Table of Content

28 June 2017, Volume 38 Issue 3

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Mud debris flow fan deposits in Dainan Formation of the Subei Basin

Liu Yurui

2017, 38(3):  419-429.  doi:10.11743/ogg20170301

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Several features such as sedimentary system,lithology and lithofacies and sandbody distribution in the Dainan Formation of Subei half-graben fault basin neither follow the general patterns of the half-graben faulted basins in Eastern China,nor follow the common patterns that trench belts are the main locations where fans and sandbodies occur in continental basins.Through a large number of core data observation,it is found that there is a special type of sedimentary rock composed of sand grade coarse debris of mudstone,which shows a certain universality and regularity.The special lithologic section is comprehensively identified on the basis of core observation and description and in combination with the thin section observation,paleontological assemblage and some peculiar phenomena of logging responses.The study shows that this set of strata belongs to fans resulted from mud debris flow,a type of gravity flow.The fan bodies can be divided into 3 belts including root-fan,middle-fan and outer-fan.Laterally,this kind of fan develops in the slope break zone of faulted lake basins which is trench zone of lacustrine basin.Vertically,these fans are mainly found in LST of the Dainan Formation of Subei fault basin.No such fans occur in TST,and some occur in HST.Subei half-graben fault basin was formed in a special geological setting.A large complete depressed lake basin in the lower tectonic zone disintegrated into several mutually separated half graben fault depressions.Multi factors including confining of thick mudstone in the 4^th Member of Funing Formation,old and new dual provenances,tectonic difference,various transportation flow patterns and mutiple types of slope break zones coupled together and controlled the sedimentary characteristics of Dainan Formation.The rules are found that the mud debris flow fans and mud-rich fans are developed in slope break zone on the concave side of fault depression and sand-rich fans occur in slope break zones on the convex side of the fault depression.

Overpressure structure dissimilarity and its controlling factors in the Jiyang Depression

Wang Yongshi, Qiu Yibo

2017, 38(3):  430-437.  doi:10.11743/ogg20170302

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Jiyang depression,one of the important petroliferous depressions in Bohai Bay Basin of eastern China,has the most complex and diverse pressure systems.Based on statistics of the measured pressures,log responses of mudstone overpressures and characteristics of over-pressure profiles of many sags in Jiyang depression,we divide the overpressure systems in Dongying,Chezhen,Zhanhua and Huinin sags into three types,namely the single high overpressure system,the compound overpressure system and the single low overpressure system.The single overpressure system distributes laterally in Dongying and Chezhan sags and occurs vertically as one overpressure zone in overpressure Es³-Es⁴,with a relatively large overpressure amplitude and a pressure coefficient over 1.8.The compound overpressure system distributes laterally in Chezhen sag and occurs vertically as three separate overpressure zones in Es¹,Es³ and Es⁴ respectively.The single low overpressure system distributes laterally in Huimin sag and occurs vertically as only one overpressure zone in Es³ with a comparatively small overpressure amplitude and a pressure coefficient less than 1.4.The dissimilarities of the overpressure structures in different sags are closely related to the sequences and the thermal maturities of source rocks,the distribution of pressure sealing layers,and the fault activities,in the sags.The distribution and thermal evolution of source rocks control the formation and distribution of the overpressure system.The pressure sealing layers control the top and bottom boundaries of an overpressure system.The deep-rooted faults control both the inside and outside horizontal boundaries.

Shale gas reservoir characteristics and its exploration potential-analysis on the Lower Jurassic shale in the eastern Sichuan Basin

Nie Haikuan, Ma Xin, Yu Chuan, Ye Xin, Bian Ruikang, Liu Zhongbao

2017, 38(3):  438-447.  doi:10.11743/ogg20170303

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Through core and outcrop observation,thin section examination,argon-ion milling,scanning electronic microscope and mercury data et al,this paper studied the shale rock types,Microscopic types and characteristics of the Jurassic Ziliujing Formation. The reservoirs are dominated by black shale which mainly developed interparticle pores, the micro fractures and the organic matter pores are less developed. The shale porosity is between 1.4%-4.9% and the nanoscale micro-pores with diameter less than 100 nm account for more than 85% of total porosity. Compared with the marine shale gas reservoir in the study area,the organic matter pores are less porous and pore diameters are smaller in the Jurassic shale gas reservoir. Considering the large area and thick interval with oil/gas show of the Jurassic shale,and the success of shale oil/gas exploration in some spots,the favorable areas are suggested where the shales (source rocks) are thick and thermal maturity is high. These places may have good source charge and reservoir quality as shale oil and gas reservoirs. The tight sandstone or limestone in the vicinity of the shale may also have good possibility for hydrocarbon accumulation of tight oil or conventional reservoir.

