Table of Content

28 October 2015, Volume 36 Issue 5

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Origin of the Ordovician gas and its accumulation patterns in Northwestern Ordos Basin

Zhao Jingzhou, Wang Daxing, Sun Liuyi, Bao Hongping, Xiao Hui, Wu Weitao, Chen Yongbo

2015, 36(5):  711-720.  doi:10.11743/ogg20150501

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In order to determine the source of natural gas and to reveal the factors controlling the accumulation and distribution of natural gas in the Ordovician reservoirs of the study area,we studied the composition of the natural gas through geochemical analysis and comprehensive geologic studies.The Ordovician gases are revealed to be entirely dry gas and their dryness indices range from 0.958 to 0.986.Geochemical analysis indicates that the Ordovician gases are mostly derived from the Carboniferous-Permian coal measure source rocks,whereas the marine Ordovician source is assumed to be a minor contribution.This is also supported by the Ordovician source rock analysis,which has revealed that the organic matter content of the mudstones and the carbonates is commonly not high.The average TOC of the mudstones in the Wulalike,Kelimoli,Lashizhong formations are only 0.54%,0.52% and 0.43%,respectively,and that of the carbonates in the above three formations are 0.35%,0.31% and 0.25%,respectively.Comprehensive studies suggest that the gas accumulation and distribution is principally controlled by structure,reservoirs,cap rocks,quality source rocks and migration pathways.Gases mainly enrich in places where the following 5 factors couple,namely structural highs of the slope,highly developed reservoirs with dissolution fractures and vugs,cap rocks of the Wulalike and Lashizhong formations,quality source rocks,and migration pathways for downward charging of gas from the Upper Paleozoic source rocks.

Unconformity of the Duyun Movement in the Sichuan Basin and its significance of petroleum geology

Sun Dongsheng, Li Shuangjian, Zhu Dongya, Zhang Dianwei, Wo Yujin, He Zhiliang

2015, 36(5):  721-728.  doi:10.11743/ogg20150502

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The Duyun Movement is the most prominent in the south of Guizhou Province and has been studied and accep-ted widely.However,the study on its geological performance and effect within the Sichuan Basin were seldom documented.Unconformity description based on outcrop,drilling and seismic data shows that there is an obvious angle unconformity between the Upper Ordovician Baota Formation and its overlying strata but no obvious angle unconformity is recognized between the Permian and the Silurian in the central and west of the Sichuan Basin,which means the Duyun Movement was the key tectonic movement effecting the formation of Chuanzhong paleo-uplift,the Kwangsian Orogeny occurred at the end of the Silurian did not cause any folding in central even the whole Sichuan Basin.The Duyun movement caused the exposure and erosion of the Ordovician Baota Formation in northwest of the Sichuan Basin,which constitute,together with the overlying Lower Silurian source rock,a new hydrocarbon accumulation model of upper source rock and lower reservoir.The movement also caused the formation of fractures in the core and flank area of Chuanzhong paleo-uplift and strengthened the formation of karst in the Sinian-Ordovician.It is proved that the Sinia-Ordovician karst reservoirs are well developed in areas covered by the Silurian around the Chuanzhong paleo-uplift,which are favorable exploration areas.

Characteristics of collapse breccias filling in caves of runoff zone in the Ordovician karst in Tahe oilfield,Tarim Basin

Jin Qiang, Tian Fei, Lu Xinbian, Kang Xun

2015, 36(5):  729-735,755.  doi:10.11743/ogg20150503

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Fractures and vugs are the major reservoir spaces in Tahe oilfield,but more than 70% of their spaces were filled by underground river sediments and cavern-roof collapse breccias,of which the latter accounts for over 30% in the cavern fillings.It is of great significant for reservoir geology of karst carbonates to study the formation and distribution of the collapse breccias.Combined with investigations on the Ordovician karst outcrops,observations of core samples and interpretations on well logging and seismic data,it was found that the collapse breccias are various in sizes and shapes in which small size of the breccias are transported by underground river and interbedded with sedimentary debris layers in caverns.The larger the caverns were(such as chamber caverns and main channel caverns)and the closer to active faults and epi-karst zone,the larger the thickness of the collapse breccias are.Porosities and permeabilities of the collapse breccias are low when the grain size of the breccias is smaller than 2cm.The breccias with bigger grain size would have better reservoir physical property and oil-bearing property,and thus should be one part of reservoir rocks in Tahe oilfield.

