the Neoproterozoic-Lower Paleozoic sequences,is one of the largest old and deep gas pools in China.Since the paleo oil-pyrolysis gas pools are high in degree of evolution and very old,and there is in lack of geochemical evidences,the accumulation and evolution,especially the timing of accumulation,remains a controversial issue in study.Starting from the newly-measured terrestrial heat flow history and threshold depth of hydrocarbon generation from source rocks in similar areas,we apply geological analogy method,instead of microthermometry of fluid inclusions,to the analysis of paleo-oil reservoir evolution:reconstructing the burial-geothermal history of key wells,and then determining the timing of hydrocarbon generation in the Qiongzhusi Formation source beds in Deyang-Ziyang rift and of paleo-oil reservoirs' formation and destruction.The results of geochemical experiments and relevant geological analyses indicate that hydrocarbon generation in the Qiongzhusi Formation source rock and paleo-oil reservoir formation occurred around 275-263 Ma,corresponding to the late Early Permian-Late Permian.The oil pyrolysis occurred since the early Triassic with deep burial,and the paleo-oil reservoirs were entirely transformed to a pyrolysis gas pools during the late Middle Jurassic-early Early Cretaceous.

Distribution and controlling factors of natural fractures in deep tight volcanic gas reservoirs in Xujiaweizi area,Northern Songliao Basin

Gao Shuai, Gong Lei, Liu Xiaobo, Liu Bo, Gao Ang, Su Xiaocen, Wang Jie, Li Jing

2020, 41(3):  503-512.  doi:10.11743/ogg20200307

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The natural fracture system controls the hydrocarbon migration,accumulation and enrichment in the tight volcanic reservoirs,Northern Songliao Basin.It is of fundamental significance to illuminating the factors controlling natural fracture development in tight volcanic reservoirs for an accurate prediction of fracture distribution pattern.Using data from outcrop,core and thin section observation,as well as fracture interpretation with image logging,we finely characterize the development features of natural fractures in the tight volcanic reservoirs of the study area,and make a systematic research on the fracture distribution and its controlling factors.The results show that the fractures developed therein can be grouped into two types,including primary and secondary fractures.Among others,the secondary tectonic fractures are dominant,with a linear density ranging from 1.0 m^-1 to 5.0 m^-1,averaging at 2.45 m^-1.The fracture intensity varies greatly,mainly controlled by rock type,mechanical property,brittleness and tectonic position.The natural fracture orientation is complex and variable,a result of the combined action of paleotectonic stress,fault and rock heterogeneity (foliation).

Architecture of channel reservoirs of shallow-water delta in the 4th member of Cretaceous Quantou Formation in southern Sanzhao Sag,Songliao Basin

Deng Qingjie, Kang Dejiang, Hu Mingyi, Shen Jiao, Lei Ming

2020, 41(3):  513-524.  doi:10.11743/ogg20200308

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The channel reservoirs in the 4th member of Cretaceous Quantou Formation (Quan 4 member) were mostly deposited in a lake-shallow-water delta system on a gentle slope,and are targets for hydrocarbon exploration and development in southern Sanzhao Sag of Songliao Basin.We study the reservoir architecture via core,logging and seismic data based on Miall's architecture theory,and identify two types of fourth-order architecture interface,namely the interfaces of fine-grained conglomerate or mud pebble-bearing fine sandstone,and of mudstone or silty mudstone.In turn,6 categories of architecture units in accordance are identified according to 17 types of lithofacies assemblages,and 4 vertical combinations of architecture units are defined as Type CH1,Type CH2,Type CH3,and Type CH4 by their contact relations.The 4 lateral combinations of composite channel architecture units are described via flattening on top of the composite channel deposits and well-to-well correlation in four development blocks.Following well-to-seismic tie and 90° phase transformation,we count the key locations of the composite channel growth,and study the architecture evolution of the Quan 4 member reservoirs in southern Sanzhao Sag by extracting volume attribute of time-frequency in three primary color display.The results show that Type CH1 is composed of 3-7 GB-DA in vertical accretion;Type CH2 is composed of 2-5 GB-SL-OF or SB-SL-OF in lateral accretion; Type CH3 is composed of 2-4 SB-SL-OF in lateral accretion; and Type CH4 is composed of 1-3 SB-DA-LS in vertical accretion.The fourth-order architecture interfaces of Types CH1 or CH4 horizontally extend forward,whereas those of Types CH2 or CH3 extend a short distance with low-angle inclination.In addition,from Q4-sq1 to Q4-sq4 periods,the architecture unit assemblages in the channel reservoirs evolve in a pattern of Type CH1 -Type CH2 -Type CH3 -Type CH4 as the lake level keeps rising.