Oil content and occurrence in low-medium mature organic-rich lacustrine shales: A case from the 1^st member of the Eocene-Oligocene Shahejie Formation in Well Luo-63 and Yi-21,Bonan Subsag, Bohai Bay Basin

Li Zhiming, Qian Menhui, Li Maowen, Jiang Qigui, Liu Peng, Rui Xiaoqing, Cao Tingting, Pan Yinhua

2017, 38(3):  448-456,466.  doi:10.11743/ogg20170304

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This paper studied oil content and occurrence in low to medium mature organic-rich lacustrine shales by taking the cored section of the 1st Member of the Eocene-Oligocene Shahejie Formation in Well Luo63 and Yi21 in Bonan subsag,Zhanhua sag,Bohai Bay Basin,as an example.Oil content was evaluated qualitatively by using chloroform bitumen "A" and oil saturation indices (OSI) after light hydrocarbon correction,and oil occurrence was studied both qualitatively and quantitatively through thin section observation,SEM observation,and thermal desorption.The results show that organic-rich shales in the 1^st Member of Shahejie Formation (Es₁) contain over 1% chloroform bitumen "A",whereas the oil saturation indices (OSI) indicate modest oil content to oil show,with a few samples display high potential for shale oil production,after light hydrocarbon correction.Oils retained in the organic-rich shales occur both in association with organic macerals (adsorption or dissolution in bitumenite and laminate alginate),and in bedding fractures,stress induced microfractures,intergranular and intercrystalline pores,grain boundary fractures and mineral dissolution pores.High molecular weight hydrocarbons occur mainly in association with insoluble organic matter,by adsorption or in dissolution,accounting for 68%~84% of total oil content (with an average of 75%).The free low to medium molecular weight hydrocarbons occur mainly in various fractures and pores,accounting for 16%~32% of total retained oil (with an average of 25%).The contents of practicable movable oil (sum contents of light hydrocarbon correction and thermal desorption at 200℃) are in the range from 0.62 mg/g to 1.76mg/g (with an average of 1.12 mg/g) and the movable oil ratio (the percentage of practicable movable oil content in total oil content) from 5.39% to 9.10%,with an average of 7.06%.The sum of light oil thermal desorption at 200℃ and light hydrocarbon correction can be used reasonably to evaluate shale oil resource potential in organic-rich shales.

Discovery of double slope breaks and its significance in Chang 8¹ sand group in Zhenyuan area,Ordos Basin

Wang Changyong, Zhang Yi, Lin Shiguo, Zheng Rongcai, Wei Yaqiong, Wang Haihong, Hou Changbing, Gou Xingfu

2017, 38(3):  457-466.  doi:10.11743/ogg20170305

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Through the detailed core observation and lithologic interpretation of logging data,rock association characteristics,sedimentary architectures,sedimentary facies and sequence stratigraphy of Chang 8 reservoir in Zhenyuan area were studied.The results show that a shallow braided-river delta sediments with two slope breaks was developed during the deposition Chang 8¹,and the paleo-shoreline moved frequently within tens of kilometers above the slope breaks.These slope breaks were mainly identified by logging and sedimentary architecture features.Slope breaks of Chang 8¹ sand group in Zhenyuan area were mainly controlled by basement faults affected by violent volcanic and tectonic activity in the southern and western Ordos Basin.During deposition of Chang 8¹,the study area can be divided into three geomorphology units,including platform-like plain (front) above the upper slope break,central-slope between the two slope breaks and lower-slope below the lower slope break.The distribution of sand body in transgression and regression stages was controlled by the ancient geomorphological features.The platform-like plain which was close to the provenance received more sandstone during transgression stage,and mudstone and siltstone deposited during regression stage.The central-slope that was farther from the provenance developed thicker sand bodies during lake transgression stage,while sands preferentially accumulated around the first slope during the lake regression stage.The lower-slope that was far away from the provenance developed small amount of sand bodies in anytime.The shoreline moved to the first slope in low water stand,and sand bodies around the slope break may feature "coastal sand bar" because of the transportation and transformation of waves and coastal current.

