Table of Content

08 October 2013, Volume 34 Issue 5

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Petroleum Geology

Hydrocarbon accumulation patterns of large tight oil and gas fields

Zhao Jingzhou, Li Jun, Cao Qing, Bai Yubin, Er Chuang, Wang Xiaomei, Xiao Hui, Wu Weitao

2013, 34(5):  573-583.  doi:10.11743/ogg20130501

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Three patterns of hydrocarbon accumulation in large tight oil/gas fields are proposed,namely,continuous accumulation (or basin-centered gas type or deep-basin gas type),quasi-continuous accumulation,and discontinuous (conventional trap type) accumulation.Like the continuous accumulation,the quasi-continuous accumulation also features in large area,without clear boundaries and water lags,reservoir being adjacent to source rocks and pervasive distribution.In addition,non-buoyancy-driven hydrocarbon migration is dominant.The major driving forcers of migration are overpressure,diffusion and capillary pressure,while buoyancy is limited.The migration is largely in the form of non-Darcy surge flow and diffusion flow.However,the quasi-continuous accumulation is different from the continuous accumulation in many aspects.The quasi-continuous accumulation commonly consists of many adjacent medium to small oil/gas reservoirs and the hydrocarbons are in quasi-continuous distribution.Oil/gas and water contact is complex and no regional updip water present.Hydrocarbon charging is mainly vertical and pervasive expulsion.Hydrocarbon accumulation is characte-rized by direct accumulation via primary migration or short-distance secondary migration.The reservoirs get tight prior to or contemporaneous with the hydrocarbon charging and are highly heterogeneous.The traps have certain controlling effect on hydrocarbon accumulation.The continuous unconventional accumulation represented by the basin-centred gas or deep-basin gas and the typical discontinuous conventional accumulation are representative of two end-member types of tight hydrocarbon accumulations in complicated geological settings.Hence some transitional varieties must exist between them and the quasi-continuous accumulation is just such a transitional type that might be a universal and predominated pattern for large oil/gas accumulation to occur in tight formations.In reality,a typical continuous accumulation ought to be that occurs within a source rock interval like shale gas or coalbed methane,while a typical discontinuous accumulation is that occurs within the conventional reservoirs outside or even far away from source rocks,and it is not only strictly confined by conventional traps but also possesses bottom or edge water.The hydrocarbon accumulations in tight reservoirs out of but close to source rocks are predominantly quasi-continuous accumulation,followed by atypical discontinuous accumulations.In contrast,continuous accumulations such as basin-centered gas or deep-basin gas are,if any,uncommon.

Gas source conditions of quasi-continuous accumulation of the Upper Paleozoic in Ordos Basin

Cao Qing, Zhao Jingzhou, Fu Jinhua, Yao Jingli, Liu Xinshe, Zhao Huitao, Hou Yundong, Fan Liyong

2013, 34(5):  584-591.  doi:10.11743/ogg20130502

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A comprehensive analysis of the Upper Paleozoic gas source conditions in Ordos Basin shows that high quality source rock is the major factor controlling the formation and distribution of large tight sandstone gas field in the study area.The lower limit of gas generation intensity for large tight sandstone gas field is determined based on analysis of gas production test results.A correlation analysis is first performed on the thickness,total organic carbon content (TOC) and maturity of the Upper Paleozoic source rocks in Ordos Basin,and gas generation intensity of the sources rocks is then calculated based on the correlation analysis.The calculation results are finally integrated with gas production tests to perform comprehensive analysis.The analysis of gas source conditions indicates that the Upper Paleozoic source rocks are characterized by high quality,wide distribution,high maturity,high intensity of gas generation and wide gas supply.In general,the intensity of gas generation ranges from 10×10⁸ m³/km² to 40×10⁸ m³/km².However,the gas generation intensity is not equilibrium in the whole basin.It ranges from 10×10⁸ m³/km² to 20×10⁸ m³/km² in the western basin,whereas,it is more than 16×10⁸ m³/km² in most part of eastern basin.In conclusion,high quality source rock is one of the major factors controlling the formation of large tight sandstone gas field,and it also controls the distribution of gas-water contact in the Upper Paleozoic tight sand reservoirs.As gas accumulation is closed to the source rocks and the accumulation efficiency is high,the lower limit of gas-generation intensity can be reduced to about 10×10⁸ m³/km² for Upper Paleozoic quasi-continuous tight gas accumulation in Ordos Basin.