Characteristics of natural fractures and their influencing factors in the paleo-buried-hill reservoirs of the Western Sag in the Liaohe Basin,China

Liu Guoping, Dong Shaoqun, Li Hongnan, Peng Shimi, Guan Cong, Mao Zhe

2020, 41(3):  525-533.  doi:10.11743/ogg20200309

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The paleo-buried-hill reservoirs are important targets for hydrocarbon exploration in the Western Sag of Liaohe Basin,China.The heterogeneity in natural fracture development is one of the main factors affecting the hydrocarbon enrichment and single well productivity in the area.Based on the analysis of analogous outcrops,drilling cores,thin sections,scanning electron microscopy (SEM) and image logging data,we study the types and developmental characteristics of natural fractures therein,and discuss the main factors affecting the development of natural fractures in the area.According to the geological genesis,natural fractures in these buried hills can be divided into 3 groups,namely tectonic,weathering,and dissolution fractures with the dominant orientation of NNE-SSW strikes.Thin section and SEM observation indicates that mineral intercrystalline fractures,cleavage fractures,cracks and mechanical fractures were developed in these reservoirs.Among others,mineral intercrystalline fractures have the largest aperture and scale with an aperture of 20-40 μm and the scale unlimited by mineral particles.The heterogeneity of natural fractures is controlled by tectonic movements,lithology,and diagenesis of the paleo-buried-hill reservoirs.The Caledonian,Indosinian and Yanshan movements are the main periods of natural fracture development.The granitoids in the metamorphic rocks,dolomites in the carbonate rocks and medium-acid rocks in the volcanic rocks are advantageous to the development of natural fractures.The supergene weathering,dissolution,and metamorphism are the constructive diageneses for natural fracture development,while the filling and metasomatic dissolusion are the destructive ones.The results provide a sound geological basis for the exploration and development of the paleo-buried-hill reservoirs in the Western Sag of Liaohe Basin.

Fracture network in the low-permeability fault block reservoirs in deep-buried Gaoshangpu oilfield,Bohai Bay Basin,and its controlling factors

Wang Zhaosheng, Dong Shaoqun, Meng Ningning, Liu Daojie, Gao Wei

2020, 41(3):  534-542,626.  doi:10.11743/ogg20200310

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The deep-buried,low-permeability fault block reservoir has considerable petroleum resource potential in Gao-shangpu oilfield,Bohai Bay Basin.The fracture network composed of hydraulic fractures and natural fractures controls the seepage performance of reservoirs after hydraulic fracturing.Analysis of the fracture network and its controlling factors is of great significance to developing the reservoir fracturing scheme and improving the development performance.In the study,an integration of core observation,imaging logging,stratigraphic dip logging and micro seismic monitoring data is applied to describe the development characteristics of natural and hydraulic fractures,as well in-situ stress,and to reveal the distribution pattern and major controlling factors of the fracture network in the study area.Results show that the reservoirs herein feature well developed natural fractures with high-angle shear fractures as the primary type,and the natural fractures were mainly developed under tectonic events during the Paleogene and the Late Neogene.The fractures therein can be divided into 3 groups,striking to NEE-SWW,NW-SE and near E-W respectively,with the first group,NEE-SWW-striking fractures being dominant.The dominant direction of maximum horizontal principal stress is N75°-85°E at present.Consequently,the NEE-SWW-trending natural fractures have the biggest aperture and the highest permeability under in-situ stress,acting as the main seepage channels.Hydraulic fractures are approximately vertical,with a primary orientation of N80°-90°E,as well as an average length and height of 136 m and 17.4 m,respectively.The distribution pattern of fracture network in the deep-buried fault block reservoir in Gaoshangpu oilfield is controlled by the coupling of hydraulic fractures with in-situ stress and natural fractures,while the controlling factors of fracture network growth are natural fractures,in-situ stress,and faults.

