Table of Content

28 August 2018, Volume 39 Issue 4

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Comparative analysis of characteristics and controlling factors of fractures in marine and continental shales: A case study of the Lower Cambrian in Cengong area,northern Guizhou Province

Wang Ruyue, Hu Zongquan, Liu Jingshou, Wang Xinghua, Gong Dajian, Yang Tao

2018, 39(4):  631-640.  doi:10.11743/ogg20180401

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The study analyzed the shale fracture characteristics,controlling factors and differences of fractures developed in marine and continental shales in the second Member of the Lower Cambrian Bianmachong Formation(C₁b²) based on fracture observation and experimental data.The results show that the C₁b² shales show certain features of both marine and continental shales.Its fractures are dominated by fractures at low angle with bedding,interlayer and slip fractures,which are similar to those in continental shales.The fractures in sandstone-shale alternating intervals and sandstone intervals are highly developed,with the high dip-angle structural fractures being dominant.The controlling factors of fracture development are mainly represented as follows:lithology and rock mechanical property control structural fracture development; stratum thickness controls the density and scale of fractures; TOC influences the development of interlayer overpressure fractures; and reservoir heterogeneity controls the development of bedding-parallel fractures.In addition,the TOC,quartz content,brittleness and fracture have different coupling relationships in marine and continental shales.Although the greater brittleness and single layer thickness of marine shale are favorable for reservoir stimulation,cross-layer fractures may form easily under strong deformation,thus are unfavorable for hydrocarbon preservation.However,continental shales have lower single layer thickness,stronger ductility and heterogeneity.They can effectively prevent the development of cross-layer fractures,thus can facilitate the preservation of shale gas.In general,the different lithologic combinations and their corresponding characteristics and controlling factors of fracture development in the C₁b² are of referential value in the aspects of gas preservation,enrichment and reservoir stimulation in marine and continental shale gas.

Mechanism and distribution prediction of abnormal high pressure of the Paleocene Shahejie Formation in Linnan Sag,Huimin Depression

Wang Bing, Zhang Likuan, Li Chao, Chen Kaiyuan, Song Guoqi, Luo Hongmei

2018, 39(4):  641-652.  doi:10.11743/ogg20180402

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In the process of oil and gas exploration,various abnormal high pressures were tested in the Paleogene Shahejie Formation in Linnan Sag,Huimin Depression.The uncertainty of overpressure origin would reduce the reliability of pressure prediction.This study elaborated both the characteristics of fluid pressure in porous sandstones and the well log responses of overpressured mudstones through such data as drill stem tests (DSTs),mud density and logging. Besides,based on acoustic and density logs in normal and abnormal high pressured intervals,we established an effective stress-log response plot to deduce the origin of overpressure in the Shahejie Formation,and predicted the distribution of abnormal pressures in the study area.The results show that overpressures in the sandstone reservoirs of the Shahejie Formation mainly occur in the 3^rd (Es³) and 4^th (Es⁴)members of the formation with a depth over 3 000 m,and their maximum excessive pressures are 23.82 MPa and 14.04 MPa,respectively.The logging responses of overpressured mudstones typify undercompaction,usually exhibiting abnormally high interval transit time,abnormally low density,and high neutron poro-sity.Overpressures in the formation mainly originate from overpressure transmission among neighboring mudstones induced by disequilibrium compaction.Most overpressure data points follow the trend of loading curves,and only the overpressure data points in local deep depression zones(with burial depth being more than 4 000-4 300 m)follow unloading curves, possibly due to the pressure increase with hydrocarbon generation resulting from a higher maturity (R_o is within 0.9%-1.05%)of the deeply buried organic-rich mudstones.Besides,fluid pressures calculated with equilibrium depth method match well with the pressure data obtained from DSTs. This verifies the conclusion that the abnormal high pressures in the formation are fundamentally caused by disequilibrium compaction. The excessive pressures of the Es³ show a ring-like distribution surrounding the core of the sag and progressively lower towards and become normal in its neighboring slopes and uplifts. These research results will be of valuable guidance for pre-drilling pressure assessment in Linnan Sag.

Control of tectonics on sedimentation of sandstone and process of sediment filling in multi-fault lacustrine basins: A case study on the Eocene in eastern Zhanhua Sag,Jiyang Depression in Bohai Bay Basin

Dong Daotao, Qiu Longwei, MA Yongda, Yang Yongqiang, Zou Yu, Dai Li, Teng Baogang, Zhang Zaipeng