Gas migration mechanism of quasi-continuous accumulation in the Upper Paleozoic of Ordos Basin

Li Jun, Zhao Jingzhou, Fan Yuanfang, Cao Qing, Wang Qian, Hu Weiqiang

2013, 34(5):  592-600.  doi:10.11743/ogg20130503

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Based on the natural gas geochemical behaviors and basic geological conditions for gas migration,this paper studied the gas migration characteristics of the quasi-continuous tight sand gas field of the Upper Paleozoic in Ordos Basin.Laterally,the variations of gas components and carbon isotope composition are mainly controlled by thermal maturity of source rocks (R_o),while vertically,they are affected jointly by thermal maturity of source rocks and gas migration fractionation.Further analysis of the migration driving force and pathway conditions indicates that the large tight gas field of the Upper Paleozoic is characterized by near-source accumulation via primary migration and short-distance secondary migration.The principle driving force for migration are diffusion force generated by the gradient of gas molecule concentration and abnormal high pressure,while buoyancy force has little or no contributions to gas migration.There are two main migration styles:surge flow caused by overpressure and diffusion flow resulted from diffusion.The overpressure-dri-ven surge flow is predominant in the Taiyuan Formation and the 2^nd Member of Shaanxi Formation within source rock interval,while the diffusion-driven diffusion flow is predominant,in the 1^st Member of Shaanxi Formation and the 8^th Member of Xiashihezi Formation as well as reservoirs above them.The vertical regular variations of gas composition and carbon isotope are the direct responses to these gas migration patterns in Sulige gas field.

Geochemical characteristics of primary gas in the Ordovician and their significance for the gas source of Jingbian gas field,Ordos Basin

Xiao Hui, Zhao Jingzhou, Wang Daxing, Ren Junfeng, Ma Zhanrong, Wu Weitao

2013, 34(5):  601-609.  doi:10.11743/ogg20130504

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Gas source of the Ordovician weathering crust gas reservoirs in Jingbian gas field is restudied by analyzing gas source of Well Yutan1 in Ordos Basin.An analysis of the geochemical features of source rock and a comparative analysis on carbon isotopes of natural gases are performed.The total organic carbon (TOC) of the Middle Ordovician Wulalike Formation is 0.30%-1.16% (averaging at 0.51%) and the thickness of dark mudstone is 52.59m.These two parameters indicate that this formation can be effective source rock.The carbon isotopes of methane in natural gas from Yutan-1 well are significantly lighter with δ¹³C₁ value in the range of -39.11‰--38.92‰,which is similar with the pyrolytic methane from carbonate source rocks.In contrast,the carbon isotopes of ethane are relatively heavier with δ¹³C₁ value in the range of -27.26‰ and -27.17‰,which seems to be the characteristics of coal-derived gas.However,the calculated gas maturity (R_o=1.86%-1.89%) from source rock thermal simulation experiment is coincided with the measured gas maturity (R_o=1.83%-1.92%) from source rock,both indicating high thermal maturity.In addition,the overlying thick Ordovician mudstone of the gas reservoir makes it difficult for the coal-derived gas from the Upper Paleozoic to migrate into the reservoir.All these evidences show that the Ordovician gas of Yutan-1 well should be oil-generated gas.Using the Ordovician gas of Yutan-1 well as the end member of oil-generated gas in Ordos basin,a comparative analysis is performed on the carbon isotopes of gas from the central-northern and southern Jingbian gas field.The following results are obtained.(1) Carbon isotope of methane should be considered as the major gas source index for the Ordovician gas,and δ¹³C₁<-38‰ is a index for identifying oil-generated gas in Jingbian weathering crust gas field;(2) The gas source in the entire Jingbian gas field is still dominated by mixed coal-derived gas of high maturity,but the proportion of oil-generated gases is higher in the southern Jingbian gas field than that in the central-northern Jingbian gas field;(3) Secondary pyrolysis of ethane may be the major factor causing heavier ethane carbon isotope of the oil-generated gas in the Ordovician.