Control of fault-related folds on fracture development in Kuqa Depression,Tarim Basin

Feng Jianwei, Sun Jianfang, Zhang Yajun, Dai Junsheng, Wei Hehua, Quan Lianshun, Ren Qiqiang, Zhao Libin

2020, 41(3):  543-557.  doi:10.11743/ogg20200311

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The fracture prediction has always been one of the hotspots and difficulties in the petroleum geology study,and the key reason lies in the ambiguous understanding on the main controlling factors and mechanisms of fracture development.In the multi-stage evolution process,a large number of structural fractures,complicated in spatial distribution,were induced from or associated with the fault-related folds in the Kuqa Depression.The study takes the Keshen,Dabei,Dina and Kela gas fields as examples,and makes a detailed statistical analysis of fractures in a single well via outcrop and core observation and imaging logging analysis combined with the regional tectonic setting.The fracture development stages are determined and the paleo-tectonic stress field is restored by structural trace analysis and rock acoustic emission test.Guided by the fault-related fold theories,we select the Kuqa river outcrop to restore the tectonic evolution process of each fold,and compare and analyze the development pattern and formation mechanisms of paragenetic fractures through finite element numerical simulation.Finally,the evolutionary geological model of the paragenetic fracture system in the relevant folded region of Kuqa Depression is established through "dynamic and static analyses",and the dominant fracture deve-lopment zone is assessed.The results show that the Keshen-Dabei gas field is in a typical pattern of paragenetic fractures with folds of top pop-up type,and the Dina-Kela gas field is in a typical paragenetic fracture pattern of top-graben type,both of which provide effective guidance for efficient exploration well emplacement of the piedmont thrust belt in Kuqa Depression and comprehensive management of old oil/gas fields.

Impact of hydrothermal activities on reservoir formation controlled by both faults and sequences boundaries: A case study from the Lower Ordovician in Tahe and Yubei areas,Tarim Basin

Sun Funing, Hu Wenxuan, Hu Zhongya, Liu Yongli, Kang Xun, Zhu Feng

2020, 41(3):  558-575.  doi:10.11743/ogg20200312

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The Lower Paleozoic carbonate rocks in the Tarim Basin are widely developed,representing an important target of hydrocarbon exploration.Taking the Lower Ordovician in Tahe and Yubei areas in the Tarim Basin as examples,we discussed the impact of hydrothermal activities on reservoir formation in the fault belts and along stratigraphic framework,through core and thin section observation,electron probe microanalysis (EPMA),seismic and logging data,as well as the analyses of C,O,and Sr isotopes and REEs.The results show that medium to fine-crystalline dolomites are strongly modified by silica-rich hydrothermal fluids.A series of characteristics occurred as a result of this process,such as:thermal fading,saddle dolomite filling,and mineral precipitation including dolomite,calcite,chalcedony,pyrite and so on,as well as higher SiO₂,FeO,MnO and BaO contents,lower SrO contents and ⁸⁷Sr/⁸⁶Sr values,more negative δ¹⁸O values,lower ∑REE contents,obvious positive Euanomalies and negative Ce anomalies.While the dolomite reservoir transformation resulted from hydrothermal fluids mainly includes dissolution,recrystallization and filling,the secondary pores with dissolution serve as the most important reservoir space.The conduit system for deep hydrothermal fluid migration is composed of faults of various scales,third-order sequences boundaries,or sequence boundaries of even shorter periods,playing a deci-sive role in controlling hydrothermal activities and reservoir development.Finally,a conceptual model displaying the impact of hydrothermal activities on reservoir formation was established under the dual control of faults and sequences boundaries in the study area.