2018, 39(4):  653-663.  doi:10.11743/ogg20180403

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Extensive research has been carried out on the control of tectonics on sedimentation of sandstone in rift basins.The tectonic-dominated mechanism controlling sandstone development and the process of sediment filling for the upper 4^th-3^rd member of the Eocene Shahejie Formation (the Es⁴ and the Es³) in eastern Zhanhua Sag,Jiyang Depression in Bohai Bay Basin,were investigated through well-seismic correlation,core description and thin section observation.The results show that (1) level-Ⅰ tectonic transition zones lay down the macroscopic tectonic-sedimentary framework,and multi-stage tectonic slope breaks and eustatic lake level control the sand body accretion of different system tracts in the third-order sequences and the cyclicity of sedimentary systems; while level-Ⅱ tectonic slope break zones and secondary faults control the local sand transport systems and sand body distribution within the system tracts,and steep slope zones are often infavorable for input of clastics into lake basins;(2) SQ1 and SQ2 have two isolated sand transport systems (Gubei and Gunan),while SQ3 has a single sand transport system with the Gubei and Gunan being combined into one,so small but numerous fan deltas without significant progradation or retrogradation are mainly developed in SQ1,while deltas,(semi-)deep lake and gravity flow sand bodies occur in SQ2,and large-scale deltas are dominant in SQ3,with gravity flow sand bodies being common in the (semi-)deep lake area; (3) Tectonic activities control,via tectonic transition zones,slope breaks,secondary faults and steep slopes,sandstone development and sediment filling process in multi-fault lacustrine basins,whereas paleoclimate can only function to strengthen or weaken the controlling effect of tectonic activities,but will not change the sediment filling process in the basin.

Abnormal pressure system and its origin in the Nanpu Sag,Bohai Bay Basin

Zhang Lei, Xiang Caifu, Dong Yuexia, Zhang Mengyuan, Lyu Yue, Zhao Zhongxin, Long Huashan, Chen Shuang

2018, 39(4):  664-675.  doi:10.11743/ogg20180404

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Abnormal pressure in the Nanpu Sag is prevalent,but its distribution in varying tectonic zones and its genetic mechanisms are rarely studied.So we depicted the architectural features of the pressure for varying tectonic zones in detail based on the data from 1 354 drill stem tests(DST),repeated formation pressure tests(RFT),and the well logging curves of 357 wells.The results show that vertically,the formation pressure system can be classified into three zones in the Nanpu sag,namely,the upper normal pressure zone(with burial depth less than 1 800 m),the middle transitional zone(with burial depth ranging from 1800m to 2 400 m),and the lower abnormal high pressure zone(with burial depth more than 2 400 m).Large part of the Shahejie Member 3 is controlled by significant abnormal overpressure with pressure coefficient up to 1.9,and the depth from ground to the top of the overpressure zone is about 2 400 m.The middle and upper layers of the Laoyemiao Formation overpressure is less intense with pressure coefficient about 1.2.However,the Dongying Member 1 and 2 offshore,develops abnormal low pressure in some area.In order to further discuss the abnormal pressure mechanisms in the Nanpu Sag,we resorted to methods such as numerical simulation,vertical effective stress-acoustic time cross-plot,and comprehensive analysis of hydrocarbon generation and expulsion for source rocks.Consequently,we propose four conclusions:(1)the large-scale overpressure in Es³ mainly originates from the sandstone disequilibrium compaction during the Dongying period,and contribution from hydrocarbon generation is relatively minor;(2)in Minghuazhen period,hydrocarbon generation becomes the main mechanism for generating pressure,while the influence from disequilibrium compaction is rather limited;(3)the lower-degree overpressure of the middle and upper layers is thought to be induced by the vertical fluid migration from the deep overpressured formations through active fault zone;(4)the regional uplifting and denudation during the late Dongying period result in pore rebound of rock matrix and fluid shrinkage,which are considered to be the main cause of the abnormal low pressure in the Nanpu sag.

Origin and accumulation characterization of petroleum in buried hill reservoirs in Shaxibei area,Bohai Sea

Wang Qi, Hao Fang, Xu Changgui, Wei Ajuan, Sun Zhongheng, Cao Yijun, Zou Huayao

2018, 39(4):  676-684.  doi:10.11743/ogg20180405

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The study discussed the origin and accumulation characteristics of hydrocarbon in buried hills in Shaxibei area,Bohai Sea,through geological and geochemical research methods.Analysis of biomarker parameters reveals that the proved buried hill oil samples in Shaxibei area originate from the Shahejie Formation source rocks.Oil samples from the CFD2-1 buried hill carbonate reservoir have a high G/H index and relatively low 4MSI,and are characterized by depleted ¹³C for n-alkanes(less than -29‰ as a whole).Such characteristics may be an indicator of the oil origin from the Es¹ source rocks.Oil samples, taken from the granite reservoir of the CFD1-6 oil fields,are typified by relatively higher 4MSI (more than 0.4) and heavier carbon isotope of n-alkanes (more than -29‰), which may indicate that CFD1-6 buried hill oil is dominated by the Es³-derived oil. The CFD2-1 buried hill is overlain by the Es¹ formation unit,and the mature Es¹ source rock may supply adequate petroleum for charging the carbonate reservoirs without long migration distance.This accumulation model can be referred to as the assemblage of generation from the draped source rock and vertically charging to the underlying carbonate reservoir.CFD1-6 granite buried hill is covered by the Ed shale,being characterized by low organic matter abundance and thermal maturity,which can be served as a seal for oil accumulation.Oil generated from the source rock in the Qikou Sag can be transported through the laterally distributed sand body and charge the CFD1-6 buried hill traps,exemplifying a hydrocarbon accumulation model of migration through lateral sand body.On the basis of core observation and casting thin section experiments,we analyzed the textural characteristics of weathered crust from Well CFD2-1-2.The results show that the weathered crust structure in Shaxibei area may not be able to receive the laterally transporting petroleum from sags to buried hill traps,while the overlying effective source rocks on the surface of buried hills or the occurrence of sand bodies communicating source and reservoir are fundamental for hydrocarbon accumulation in buried hills.