Geological conditions for hydrocarbon accumulation in middle reservoir-source rock combination of the Ordovician Majiagou Formation on the east side of the paleo-uplift in Ordos Basin

Shi Baohong, Liu Yanan, Wu Chunying, Huang Zhengliang, Ren Junfeng

2013, 34(5):  610-618.  doi:10.11743/ogg20130505

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The dolomite reservoirs in middle reservoir-source rock combination of the Ordovician Majiagou Formation on the east side of paleo-uplift in Ordos Basin are obviously different from the weathering crust reservoirs at the top of the Ordovician in Jinbian gas field.A systematic study had been carried out on the geological conditions for hydrocarbon accumulation in these dolomite reservoirs in respect of source rocks,reservoirs,sedimentation based on results of reservoir lithology characterization and fluid inclusion testing as well as analysis of drilling data,geochemical and testing data.The results show that the middle reservoir-source rock combination of the Formation experienced strong hydrodynamics against a brief regression sedimentary settings.Beach facies were well developed and formed dolomite reservoirs.The dolomite features in highly automorphic coarse powder and fine-crystal texture,uniform grains,excellent connectivity and well-developed intercrystal pores.Affected by tectonic uplifting during the late Caledonian,the top Ordovician at eastern side of the paleo-uplift were denuded and came into contact with coal-measure source rocks,providing favorable source-reservoir combination.Controlled by lithofacies transformation at updip direction,the dolomites of beach facies in the middle reservoir-source rock combination of the Formation change gradually into micrite,forming discontinuous lithological traps.

Paleogeomorphology characteristics and oil accumulation patterns of Chang 1 oil reservoir in the Yanchang Formation,Ordos Basin: a case study of W area in the northern of Shaanxi

Zhang Fengqi, Yan Xiaoyong, Wu Fuli, Li Zhao, Wang Weixi, Bai Erlin

2013, 34(5):  619-624.  doi:10.11743/ogg20130506

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Revealing the accumulation patterns of Chang 1 oil reservoir of the Yangchang Formation in Ordos Basin is very significant for oil and gas exploration in this area.Based on stratigraphic division and correlation,the paleogeomorphology of Chang 1 oil reservoir is restored by using residual thickness methods combined with previous research of regional paleogeomorphology.On this basis,its relationship with the distribution of hydrocarbons is analyzed.The patterns of hydrocarbon accumulation of this formation in this region are summarized through a comprehensive analysis of various controlling factors on hydrocarbon accumulation such as structures,sedimentary facies,sandstone thickness,sealing conditions,and so on.Three geomorphic units are recognized in Chang 1 oil reservoir,namely erosion valley,erosion slope and erosion monadnock.The paleogeomorphology features in "one valley,two slopes and two monadnocks".The erosion monadnocks in relatively higher terrain,the ridges of nose-like uplift,channel sands with larger thickness and higher net-to-gross ratio all have significant controlling effects on distribution of hydrocarbons of this formation.Moreover,cyclical mudstone,tight sandstone,erosion unconformity surfaces and hydrodynamic force all have large contributions to the sealing of the Chang 1 oil reservoir.Structural-lithologic reservoir and lithologic reservoir are the primary reservoir types,while lithologic-hydrodynamic reservoir is the secondary reservoir type.The overlapping regions of the main factors,such as the higher paleogeomorphology and the favorable sedimentary facies zones with larger sandstone thickness are the key exploration targets of this formation in Ordos Basin.

Geochemical behaviors and genesis of formation water in 8^th Member of Xiashihezi Formation in western Sulige gas field,Ordos Basin

