Table of Content

28 December 2017, Volume 38 Issue 6

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Mechanisms for gas accumulation and enrichment in tight sandstone reservoir in rift basins:Cases from the Songliao Basin and the Bohai Bay

Li Jian, Jiang Xiaohua, Wang Xiuqin, Cheng Honggang, Hao Aisheng

2017, 38(6):  1005-1018.  doi:10.11743/ogg20170601

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Tight sandstone gas in rift basins is a new play concept for exploration.This paper takes the Songliao Bbasin and the Bohai Bay Basin as case areas to systematically investigate the mechanisms for gas accumulation and enrichment in tight sandstones.The reservoir characteristics of tight sandstone gas in rift basins have been studied,which are different from those of craton basins.As to gas accumulation mechanism,it is demonstrated that multiple factors have accelerated the generation of gas by source rock,and multiple sediment provenance have ensured deposition of favorable reservoirs.Both the long gas migration distance and tightening of the reservoir before gas charge favors gas accumulation in multiple sections over large areas.For the enrichment controlling factors,it is concluded that the distribution of gas pools and source kitchens are located along the faulted troughs,the reservoir quality is controlled by facies and tectonic belts,while the vertical distribution of flowable gas sections is controlled by development of secondary porosity zones.Finally,the exploration potential and future exploration concepts of these two basins is analysed.

Characteristics of reservoir sand bodies within high-resolution sequence framework:A case from the 3^rd and 4^th members of Quantou Formation in Zhou 311 area,northern Songliao Basin

Sun Chunyan, Hu Mingyi, Hu Zhonggui, Deng Qingjie

2017, 38(6):  1019-1031.  doi:10.11743/ogg20170602

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To provide more information for exploration and development of the subtle sandstone oil and gas reservoirs in Zhou 311 area of Northern Songliao Basin,it is necessary to describe the sedimentary characteristics of reservoir sand bodies in the 3^rd and 4^th members of Quantou Formation.Six short-term base-level cycles were identified under the guidance of high resolution sequence stratigraphy and reservoir sedimentology theories and through integration of core,logging and regional seismic data,and were used to establish a sequence stratigraphic division scheme.The types and characteristics of reservoir sand bodies were analyzed within the established sequence framework.The 3^rd and 4^th members of Quantou Formation can be divided into two long-term base-level cycles (Q3-LSC1-Q4-LSC1),five medium-term base-level cycles (Q3-MSC1-Q4-MSC3),and twelve short-term base-level cycles (Q3-SSC1-Q4-SSC7).The short-term base-level cycles mainly develop asymmetric structure dominated by ascending half cycles (A₁、A₂、A₃),almost complete-complete symmetrical structure with ascending and descending half cycles being nearly equal (C2),incomplete symmetrical structure dominated by either ascending half cycles or descending half cycles (C1,C3).Vertically,the evolution of the 3^rd and 4^th members of Quantou Formation shows a pattern ofbase level rising,falling,and then rising.The vertical sedimentary evolution sequence is distributary channel-meandering channel-distributary channel-underwater distributary channel.The channel sand body types develop from less overlap to more vertical or lateral overlap,and then gradually change to isolated type.Characteristics of sedimentary section and internal sedimentary cycle structure of the meandering channel sand bodies,distributary channel sand bodies,underwater distributary channel sand bodies are analyzed on the basis of high-resolution sequence stratigraphy division and correlation and under the control of short-term base-level cycles.The sand bodies were mostly oil-immersed and thick during the deposition of Q4-SSC1 and Q3-SSC5,thus they are most favorable for quality sand body development.In the process of fine exploration and development,fine description of channel sand bodies is imperative due to the complex heterogeneity within the reservoir.