Zircon U-Pb dating of volcanic rocks of Changling Fault Depression in the southern Songliao Basin and its geological significance

Li Ruilei, Zhu Jianfeng, Liu Wei, Liu Manli, Lu Jianlin, Wang Baohua, Chen Guangyu

2015, 36(5):  736-744.  doi:10.11743/ogg20150504

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Volcanic rocks are the main basin fillings during the formation of fault depression in the Songliao Basin,and their accurate dating is the basis of the rifting timing and layer subdivision.Twenty volcanic rocks samples were collected from the Changling Fault Depression and dated with LA-ICP-MS zircon U-Pb ages.Zircon U-Pb dating shows that the previous recognized Huoshiling Fm volcanic rocks formed between 110～119 Ma and should be grouped into the 1^st Member of the Yingcheng Fm.While the previously recognized Yingcheng Fm volcanic rocks formed between 100～109 Ma and should be grouped into the 3^rd Member of the Yingcheng Fm.Petrologically,the volcanic rocks are composed dominantly of andesite,trachyte,rhyolitic tuff and rhyolite,which are displayed in TAS diagram.Geochemical data show that the volcanic rocks have similar features such as light rare earth elements enrichment and negative Eu anomalies,indicating their affinity.As the deepest structure unit in the southern Songliao Basin,new layer subdivision and correlation strategy need to be proposed.And the results also indicate that the syn-rift stage of the southern Songliao Basin started at the Early Cretaceous rather than the previously believed Late Jurassic,which means the corresponding structure-sedimentary and hydrocarbon formation history need to be further recognized.

Types,carbon/oxygen isotope characteristics and depositional environments of the Lower Ordovician contourites in Jiuxi and Yuanguping of North Hunan Province, China

Luo Shunshe, Lyu Qiqi, Xi Mingli, He Youbin, Xiang Ji

2015, 36(5):  745-755.  doi:10.11743/ogg20150505

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The literature on deep-water contourites is extensive after more than 50 years of research.However,many unknowns exist in terms of their forming mechanism and sedimentary settings.The article explores the characteristics of circulation and environment for forming various contourites in the Panjiazui Formation of the Lower Ordovician in the study area based on field measured section,detailed description and analyses of carbon and oxygen isotope characteristics.Lithologic analyses show that the formation contains 5 kinds of contourite lithofacies: calearenitie,calcilutite,caleilutitie,gravel and bioclastic contourities.Vertically,three types of incomplete vertical contourite sequences and some complete contourite sequences are recognized.The incomplete sequences include superimposed single calearenitie contourite sequence,interbedding calearenitie contourites and bioclastics contourite sequenceand interbedding calearenitie contourites and caleilutitie contourites sequencece.Systematically sampling and carbon/oxygen isotope analyses of the various contourites and autochthonous deposits indicate δ¹³C values varying between 0.18‰ and 2.31‰,δ¹⁸O values changing from -10.93‰ to -9.54‰,salinity index Z shifting from 122.91 to 127.14,and temperature(T) ranging between 16.98 and 24.81 ℃.Compared with the autochthonous deposits,the contourites in the formation have higher values of δ¹³C,Z and T.With the grain size of the rock getting bigger,these values continue to increase,showing that the sedimentary environment of contourites is totally different from that of the autochthonous deposits.The contourites were mainly formed in relative deep water environment with high salinity,and higher salinity and temperature were more favorable for the formation of certain high-energy contourites,such as caleilutitie and bioclastic contourities.The forming periods of contourite are all characterized by the periodical changes of first increasing and then decreasing palaeo-salinity,palaeo-temperature,and sea level.