Liang Jiwei, Li Rongxi, Chen Yuliang

2013, 34(5):  625-630.  doi:10.11743/ogg20130507

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Genesis of formation water of 8^th Member of Xiashihezi Formation (H8) and their relationships with gas accumulation in western Sulige gas field of Ordos Basin in central China was studied by means of analysis of the geochemical behaviors of formation water samples from 39 gas wells.The distribution regularity of oil,gas and water was investigated.There is an obvious difference in the concentration(r)of anion and cation of formation water.Content of Cl^- is very high and dominant among the anions of the water,while that of HCO₃^- and SO₄^2- are very low.Content of Ca²⁺ is the highest among the cations of formation water,followed by K⁺+Na⁺,while the content of Mg²⁺ is low.The formation water show property of weak acidity with medium-high salinity.The calcium chloride Ⅴ-type water is predominant,with the calcium chloride Ⅲ-type and Ⅳ-type water occurring locally.The chemical characteristic parameters of the formation water are characterized by low sodium-chloride ratio (r_Na⁺/r_Cl^-),high coefficient of desulfurization (2×100×r_SO4^2-/r_Cl^-),high metamorphic coefficient[1/2(r_Cl^--r_Na⁺)/r_Mg²⁺]and low magnesium-chloride ratio (r_Mg²⁺/r_Ca²⁺).The formation water of H8 in western Sulige gas field has the characteristics of gas-associated water.It could be residual formation water formed when gas was charged under a closed and reducing environment.The distribution of formation water is controlled by sand body and reservoir conditions.Three genetic types of formation water are identified,namely the closed formation water within isolated sand body,the residual water with weak dynamic and incomplete gas-water displacement,and the stagnant water at lower position of large sand body.

Accumulation conditions and characteristics of the Chang 7 tight oil reservoir of the the Yanchang Formation in Zhidan area,Ordos Basin

Bai Yubin, Zhao Jingzhou, Zhao Zilong, Yin Yueyue, Tong Jiangnan

2013, 34(5):  631-639.  doi:10.11743/ogg20130508

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Through integration of geological and geochemical data,this paper comprehensively analyzed the conditions,main controlling factors and characteristics of oil accumulation in the Chang 7 tight oil reservoir from such aspects as the tight oil distribution,physical properties,geochemical features of the oil source,reservoir characteristics and heterogeneity,trap conditions,and oil and water distribution.The following results were obtained.The Chang 7 tight oil reservoirs own the geological conditions and characteristics for the formation of quasi-continuous oil and gas accumulation.The Chang 7 reservoir interval contains high-quality source rocks,of which the Zhangjiatan shale in the Chang 7³ layer is the best one.The outer boundary of the tight oil reservoirs is controlled by the distribution of the source rocks.The favorable area of hydrocarbon accumulation occur in where the thickness of hydrocarbon source rocks is over 20 m.The Chang 7 tight oil accumulation has favorable source-reservoir-seal assemblages and the gravity flow sandbodies occur in the mudstone or source rocks mainly in lenticular or stripped shapes,thus are advantageous in trapping oil and gas.The importance of trapping conditions is relatively low for the accumulation of oil in the Chang 7 tight reservoir,as the main seals are lithological,physical property and diagenesis boundaries.Dynamic trap is also an important trap type.Oil enrichment in the Chang 7 tight reservoir has no direct relationship with the local tectonic background,instead is mainly controlled by reservoir distribution and their physical properties.The high intensity of hydrocarbon generation,expulsion and charging results in the relatively high original oil saturation in the Chang 7 reservoir.Due to the influences of the origin material composition and diagenetic intensity,the Chang 7 reservoir is of strong heterogeneity,resulting in the poor differentiation of oil,gas and water,and a complicated oil-water contact in the tight reservoir.

Phase state changes of formation water in gas accumulation process: a case study from the Upper Paleozoic,Ordos Basin

Zhang Chunlin, Pang Xiongqi, Tian Shicheng, Xu Huazheng, Zhang Fudong, Liu Rui'e

2013, 34(5):  640-645.  doi:10.11743/ogg20130509

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According to the production characteristics of the abnormal high temperature gas reservoir,the producing me-chanism of condensate water in gas wells,and the salinity characteristics of the produced water in the Qianmiqiao buried hill,we believe that the gas reservoir of the Upper Paleozoic was formed during the abnormal high temperature phase in Ordos Basin.The abnormal high temperature resulted in not only the generation of methane but also the vaporization of some formation water and the formation of abnormal high pressure.The gas (vapor) diffused upward under the driving of the abnormal high pressure.This gas migration led to the decreasing of pressure in the lower strata and the speeding up of formation water vaporization.The vaporization of formation water may in turn increase the pressure of the lower strata,leading to a new round of hydrocarbon migration.This process may repeat again and again,gradually making the me-thane,the vapor,the associated temperature and pressure diffuse to the whole compartment,realizing local balance of potential field in the compartment,and finally resulting in the formation of gas reservoirs of high temperature and pressure in the basin.After that,uplifting and erosion cause the lowering of temperature and pressure,while liquefaction of vapor leads to the decreasing of vapor density,methane concentration and gas column pressure,resulting in the formation of underpressured gas reservoirs.