Genetic mechanisms of superimposed fault systems in Block 3 of Nanpu Sag,Bohai Bay Basin

Feng Jianwei, Wang Zhikun, Shang Lin

2017, 38(6):  1032-1042.  doi:10.11743/ogg20170603

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This research focuses on investigating the genetic mechanisms and evolutionary processes of the superimposed multi-stage fault systems in Block 3 of Nanpu Sag,Bohai Bay Basin.The workflow herein adopted a variety of modern techniques such as 3-D seismic interpretation,physical simulation of structure and finite element numerical simulation,and took the theories of structural geology and geomechanics as guidance.We recognized the geometry and combination modes of faults,restored the paleostress state of different tectonic periods,analyzed the development and evolution patterns of faults under variable stress field,discussed the control of pre-existing structures on later fault formation and evolution,and established geodynamic models of the superimposed multi-stage fault systems in the study area.The results showed that,bounded by unconformity on the top of Shahejie Formation,the X-pattern normal faults could be divided into two vertically superposed fault systems with short-term inversion characteristics.The upper tone is a SN-trending composite extensional and sinistral strike-slip fault system with pure shear deformation being predominant,while the lower tone is a NW-SE-trending extensional fault system with single shear deformation being predominant.Distribution of geomechanical layers,later tectonic stress intensity,and angle between pre-existing fault strike and later principle stress direction were regarded as the three main factors controlling the development of superposed fault systems.The pre-existing faults could influence the redistribution of local stress field and the development of later faults,which either continued to expand along early faults,or created new faults.As a mechanical weak surface,the unconformity was easy to generate uncoordinated deformation or slipping,resulting in significant vertical discontinuous development of faults.Regionally,the formation and evolution of the superimposed fault system in Block 3 of Nanpu Sag was directly controlled by transition of tectonic stress field by Tan-Lu fault since the Paleogene.

Geochemical characteristics and secondary hydrocarbon generation of coal-measure source rocks in Upper Paleozoic of Dagang oilfield

Zhou Lihong, Hua Shuangjun, Sun Chaonan, Dai Kun, Mu Zhiquan

2017, 38(6):  1043-1051.  doi:10.11743/ogg20170604

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Coal-measure formations in the Upper Paleozoic mainly distributes in central-south part of Dagang Oilfield and covers an area of nearly 10 000 km².Source rocks there consist of coal beds,carbonaceous mudstone and mudrock,and have a gross thickness of 100 to 450 m.Vertically,they distribute mainly in the Taiyuan and Shanxi Formations with some developed in the Benxi Formation.The coal beds are rich in organic matter (more than 85%) and are high to very high quality source rocks,while the organic matter content in mudstone varies greatly from place to place.Vitrinite dominates the maceral of the source rocks with a content of more than 50%,indicating favorable conditions for gas generation during the mature stage of the rocks.Meanwhile,affected by coal-forming environment,the coal-measure strata in the main coal-forming centers are also contain some hydrogen-rich macerals,hinting large oil generation capacity during low maturity to mature stage.The Upper Paleozoic strata experienced multiple phases of tectonic activities,so were the coal-measure source rocks,which went through two stages of hydrocarbon generation.The secondary hydrocarbon generation was cha-racterized by large area and high resource potential and has been the focus of study during the buried hill exploration.Four types of burial history can be recognized in the oilfield based on the tectonic evolution:the early-burial and late-lift type,the early-lift and late-burial type,the continuous deep-burial type and the steady-burial type.Except for the first type,all the other three types possessed favorable conditions for secondary hydrocarbon generation.Years of exploration in the area of secondary hydrocarbon generation yielded primary oil and gas discoveries in buried hills one after another:Kongxi,Wumaying,Chenghai and Wangguantun,to name just a few.

Cenozoic transtensional structural styles and their controls over hydrocarbon accumulation in Dongying Depression

Zhang Weizhong, Zha Ming, Zhang Yunyin, Chen Cheng, Qu Zhipeng, Zhang Lin, Gao Ping

2017, 38(6):  1052-1058.  doi:10.11743/ogg20170605

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Various methods including stratigraphic slicing and seismic coherence are adopted based on fine interpretation of seismic data to analyze the styles and distribution patterns of transtensional structures and their controls over hydrocarbon accumulation in Dongying Depression.The results show that there are at least 11 transtensional structural belts,including Wang Jiagang,Ba Mianhe,Gaoqing,Qingxi,Haoxian and etc.They can be categorized into three types of compound structures according to the features of fault complexes on map and section and the characteristics of formation deformation there:the broom-half-flower transtensional structure,the en echelon-tree-shaped flower transtensional structure and the juxtaposed-curled flower transtensional structure.The first style is mostly found in the steep slope of the basin,the second one is mainly observed in the slope of the basin and the last style is distributed in the central uplift belt of the depression.The three styles controlled the trap types,and formed their own individual types of oil and gas accumulation.