Organic matter occurrence and microscopic mechanism of pore formation in the lacustrine tight carbonate reservoirs

Xiong Jinyu, Li Sitian, Tang Xuan, Chen Ruiyin, Wang Min, Huang Zhenglin, Sun Xining, Du Kefeng

2015, 36(5):  756-765.  doi:10.11743/ogg20150506

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Pore formation mechanism and prediction is one of the difficult scientific problems for study of lacustrine tight carbonate reservoirs.This paper aims to study the thermal evolution of pores during burial through hydrous pyrolysis experiments,in which the samples were collected from the lower part of the 3^rd Member of Shahejie Formation in the Shulu sag,Bohai Bay Basin.The geochemical analysis,focus ion bean milling Scanning Electronic Microscope(FIB-SEM)and lower temperature nitrogen adsorption were performed to examine the impacts of organic matter occurrence and thermal maturation on pore development and to reveal thermal evolution mechanism of organic pores in the tight reservoirs.The experimental result shows that the organic matter mainly occurs as combination of organic matter-argillaceous or marl matrix in the lacustrine carbonates,and the maturation has significant impact on the organic pore formation and evolution.With thermal maturity increasing from 0.5% to 1.5%(R_o),the total organic carbon(TOC)lowers more than half from 2.07% down to 0.85%,and porous volume and total specific surface area increase by 3 to 4 times.The organic pores is poor developed during the low thermal maturity-oil window(R_o=0.5%-1.0%),but increases dramatically during high thermal maturity-gas window stage(R_o≥1.5%)due to the development of pores and fractures.The decomposition of organic matter should be the primary explanation for the increase of pore volume and specific surface area.The newly generated pores are dominated by organic pores,with the micro-and meso-organic pores being most sensitive to maturity.The pores with diameter less than 5nm make the biggest contribution to the additional total pore volume and specific surface area.The increase of total pore volume and specific surface area mainly comes from the formation of new pores instead of the enlargement of the existing pores.

Controlling factors of natural gas accumulation in the Amu Darya Basin, Central Asia

Zhang Changbao, Luo Dongkun, Wei Chunguang

2015, 36(5):  766-773.  doi:10.11743/ogg20150507

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Amu Darya Basin in Central Asia features in complex structural deformations and multiple types of traps for natural gas accumulation.The major factors controlling gas accumulation in various reservoirs have both similarities and differences.In order to reveal reservoir distribution and select new targets for next phase of exploration in the Basin,reservoir and single well production data and geophysical data are integrated with regional geological characteristics,depositional settings and reservoir property to analyze the reservoir characteristics in the targeted area.It is suggested that two common controlling factors have played main roles in the formation of the natural gas reservoirs in the region.Laterally,gas reservoir distribution were controlled by the development of paleo-highs and reef complex,and the paleo-highs provided favorable structure settings for hydrocarbon accumulation and formation of effective reservoir spaces from exposure and dissolution.Meanwhile,the structural highs are the preferential target areas for hydrocarbon migration and accumulation.Vertically,gas reservoir distribution was controlled dominantly by trap conditions and carrier systems like faults,unconformities and permeable sandstones.These factors jointly controlled the spatial distribution of natural gas reservoirs.Based on the analysis,it is recommended that exploration of the lower play should be focused on the target areas with well-develo-ped reef complex within paleo-highs,fault/unconformity carrier systems and evaporite cap rocks.In contrast,exploration of the upper plays should be focused on areas where well-developed reef complex within paleo-highs are overlain by the Cretaceous mudstone seals outside the pinch-out areas of the lower evaporates.