Characterization and formation of the Ordovician tight paleokarst carbonates in the eastern Ordos Basin and its gas accumulation

Wang Guoting, Cheng Lihua, Meng Dewei, Zhu Yujie, Sun Jianwei, Huang Jinxiu, Peng Yanxia

2018, 39(4):  685-695.  doi:10.11743/ogg20180406

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The Ordovician palaeokarst carbonate reservoirs in the eastern Ordos Basin are poorly understood at present.Thus it will be fundamental for discovering gas reserves and evaluating its developmental prospects to analyze the formation mechanism of the palaeokarst reservoirs and the gas accumulation potentials of the Ordovician carbonates.We elaborated the reservoir characteristics,controlling factors for the reservoir formation,major controlling factors of gas accumulation and storage capacity of the Ordovician carbonates in the study area.The conclusion is that in the eastern Ordos Basin,the weathered crust reservoirs of carbonates are well developed,gas source is sufficient and the timing of source,migration and reservoir is good,so the area is favorable for large-scale gas accumulation. In terms of the reservoir characte-rization,it is featured by low porosity and low permeability as a whole,with the porosity of 3% and permeability of 0.05×10^-3 μm² being regarded as the lower limit of physical property for effective reservoir,and with the pore diameter of 30 μm and throat diameter of 5 μm regarded as the lower limit of reservoir pore-throat sizes.The generation of high-quality carbonate paleokarst reservoir is mainly effected by favorable sedimentary microfacies,effective karstification,and integrated diagenesi,etc.Dolomite with anhydrite concretions of half-filling type is the most important high-quality reservoirs.The controlling factors for gas accumulation are summarized as follows:① the development of high-quality reservoirs lays a substantial basis for gas accumulation;② the favorable timing of source,migration and reservoir is the key.In general,the Lower Paleozoic carbonate karst reservoirs in the eastern Ordos Basin are of great potential for exploration.

Characteristics and formation mechanisms of waterflood induced fractures in low-permeability reservoirs: A case study from Chang 6 reservoir in Ansai oilfield,Ordos Basin

Zhao Xiangyuan, Zeng Lianbo, Jin Baoguang, Wang Ce, Li Shujun

2018, 39(4):  696-705.  doi:10.11743/ogg20180407

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The study took Chang 6 reservoir in Ansai oilfield of Ordos Basin as an object to demostrate the essential ch-aracteristics and the formation mechanisms of waterflood induced fractures,based on the analysis of dynamic production features of the water-injecting lowpermeability reservoir.And the main formation mechanisms of waterflood induced fractures in the study area were numerically simulated in the end.Waterflood induced fracture is an injector-centered open fracture or a fast flow channel,which is formed when the water injection pressure exceeds the opening pressure of various fractures or the formation fracture pressure during the long-period waterflooding of low-permeability reservoirs.The fractures are an artificial geological attribute and the main cause of heterogeneity for waterflood development of low-permeability reservoirs,and they are widely occurring and inevitable in these reservoirs.Three formation mechanisms of waterflood induced fractures were summarized as follows:(1) when the water injection pressure exceeds the opening pressure of natural fractures,the natural fractures become open and are extended; (2) When the water injection pressure exceeds the formation fracture pressure,new fractures are developed throughout the injection life; (3) Other types of artificial fractures are generated by production or stimulation measures such as perforating and fracturing around the injector.In Chang 6 reservoir,high angle structural shearing fractures,in en echelon arrangement,are widespread approximately along the direction of major principal stress,and the dominate fractures are the first type of waterflood induced fractures.According to numerical simulation results,the bottom hole pressure of the injeciton well exhibits a continuous irregular periodic change with the continuous expansion of the waterflood induced fractures.