Microbial characteristics of Shunbei faulted-karst reservoirs and prediction of play fairways,Tarim Basin

Yan Liang, Ji Miao, Jia Baoqian, Chen Xiaotong, Gao Ping

2020, 41(3):  576-585.  doi:10.11743/ogg20200313

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Microbial prospecting of oil and gas (MPOG) is drawing more and more attention of explorationists due to its advantages including low cost,immediacy and high efficiency.Taking the 3D seismic acreage of northern Shun 8 block,Shunbei area,Tarim Basin as the research object,we carried out researches on abnormal characteristics of hydrocarbon-degrading microorganisms,mainly adopting improved plate culture counting method,16S rRNA gene cloning library me-thod and high-throughput sequencing method to quantitatively detect hydrocarbon-degrading microorganisms in soil samples.The former serves to identify the characteristics of microorganisms in respect of quantity in Shunbei area;while the latter two function to ascertain the abnormalities of hydrocarbon-degrading microorganisms in respect of types therein.According to the obtained results,we built models for the abnormal quantity and types of microorganism in the known hydrocarbon reservoirs in Shunbei-Yuecan area,and in turn identified 4 abnormal zones of microbial quantity in 3D seismic acreage of northern Shun 8 block,which are considered to the potential exploration target zones(abnormal zones).

Major factors controlling waterflooding-induced fracture development in low-permeability reservoirs—A case study of Chang 6 reservoir in W block in Ansai oilfield,Ordos Basin

Zhao Xiangyuan, Lyu Wenya, Wang Ce, Zhu Shengju, Fan Jianming

2020, 41(3):  586-595.  doi:10.11743/ogg20200314

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Waterflooding-induced fractures are the by-products during long-term water-injection in low-permeability reservoirs,seriously affecting the performance of oil reservoir.We study the major factors controlling the formation and development of these factures in Chang 6 reservoir in Ansai oilfield,Ordos Basin,using geological,logging,testing and production performance data.The formation of these fractures is mainly controlled by both geological and engineering factors.The geological factors refer to natural fractures,current in-situ stress,reservoir architecture and rock mechanical properties,and the engineering factors mainly include artificial fractures and injection-production parameters (such as injection time and volume,injection-production ratio or injection-production volume,etc.).Natural and artificial fractures serve as physical stimuli to the formation of waterflooding-induced fractures; current in-situ stress controls the dominant direction of the fractures,and indirectly affects their propagation; architecture units and architecture interfaces of various levels control the scale and propagation extent respectively; reservoir rock mechanical parameters constrain,to a certain extent,the propagation routes; and injection-production parameters control the formation and propagation speed to the most.

Characteristics and genesis of bedding-parallel fractures in tight sandstone reservoirs of Chang 7 oil layer,Ordos Basin

Ju Wei, You Yuan, Feng Shengbin, Xu Haoran, Zhang Xiaoli, Wang Shengyu

2020, 41(3):  596-605.  doi:10.11743/ogg20200315

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The development degree and type of natural fractures are key to obtaining high and stable yield in tight sandstone reservoirs.Currently,tectonic fractures have been highlighted in exploration and development of tight sandstone reservoirs in the Yanchang Formation,Ordos Basin,whereas researches on bedding-parallel fractures are rarely seen.However,recent exploration and development practices indicate that bedding-parallel fractures widely developed in tight reservoirs have great effects on hydrocarbon accumulation.In this study,characteristics and genetic mechanism of bedding-parallel fractures in Chang 7 oil layer,Ordos Basin are intensively analyzed based on outcrop,core and thin section observation.The results indicate that the bedding-parallel fractures are commonly in low dip angles or sub-horizontal with small aperture.They are mostly discontinuous and short horizontally and isolated vertically,featuring strong heterogeneity in distribution,but can be cut by tectonic fractures.Oil immersion and stains are common within bedding-parallel fractures of drill cores.It is considered that the bedding-parallel fractures of high density in Chang 7 oil layer were generated in large scale since the Late Yanshanian Movement,and their distribution is jointly controlled by multiple geological factors,including fluid pressure,tectonic stress and dissolution.The accumulation effect of bedding-parallel fractures therein is of geological implication to tight oil exploration and development in the Ordos Basin.The differential development of bedding-parallel fractures is an important factor for the heterogeneity of tight oil distribution and large variability of single well productivity in the Ordos Basin.In all,regions of well-developed bedding-parallel fractures may be the "sweet spots" for tight oil exploration and development.