Pore structure of tight sand reservoir and its influence on percolation —Taking the Chang 8 reservoir in Maling oilfield in Ordos Basin as an example

Ren Xiaoxia, Li Aifen, Wang Yongzheng, Wu Songtao, Wang Guijuan

2015, 36(5):  774-779.  doi:10.11743/ogg20150508

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The percolation of oil and gas in tight sandstone reservoirs is different from the conventional reservoirs because of its tight lithology,tiny pore and throat,Jamin effect and strong stress sensitivity.In order to understand the influences of pore structures on the percolation of oil and gas,the structural characteristics of micro pores of the Chang 8 reservoir were analyzed based on core and flake observation,SEM (scanning electron microscope) and mercury intrusion porosimetry.The results indicate that the reservoir has low surface porosity,complicated pore types and strong heterogeneity.The percentage of nanopore throats and submicron pore throats in the total pore throats is high(30%～55%)in the cores with permeability lower than 1×10^-3 μm²,while the percentage of micron pore throats in the total pore throats is higher in the cores with permeability higher than 1×10^-3 μm².The contributions of the pores with different throat sizes to the permeability were analyzed with the capillary's model.The result shows that the submicron size pore throats play a major role in the permeability of reservoir in this study.The result of core displacement experiments shows that the minimum starting pressure gradient of oil(S_wc)is negatively correlated with the largest pore sizes.The minimum starting pressure gradient of oil(S_wc)increases rapidly along with the decreasing of pore throat size when the largest pore throat radius is less than 1.0 μm.As the core permeability decreases,the throat distribution curve is shifted to the left,the pore throat radius lowers,and the nonlinearity section of the velocity-pressure gradient curve becomes longer.

Evaluation of effectiveness of metamorphosed basement buried hill reservoirs

Tong Kaijun, Cheng Qi, Nie Lingling, Fang Na

2015, 36(5):  780-787.  doi:10.11743/ogg20150509

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Evaluation of effectiveness of crystalline basement buried hill reservoirs has long been a difficult geological problem.In this paper,JZ25-1S metamorphosed basement buried hill in Bohai Bay Basin is taken as an example.First,fracture characteristic parameters are studied by using various data such as core,thin section,well logging,production test and experimental analysis.Then,the effectiveness of fractures and matrix in the buried hill reservoirs are evaluated based on the analysis of both dynamic data and static data.The research results show that fractures are highly developed in JZ25-1S and their effectiveness is high.The most effective fractures are open oblique fractures which are formed in later period,followed by the half-filled and full-filled vertical fractures,and the worst fractures are the filled horizontal fractures which are formed in early period.With the characteristics of rock physics,PLT and core centrifuge test,the lower limits of physical properties in buried hill reservoirs are determined.By using static imbibition experiment,it is possible to quantitatively evaluate the development potential of Matrix system.Capillary force is the main driving force for drainage process in matrix blocks.And the main factors influencing drainage process are the distribution and growth level of micro-fractures.

Factors controlling formation of effective reservoirs in deep of the Gaoyou Sag

Qin Weijun, Li Na, Fu Zhaohui

2015, 36(5):  788-792.  doi:10.11743/ogg20150510

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There developed three petroleum systems in the Gaoyou Sag,Subei Basin,including the upper,the middle and the lower petroleum system.The deep reservoirs belong to the lower petroleum system.The basic characteristics of hydrocarbon accumulation and the key factors controlling the formation of deep reservoirs are studied.Hydrocarbon accumulation in the lower petroleum system features in “upper source rock-lower reservoir”.That is,hydrocarbons sourced from the upper E₁f² migrated downward into the E₁f¹ under abnormal high pressure.The E₁f¹ reservoirs are mainly delta front sands with large volume but low porosity and low permeability.The result shows that development of the deep effective reservoir was controlled by sedimentary microfacies,burial depth and diagenesis as well as abnormal pressure and hydrocarbon charging.Underwater distributary channel and mouth bar are favorable sedimentary microfacies for effective reservoirs;The reservoirs are in the late diagenetic stage A1 and the secondary pores improved reservoir quality;The abnormal high pressure in the deep reservoirs is favorable for hydrocarbon charging,and the charging is earlier than sandstone densification,being conducive to reservoir space preservation.Among these factors,the favorable sedimentary microfacies and the development of secondary pore zone are the main factors controlling the development of deep effective reservoirs.