Modification of dolomitization on pores in oolitic shoal reservoirs of the Feixianguan Formation in the northeastern Sichuan Basin

Li Kaikai, Zhang Xuefeng, He Xunyun, Fan Junjia

2018, 39(4):  706-718.  doi:10.11743/ogg20180408

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A progressive increase in dolomite content over a certain range is usually accompanied by an increase in poro-sity of dolostone.It is challenging to clarify whether dolomitization is capable of creating new pores or not,and this deserves special attention.We assessed this process in the oolitic shoal reservoirs in the Lower Triassic Feixianguan Formation in the northeastern Sichuan Basin,China,through petrographic observations and comparisons of petrologic and mine-ralogical features of the limestones and dolostones.The results show that the content of intergranular calcite cement of phase 3,the product of pressure solution and re-precipitation in oolite,continuously decreases with the increase of dolomite content after the content of dolomite exceeds 40%.Then the intergranular pore space,which was supposedly occupied by the calcite cements in the precursor lithologies,would have increased and developed into the present porosity.This observation explains well the trend of porosity increasing with dolomite content.That is,rather than creating new pores,dolomitization in the oolitic grainstone layers was more likely to have helped to preserve the early pores.We also found that the variation of petrophysical properties of the dolostones in different parts of the present oolitic shoal bodies originated from the differential evolution of dolostone fabric,which was initiated by the difference in lithologies and their distribution patterns in one depositional cycle.The effects of eogenetic freshwater infiltration on oolitic fabric,mineral composition and stability have exerted substantial influence on preservation of the original textures of the oolites.Recrystallization and deep buried dolomite dissolution further obliterated the oolites and the original fabrics of the precursor lithologies,resulting in the final crystallized dolomites.

Characteristics and effectiveness of structural fractures in ultra-deep tight sandstone reservoir: A case study of Keshen-8 gas pool in Kuqa Depression,Tarim Basin

Wang Ke, Yang Haijun, Zhang Huiliang, Li Yong, Zhang Ronghu, Yang Xuejun, Wang Junpeng

2018, 39(4):  719-729.  doi:10.11743/ogg20180409

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Characteristics analysis and effectiveness evaluation of structural fractures are of great significance to predict favorable tight sandstone reservoir and to select development strategy in Keshen-8 gas pool,Kuqa Depression,Tarim Basin.We analyzed the basic characteristics and forming sequence of the structural fractures,and the factors influencing their growth through such data as core,image logging and single well productivity.In the end,the planar and vertical effectiveness of structural fractures was evaluated,and some suggestions for development were given.The results show that (1) the structural fractures,partly filled with anhydrites,dolomites and seepage sand particles,are mainly upright or have a high dip angle.The third-phase structural fractures developed during the deposition of the Pliocene Kuqa Formation are the most important ones for gas preservation in Keshen-8 gas pool.(2) Faults,lithology,stratum thickness and sedimentary microfacies function to control the development of structural fractures.Structural fractures on the upper part of anticline are highly effective,but their effectiveness would decrease dramatically when their dominant orientation intersects with the direction of present maximum principal stress at a large angle.(3) The controlling effect of unconformity on structural fracture effectiveness is limited within 70 m to the unconformity.The Bashijiqike Formation reservoirs in Keshen-8 gas pool should be produced primarily through hydraulic fracturing,and top priority should be given to the 3^rd sand unit in development,followed by the 4^th sand unit and its underlying reservoirs,and the 1^st and 2^nd sand units being the last ones.

Effect of TSR on the crude oil in Ordovician reservoirs of Well Luosi-2 from Maigaiti Slope,Tarim Basin: Evidences from molecular markers

Ma Anlai, Jin Zhijun, Zhu Cuishan, Gu Yi, Li Huili, Lu Qinghua

2018, 39(4):  730-737,748.  doi:10.11743/ogg20180410

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The study made a quantitative analysis of diamondoid, dibenzothiophenes and thiadiamondoid series in the crude oil from Well Luosi-2 via the GC-MS method with internal standards.The concentrations of diamondoid and 4-+3-methyldiamantane(4-+3-MD)are 10818 and 331 g/g in the study area,suggesting that the crude oil suffered serious cracking and the proportion of cracking oil is up to about 90%.The diamondoid indexes show that the maturity of crude oil from Well Luosi-2 is above 1.6% R_c.Complete thiaadamantane,thiadiamantane,thiatriamantane series were detected from the Well Luosi-2 crude oil,with the contents of thiadiamondoid,thiaadamantane,thiadiamantane,thiatriamantane being 192,160,26 and 6 μg/g respectively.The high concentration of thiadiamondoid is a sign that the degree of thermochemical sulfate reduction(TSR)of oil from Well Luosi-2 is higher than that of most crude oil from the Lower Ordovician Yingshan Formation in Tazhong uplift.TSR results in the higher DBTs, about 8 201 μg/g,in the oil from Well Luosi-2, and in turn the higher ratios of DBT/P,C₀-/C₁-DBT and C₁-/C₂-DBT.