Distribution characteristics and genesis of present formation pressure of the Upper Paleozoic in the eastern Ordos Basin

Wang Xiaomei, Zhao Jingzhou, Liu Xinshe, Zhao Xiaohui, Cao Qing

2013, 34(5):  646-651.  doi:10.11743/ogg20130510

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Distribution of present formation pressure is the final result of joint action of many factors on pore fluid,such as basin tectonic evolution,sedimentary environment and thermal evolution of organic matter in source rock in geologic history.Analysis of pressure data obtained from pumping tests and well tests shows that both the Cretaceous and Triassic in the Ordos basin feature in lower pressure.In contrast,the Upper Paleozoic in the eastern Ordos Basin shows normal pressure,abnormal low pressure and abnormal high pressure in different places and intervals.The normal pressure type is dominated,accounting for 56.76%,followed by the abnormal low pressure type which accounts for 30.63%,while the abnormal high pressure type is the most rare.The pressure coefficients of the Lower Shihezi Formation,Shanxi Formation,Taiyuan Formation and Benxi Formation in the eastern Ordos Basin drop with increasing burial depth.Taken the Zizhou gas field as an example,several separated pressure systems are identified in the 2^nd member of Shanxi Formation base on the pressure gradient method.These pressure systems are separated and disconnected.According to fluid inclusion analysis,the formation was over pressured with pressure coefficients ranging from 1.14 to 1.66 at the end of Early Cretaceous.Current normal pressure in the eastern Ordos Basin is the result of the last stage of pressure evolution.The formation temperature decreases due to basement uplift and subsequent erosion as well as regression of the tectonic-thermal event,leading to formation pressure reduction by 8.6-11.1 MPa,which accounts for 32%~40% of the total pressure drop.Pressure drop caused by gas dissipation accounts for 20%~30% of the total pressure drop.The higher pressure coefficients in the study area are mainly resulted from its shallower burial depth.The high-relief landform and difference of gas enrichment cause different pressure coefficients among different gas fields in the study area.

Sequences and Reservoirs

Carbon and oxygen stable isotopic features of diagenetic facies of Ordovician carbonate rocks in Ordos Basin

Wang Qicong, Zhang Yang, Xiao Ling

2013, 34(5):  652-658.  doi:10.11743/ogg20130511

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Based on thin section and cathodoluminescence imaging,this paper discussed the types of diagenetic facies and the characteristics of lithofacies of Ordovician carbonate rocks in Ordos Basin.In combination with carbon and oxygen stable isotopic characteristics,we also studied the geochemical characteristics of each diagenetic facies.During the early Paleozoic,the Ordovician carbonate platform was in a near-surface diagenetic tract and can be divided into four diagenetic environments and facies including normal seawater,evaporative seawater,early meteoric freshwater and hot seawater according to the diagenetic medium.The δ¹⁸O and δ¹³C of diagenetic facies of normal seawater,hot seawater and evaporative seawater are generally high,but tend to decrease along with the increasing bioclastic content.In contrast,the δ¹⁸O and δ¹³C of the early freshwater diagenetic facies,are generally low.During the late Paleozoic,the top of the Ordovician was in the epidiagenetic tract.According to the rock types of the current residual strata,the epidiagenetic tract can be divided into limestone weathering diagenetic facies in the Fengfeng Formation and dolomite weathering diagenetic facies in the Majiagou Formation.Their δ¹⁸O and δ¹³C values are significantly lower than that of the early freshwater diagenetic facies.At the end of the Late Paleozoic,the Ordovician Formation was in the moderate-deep burial diagenetic tract at a depth of 3 000-4 000 m,which can be divided into a closed system diagenetic facies and an open system diagenetic facies.The δ¹⁸O and δ¹³C values of the closed system diagenetic facies are similar with their original rocks.In contrast,the δ¹⁸O and δ¹³C values of the open system diagenetic facies are much lower than their original rocks because of the isotope fractionation effect in the hot environment.These results indicate that the petrographic characteristics and the δ¹⁸O and δ¹³C va-lues of carbonate rocks are important indicators for identification of the diagenetic environment and classification of diagenetic facies of carbonate rocks.