Reservoir prediction of the Cambrian reef flat facies of platform margin in Lunshen area

Fan Xingyan, Zhang Yan, Xiao Gaojie, Tang Xian

2015, 36(5):  793-803.  doi:10.11743/ogg20150511

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An extensive research has been performed on the lithofacies and carbonate platform types of the Cambrian in the Tarim Basin during the past decades.However,studies on the development characteristics,distribution patterns and high-quality reservoirs of the reef-flat facies are rarely documented because of the deep burial of the Cambrian,the sparse and limited exploratory wells that penetrate and reveal a relatively whole Cambrian as well as the limited quantity,cove-rage and quality of seismic data.Lunshen area located in the center of Lun Nan low salient ais taken as an example to analyze the drilling data and regional stratigraphy based on the previous study.It reveals that the Cambrian carbonate reservoirs have close relationship with sequence stratigraphy which has typical progradation characteristics of platform margin.Under the constraint of sequence stratigraphic framework and in combination with the analysis on seismic facies,a detailed description is performed on the characteristics of the Cambrian sedimentary strata in platform area,platform margin and basin area in different periods.Development characteristics,distribution and location of high-quality reservoirs of reef flat facies on platform margin are confirmed.It is of great significance for understanding the characteristics of deep reef flat facies on platform margin and selecting deep targets in Lunshen area.

Genesis of secondary pore of quartz dissolution type and its influences on reservoir: Taking the tight sandstone reservoir in the Upper Paleozoic of Daniudi gas field as an example

Qu Xiyu, Chen Xiu, Qiu Longwei, Zhang Manli, Zhang Xiangjin

2015, 36(5):  804-813.  doi:10.11743/ogg20150512

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By virtue of polarizing microscope,scanning electron microscope,cathode luminescence microscope,laser scan confocal microscope in combination with X-ray diffraction data and water rock interaction experiment,we studied quartz dissolution in tight sandstone reservoirs in the Upper Paleozoic of Daniudi gas field in Ordos Basin,and summarized the typical morphology features,genesis and its influences on tight sandstone reservoir.Results show that:(1)Quartz dissolution is common and has typical SEM morphology features as “rain print” and “honey comb” solution pits and/or vugs.(2)Quartz dissolution in Daniudigas field develops in alkaline environment,and the detailed evidences are as follows:①hydrothermal experiments verify that quartz dissolves more easily in alkaline environments than in acid conditions;②calcite cementation and metasomatism associated with quartz dissolution are common;③some horizons are rich in illite but poor in kaolinite,with some chlorite coatings occurring sporadically.(3)Quartz alkaline dissolution results in secondary pores such as intergranular dissolution pores and enlarged pores,leading to an average incremental porosity of about 6%.This is the cause of quality reservoirs in some tight sandstone intervals of Daniudi gas field.

Characteristics and geophysical prediction of lithology heterogeneity of oil shale: Taking the 2^nd Member of Funing Formation of Qintong sag, Subei Basin,as an example

Wang Chao, Lu Yongchao, Du Xuebin, Liu Zhanhong

2015, 36(5):  814-821.  doi:10.11743/ogg20150513

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In order to illustrate the lithologic heterogeneity characteristics and spatial-temporal distribution of oil shale of the 2^nd Member of Funing Formation in Qintong sag,Subei Basin,this paper makes research on lithology heterogeneity of shale through cores,well logging,geochemical tests and 3D seismic data.After analyzing rock type,well logs,TOC and mineral contents,the oil shale is divided into black shale,silty mudstone and lime mudstone.After the research on lithology division and isochronous sequence stratigraphic framework,Facies and Fluids Probabilities(FFP)method is used to predict the spatial-temporal distribution of various lithologies within the oil shale,and the prediction results are confirmed by the real logging data.The results indicate that in the early expansion system tract,lime mudstone widely deve-loped in the research area,meanwhile silty mudstone only developed in the southwest of Qintong sag.With the lake level rising,lime mudstone developed in the central sag and silty mudstone developed in the southwest and northeast of the sag.Black shale deposited in the entire area.In the late expansion system tract,black shale was widely deposited in the sag,while lime mudstone developed sporadically and silty mudstone only developed in the southwest and northeast of the sag.In conclusion,FFP can accurately predict the temporal and spatial distribution of various lithologies within oil shale,which has important significance for the exploration and development of oil shale.