Fluid sensitivity evaluation of ultra-deep tight sandstone gas reservoirs,Tarim Basin

Kang Yili, Zhang Dujie, You Lijun, Wang Zhe, Tian Jian

2018, 39(4):  738-748.  doi:10.11743/ogg20180411

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It is critical to understand the fluid sensitivity of reservoir and its corresponding evaluation method to optimize the selection of working fluids in the wells and their properties.The fluid sensitivity,which is influenced by mineral composition,pore throat characteristics,high-temperature fluid environment,etc,refers to damage of ultra-deep tight sandstone reservoirs.So the conventional evaluation methods in the industry seem to be not applicable any more.The ultra-deep tight sandstone gas reservoirs located in Tarim Basin are taken as the objects in this paper,and a modified fluid sensitivity evaluation method is proposed,which requires the simulation of the formation temperature and maintaining high back pressure on the outlet throughout the experiments.Twelve typical ultra-deep tight sandstone base blocks are chosen to carry out water,salt and alkali sensitivity experiments.The results show that the index of water,salt and alkali sensitivity is 0.41-0.52,0.72-0.73 and 0.83-0.92,respectively.By comparison,the degree of damage derived through this method,which maches well with the field measured data,is higher than that obtained at the room temperature experiments in the past.After analysis the paper indicates that (1) the modified evaluation method could reflect the actual condition of reservoirs and also reduce experimental errors, (2) the narrow pore throat and abundant clay minerals are the fundamental causes of fluid sensitivity, (3) the major mechanisms for increased fluid sensitivity for ultra-deep tight sandstone gas reservoirs,can be summarized as follows:in higher temperature,the thickness of the water film on mineral surface increases,reducing the effective pore throats' radius,and the intensified clay hydration and swelling accelerate the expansion/migration of formation particles and the dissolution/deposition of minerals.

Milankovitch cycles in the Silurian Kepingtage Formation in Shuntuoguole area,Tarim Basin

Jia Dongli, Tian Jingchun, Lin Xiaobing, Yang Guohua, Feng Wenxin, Zhang Xiang, Tang Yan, Yang Chenyu

2018, 39(4):  749-758.  doi:10.11743/ogg20180412

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The Silurian Kepingtage Formation in the Tarim Basin is mainly deposited in the tidal flat environment during a relatively stable period of tectonic activity.It is one of the typical marine clastic sequences in the basin.And it is composed of frequently alternating and superimposing medium-fine sandstone and mudstone,and can be obviously divided into three intervals.According to previous research results,the Kepingtage Formation can be approximately correlated with the Telychian,Aeronian and Rhuddanian Stage of the International Stratigraphic Chart,and its deposition period should be within 8.1-12.7 Ma.However,its Milankovitch high-frequency stratigraphic cycle framework is rarely discussed.Ta-king the Gamma-Ray logs of Well Shun-9 as proxies and using spectral analysis method for computation,we found that the Kepingtage Formation in Well Shun-9 recorded the complete Milankovitch cycle.The astronomical cycle significantly controls the cycle thickness,with the cycle thickness of 16.73 m corresponding to the long eccentricity of 405 ka,the cycle thickness of 5.24 m and 3.92 m corresponding to the short eccentricity of 125 ka and 95 ka respectively,and the cycle thickness of 1.59 m and 1.26 m corresponding to the obliquity of 38.1 ka and 31.1 ka respectively.The average sedimentary rate was estimated as 4.15 cm/ka and the depositing time as 10.4 Ma.And a "floating" astronomical time scale was established for the Kepingtage Formation in Well Shun-9 based on Empirical Mode Decomposition(EMD).The Kepingtage Formation was found to have 83 cycles with short eccentricity of 125 ka and 334 cycles with obliquity of 38.1 ka.Finally,we analyzed the superposition differences and their causes of each high-frequency cycle by using the Fischer plots and a standard model for fifth-order cycle division under the control of the short eccentricity of 125 ka.This result provides a new method that is unified and unbiased for high-frequency cycle division of marine clastic strata in this basin and similar basins.

Identification of igneous reservoir lithology based on empirical mode decomposition and energy entropy classification: A case study of Carboniferous igneous reservoir in Chunfeng oilfield

Han Yujiao, Yuan Chao, Fan Yiren, Ge Xinmin, Fan Zhuoying, Yang Wenchao

2018, 39(4):  759-765.  doi:10.11743/ogg20180413

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Igneous reservoirs are characterized by diversity of eruption patterns and lithology geneses,and variation in mineralogy assemblage.It is really difficult to identify lithology in details,which in turn will greatly hinder the correct calculation of reservoir parameters,as well as subsequent hydrocarbon development strategies.Therefore,the study took a case study of the carboniferous igneous reservoirs in Chunfeng oilfield,Junggar Basin.The lithological categories of the reservoirs are basalt,basaltic andesite,andesite,tuff and volcanic breccias.A combination of core,thin section and other tests' data were used in the study.Then the logging response characteristics of different lithologies were clarified.The tuff and volcanic breccia were identified with the cross plot technique and "progressive stripping" concept.For the hard-to-identify volcanic lava,we used the empirical mode decomposition algorithm to convert the conventional logging data into multiple band sets of intrinsic mode functions,and attained the energy entropy of empirical mode function of various logging parameters for lavas.Then the precise identification of igneous lithologies was realized by using the discriminant algorithm.In applying the method to the block, we found that the overall matching rate of the proposed method is 93.7%, which has greatly improved the accuracy of lithological identification.