Occurrence,genesis and evolution of evaporite minerals in dolomite reservoir: a case study of the Majiagou Formation in gas fields of the central Ordos Basin

He Jiang, Zhao Zhongjun, Qiao Lin, Wei Wei, Wang Yajie, Feng Chunqiang, Chen Bo, Chen Chao

2013, 34(5):  659-666.  doi:10.11743/ogg20130512

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Anhydrite columnar crystal and nodules of millimeter to centimeter sizes are widely developed in the crystal powder dolomite of Ma 5¹-5⁴ (the 1^st to 4^th submembers of the 5^th member of Majiagou Formation) in the gas fields of the central Ordos Basin,and are critical to reservoir development.The occurrence,genesis and evolution of anhydrite were finely analyzed for the first time through detailed subsurface geological survey,observation of typical core section,system sampling and testing,indoor comparative analysis,and so on.The anhydrite columnar crystals and small nodules occur in crystal powder dolomite.The columnar crystals are scattered in the dolomite,while the small nodules have three typical distribution forms,namely medium nuclear diameter in middle-high nucleation number,small nuclear diameter in middle-high nucleation number and large nuclear diameter in low nucleation number.All of the anhydrite minerals mentioned above are the products of metasomasis of early dolomite during penecontemporaneous or shallow burial periods in an restricted (inside) continental shelf environment.of the amount of anhydrite minerals depends on the seawater salinity at the beginning of anhydrite precipitation,while their sizes are controlled by the duration of seawater salinity that is suitable for the growth of anhydrite minerals.Moreover,different evaporite mineral associations may experience different process of modification.Residual moldic pores and associated cracks are highly developed in the coarse crystal powder dolomite of medium nuclear diameter in middle-high nucleation number,which finally evolved into the high quality fractured-vuggy reservoir in the area.

Oil accumulation patterns of Chang 4+5 and Chang 6 in Yan’an delta,Ordos Basin

Li Airong, Wang Weixi, Wu Fuli, Zhao Jingzhou, Zhang Hui

2013, 34(5):  667-671.  doi:10.11743/ogg20130513

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Yan'an Delta is located in the southeastern part of Shanbei slope in Ordos Basin and contains several oil reservoirs.Chang 4+5 and Chang 6 are the main pay zones.According to sedimentary facies,reservoir-seal assemblage and distance from source,this paper established several hydrocarbon accumulation patterns.The superimposed channel sandbodies in the delta are the major migration pathways for the hydrocarbon,and they provided conditions for long distance lateral and vertical migration of hydrocarbons.According to the distance between source and reservoir and the ability of oil accumulation,the reservoirs can be divided into three types,i.e,proximal accumulation,distal accumulation and pro-ximal dissipation.The former two types of accumulation areas have better oil enrichment conditions as the hydrocarbon migration and/or sealing conditions are favorable,while the later is limited in hydrocarbon accumulation capacity despite its shortest distance to oil source due to the migration of hydrocarbons to structural high.According to the migration pathways'quality and reservoir location,the accumulation pattern of Yan'an Delta are divided into five types,i.e.the proximal accumulation pattern with main migration pathways,the distal accumulation pattern with main migration pathways,the proximal accumulation pattern with sub-migration pathways,the distal accumulation pattern with sub-migration pathways and the proximal dissipation pattern.