Evolutionary patterns of non-tectonic fractures in shale during burial

Yuan Yusong, Zhou Yan, Qiu Dengfeng, Wang Qianqian

2015, 36(5):  822-827.  doi:10.11743/ogg20150514

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The main types of non-tectonic fractures and their depth distribution range are discussed based on the previous research and considering the inherent special properties of shale such as bedding,heterogeneity,anisotropy in mechanical properties,under-compaction and hydrocarbon generation.The critical confining pressure of shale fracture closing and its corresponding depth range are calculated by making use of porosity and permeability data measured under overburden pressure.A comprehensive analysis reveals that the non-tectonic fractures of shale can be categorized into five types,including suncrack,underwater shrinkage crack,bedding crack,overpressure fracture and modern hypergenetic fracture.The permeability data measured under confining pressure reveals that shale fracture should be closed under a vertical effective stress of about 15 MPa.The depth threshold of shale fracture closing is in the range of 870-1 138 m,averaging at 1 000 m.The depth of bottom boundary of overdressed shale is consistent with the depth of fracture(in horizontal direction)closing under overlying pressure and the fractures in shale under the bottom boundary of overpressure zone are no longer open.The formation and evolution of non-tectonic fractures of shale in burial history feature in time sequence/episode,i.e.different types of non-tectonic fractures develop in different stages from the deposition to maximum buried depth.Surface weather-shack,shallow subaqueous shrinkage cracks,mid-deep under-compaction overpressure fractures,deep hydrocarbon-generating overpressure cracks and super-deep closed fracture occur successively from shallow to deep.

A discussion on formation and evolutionary pattern of shale structural fracture: A case study of shale in western Sichuan Basin

Zhou Yan, Yuan Yusong, Qiu Dengfeng

2015, 36(5):  828-834.  doi:10.11743/ogg20150515

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Taking the shale in the eastern Sichuan Basin as examples,we studied the formation and evolutionary patterns of structural fractures in shale through field observation,rock mechanics experiment,physical simulation of photo elastic and finite element simulation in combination with statistical data of development via hydraulic fracturing. It is revealed that the shale has rich deposit rhythm,pre-existing weak surfaces and strong heterogeneity. Shale bedding is the preferred structure surface for fracture development and controls the formation and evolutionary characteristics; Stress will adjust its direction when encountering shale bedding during structural deformation. The compressive stress tends to parallel weak surface,while the tension stress tends to be vertical to weak surface,which leads to weak plane opening and preferred development bedding-parallel fractures. Bedding-crossing fractures would develop when the bedding-parallel fractures are not enough to accommodate tectonic deformation. Characteristics of shale fracture distribution have sharp distinction in different locations of tectonic deformation which can be divided into five areas such as anticlinal top high angle fractures,extension of bedding-crossing fracture to bedding-parallel fracture,slip conversion,bedding-crossing fracture climbing to bedding-parallel fracture and synclinal bottom high angle fractures. It is proposed that areas with multiple sets of cap rocks overlapping is a favorable condition for exploration on structural highs,while the development and expansion of fractures is the key factor for exploration in sweet spots.

Fine characterization of thick reservoirs with complex lithologies and its influence on oil recovery —A case from reservoirs of the 1^st and 2^nd Members of Shahejie Formation in A oilfield, Shijiutuo uplift in Bohai Bay Basin

Hu Xiaoqing, Fan Tingen, Wang Hui, Zhao Weiping, Zhang Yukun, Niu Tao, Xiao Dakun, Zhang Xianwen