Characteristics of tight sandstone gas reservoir distribution under control of sedimentary facies: A case study from the Penglaizhen Formation in the central western Sichuan Depression

Chen Gongyang, Jin Jing, Luo Yingchun, Yin Senling, Zhang Ling, Zhang Shihua

2017, 38(3):  467-477.  doi:10.11743/ogg20170306

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To understand the influences of sedimentary facies on tight sandstone gas reservoirs,this paper studied in-detail the sedimentary facies and their control on tight sandstone gas reservoir distribution of the Penglaizhen Formation in the central western Sichuan Depression by using outcrop,core,logging and seismic data.The Penglaizhen Formation can be stratigraphically divided into 2 long-term,4 middle-term and 29 short-term sedimentary cycles.The deposition of Penglaizhen Formation is controlled primarily by the short-axis provenance in the northwestern Longmen Mountain and secondly by the long-axis provenance in the eastern part.Alluvial fan,alluvial plain,braided river delta and lacustrine depositional systems occur successively from piedmont of Longmen Mountain to Chengdu sag.The eastern area is dominated by a fluvial depositional system.The gas-bearing sand bodies of the Penglaizhen Formation mainly occur in the western delta plain,delta front distributary channels and river mouth bars,as well as the eastern fluvial point bars and mid-channel bars in the forms of discrete sands.The potential gas-bearing traps are jointly controlled by the superposition of sand bodies and slope structural setting.

Formation conditions of the Jurassic Xinhe Formation tight oil reservoirs and resource potential in the Yabrai Basin

Gao Gang, Zhao Leyi, Ma Guofu, Li Tao, Wang Jianguo, Yang Jun, Li Yudong, Xujin

2017, 38(3):  478-488.  doi:10.11743/ogg20170307

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The Yaburai Basin is a Jurassic lacustrine petroliferous basin in western China. In order to assess tight oil potential of the Xinhe Formation in the basin,we employed a geological factor superimposition method to select tight oil play fairway and a small cell volumetric method to calculate the resources based on analyses of source rocks and reservoiring conditions as well as their coupling relationship in the basin. The research shows that high-quality source rocks are mainly located in the lower Xinhe Formation in the Middle Jurassic. Tight oil reservoirs are mainly developed in this source rock interval featuring in large cumulative thickness,rich organic matter of favorable types,and being within oil window. Low-porosity and low-permeability tight reservoirs occur as thin interbeds and interlayers inside the source rocks. Crude found there shows clear signs of self-generation and self-preservation. The large area of contact between matured organic-rich source rocks and reservoirs and the effective sealing of massive mud rock cap are the most important controlling factors of tight oil reservoirs in the basin. The sub-sag Xiaohu was considered to be a play fairway based on analyses of maturity of source rocks. A comprehensive study of mud and wireline logging data as well as oil and gas shows were used to establish a standard (TOC>1%,0.6%<R_o<1.3%,reservoir porosity ≥ 4%,accumulative thickness>10m) to pinpoint the location of play fairway at the central and western part of the subsag. Calculation shows that the target holds an oil-in-place of 92 million tons and resource abundance of 345 thousand tons per square kilometers.

Characteristics and main controlling factors of the Cambrian pre-salt dolomite reservoirs in Tazhong Block,Tarim Basin

Ni Xinfeng, Huang Lili, Chen Yongquan, Zheng Jianfeng, Xiong Yixue, Zhu Yongjin, Yang Pengfei, Li Chang

2017, 38(3):  489-498.  doi:10.11743/ogg20170308

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Studies on the Cambrian pre-salt dolomite reservoirs is still rare in eastern Tarim Basin,although the exploration there has made great discovery. Research on reservoir types and characteristics,main controlling factors and distributions of the Cambrian pre-salt dolomite reservoirs were presented on the basis of drilling data,cores,thin section SEPM,logging and 3-D seismic data. The results show that grain dolomite reservoir of beach facies and gypsodolomite reservoir of tidal flat facies are two main reservoirs in the pre-salt dolomite of Cambrian in Tazhong Block,Tarim Basin. The former exists in the Lower Cambrian Xiaoerbulake Formation and the Middle Cambrain Upper Shayilike Formation. Lithologies include algae granular dolomite from shoals in platform proximate,algae dolomite and silty crystalline dolomite,and were mainly controlled by facies. The reservoir quality are mostly inherited from deposition and the porosity was improved by the later interlayer Karst processes. The latter mainly exists in the Middle Cambrian Awatage Formation and Middle Cambrain Lower Shayilike Formation. Lithology include gypsiferous dolostone,silty crystalline dolomite,gypsodolomite breccia and they were controlled by both petrography and later fracturing. For its microporous reservoir,the matrix formed during deposition while micropores formed due to later organic acid dissolution. Nowadays,the favorable objectives are most likely to be grain dolomite reservoirs of beach facies in Lower Cambrian Xiaoerbulake Formation.