Sedimentary characteristics of the shore-shallow lacustrine beach bar system of Yan 7+8 oil reservoirs in southern Ordos Basin

Wang Jianmin

2013, 34(5):  672-678.  doi:10.11743/ogg20130514

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Retrograding sedimentary sequence and shore-shallow lacustrine beach bar system are developed in the Mesozoic Jurassic Yan'an Formation in southern Ordos Basin,and the latter has close paragenetic relationship with the meandering river delta around the lake basin of the Yan'an stage.Retrogradation and the destruction of the meandering delta provide clastic input for the development of the low-shallow lake beach bar system.During the depostion of Yan 7+8,An'sai and Zhidan areas in the north of Shaaxi province were in shore-shallow lacustrine environment.Beach bar deposits were widely developed as the major part of sedimentation,and can be divided into four microfacies types including beach sand,sand bar,sand sheet and mudstone.Generally,the beach bar sand bodies occur at the side of the river mouth and the flat open shore-shallow lake environment,and are parallel with the lake shoreline.They are dominated by medium and fine sandstone,and have massive bedding,wavy bedding,inclined wavy bedding and lenticular bedding,etc.Beach sand is usually in sheet-like shape and is relatively thin.Bar sand,which is the most representative deposit of the beach bar system,usually has banded shoreline-parallel distribution,large thickness,and lenticular shape on section,reverse grading at the bottom and transgressive sequences in the middle and upper parts.A series of beach bar sandbodies superimposed with each other in different phases,forming large-scale shoreline-parallel clustered reservoirs with high net-to-gross ratio.The mudstones of shore-shallow lake facies between the beach bars act as lateral barriers or overlying seals of the beach bar sandbodies.The sedimentary characteristics of the shore-shallow lacustrine beach bar system have significant influences on reservoir development and oil/gas accumulation of the Yan'an Formation.

Characteristics of the Chang 8 tight sandstone reservoirs and their genesis in Huaqing area,Ordos Basin

Zhang Jizhi, Chen Shijia, Xiao Yan, Lu Jungang, Yang Guping, Tang Haiping, Liu Chaowei

2013, 34(5):  679-684.  doi:10.11743/ogg20130515

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Chang 8 sandstone of the Upper Triassic Yanchang Formation in Huaqing area,Ordos Basin,is a typical set of reservoirs with low-ultra low porosity and permeability.Its average porosity is 8.27% and average permeability is 0.51×10^-3 μm².The main type of pore space is residual inter-granular pore,and the main type of pore structure is tiny and micro throat.The Chang 8 reservoir has experienced complex diagenesis,such as compaction,cementation,and dissolution.The destructive diagenesis of compaction and cementation sharply reduce the primary pores of the reservoir,besides,some pores are filled with bitumen,resulting in the lowering of porosity and permeability and the tightness of the reservoir.Diagenis is a determining factor in reservoir tightening,while depositional environment is the basic controlling factor of the reservoir physical property.The underwater distributary channel sands in delta front have the best physical property,and the central part of the sandbody is better than its two sides.With the increasing of interstilital matter,the physical properties get poorer.

Sedimentation and diagenesis of Chang 8² reservoir in the Yanchang Formation in Jiyuan region, Ordos Basin

Chen Zhaobing, Zhu Yushuang, Chen Xinjing, Niu Xiaobing, Zhou Shuxun, Wang Xiujuan, Zhao Aibing

2013, 34(5):  685-693.  doi:10.11743/ogg20130516

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The structure of Jiyuan region in the Ordos Basin was relatively stable during the deposition of Chang 8².In this period,a shallow water delta depositional system was developed,the lake shore line slightly migrated in the southeast,and distributary channel and underwater distributary channel constituted the deposition framework.This paper studied the diagenesis of Chang 8² reservoir based on large amount of thin sections,core observation,SEM samples,X-ray analysis,and Cathodoluminescence analysis.The study shows that the diagenesis has double effects on the reservoir.On one hand,early development of chlorite film has a positive effect on anti-compaction.The early calcite is easy to be dissolved by organic acid,resulting in secondary pores and improving poroperm characteristics.Dissolution is the major mechanism of secondary pore formation and can greatly improve reservoir poroperm characteristics.On the other hand,compaction destroys the primary porosity formed in early digenetic process,and late ferrocalcite fills pores,resulting in local strong calcareous cementation zone.In general,the sedimentary facies and diagenetic facies control the distribution of oil and gas.The former determines the reservoir shape and dimensions.While the later modifies reservoir pore space.Late diagenesis-modified chlorite-cemented sedimentary facies with residual inter-granular pores and that with feldspar-dissolution plus residual inter-granular pores are the major reservoir spaces for hydrocarbon accumulation.