Tight oil accumulation mechanism and controlling factors for enrichment in mixed siliciclastic and carbonate sequences in the Xiagou Formation of Qingxi Sag,Jiuquan Basin

Guo Yingchun, Song Yan, Fang Xinxin, Jiang Zhenxue, Chen Jianjun, Guo Jigang

2018, 39(4):  766-777.  doi:10.11743/ogg20180414

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Tight oil reservoir in mixed siliciclastic-carbonate strata is an important type of tight oil resources.This type of tight oil reservoir is widely distributed across China and North America,and is believed to have great resource potential.Study on tight oil accumulation mechanism and main controlling factors for enrichment in mixed siliciclastic-carbonate strata are of great significance.The mixed strata of the Xiagou Formation (K₁g),Qingnan subsag,Qingxi Sag,Jiuquan Basin,are the case of study.We restored its burial history,thermal evolution history and hydrocarbon generation history via basin simulation techniques,investigated the formation of abnormal overpressure,pressure difference evolution between reservoir and source rock,and corresponding lower limit of pore throat for tight oil charging,and discussed the controls on hydrocarbon charging,seepage and enrichment in the mixed strata.Results show that the timing of hydrocarbon generation varies from different members in the formation:K₁g⁰ source rocks generated hydrocarbons early at the end of Early Cretaceous due to the high heat flow value in the early stage,while source rocks of K₁g² and K₁g³ started to generate hydrocarbons late in the Neogene,and K₁g¹ source rocks generated hydrocarbons in two periods(one early and one late period).Overpressure was believed to be attributed to hydrocarbon generation.Ever since 20 Ma,the remaining pressure on the boundary of source and reservoir rocks increased from 2 MPa to 32 MPa.The lower pore throat limit for oil charging on the boundary of source and reservoir rocks is 6 nm.Tight oil charging in the mixed strata of the Xiagou Formation and its short-distance migration accord with the invasion-percolation way,resulting in the pervasive accumulation of tight oil within and nearby the source rocks.The source-reservoir configuration and their internal architectures are the main controls on tight oil enrichment:the self-sourcing reservoirs and "sandwich-type"reservoirs are more oil-enriched than the lower-source upper-reservoir ones and thinly interlayered ones.

Classification and origin of “sweet spots” in deep low permeability tight gas reservoirs,Xihu Sag,East China Sea Shelf Basin

Zhao Zhongxiang, Dong Chunmei, Lin Chengyan, Zhang Xianguo, Duan Dongping, Huang Xin, Zeng Fang

2018, 39(4):  778-790.  doi:10.11743/ogg20180415

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"Sweet spot" identification and prediction is the key to develop tight gas reservoirs with low permeability.In order to explore the types and genesis of "sweet spots" in deep low permeability tight gas reservoirs with fewer wells in the sea,and to guide the exploration and development of oil and gas fields,the study investigated the "sweet spots" in X gas field in the Xihu Sag,East China Sea Shelf Basin,through multiple methods,such as core observation,cast thin section,cathodoluminescence and scanning electron microscopy thin section identification,grain size analysis,nuclear magnetic resonance,high pressure and constant pressure mercury intrusion porosimetry(MIP),porosity and permeability test and elemental analysis.First of all,the definition of "sweet spot" was explored,and three classes of "sweet spots" were identified based on the theoretical production capacity,physical property,pore structure and bound water content:"Sweet spot" Class Ⅰ is high yield gas reservoirs with Type Ⅰ pore structure being dominant and the irreducible water saturation mostly in the range of 22% to 57%; "Sweet spot" Class Ⅱ is moderate yield gas reservoirs with Type Ⅱ pore structure being dominant,followed by Type Ⅰ pore structure and Type Ⅲ pore structure being rare,and irreducible water saturation varies mostly between 45% and 55%; "Sweet spot" Class Ⅲ is low-yield gas reservoirs,with Type Ⅲ pore structure being dominant and a small number of IV pore structures developed,and irreducible water saturation mostly between 45% and 55%.It is believed that Class Ⅰ and Ⅱ "sweet spots" can be efficiently and economically developed at present,but Class Ⅲ "sweet spot" has "potentials" for future development.According to the research,we propose that (1) burial depth is the decisive factor in controlling the development of "sweet spot",thus no Class Ⅰ and Ⅱ "sweet spots" occur in areas with burial depth larger than 4 000 m in general; (2) sedimentary environment is the root cause controlling the development of "sweet spot"; (3) event sedimentation and diagenesis greatly impact the formation of Class Ⅰ and Ⅱ "sweet spot"; (4) dissolution is the main controlling factor for the development of Class Ⅲ "sweet spot"; (5) tectonic movement is a necessary condition for the formation and preservation of "sweet spots" in the study area.