Paleogene fault activities and their control over sedimentation at southeastern margin of Qinnan Sag,northern Bohai Sea

Pang Xiaojun, Wang Qingbin, Dai Liming, Li Huan, Chen Jing

2017, 38(6):  1059-1067.  doi:10.11743/ogg20170606

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Drilling/logging,3D seismic data and other information were combined with fault activity rate statistics to analyze quantitatively the characteristics of the Paleogene boundary fault activities and their control over sedimentation in southeastern margin of Qinnan Sag,Bohai sea.The result shows the boundary faults in the study area were significantly segmented into four sections:west Shibei1,east Shibei1,west Shibei2 and east Shibei2.Vertically,the faulting initiated during the deposition of Ek to Es⁴,and then went through two cycles of enhanced and subdued stages during the deposition of Es³,Es¹⁺²,Ed³, and Ed²,and finally inactivated during the deposition of Ed¹.Laterally,the boundary faulting was characterized by being intensive in the central part but weak at both ends during the same geological period.From a space angle,faulting activities were successively carrying on at the same location.The differences of boundary faulting controlled the differential rise and fall of basins,resulting in differential erosion in provenances and the formation of various deposition patterns and tectonic transfer zones,which further affected erosion rate of the provenances,subsidence rate,types and distribution of sedimentary facies,delta extension distance,as well as lithologies in depositional basins.

Fluid state in the 5^th Member of the Upper Triassic Xujiahe Formation in the Xinchang gas field,Western Sichuan Depression in Sichuan Basin

Wu Xiaoqi, Luo Kaiping, Wang Ping, Chen Yingbin, Wang Yanqing, Hu Ye, Li Huaji

2017, 38(6):  1068-1078.  doi:10.11743/ogg20170607

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Gas and water are simultaneously produced from the reservoirs in the 5^th Member of the Upper Triassic Xujiahe Formation (T₃x⁵) in the Xinchang gas field in the Western Sichuan Depression,and there is no consensus on whether the gas is free or dissolved.For this purpose, methane solubility experiments were conducted under different temperatures and pressures on the T₃x⁵ formation water,and the results were compared with the methane solubilities in 1 mol/kg NaCl solution.The fluid state in T₃x⁵ was demonstrated in combination with the gas/water ratio of the produced fluid and ideal gas state equation.The results indicate that methane solubilities in T₃x⁵ formation water under present average depth and historical largest depth are 2.260 m³/m³ and 3.194 m³/m³,respectively.The water-soluble gas in T₃x⁵ formation water was mainly released by reduction of pressure and temperature during uplifting since the end of Late Cretaceous.The volumetric proportions of free gas and formation water in the T₃x⁵ pores were generally lower and higher than 50%,respectively.The volumetric proportion of free gas increased slightly due to the volumetric expansion caused by reduction of pressure and temperature since the end of Late Cretaceous,whereas that of formation water decreased as the water was displaced.Due to the partial degasification of water-soluble gas by reduction of pressure and temperature,the proportions of free gas in the total natural gas increased while that of water-soluble gas decreased.The T₃x⁵ natural gas was dominated by free gas,with water-soluble gas accounting for less than 5%.Moreover,about 29.3% of the T₃x⁵ water-soluble gas was released and turned into free gas by reduction of pressure and temperature during uplifting,and the total amount of gas released from the T₃x⁵ formation water reached 1.8×10⁹ m³.

Seismic recognition and zonation of the platform margin facies of the Sinian Dengying Formation in central Sichuan Basin