Discovery of shallow-water delta in the Neogene Guantao Formation in the Liaodong Bay Depression and its significance for oil and gas exploration

Zhong Yijiang, Chen Hongde, Xu Changgui, Wang Jun, Zou Hao, Wei Peng, Du Xiaofeng, Huang Xiaobo

2017, 38(3):  499-507.  doi:10.11743/ogg20170309

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In order to break through the constraint of the earlier understandings of sedimentary facies on oil and gas exploration in the Neogene Guantao Formation in Liaodong Bay Depression,this paper studied the geological background and the evidences of the occurrence of lake and shallow-water delta through integration of sedimentary geology,geophysics and paleontology.The sedimentary patterns of the Guantao Formation and the Lower Minghua Formation in the Liaohe Depression,as well as the modern sedimentary patterns of the Zhuonai-Kusai Lake area were taken as analogies for comparison,and a semi-quantitative analysis method was used to determine paleohumidity and paleotemperature.It is believed that the Guantao Formation was deposited under subtropical climate conditions with increasing humidity,dominated by alluvial fan and braided river facies,and characterized by sedimentary pattern of shallow-water delta with a large plain and a small front.The delta plain consists of the upper plain dominated by alluvial fan and the lower plain dominated by braided river.The delta front features in small scale,instability,as well as underdeveloped mouth bar and distal bar.The area of closed lake was small during deposition of the lower Guantao Formation and delta plain deposits predominated.In contrast,the area of the open lake was enlarged during deposition of the upper Guantao Formation,delta front was well developed,and the subsags were connected by the braided river system.According to the new understandings on the shallow-water delta sedimentation system of the Neogene Guantao Formation in Liaodong Bay Depression,the front delta mudstone may act as a regional cap rock,providing an important geological guidance for future oil and gas reservoir exploration.

Diagenetic sequence and hydrocarbon accumulation phases of the Fuyu layer tight reservoir in the southern Songliao Basin

Huang Wenbiao, Deng Shouwei, Lu Shuangfang, Wang Weiming, Zhao Jing

2017, 38(3):  508-516.  doi:10.11743/ogg20170310

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The tight oil exploration of the Fuyu layer has achieved a significant breakthrough in southern Songliao Basin,but the tight oil charging characteristics and accumulation history are still unclear.Based on thin section observation and fluid inclusions occurrence characteristics,the growth sequence of various secondary mineral was analyzed.The time sequence of different diagenesis stages and hydrocarbon charging phases were established.According to the homogenization temperature of fluid inclusions in combination with geologic history and thermal history in the study area,the hydrocarbon accumulation phases were established in different blocks.The results show that the minerals mainly experienced compaction,cementation,metasomatism,dissolution processes in the Fuyu layer tight reservoir and the compaction runs through the whole diagenesis history.Hydrocarbon began to charge after the early calcite cementation and before the quartz overgrowth.And a large scale of hydrocarbon charging happened during the period of quartz overgrowth and feldspar dissolution.In the period of late calcite cementation,hydrocarbon charging entered the later stage.Liquid hydrocarbon continued to charge during the deposition of the Nenjiang Formation and Mingshui Formation in Changling Sag,while in Fuxin Uplift charging happened in the early period deposition of the Nenjiang Formation.There were two periods of gaseous hydrocarbon charging in uplifted area:the time of early Nenjiang Formation deposition and of Sifangtaizi-Yian Formations,respectively.

Comparative study on lacustrine shale and mudstone and its significance: A case from the 7^th member of Yanchang Formation in the Ordos Basin

Lin Senhu, Yuan Xuanjun, Yang Zhi

2017, 38(3):  517-523.  doi:10.11743/ogg20170311

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The 7^th member of Yanchang Formation (Chang 7) in Ordos Basin is rich in shale oil resources.However,a poor understanding of the differences of characteristics between shale and mudstone has hindered the exploitation of shale oil.Various methods including direct observation of core and thin section and analyses of XRD,Rock-Eval,TOC,chloroform bitumen "A" extraction as well as major and trace element analysis are used to study the sedimentological,mineralogical and geochemical characteristics of shale and mudstone in Chang 7 Member of Triassic Yanchang Formation in Ordos Basin,Northwest China.The results indicate that the shale shows darker color,obvious lamination and smooth surface,while the mudstone has grey color,no fissiles,and non-smooth cut surface.The organic matters in shale are mainly enriched along beddings,the organic matters in the mudstone are scattered.The clay content is high in the shale,while quartz content is relatively high in the mudstone.The average TOC of the shale (18.5%)is more than 5 times that of the mudstone and the hydrocarbon generation potential in shale is 5~8 times that in mudstone.More aromatic hydrocarbons and less saturated hydrocarbon is contained in the component of chloroform bitumen "A" of shale than that of mudstone.The shale was deposited in an environment with lightly lower paleosalinity and shallower paleo-water depth than that of the mudstone.The shales in Chang7 strike NW-SE and have thickness in the range of 15-30 m.The macroscopic distribution of shale oil is controlled by shale facies,therefore the comparative study between shale and mudstone is of great significance for accurate prediction of organic-rich shale and guiding shale oil/gas exploration.