Differential deposition and controlling factors of deep-water fan around 13.8 Ma in the Baiyun Sag,Pearl River Mouth Basin

Yang Lu, Wang Yingmin, He Min, Chen Weitao, Xu Shaohua, Zhuo Haiteng, Wang Xingxing, Li Wenjing

2018, 39(4):  791-800.  doi:10.11743/ogg20180416

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The deep-water zone in the Baiyun Sag,Pearl River Mouth Basin,is an important prospect area for oil and gas exploration in the northern South China Sea,but the understanding of the system tracts' composition and the depositional evolution process of the deep-water fans is still to be strengthened. Based on the comprehensive analysis of 3-D seismic profile,well logging and drilling data and plane properties,we discussed the evidences to identify FSST and LST,described and compared the sedimentary facies distribution characteristics and spatio-temporal evolution processes of the deep-water fans in the two system tracts. The result suggests that the fan in FSST is a small tongue-shaped fan,supplied by single sandy source vertically,whereas the fan in LST is a large irregular superposed fan with multi-line sources of mixed sand and mud,getting rid of major provenance supply from the delta. During the transformation from the former to the latter with multi-factors coupling,we speculate that such synthetic factors as sufficient sediment supply at the continental shelf break and relative sea level changes,function to provide a fundamental background;simultaneously,the strong coastal currents from southwest to northeast can be a favorable factor for the transformation.

Characteristics and 3D distribution simulation of architecture elements in deep-water turbidity channels

Zhang Wenbiao, Duan Taizhong, Liu Yanfeng, Li Meng, Xu Rui

2018, 39(4):  801-810.  doi:10.11743/ogg20180417

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The turbidity channel sandstone is noted for its importance and complexity in deep-water oil and gas fields' exploration,evaluation and development.Due to its high heterogeneity and limitations in current study,various contradictions exist between the existing sedimentary models and observations in the practical application.Based on the information derived from core and outcrop observation,well logging,and high-frequency seismic tests,we categorized the internal facies of restricted incised channel system(Grade 3 configuration)and described their features.The turbidity facies model was summarized from the perspective of lithological cycles,and the lithologic 3D distribution was simulated by means of geologic modeling.The results show that, (1) four types of turbidite architecture elements could be identified from the seismic data,including lag deposits,slump debris flow deposits,superimposed channel sand deposits with high net-to-gross ratio(NTG)and natural levee channel deposits with low NTG,occurring from the bottom of the channel to the top.The turbidite channel sand bodies at every cycle are composed of these four architecture elements in varying proportions. (2) Grade 4-5 channels within the turbidite channel system(Grade 3 configuration)incise and fill repeatedly in the process of migration and swinging,which is the immediate reason for the high reservoir heterogeneity at present. (3) The 3D simulation of the turbidite channel architecture reconstructs the rhythmicity of lithological change and features of facies-controlled constraints,which can guide the design of development scenarios and optimization of well patterns.

Lateral variation of sequence stratigraphic architecture on passive continental margin and its enlightenment: A case from the Middle Miocene in Pearl River Mouth Basin in 13.8 Ma

Xu Shaohua, He Min, Pang Xiong, Chen Weitao, Wang Yingmin, Zhuo Haiteng, Qin Chunyu

2018, 39(4):  811-822.  doi:10.11743/ogg20180418

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Current construction of sequence stratigraphic framework is mainly based on the analysis of stratigraphic superposition patterns near the estuary,with their lateral variation in the sedimentary body neglected.So the division standards of system tracts are restricted by the range of sedimentary bodies,and the sequence stratigraphic division scheme is not unique.As a result,it's difficult to carry out stratigraphic correlation in a larger scope in a sag. The study took a case of Pearl River Mouth Basin clinoform in 13.8 Ma(belongs to the Middle Miocene),and elaborated the characteristics of sequence boundary(SB),and the lateral variations of stratigraphic superposition patterns in Falling Stage System Tract(FSST)and Low stand System Tract (LST),through combination analysis of well and seismic data.The results show that a series of onlapping strata, which can be laterally tracked and correlated with LST progradation systems on seismic sections,should be named as LST rather than TST.Based on the principle of regional isochronous correlation,the definition of LST should be revised as a set of strata that are developed in the early of base-level rising cycle,marked by accommodation increasing rate being lower than the sediment supply rate,and contain lateral equivalent beds.While the definition of HST is revised as a set of strata that are formed in the late of base-level rising cycle,marked by the accommodation increasing rate being less than the sediment supply rate,and include a set of lateral equivalent beds. Meanwhile,the TST is defined as a set of strata with facies marker of the accommodation increasing rate being greater than the sediment supply rate and the lateral equivalents of LST and HST being excluded.Thus we proposed that the reconstruction of sequence stratigraphy framework should take estuary,shelf and deep water as an integrated whole,looking for the unique facies marker of each system tract,and carrying out systematic comparison and checking in three-dimensional space,rather than factitiously splitting a sedimentary system and then establish sequence stratigraphic frameworks separately.