Zeng Fuying, Yang Wei, Sun Ai, Jin Hui, Xie Wuren, Ma Shiyu, Su Nan

2017, 38(6):  1079-1086.  doi:10.11743/ogg20170608

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The Sinian Dengying Formation platform margin facies is developed in east and west of the Mianzhu-Changning Rift,Sichuan Basin,where oil and gas accumulated.Seismic recognition,fine characterization and distribution features of the platform facies zones are carried out by means of detailed interpretation of seismic facies and horizon flattening.Seismic reflection features of facies zones on the platform margin are "dome-shaped" or brackish-shaped,with internal messy or earthworm-like reflections,moderate to weak amplitudes,and low to intermediate frequencies.Three stages of platform margin zones,Z₂dn¹,Z₂dn² and Z₂dn⁴,developed in east and west of the Mianzhu-Changning Rift,and two stages of platform margin zones developed in Z₂dn².The Z₂dn¹ platform margin zone is distributed in eastern area of the Anyue-Lezhi,trending in near North-South direction.The Z₂dn² platform margin zone is distributed in a semi-circle shape.The Z₂dn⁴ platform margin zone trends in near north-south direction to the south of the Anyue,and in northeast-southwest direction in the Suining-Shehong area.The distribution pattern of the platform margin facies of the Sinian Dengying Formation in the central Sichuan Basin indicates that the Sinian Dengying Formation has favorable exploration zones.This characterization work gives some guidance for further oil and gas exploration.

Evaluation of Ordovician source rocks and natural gas accumulation patterns in west flank of a paleo-uplift,Ordos basin

Xiao Hui, Zhao Jingzhou, Xiong Tao, Wu Weitao, Mi Jingkui, Liu Sutong

2017, 38(6):  1087-1097.  doi:10.11743/ogg20170609

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The existence of a new gas accumulation type sourced from the source rocks within the Ordovician was confirmed at the lower slope in the west flank of a paleo-uplift in Ordos Basin based on effective source rock assessment results.Analyses of 347 samples taken from the source rock intervals in 11 wells revealed high-quality source rocks developed in marginal slopes of the middle and upper Ordovician platform in the Basin.Measurements showed that the source rocks in the Wulalike and Kelimoli Formations had relative higher TOC values,with marlite TOC averaged at 0.33% and 0.39%,and mudstone TOC averaged at 0.51% and 0.65%,respectively.The accumulative effective thicknesses of source rocks of the two formations were 12 to 40 m and 7 to 14 m,respectively.Areas with thick source rocks were assessed to have a gas generation capacity of 14.0×10⁸ to 15.5×10⁸ m³/km².Gas accumulations in the Kelimoli Formation experienced two charging stages:the early stage in the Middle Jurassic,during which paleo-oil reservoirs were formed;and the late stage in the Late Jurassic to Early Cretaceous,during which the paleo-oil reservoirs were cracked into petroliferous gas reservoirs.Carbon isotopes of gas-bearing inclusions from the later stage were measured to be -38.84‰ in average,indicating late petroliferous gas charging characteristics.Coal-derived gas reservoirs were distributed widely in the upper slope in the west flank of the paleo-uplift.The fact that the massive detrital calcareous berccias formed in the Kelimoli Formation at the lower slope had a close contact with the overlying effective source rocks of the Middle and Upper Ordovician and that they stretched away laterally from the top Ordovician weathering crust,made it possible for the formation of self-sourced gas accumulations,like Yutan1 gas reservoir in the area.Fractured-vuggy karst reservoirs developed in the lower slope of the platform margin were considered to be potential exploration targets for such kind of gas reservoirs.

A discussion on the influence factors of dryness coefficient of coal-derived gas: A case from the Upper Paleozoic reservoir in northern Ordos Basin

Ni Chunhua, Bao Jianping, Liu Guangxiang, Zhu Jianhui, Wu Xiaoqi, Zhai Changbo

2017, 38(6):  1098-1104,1216.  doi:10.11743/ogg20170610

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The dryness coefficient is widely used in the studies on the geochemical characteristics,genetic types and accumulation of coal-derived gas,but only a few of them have addressed the factors affecting the dryness coefficient of coal-derived gas.The major coal-derived gas fields of the Upper Paleozoic reservoir in northern Ordos Basin,such as Dongsheng (Hangjinqi),Daniudi and Sulige,are chosen.Based on the measured and collected data of chemical components,lateral and vertical distribution characteristics of dryness coefficient of coal-derived gas are analyzed,and a two-stage vertical distribution pattern is established.The main controlling geological factors are discussed within the context of regional geological background.The results indicate that ① the dryness coefficient of the Upper Paleozoic coal-derived gas decreases from the center to the margin of the basin,and it displays a two-stage vertical distribution pattern in which the dryness coefficient firstly increases and then decreases from bottom to top.② The dryness coefficient of coal-derived gas is influenced by maturity,geochromatograph effect and leakage effect.③ The maturity of the Upper Paleozoic source rocks determines the regional distribution characteristics of the dryness coefficient.The main controlling factors affecting the "two-stage" vertical distribution of the dryness coefficient are different in the different depth.The source layer and geochromatograph effect the lower and upper reservoirs,respectively.The dryness coefficient is also affected by leakage effect in the areas with poor preservation conditions.