Sedimentary boundary markers and geochemical indexes of shale sequence stratigraphy

Wei Lin, Xu Wenguo, Yang Cang, Huang Yizhou, Wang Qianyou, Wang Xinglong

2017, 38(3):  524-533.  doi:10.11743/ogg20170312

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One of the key issues in shale oil and gas exploration is to establish a high-precision sequence stratigraphy to accurately predict sweet spots.The relative lithological continuousness and subtle character varations of shale sediments have made it difficult to define key boundaries between sequences.This paper,based on a comprehensive investigation of case studies of shale reservoirs both in the North America and China,suggests that occurrences such as relatively shallow water facies,small scour surface,low-angle shale beds truncation,deformed soft sediments,shelly laminae and thrust clastics in shale,can be viewed as important sedimentary markers of sequence boundaries.For formations without the above-mentioned markers but of the same geological timing,the paper suggests that to recognize the sequence boundaries or the maximum flooding surface,the following proxies and methods should be applied:geochemical indexes such as redox conditions (e.g.organic geochemical parameters,inorganic trace/dominant element ratios) and organic matter/mineral accumulation patterns (e.g.organic matter content,sediment density,and mineral content),combined with observations of lithology facies interfaces,different types of lag deposits,pyrite ooids and phosphate nodules.The paper summaries the commonly acknowledged sedimentary features and geochemical indexes for sequence stratigraphy division with the help of well-logging data analyses,and proposes that,by using well-logging data,lithofacies combinations,sedimentary features and geochemical parameters,it is workable to divide third-order sequences and establish higher order of sequence stratigraphy.

Hydrocarbon expulsion of stylolite and matrix in carbonate rocks:A case study from the Ordovician and Carboniferous carbonate rocks in Eastern Sichuan Basin

Wang Xuan, Gao Gang, Li Jiaye, Ge Daiwei, Zhang Weiwei

2017, 38(3):  534-542.  doi:10.11743/ogg20170313

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This paper discusses the differences of hydrocarbon expulsion and compositions between stylolite and matrix in carbonate rocks.Samples were taken from the Ordovician and Carboniferous carbonate rock cores containing stylolites of Wells Zuo 3,Chi 53 and Wuke 1 in the Dachigan Structure Belt of the Eastern Sichuan Basin.The stylolite and matrix were studied through observation of core and cast thin sections and various tests such as whole rock X-Ray diffraction for mineral components and TOC content,Rock-Eval pyrolysis,solvent extraction and fractionation of extract and so on.The results indicate that there is a relatively large compositional difference between stylolite and matrix in carbonate rocks,and this is intimately related to pressure solution of carbonate mineral.Because of pressure solution,the stylolite contains more indissoluble materials such as dolomite,pyrite,and organic matter than the matrix.The mineral grains in stylolies mainly display basal cementation and matrix-supported fabric.While the mineral grains in the matrix mainly represent linear contact and close contact in concave and convex shapes.This makes stylolite to have better physical properties compared with the matrix,thus acting as important passages for expulsion of fluids in carbonate rocks in geological history.Therefore,the hydrocarbon expulsion effect of stylolite is more evident than that of the matrix.For the stylolite-bearing carbonate rocks with low organic abundance,the organic matter in the stylolite can be a prominent supplement of hydrocarbon generation in carbonate source rocks.In addition,the soluble organic matters relatively abundant in the stylolite have certain effects on Rock-Eval pyrolysis parameters.

Characteristics and diagenesis of tight sandstone reservoirs in the 4^th member of Xujiahe Formation,northern Sichuan Basin