Characteristics and genetic development of a low-angle boundary normal fault in Xijiang Main Sag,Pearl River Mouth Basin,China

Deng Peng, Mei Lianfu, Du Jiayuan, Ye Qing, Xu Xinming, Wu Jing, Dong Xiaoyun

2020, 41(3):  606-616.  doi:10.11743/ogg20200316

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The Xijiang Main Sag,located in the west-central Zhu-1 Depression,is one of the most important hydrocarbon-rich sags in the Pearl River Mouth Basin,China.To recognize the properties of the boundary fault (Fxj33) in the Xijiang Main Sag,we comprehensively analyze the geometry and kinematics of Fxj33,using high-resolution 3-D seismic data under the constraints of well data.The results show that the boundary fault Fxj33 of about 55 km long and up to 9.5 km displacement at most,is a NEE-trending low-angle normal fault with a dip of 5°-36°.During the evolution of multistage rifting,the fault Fxj33 is characterized by differential activities of different segments along the strike,and jointly controls the evolution of the sub-sags in the Xjiang Main Sag together with the two lines of EW-trending high-angle normal faults.Various structures developed in the Mesozoic basement,including imbricate structures,residual hills and pre-existing fault planes,indicate that the boundary fault Fxj33 was developed by the inversion of the pre-existing Mesozoic thrusting system in the Cenozoic,and subsequently reformed by the EW-trending high-angle fault planes cutting into it during the middle-to-late rifting stage.The research results are expected to be of referential meaning to the study of the boundary faults during rifting in passive margin faulted basins.

Effectiveness of natural fractures in CBM reservoirs and its influence on CBM development in the southern Qinshui Basin

Shi Jinxiong, Zeng Lianbo, Tan Qingsong, Wang Jipeng, Zhang Yunzhao, Li Hongwei

2020, 41(3):  617-626.  doi:10.11743/ogg20200317

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Natural fractures were well developed in coalbed mathane (CBM) reservoirs in the southern Qinshui Basin,and their effectiveness is an important factor affecting the seepage ability of CBM reservoirs and the high production of CBM wells.In this study,the types and development characteristics of natural fractures in the Upper Paleozoic CBM re-servoirs,southern Qinshui Basin,are investigated by using outcrop,core and scanning electron microscopy (SEM) data.Then,the fracture effectiveness and its main controlling factors are analyzed.Finally,the influence of effective fractures on CBM development is discussed in combination with single-well dynamic production performance.The results show that cleats and tectonic fractures were mainly developed in the study area.The cleats include face and butt cleats,and the tectonic fractures include shear and tension fractures.There are marked differences in the fracture effectiveness of various types under the influence of multiple geological factors.The cleats were formed at the stage of coalification and mostly filled or semi-filled by calcites and clay minerals,resulting in poor effectiveness of fractures.Most tectonic fractures were not filled with minerals,and their effectiveness is mainly controlled by fracture size,dip angle,reservoir depth and their included angle with the direction of the in-situ maximum principal stress at present.The tectonic fractures feature large aperture,longer propagation and good fracture effectiveness.High-angle tectonic fractures are more effective than oblique fractures.Affected by the change of in-situ stress state,the fracture effectiveness varies systematically with the increasing burial depth,i.e.,from fair to good and then fair again.The NE-SW-trending fractures are the most effective,followed by nearly EW-trending and nearly NS-trending fractures,and the NW-SE-trending fractures coming at last.The tectonic fractures,better in terms of both scale and fracture effectiveness compared with the cleats,are the main seepage and production channels of coalbeds,and play a positive role in the CBM development.However,over-sized tectonic fractures,especially when cutting through the coal seam and extending into the roof and floor,will result in the loss of CBM,the decrease of reservoir pressure,and the incursion of external formation water.Therefore,it is not conducive to the preservation and effective exploitation of CBM.

Methods and Technologies

An approach to 3D geological modeling of multi-scaled fractures in tight sandstone reservoirs

Dong Shaoqun, Lyu Wenya, Xia Dongling, Wang Shijia, Du Xiangyi, Wang Tao, Wu Yue, Guan Cong