Key geological factors influencing coal-bed methane exploitation

Feng Lijie, Jia Yibo, Yue Junju, Wang Jinfeng

2017, 38(6):  1105-1112.  doi:10.11743/ogg20170611

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Coal-bed gas resources are huge in China.However,its scaled exploitation is limited due to unique occurrence status.Scientific methods are needed to identify the key geologic factors influencing its effective exploitation.In view of that the influencing factors are large in number and high in dynamic complexity,this paper established a model to identify and analyze the key geological factors based on rough set attribute reduction theory.First,sixteen original geologic factors were selected in respects of coal petrologic characteristics,coal seam features and geologic influences and were optimized according to the available data from 40 CBM wells selected for study in different regions of China.Then,decision data were constructed and attribute reduction of decision table was performed through rough set data analysis.Finally,sorting was performed according to their relative importance and the key influential geologic factors were identified.Eight geolo-gic factors including,geological structure type,in-situ stress,coal seam pores and fractures are relatively high in importance.Overall,external influential geologic factors have stronger power than internal ones such as coal seam and coal rock.Therefore,more attentions should be paid to the influence degree and action mode of the external geologic factors to enhance the level of mining gas hazard management in practice.

Differential compaction of deep sandstones in compressive tectonic setting: A case study of Cretaceous reservoirs in Kuqa foreland thrust belt,Tarim Basin

Mao Yakun, Zhong Dakang, Li Yong, Wang Tengyu, Sun Haitao, Wang Yuan

2017, 38(6):  1113-1122.  doi:10.11743/ogg20170612

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Compaction,a key factor for reservoir quality evolution and prediction,is generally considered as one of the major diagenesis of sandstones during shallow and intermediate burial instead of deep burial.However,prominent differential compaction is observed in the deep Cretaceous sandstone reservoirs of Kuqa foreland thrust belt and studied in-detail through thin section observation,CL and grain size analysis.Differential compaction of deep sandstone is mainly controlled by grain diameter,cementation and tectonic compression.The compaction degree of the deep sandstone reservoirs shows certain regularities from north to south on different tectonic positions and appears to change inversely with burial depth.Moreover,differentiation of compaction enhances with decreasing cementation.For medium-grained sandstone and fine sandstone,differentiation of compaction lowers along with their compaction degree.The existence of significant diffe-rential compaction in the deep-buried sandstone reservoir implies that much attention should be paiyed to the influence of compaction to reservoir quality differentiation when predicting deep-buried sandstone reservoirs.

Sequence stratigraphy and sedimentary evolution of Donghe Sandstone in Tarim Basin

Jia Dongli, Tian Jingchun, Zhang Xiang, Zhu He, Lin Xiaobing, Su Bingrui, Liang Qingshao, Yu Hang

2017, 38(6):  1123-1134.  doi:10.11743/ogg20170613

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The Donghe Sandstone is constituted of the Donghetang Formation from the Upper Devonian and the Bachu Formation from the Lower Carboniferous.A basin-wide tracing and correlation of the sandstone combined with a fine study of its sedimentary evolution is currently a major task for further oil and gas exploration in the basin.Guided by sequence stratigraphy theories,we analyzed in detail the sequence stratigraphy and sedimentary evolution of the sandstone based on a wealth of data,including drilling data,mud and wire logging data,cores and seismic data.Two types of sequence boundaries together with two second-order sequences and six third-order sequences (SQ1-SQ6) were recognized in the sandstone and a basin-wide sequence stratigraphy framework was also built.The result showed great influence of palaeogeomorphology upon the distribution of the sequences and an almost intact SQ4 to SQ6 in the whole basin.Within the sequence stratigraphy framework,there were five sedimentary facies,such as delta facies,longshore facies and etc..A study of plane sedimentary evolution of the sedimentary facies showed that during the deposition of SQ1 to SQ6,the deposition area of the sandstone had gradually enlarged,with favorable conditions for the development of potential lithologic traps in SQ4 sandbody of Shunnan area,in SQ5 sandbody of Shunxi area and in SQ6 sandbody of Shuntuo area.Further exploration shall be focus on these sandbodies.