Zhou Lin, Chen Bo, Fan Rui, Hao Jingyu, Wang Xin

2017, 38(3):  543-550,560.  doi:10.11743/ogg20170314

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Analyses based on thin section,SEM,mercury intrusion and logging data were combined with outcrop and core observations to study the characteristics and diagenesis of tight sandstone reservoirs in the fourth member of the Xujiahe Formation in northern Sichuan Basin.The results show a calcareous sandstone-dominated west part,a lithic sandstone-dominated north part and a feldspar/lithic sandstone-dominated east part,in the area of interest.The rock type zoning was found to be closely linked to the distribution of the three provenances at the periphery of northern basin.Physical properties can be summarized as ultra-low porosity and permeability.Types of pores were mainly secondary solution pores with small diameter and fine throat and micro fractures.Diagenesis were complicated and varied from compaction and pressure solution,to cementation,dissolution,replacement and alteration as well as fracturing.And all the processes had once played important roles in the formation of ultra-low permeability tight sandstone reservoirs.Compaction caused loss of primary pores by 77.52% and cementation destroyed reservoir properties through porosity reduction by 30%.Dissolution generated secondary pores and increased porosity by 3.28%.Later tectonic activities also helped shaping the reservoirs with secondary solution pores-fractures being the main accumulation space.

Reservoir quality differences and major factors controlling low-permeability reservoirs of Oilfield A in the Lufeng Sag,Pearl River Mouth Basin

Wan Qionghua, Liu Weixin, Luowei, Wang Xiuling, Chen Chen, Heng Liqun, Cao Pei

2017, 38(3):  551-560.  doi:10.11743/ogg20170315

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Since the initiation of oil and gas exploration and development in the Pearl River Mouth Basin,South China Sea,understanding of the Neogene marine reservoirs has been deepening.However,understanding of the deep reservoir quality is still weak due to the late initiation of deep exploration and development and the sparseness of offshore wells,thus is inefficient to guide exploration and development.By taking Oilfield A of Lufeng sag as an example,we compared the reservoir quality between the braided river delta facies in the Paleogene Wenchang Formation and the shorefacies in the Neogene Zhujiang Formation in respects of microscopic pore structure and macroscopic physical properties.In addition,we also analyzed the main controlling factors of the low porosity and permeability reservoirs in the Wenchang Formation,so as to guide development planning of this kind of oilfield.Results show that the reservoir quality of Oilfield A is generally controlled by the tectonic sedimentation,the sedimentation in turn controls the late diagenesis to some extent,resulting in the reservoir quality difference between the Zhujiang Formation and Wenchang Formation.Diagenesis is the major controlling factor of the low porosity and permeability reservoirs in the Wenchang Formation.Diagenesis weakens the control of original sedimentary fabric on reservoir quality.It lowers porosity and permeability of the reservoirs and influences the later development of the oilfield.

Main factors controlling hydrocarbon accumulation in northern subsag belt of the Zhu-1 Depression，Pearl River Mouth Basin

Liu Zhifeng, Wu Keqiang, Ke Ling, Wang Shenglan, Yu Kaiping, Zhu Wenqi

2017, 38(3):  561-569.  doi:10.11743/ogg20170316

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The dry wells in northern sub-sag belt of the Zhu-1 Depression,indicates that the "source" and "migration" account for a large proportion of well failure and are the two key controlling factors of hydrocarbon accumulation.For the source presence,based on seismic facies and geochemical analysis,five of the eight subsags,developing deep or semi-deep lacustrine source rocks,are identified to be favorable for hydrocarbon generation.For the migration,according to structural belt analysis on the plane view,five subsags in northern subsag belt feature gentle slope belt favorable for hydrocarbon enrichment.In vertical perspective,according to analysis of faults and Enping Formation mudstone sealing conditions of the five subsags,three of them belong to the shallow strata sealing type and the other two belong to the deep strata sealing type,favorable for shallow and deep accumulations respectively.

Coupling relationship between abnormal overpressure and hydrocarbon accumulation in a central overturned structural belt,Xihu Sag,East China Sea Basin

Chen Zhiyuan, Xu Zhixing, Xu Guosheng, Xu Fanghao, Liu Jinshui

2017, 38(3):  570-581.  doi:10.11743/ogg20170317

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Abnormal overpressure is generally developed in the Huagang and Pinghu Formations of the central overturned structural belt in Xihu Sag,East China Sea Shelf Basin.Oil & gas shows have been reported frequently in these overpressured formations.Formation pressure and pressure profiles of single wells in the formations were calculated and established through Eaton method and based on acoustic well logging,geochemical and testing data,and pressure distribution (both lateral and horizontal) of the area was also defined under the constraint of measured pressure data of the formations.Vertically,the abnormal pressure mainly initiates in the lower member of the Huagang Formation,and increases in stepwise way upwards;and laterally,its top boundary deepens from south to central of the belt.The PetroMod software was employed to simulate the evolution of the formation residual pressure.The results suggest that periods of 13-9 Ma and 4-0 Ma be the key pressure incremental stages.A combination of the above mentioned research with the history of hydrocarbon charging in the area indicates a well match between the times of large scale hydrocarbon charging and pressure increase.A coupling relationship between the abnormal high pressure evolution and hydrocarbon accumulations is finally defined.