2015, 36(5):  835-841.  doi:10.11743/ogg20150516

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The 1^st and 2^nd Members of Shahejie Formation in A oilfield in Shijiutuo uplift,Bohai Bay Basin,are rare proli-fic thick reservoirs with complex lithologies.Six lithologies are identified,mainly including limy sandstones,oolitic dolomites,dolomitic sandstones.There are various factors challenging reservoir description of the A oilfield,including complex lithologies,poor quality of seismic data,and sparse appraisal wells,large difficulty of fan delta reservoir description,and poor understanding of reservoir spatial distribution.This paper applied a well-logging and seismic comprehensive method,i.e.‘four step’ method,using ECS logging,core and seismic data.It consists of reservoir rock typing based on sedimentary facies,sedimentary modeling based on paleo-geomorphy restoration,reservoir framework reconstructing based on fine isochronous stratigraphy and reservoir prediction based on facies constraints.By using this four step method,sandbodies of 5 phases are identified and mapped in the 1^st and 2^nd Members of Shahejie Formation.In addition,good-quality reservoirs are predicted by using seismic technique,and 9 new development wells are proposed for emplacement on the Ⅰ-2 sand body with type Ⅰ reservoirs.This ‘four step’ method is very useful in estimating producible oil reserves,layer division,and well pattern optimization in similar oilfields.

Opening mechanism of dynamic fractures caused by water injection and effective adjustments in low permeability reservoirs, Daqing oilfield in Songliao Basin

Wang Wenhuan, Peng Huanhuan, Li Guangquan, Lyu Wenfeng, Wei Chenji, Qin Yong

2015, 36(5):  842-847.  doi:10.11743/ogg20150517

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The performance of water flooding in low permeability reservoirs of Daqing oilfield is not satisfied due to the dynamic induced fractures.Understanding of the opening mechanisms of the dynamic induced fractures and optimization of development adjustment schemes are critical to improving water flooding recovery.Geomechanics, reservoir engineering and numerical simulation methods were utilized to establish a new method to calculate the fracture opening pressure and reveal propagation mechanism.Corresponding adjustment measures are proposed for different fracture opening scenarios.Results indicate that when injection pressure exceeds the current minimum horizontal principle stress, dynamic fractures will open first in the direction of the maximum horizontal principle stress.Then, they may open in the direction of line connecting injector and producer that has the smallest included angle with current maximum principle stress along with the rising injection pressure.Based on calculation methods of fracture opening pressure and in combination with the conditions of A reservoir in the periphery of Daqing oilfield, a critical injection pressure of 9 MPa is determined for fracture opening.Actually the injection pressure of A oilfield is up to 12～14 MPa.When the line of injection wells is in parallel with the current maximum horizontal principle stress, fractures only open in the direction of the maximum horizontal principle stress, and remaining oil are distributed along the fractures on both sides.Otherwise, fractures open in multiple directions, and the remaining oil distribute irregularly.Based on the remaining oil distribution under different well patterns, a new concept is proposed that 'controlling the opening of multi-directional dynamic fractures through limiting injection pressure and line flooding along the dynamic fracture in parallel with the maximum horizontal principle stress and lateral matrix displacement'.Water injection pressure limits and well pattern adjustment strategies are proposed under the gui-dance of this concept, and their field application is very successfully.

Relationship between characteristics of tight oil reservoirs and fracturing fluid damage: A case from Chang 7 Member of the Triassic Yanchang Fm in Ordos Basin

Wang Minglei, Zhang Sui'an, Guan Hui, Liu Yuting, Guan Baoshan, Zhang Fudong, Cui Weixiang

2015, 36(5):  848-854.  doi:10.11743/ogg20150518

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Fracturing stimulation is the main method for development of tight oil.It can improve reservoir quality,but also can cause damages to reservoirs.Taking the Chang 7 Member of Yanchang Formation in Ordos Basin as an example,the relationship between the characteristics of tight oil reservoirs and fracturing fluid damage is studied based on experimental analysis,such as physical properties of reservoir,casting thin sections,scanning electron microscope(SEM),X-ray diffraction(XRD),rate-controlled mercury penetration,NMR,CT and sensitivity test.The Chang 7 Member is a typical tight oil reservoir in which the content of interstitial materials is as high as 15%.Illite,which is easy to transport and expand,accounts for a large proportion in the interstitial materials; Both pores and pore throats are of micro-nanometer scale,and the pore throats have poor connectivity.Large pores are generally controlled by small throats.The tight oil reservoir of the Chang 7 Member has moderate to weak velocity sensitivity(the damage rate of core permeability is in the range of 0.33-0.48),strong water sensitivity(the damage rate of core permeability is 0.14-0.28),and strong tendency of water blocking.Therefore,clay inhibitors,anti-swelling agents and cleanup additives should be added to the fracturing fluid formula to reduce the damage of fracturing fluid to the reservoir;salt sensitivity damage would occur when the salinity of the fracturing fluid running into the well is lower than 10,000 mg/L;the particle size of the fracturing fluid residue is 2.25-8.39 μm.For tight oil reservoirs,various damages such as filter cakes,sediments,absorption blocking and bridge plug can occur.It is considered through integrated study that using low damage fracturing fluids is the main way to reduce the residual damage.