Formation pressure anomalies and controlling factors in central Juggar Basin

Wu Haishen, Zheng Menglin, He Wenjun, Yang Tongyuan, Chen Lei, Guo Jianchen, Yang Yibo

2017, 38(6):  1135-1146.  doi:10.11743/ogg20170614

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Formation pressure measurements and logging and seismic data were combined with basin sedimentary texture to analyze the spatial distribution of formation overpressure and its controlling factors.The results show that 1) overpressure zone is widely distributed in the central part of the basin.It has a diachronous top surface,shallows overall in NW direction and deepens toward the centre of the sag.In Mosuwan area,the top surface of the overpressure zone corresponds to the Jurassic Sangonghe Formation and has a burial depth of about 4 400 m.In Xiayan-Dabasong and the Mahu areas to the west,it corresponds to the Triassic and shallows to 3 500 m.In an oil field called Yongjin in Munan uplift,it corresponds to the bottom of the Cretaceous and deepens to about 5 800 m. 2) Overpressure can be observed in layers at and below the Jurassic in Mosuowan area with the highest values measured in the Jurassic and some lower values measured in the Permian-Carboniferous.Two types of overpressure are classified based on profile characteristics of pressure coefficient structure. 3) Spatial characteristics of the overpressure anomalies in central Basin reveal the roles of different controlling factors upon the forming of overpressure in different tectonic locations in the basin.

Characteristics and controlling factors of reservoir development in the Wenchang Formation,Southern Lufeng Sag,Pearl River Mouth Basin

Wang Xudong, Zhang Xiangtao, He Min, Zhang Sufang, YuYixin, Wuchen Bingjie

2017, 38(6):  1147-1155.  doi:10.11743/ogg20170615

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The Wenchang Formation in the Lufeng Sag,Pearl River Mouth Basin,is sedimentary fill deposited in the rifting stage.High quality reservoir was developed in the 5^th Member of Wenchang Formation which was deposited during the early stage of rifting.To uncover the genetic mechanisms of high quality reservoir,various data including seismic,drilling,core,thin section,scanning electron microscope (SEM) images and mercury experiments were integrated to analyze the sedimentary facies,reservoir characteristics and diagenesis of the 5^th Member of Wenchang Formation in the LF-A Sub-sag,Southern Lufeng Sag.The research results indicate that the 5^th Member of Wenchang Formation is dominated by deposits of braid river delta front subfacies.Lithic quartz sandstones and feldspathic quartz sandstone are the dominatant rock types.The remnant primary intergranular pores are the main reservoir spaces.The 5^th Member of Wenchang Formation is characterized by low porosity,low-medium permeability,and low-medium displacement pressure.High quality reservoirs mainly present in thick channel sandstones with low lithic content and high maturity,reflecting the control of high energy environment on reservoir formation.The cementation and compaction processes in the early stages of diagenesis decreased porosity,while the dissolution process in the late stages of diagenesis enhanced reservoir property.Structural uplifting process in the early stages prohibited strong compaction by the overburden,making it possible for the preservation of large amount of primary pores,thus was the crucial factor for the formation of high quality reservoirs.

Genesis of mudstone dikes and their impact on oil accumulations in D-F oilfield of Oriente Basin,Ecuador

Yang Jinxiu, Zhang Kexin, Chen Heping, Lu Shuangfang, Wan Xuepeng, Tang Mingming, Xiao Dianshi, Zhang Chaoqian

2017, 38(6):  1156-1164.  doi:10.11743/ogg20170616

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Two mudstone dikes formed in the M1-SS layer of D-F oilfield,Oriente Basin,Ecuador,were studied to characterize their vertical and planar distribution based on drilling,logging and seismic data,and to analyze the factors that controlled their formation in respects of sedimentary setting and differential compaction process.Their impact upon the distribution of oil accumulations in the oilfield was also discussed in the paper.Both NW-SE-trending dikes are characterized by higher clay content in well logging and low amplitudes in seismic data.The study suggests that the dikes correspond to hybrid sand-mud flat microfacies in a tide-dominated estuarine environment and have higher clay content than surrounding sand flat and sand bar microfaices.Therefore,after differential compaction,the sand-mud flat ended up with a thinner thickness of M1-SS and even formed tiny faults at the edge of dikes.Besides,the presence of the dikes affected the reservoir heterogeneity and decreased the continuity of the sandbodies in M1-SS,thus blocked lateral migration of hydrocarbons.As a result,the oil accumulation to the east of the source rock-rooted fault was compartmentalized into five small reservoirs.