Calculation and analysis of unsteady production of different fractured injection-production patterns in low permeability reservoirs

Pu Jun, Qin Xuejie, Gou Feifei, Fang Wenchao

2018, 39(4):  823-832,838.  doi:10.11743/ogg20180419

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Waterflooding in fractured reservoirs,an important way to develop low permeability reservoirs,can effectively improve the flow field effectiveness and increase oil discharge area. Rapid and accurate prediction of production of low permeability reservoirs with water injection can establish a foundation for optimal design of development. However,the flow field of water injecting into low permeability reservoirs presents unsteady and nonlinear flow characteristics,so the calculation methods based on Darcy's law are not applicable. After the analysis of flow fields,the equivalent flow units are divided. Taking the threshold pressure gradient of oil-water two-phase flow into consideration,we established the analytical calculation method for unsteady oil production with injection in different fracturing modes,based on the flow line integral method. And the method is simpler and faster compared with physical and numerical simulations,so it is applicable for optimizing fracturing injection and production well patterns and water injection development strategies in low permeability reservoirs. Besides,this method was used to calculate and analyze the effects of threshold pressure gradient,fracturing injection and production methods and fracture lengths on the productivity of oil wells. The results show that (1) the greater threshold pressure gradient leads to larger seepage resistance,so the well productivity is lower than that without considering the threshold pressure gradient; (2) the well productivity is controlled by flow units,so with different fracturing injection and production modes,the effects of production increase and the time of water breakthrough are completely different. Although the increase of production with simultaneous fracturing injection and production is the best,being 3.1 times that of other modes,the water breakthrough time is just 24 months; (3) with the increase of fracture length,the oil production increases,whereas the effect of increased oil production is not obvious any more when the fracture length exceeds the optimum length.

Characteristics and influential factors of water cut change in low permeability water-drive oil reservoirs in Shengli oilfield

Wang Jianzhong, Yu Xinchang, Sun Zhigang, Li Rongqiang, Xu Jinjie

2018, 39(4):  833-838.  doi:10.11743/ogg20180420

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The characteristic change of water cut can reflect the effectiveness of waterflooding to some extent.Early water breakthrough,fast water cut rise,and low efficiency of waterflooding have always been the obstacles preventing the development of low permeability reservoirs.Variation of water cut in waterflooding oil reservoirs involves two-phase flow of oil and water in rocks,thus is controlled by petrophysical property,oil-water features and driving force.In regard to the low permeability water-drive oil reservoirs in Shengli oilfield,we analyzed the factors influencing the variation of water cut and waterflood efficiency based on larges amounts of experimental data.These factors are characteristics of reservoir pore structure,original water saturation,stress sensitivity and displacement pressure.The results show that pore connectivity is the major factor for water cut rising,while the initial water cut mainly depends on original water saturation.Appropriate control on displacement pressure is beneficial for controlling water cut and increasing waterflood efficiency.Considering the stress sensitivity of permeability in low permeability reservoirs,we suggested the implementation of advanced water injection to avoid the permeability damage caused by pressure drop,so as to improve waterflood efficiency.

Seepage characteristics and quantitative description of large pore pathways at high water cut stage

Wu Zhongwei, Cui Chuanzhi, Yang Yong, Huang Yingsong, Liu Zhihong

2018, 39(4):  839-844.  doi:10.11743/ogg20180421

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The identification and quantitative description of large pore pathways are of great significance to improve the measures of oil recovery at high water cut stages.Since the large pore pathways,usually developed in unconsolidated sandstone oil reservoirs at high water cut stage under the long-term injected waterscouring,can cause ineffective water cycle,and compromise the effect of water injection on recovery enhancement.Thus the study analyzed the seepage characteristics of high-velocity non-Darcy flow in the large pore pathways,to set up diagnostic criteria for large pore pathways according to the standards used to identify the high-velocity non-Darcy flow,and established an oil reservoir simulation system based on permeability,which changes with the pore volume expansion during scouring through water injection.And a quantitative description of the large pore pathways was given.Finally,the Ng6³⁺⁴ development unit in the upper member of the west Gudong oilfield was selected for reservoir simulation to identify large pore pathways and to carry out the quantitative description of the pathways.The results show that the proportion of large pore pathways,mainly distributed around the injectors or between injectors and producers,is relatively small in the simulated reservoir.In conclusion,the diagnostic criteria of large pore pathways,taking into account of the injector-producer spacing,injection-production pressure difference and permeability,can be effectively applied to the oil reservoir simulation system and the resultant quantitative description of the large pore pathways is accurate.