2020, 41(3):  627-637.  doi:10.11743/ogg20200318

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The fracture system in a tight sandstone reservoir is composed of multi-scaled fractures,Fractures of each scale play a different role in oil and gas recovery.Multi-scaled fracture modeling is crucial for the development of tight reservoirs.However,there are still problems in multi-scaled fracture modeling in terms of multi-scaled fractures' coupling,determination of simulation termination conditions,and efficiency improvement of constrained modeling algorithm.To solve these problems,we discuss the three-dimensional (3D) geological modeling of multi-scaled fractures in tight sandstone reservoirs in H oilfield,Ordos Basin,based on common fracture network modeling,with a view to improving the efficiency and accuracy of multi-scaled fracture modeling.The proposed multi-scaled fracture modeling method in the study includes 4 steps,namely identification of large-scaled fractures with a deterministic method,multi-scaled coupling through distribution function matching for determining the parameters required for small-scaled fracture modeling,generation of medium- and small-scaled fracture networks by an improved method of discrete fracture network modeling constrained by fracture density,and superimposition of multi-scaled fracture networks.To verify the efficiency of the proposed method,we compare the built fracture model with single-well fracture interpretation,production data,etc.in the study area,and the results indicate that the multi-scaled fracture model established by the proposed method coincides with actual geological understanding.

Numerical simulation on tight oil reservoir reverse imbibition with boundary layers

Xu Zhongyi, Fang Sidong

2020, 41(3):  638-646.  doi:10.11743/ogg20200319

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Multi-stage volume fracturing of horizontal wells is common in the development of tight oil reservoirs,where the matrix near wellbore is fully cut by and in large-scale contact with the fracture network,resulting in reverse imbibition of fracturing fluid or water into the matrix through the fracture network.Therefore,the reverse imbibition effect shouldn't be ignored in both fracturing and water-flooding.In addition,due to the difference of flow pattern between tight and ordinary reservoirs,the factors contributing to the reverse imbibition effects are accordingly different in the two distinct reservoirs.To accurately simulate the imbibition in fractured tight oil reservoirs,we apply the Dissipative Particle Dynamic(DPD)method to verify the existence of boundary layers during the imbibition in nano-scale pore throats in tight reservoirs,and discover the variation pattern of the boundary layer thickness;then calculate the critical seepage parameters with boundary layers considered via the pore network model,mainly including permeability,capillary pressure curve and relative permeability curve;and finally incorporate these parameters into the numerical simulation model to characterize complex fracture networks in tight reservoirs.The simulation results show that the boundary layers serve to greatly reduce imbibitions-the imbibition recovery in tight reservoirs with boundary layers is only 3/8 of that without considering imbibition.In conclusion,the boundary layer in tight reservoirs deserves great attention;otherwise,the flow potential of these reservoirs may be overestimated.

Dynamic evaluation of well and reservoir parameters on large-scale karst caverns encountered in drilling

Yu Qingyan, Hu Xiangyang, Li Yong, Jia Ying

2020, 41(3):  647-654.  doi:10.11743/ogg20200320

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The carbonate oil reservoirs of fractures and cavities are diverse in types with large-scale karst caverns,dissolved pores,and internal fractures of various scales.The reservoirs are characterized by extreme irregularity in morphology,large differences in scale,poor connectivity,resulting in extremely strong heterogeneity.Among others,large-scale karst caverns are the main reservoir type;fractures are the major seepage channels;and the contacts between oil and water layers are complicated.Currently,the static sculpturing of fractured-vuggy unit based on seismic interpretation usually results in large inconsistency in assessment of oil in place,and cannot be applied to evaluate other oil and reservoir parameters.To improve the accuracy in assessment,we propose a new method for the assessment of oil in place and other reservoir parameters by dynamic data based on actual production data and dynamic production analysis.In the new method,large-scale karst cavern reservoirs is simplified as a composite model of multi-porosity media with an integration of large-scale karst caverns and surrounding small-scale pores,cavities and cracks.The fluid flow in large-scale karst caverns is regarded as free flow,while that in the surrounding multi-porosity media is equivalent to Darcy flow,and then the corres-ponding mathematical model is built in coupling Darcy flow with free flow.Afterwards,the dynamic production curves and the parameter sensitivity analysis are obtained by solving the mathematical equation.Finally,the cross-plot of normalized Blasingame typical curves is built,and the fitting of the plot with actual production data solves well and reservoir parameters.The field application to the Tarim oilfield verifies the reliability and practicability of the newly-proposed method.And the parameters acquired can be applied to the assessment of dynamic oil production and to guide more reasonable infill well emplacement and recovery enhancement.