Petroleum geology and resources assessment of major basins in Middle East and Central Asia

Tian Naxin, Yin Jinyin, Tao Chongzhi, Kong Fanjun

2017, 38(3):  582-591.  doi:10.11743/ogg20170318

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Through the study of regional geology,basin geology,petroleum system,play,hydrocarbon accumulation characteristics was delineated in various type of basins,and resources potential in major basins were evaluated.The plates of the Middle East and Central Asia experienced a divergent tectonic evolution in the south and a convergent evolution in the north.Four prototype basins are recognized:Cratonic basin,rift basin,passive margin basin and foreland basin.On the plane view,oil and gas are distributed in the Mesozoic paleo-passive margin in the northwest of Arabian plate,the southeastern margin of Eastern Europe platform,and the relatively stable areas in tectonically active zones such as the Zagros fold belt and Skiff-Turan tectonic belt.Stratigraphically,petroleum reserves are largely confined to five reservoir intervals:Carboniferous,Permian,Jurassic,Cretaceous and Neogene.Carbonate and clastic rocks contain the bulk of the petroleum reserves,with the former accounting for 80.45%.Based on the integrated geological assessment,this paper evaluates resources potential of 71 plays in 8 major basins.By taking the "play" as the assessment unit,and then type Ⅰ and Ⅱ favorable accumulation areas are determined in the Middle East and Central Asia region.

Geochemical characteristics and deposition environment of the Yogou Formation mudstone in the Termit Basin，Niger

Tang Wangxin, Jiang Zaixing, Liu Ruohan, Wang Xuying

2017, 38(3):  592-601,632.  doi:10.11743/ogg20170319

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Yogou Formation mudstone is one of the main hydrocarbon source rocks in the Termit Basin.In order to reveal the sediment source provenance attribute,tectonic setting and sedimentary environment of the Yogou Formation,geochemical elements of mudstones in the Yogou Formation were comprehensively analyzed,based on experimental data of the major elements,trace elements and rare earth elements of the 20 samples from 4 wells.The results show that the provenances of the upper Yogou Formation are derived from a passive margin source,while the provenances of the lower Yogou Formation are derived from a felsic source.The tectonic setting of the Yogou Formation provenance is similar to continental island arc.Water body during the deposition middle-upper Yogou Formation appears to have relatively lower salinity,in a marine-continental transition environment.The water body during the deposition of lower Yogou Formation appears to have relatively higher salinity,marine environment.According to CIA index and Sr/Cu ratio,the climate was warm-wet and water body was in the anaerobic environment based on the ratio of V/(V+Ni)during the Yogou Formation period.

Measurement and logging evaluation of total porosity and effective porosity of shale gas reservoirs：A case from the Silurian Longmaxi Formation shale in the Sichuan Basin

Li Jun, Wu Qingzhao, Lu Jing, Jin Wujun

2017, 38(3):  602-609.  doi:10.11743/ogg20170320

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There is no consensus on the understanding and measurement of effective porosity of shale gas reservoirs under irreducible water conditions in LAB. It gives rise to a great variance of core-measurement results,even on the same sample,and brings about difficulties and confusions in evaluation of shale gas reservoirs.To cope with the problem,both total and effective porosity are measured on paired-samples from the Longmaxi Formation in Sichuan Basin according to the criteria of GRI and SY/T 5336-2006,respectively.In addition,mineralogy,clay content and TOC are also measured.The results show that the total porosity and effective porosity of the paired-samples vary by a factor of 1 to 3.The large difference is mainly caused by the different treatment approaches of clay-related irreducible water volume in shale gas reservoirs.The gas-filled porosity (effective porosity) shows strong positive correlation with TOC.The coefficient of correlation is over 0.9.It implies that the organic matter pore is the main contributor to effective porosity in shale gas reservoirs.The irreducible water-filled porosity shows significant positive correlation with clay content.It implies that the clay-related micro-pores are occupied by irreducible water and are ineffective pores.A quantitative equation of effective porosity is established based on the above-mentioned understandings.Meanwhile,a logging evaluation method of total porosity is established based on the dry clay volume model.Both total porosity and effective porosity are calculated by using actual logging data,and the results coincide well with the core-measured results.