Pore-scale simulation of gas flow in low-permeability tight sandstone with gas slippage effect and Knudsen diffusion taken into consideration

Cao Tingkuan, Liu Chengchuan, Pu Tao, Duan Yonggang, Fang Quantang, Sun Lianpu

2017, 38(6):  1165-1171.  doi:10.11743/ogg20170617

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The strong heterogeneity and complicated pore structure of low-permeable tight sandstone with nano-and micro-scale pore-throats could not be accurately covered by using current core testing methods.Micro-scale flowing simulation based on digital core theories was then introduced to study the gas flowing in such stones.Samples taken from the Shaximiao Formation in Western Sichuan Basin were scanned with CT to build up three-dimensional (3D) digital cores.Methods such as pore-throat identification,pore topology simplification and throat adjustment,were also resorted to establish 3D pore network models with various throat distribution patterns.Considering the complexities of gas flow in such nano-and micro-scale pore-throats,we also established a multi-mechanism flow equation that takes Darcy flow,gas slippage effect and Knudsen diffusion into consideration,and tried to solve discretely the equation by combining it with the pore network models.Numerical simulations were then carried out to study the influence of various transport mechanisms upon gas migration under different pressures.Results indicate that smaller pore-throat size and lower pore pressure mean a stronger non-linear gas flow.Gas flow was observed to basically follow the Darcy's law when pore pressure higher than 10MPa and average throat radius larger than 0.1 μm.While in samples with average pore-throat radius less than 0.1 μm and pore pressure less than 10 MPa,gas flow was dominated by gas slippage and Knudsen diffusion,a phenomenon worthy of considering during gas production.

A triaxial compression test of tight quartz sandstone under high temperature and high confining pressure

Deng Hui, Zhang Mi, Deng Tonghai, Zhang Qian

2017, 38(6):  1172-1179.  doi:10.11743/ogg20170618

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The impact of temperature and confining pressure upon mechanic characteristics and failure modes of the tight quartz sandstone in the second member of the Xuejiahe Formation(T₃x²) in Xinchang gas field of Sichuan Basin,was studied through a conventional triaxial compression test by using a MTS 815 servo-controlled testing machine to simulate subsurface environment of high temperature and high confining pressure.The results show that,when the temperature remains constant,the average values of compressive strength and elastic modulus of quartz sandstone samples increase with growing confining pressure.And when under a constant confining pressure,the value changes in a two-stage way below and over the temperature of 60℃.Before the temperature reaches 60℃,the samples have a quadratic nonlinear decrease in terms of cohension and a nonlinear increase for their internal friction angle,triaxial compression strength,and elastic modulus.When the temperature is higher than 60℃,the above-mentioned parameters vary in a totally reverse direction.The temperature of 60℃ is then the critical point for the mechanical properties of the samples to reverse under joint control of temperature and confining pressure.Within the ranges of pressure from 0 to 100 MPa and temperature from 15 to 150℃,the deformation or failure modes of the samples depends on temperature and confining pressure,with the latter having stronger impact than that of the former.Based on deformation process and failure modes,we roughly categorize the stress-strain curves into five types,which in turn loosely correspond to failure modes of tensile,shear,shear dilatancy,and tensile-shear,respectively.

Mechanisms of remaining oil production by plugging dominant flowing path and the application and strategies

Xiao Kang, Mu Longxin, Jiang Hanqiao, Shen Jian, Zhang Pengyu

2017, 38(6):  1180-1186.  doi:10.11743/ogg20170619

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Based on 3D physical model which can simulate growth of dominant flowing path and nuclear magnetic resonance(NMR),macro remaining oil distribution and production contribution from micro pore under different plugging methods are studied.Furthermore,controlling mechanisms of remaining oil production in dominant flowing path are summarized.Finally,strategy of remaining oil production and its adaptability are obtained through numerical simulation.The results show that,by establishing the correlation between NMR imaging and oil saturation,oil distribution in 3D model can be monitored without probes.Considering the plugging efficiency,short and deep profile controls can be applied to weak and strong dominant flowing paths respectively,and initial potential has the most important influence on the plugging.Remaining oil production in large pore contributes to most part of the overall plugging.It is also the main factor causing different recovery improvement among different plugging methods.Because channeling and initial potential in different kinds of pores are more uniform in weak path model,production of medium and small pores is slightly more efficient than in a strong model.Plugging agent content for weak path should not be too high in order to guarantee injection,and to achieve plugging objective,strong path needs relatively high injection pore volume.This study can provide some theoretical guidance for plugging design and optimization of high water cut oilfiled.