Discussion on optimal injection pressure of fractured low-permeability sandstone reservoirs—A case study from Wangyao block in Ansai oilfield, Ordos Basin

Zhao Xiangyuan, Zeng Lianbo, Jin Baoguang, Jiao Jun, Li Peihai, Zhang Yangyu, Chen Minzheng

2015, 36(5):  855-861.  doi:10.11743/ogg20150519

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Analog outcrop,core and well logging data are integrated in this paper to calculate opening pressure of fractures and formation fracture pressure in different reservoir intervals and analyzed various factors affecting opening pressure of fractures based on evaluation of natural fracture characteristics.A sensitivity analysis was performed to determine proper injection pressure for low-permeability reservoirs.High dip-angle fractures are predominant in Chang-6 reservoir in Wangyao block in Ansai oilfield,and their main orientations are NEE-SWW,E-W,S-N and NW-SE,respectively.The opening pressures vary significantly in different reservoir intervals due to the control of buried depth,fracture orientation,pore fluid pressure and current geo-stress.The reasonable injection pressure should be determined by the maturity of local natural fractures accordingly.In areas with few natural fractures,water injection pressure should not be larger than formation fracture pressure,so as to avoid large-scale fracturing that leads to water breakthough.In areas with dense fractures,if the fracture opening pressure is less than the formation fracture pressure,the rational injection pressure should be set less than the formation fracture pressure to avoid large-scale opening and connection of natural fractures; otherwise the injection pressure should be properly set according to the formation fracture pressure to avoid new large-scale fracturing.

Distribution patterns of reservoir quality based on configuration unit of mouth bar sandbody:A case study from the 8^th sand group of the 2^nd member of the Shahejie Formation in Shenger Block of Shengtuo oilfield, East China

Zhang You, Hou Jiagen, Cao Yanqin, Zheng Xingping, Shao Guanmin, Jia Junshan, Bai Xiaojia, Duan Dongping

2015, 36(5):  862-872.  doi:10.11743/ogg20150520

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Based on core,thin section,SEM,well logging data and performance analysis and guided by reservoir configuration theory,configuration analysis of thick mouth bar sandstone in the second block in Shengtuo oilfield was carried out at three levels including composite mouth bar,single mouth bar and internal accretion within single mouth bar.On this basis,reservoir quality distribution patterns of different configuration units were analyzed.Result indicates that reservoir parameters of different configuration units are significantly different.Class Ⅰ and Ⅱ reservoirs mainly occur in the major parts of mouth bars.Class Ⅲ reservoir is generally distributed on flanks of mouth bar and under interlayers in the interior of mouth bars,while Class Ⅳ reservoirs mainly occur in sheet sands.In addition,dynamic characteristics of reservoir parameters in different configuration unit during waterflooding were discussed by using performance data.The reservoir porosity increased during waterflooding as a whole,but changed slightly.The largest increase appeared in the center of mouth bar,followed by bar flank,and nearly no changes occurred in sand sheet.In contrast,reservoir permeability showed different trends in different configuration units.Some units increased while others decreased.With constant flushing of injected water,effective permeability increased in high or ultra-high permeability reservoirs such as mouth bar center,decreased in middle or low permeability reservoirs like bar flanks and underwater distributary channels and remained unchanged in low-permeability sheet sand.The amplitude of porosity increase was much smaller than that of permeability.Median grain size of different configuration units all increased to some extent at both low and high water-cut stages,while clay content decreased.