Mechanism of supercritical CO₂ flooding in low-permeability tight gas reservoirs

Shi Yunqing, Jia Ying, Pan Weiyi, Yan Jin, Huang Lei

2017, 38(3):  610-616.  doi:10.11743/ogg20170321

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The mechanism of supercritical CO₂ flooding in gas reservoirs was studied through both laboratory and numerical simulations to explore ways to increase recovery of low-permeability tight gas reservoirs.Behaviors of mixing supercri-tical CO₂ with natural gas were observed first to reveal the mixing patterns of the two phases.A laboratory quantitative measurement was carried out for an equilibrium phase behaviors to define the in-situ physical properties of both carbon dioxide and natural gas.The results show that the differences between the properties of the two gases are favorable for CO₂-based flooding and storage in gas reservoirs.A supercritical CO₂-natural gas diffusion test offered opportunities for observing behaviors of mixed gas front during flooding process.The result indicates that CO₂ does not diffuse much in na-tural gas and can form only a narrow mixing phase belt,which is good for an effective CO₂ displacement.Based on an analysis of CO₂ and natural gas mixing behaviors,we carried out a CO₂ displacement experiment on a long core from tight gas reservoirs.The results show that gas recovery rate can be increased by 12% after supercritical CO₂ flooding.Finally,based on the experiments,we used numerical simulation method to establish long core model,single injector-single producer dip mechanism model as well as anticline model.These models were then used to systematically verify the mechanism of supercritical CO₂ displacing natural gas.It suggests that a higher gas recovery is possible because of a partial mixing of natural gas with CO₂ at the flooding fronts helping maintain gas reservoir pressure and of a supercritical CO₂ "pad" formed at the lower part of gas traps favorable for gas recovery.The displacement mechanisms expounded through both laboratory works and numerical simulations provide a basis for selecting potential CO₂ flooding target in tight gas reservoirs.

Biodegradation and water washing effects on oil during water flooding

Chang Xiangchun, Zhao Wanchun, Xu Youde, Wang Tao, Cui Jing

2017, 38(3):  617-625.  doi:10.11743/ogg20170322

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Water flooding is one of the most commonly used methods for enhancing oil recovery.About 36 oil samples from 12 waterflooded wells in Quti-9 oil reservoir of Jiyang Depression were investigated for dynamic tracking and geochemical measurements,to characterize their quality alterations and component changing patterns during the water flooding production.The results show increasing saturates and decreasing aromatics in the samples through the progress of water flooding,the typical effect of water washing during water flooding.The Pr/n-C₁₇ and Ph/n-C₁₈ ratios change in two different ways:decrease first and increase later or increase first and decrease later,indicating an effect of a strata chromatography adsorption,along with the water washing effect or biodegradation.The increasing C₁₅8β(H)-drimane/C₁₆8β(H)-homodrimane ratio can equally suggest an alteration by biodegradation or water washing.The obvious increase of tricyclic terpanes/17α(H)-hopane,C₂₇-diasterane/C₂₇-sterane,sterane/17α(H)-hopane ratios,and subtle decrease of C₃₁R/C₃₀H ratios,all indicate biodegradation.The MPI₁,and DBT/P,C₂₈TAS 20S/(20S+20R) and C₂₆-/C₂₈ TAS(20S) ratios are constant,suggesting triaromatic steroid(TAS) is unaffected by water flooding.And the phenanthrene and dibenzothiophene show no clear signs of difference in solubility or biodegradation during the waterflooding.

Application of the reservoir interface prediction model in geosteering of horizontal wells

Zhang Yangyang, Li Qian, Wu Xuning

2017, 38(3):  626-632.  doi:10.11743/ogg20170323

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Current reservoir interface prediction method of pre-drilling geosteering models works well in prediction of stabile formations,but it gives a relatively big error when predicting reservoir interfaces in a complex structural setting.Selecting appropriate adjacent wells and establishing a rectangular coordinate system with north direction as the y axis we calculated the horizontal displacement of the horizontal wells and the corresponding depth of the reservoir interface in the rectangular coordinate system,and built a 2D reservoir interface prediction model by using the mathematical method of curve fitting.This model is a binary function of horizontal displacement and reservoir interface.The reservoir interface prediction model is used to determine reservoir dips and calculate apparent dip angles and the relative displacement between bit and reservoir boundaries.The model is applied to predict a horizontal well in JL block,and the results show that the final reservoir interface prediction model is a quadratic function with a fitting degree of 68%.The predicted vertical depth of reservoir interface basically coincides with the actual vertical depth,and the predicted relief of reservoir interface is consistent with the actual relief on map view.The results show that the reservoir interface prediction model with a fitting degree of 68% built based on the data of 6 adjacent wells can meet the needs of landing and geosteering.The accuracy of the prediction results is positively correlated with the number of adjacent wells chosen.The results can provide guidance for geosteering of horizontal well and effectively keep the horizontal well within the target interval.