Reservoir characteristics of Chang 8 and single-well productivity prediction of M wellblock in Huanxian oil district,Longdong area

Zhao Junlong, Yan Bo, Zhao Jingzhou, Wang Yiping, Xu Dengcai, Gao Xiuli

2013, 34(5):  694-699.  doi:10.11743/ogg20130517

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For the purpose of making rational single-well production programs for Chang 8 reservoir of M wellblock in Huanxian oil district area,Longdong and optimizing productivity of M wellblock,we performed a comprehensive study through integrating static geologic features of the reservoir such as lithologies,physical properties,oil-bearing properties with dynamic features like seepage flow patternss and production performance.Based on the geological data,core data,logging data and phase permeability data of the M wellblock,we carried out the research on reservoir characteristics,residual oil distribution and single-well productivity,and predicted the reasonable single-well production of this area.Chang 8 reservoir of M wellblock shows typical ultra-low permeability,significant logging response features and double wettability to oil and water.The reservoirs in M wellblock are structural-lithological reservoirs and the oil-water distribution is controlled by lithology.Oil-bearing properties are good in reservoirs with good poroperm characteristics along the main channel and in reservoirs on relative structural highs.Current production of M wellblock is mainly from southeastern-central part of M wellblock,with residual oil mainly occurring in the northern part and local southern-central part.The physical parameter method,test data conversion method and production test method are suitable for the prediction of single-well production in the study area.The rational single-well production is 3.5~4.0 t/d.These results provide important data-base for the further development of Chang 8 reservoir.

Genesis of illite in Chang 7 tight oil reservoir in Heshui area,Ordos Basin

Tian Jianfeng, Gao Yongli, Zhang Pengbo, Wang Xiujuan, Yang Youyun

2013, 34(5):  700-707.  doi:10.11743/ogg20130518

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Chang 7 reservoir in Heshui area,Ordos Basin is a typical tight oil reservoir with high content of illite up to 9.1%,which is the key factor controlling the quality of reservoir.Analyses of the composition,distribution and occurrence of illite through SEM,EDS,cast thin section and X-diffraction show that the Chang 7 reservoir is very fine turbidity sandstones.The pervasive dissolution of K-feldspar yield lots of secondary pore which is the main pore type.The illites are complex in chemical composition and are mainly in scattered,chaotic sheet-like shape.They only occur in residue intergranular pores.And their content increases with the decreasing of grain size.In comparison with the adjacent shales,the relative content of clay mineral is similar,but the K-feldspar/plagioclase ratio is smaller.Volcanic activities were very intense during the deposition of Chang 7,which provides the favorable condition for semctite development at the early diagenetic stage.The semctites were transformed to illites subsequently.This process may be the main genesis of illite in Chang 7,as illitization of smectite is very common.In addition,the palaeo-geotemperature in the study area is generally lower than 120 ℃ and there are no external potassium-rich hot fluid input,making it impossible for the formation of illite of other genesis.

Reservoir characteristics of the organic-rich shales of the Triassic Yanchang Formation in Ordos Basin

Er Chuang, Zhao Jingzhou, Bai Yubin, Fan Hao, Shen Wuxian

2013, 34(5):  708-716.  doi:10.11743/ogg20130519

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Reservoir characteristics study is very important to understand the hydrocarbon potential of organic-rich shales.A series of analyses and tests,including thin sections,SEM,XRD,physical property,total organic carbon(TOC),specific surface area and pore size distribution,were made to reveal the reservoir characteristics of the organic-rich shales of the Triassic Yanchang Formation(Fm)in Ordos Basin.This paper also defined the lithofacies,mineral types,diagenesis,pore types and physical properties.Three types of lithofacies are identified in the organic-rich shales,including muddy shale,silt-bearing shale and silty shale.The muddy shale has the highest TOC content,clay content of more than 50% and relatively weak diagenesis.The dominant pore types of the organinc-rich shales in the Yanchang Fm are intragranular and intergranular pores.The pore sizes of micropores are mainly within 0.4 to 1 nm and those of mesopores are mainly within 3 to 5 nm.The contribution of the mesopores to pore volume and specific surface area is larger than that of the micropores.The high content of ductile minerals and weak diagenesis of the organic shales in the the Yanchang Fm make the reservoir stimulation more challenging.