Characteristics of waterflood induced fracture in low-permeability sandstone reservoirs and its identification methods:A case study from Chang 6 reservoir in W area in Ansai oilfied,Ordos Basin

Zhao Xiangyuan, Zeng Lianbo, Hu Xiangyang, Zhu Shengju

2017, 38(6):  1187-1197.  doi:10.11743/ogg20170620

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This paper introduces the principles of waterflood induced fracture,clarifies their basic characteristics and proposes the methods to identify them.Then the waterflood induced fracture of Well A0 group of Chang 6 reservoir in W area in Ansai oilfield,Ordos Basin has been identified by these methods.With larger scale,long extension and high permeability,waterflood induced fracture are tension fractures which are not controlled by single stratum in vertical extension.Their trend direction are generally along the present maximum horizontal principal stress or the main seepage fracture direction.In a long term waterflooding process,the following are clear indications for the establishment of waterflood induced fracture between producer and injector wells:if the oil wells shows a gradually dominant direction of flooding,the water-cut curve presents step-type rise,well test interpretation has the character of fracture seepage,the water injection profile of the injection well gradually presents finger-like water absorption characteristics,inflection point appears in water injection index curve,tracer or water flooding front monitoring showed good connectivity between wells.

Improvement and application of quantitative calculation of porosity evolution of clastic rock

Feng Xu, Liu Luofu, Li Chaowei, Dou Wenchao, Zheng Shanshan

2017, 38(6):  1198-1207.  doi:10.11743/ogg20170621

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Porosity evolution of reservoirs is a key and difficult point of the study on hydrocarbon accumulation mechanism.But the present research methods of "porosity inversion and back stripping" have some flaws,including equating thin section porosity with porosity,attributing the porosity loss by compaction to early diagenetic stage,and neglecting rock bulk volume's reduction in the compacting process.In this study,compaction was corrected in every stage of diage-nesis according to the normal compaction curve.Equation of porosity variation through history is derived,accounting for the porosity loss through compaction and cementation,and also porosity increase by corrosion,also taking into account of rock bulk volume variation in every stage of diagenesis.A case study on Chang 7 and Chang 6³ tight oil reservoir of the southwest Ordos Basin was carried out,integrating the diagenetic facies of the research area and the derived formula.We reconstructed the porosity evolution from inversion and back stripping under the constraint of diagenetic sequences,and built five porosity evolution pattern and porosity evolution curve by different methods.If compaction is not corrected and rock bulk volume change is not considered,large error of the recovered porosity exist and it may be up to 13.8%.

Numerical simulation for ultra-deep sour gas reservoirs with liquid sulfur condensate

Gu Shaohua, Shi Zhiliang, Shi Yunqing, Hu Xiangyang, Cen Fang

2017, 38(6):  1208-1216.  doi:10.11743/ogg20170622

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Liquid sulfur condensate phenomenon exists in ultra-deep sour gas reservoir,which affects the reservoir permeability and reduce the gas well productivity.A numerical simulation model proposed in this paper for sour gas reservoirs considers such a two phase flow with both liquid sulfur condensate and gas fluids.On basis of the model,a numerical simulator is developed and used in research of sour gas well production.The results shows liquid sulfur mainly gathers near wellbore,and never extend further during the entire production life.Compared with solid sulfur deposition,condensation of liquid sulfur has less effect on the gas well production.However,the peak production period of gas well reduces obviously.In heterogeneous formation,the liquid sulfur mainly gathers where better reservoir property is present.The liquid sulfur preferentially occupies high permeability zones such as fractures or vugs,which leads to reservoir permeability reduction and the well production decline.Based on this technology,the liquid sulfur condensate behavior in sour gas reservoir can be predicted quantitatively.Thus the technology can provide reliable results for decision-making.