Table of Content

25 April 2007, Volume 28 Issue 2

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A discussion on dynamics and dynamic system for hydrocarbon migration and accumulation

Tian Shicheng, Sun Ziming, Fu Jinhua, Han Jun, Hu Chunyu

2007, 28(2):  129-138.  doi:10.11743/ogg20070201

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Hydrocarbon migration and accumulation dynamics studies various driving forces of hydrocarbon migration and accumulation,geodynamic background of basins,and dynamic systems for hydrocarbon migration from source rocks to traps and entrapment.The dynamic system not only serves as a carrier for hydrocarbon migration and accumulation dynamics,but also occupies a very important place in the dynamics studies.It consists mainly of hydrocarbon expulsion units and their combinations,subsystem of hydrocarbon migration and accumulation dynamics,pathway systems.The progresses in the study of dynamic systems include(1) the application of sequence stratigraphy to the determination and classification of the dynamic systems,resulting in a better understanding of the fact that the maximum flooding surface is critical for recognizing and classifying the dynamic systems;(2) achievements on the study of the dynamic system in the areas with abnormal pressure compartments;(3) researches on the role the tectonic dynamics in the process of hydrocarbon generation,expulsion,migration,accumulation,re-migration,and re-accumulation,and the destruction of the reservoirs.

Connotations of hydrocarbon accumulation geology and its position in petroleum geology

Zhao Jingzhou, Zhang Chunlin, Cao Qing, Gao Le

2007, 28(2):  139-142,180.  doi:10.11743/ogg20070202

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As a major branch of petroleum geology,the geology of hydrocarbon accumulation is focused on the research of hydrocarbon pooling and distribution,including the fundamental elements or basic conditions,geochronology,geochemistry,dynamics of hydrocarbon accumulation,and the patterns of formation and distribution of reservoirs.Currently,the focus of petroleum geology has shifted from the formation of basins and generation of hydrocarbon to hydrocarbon accumulation.The understanding of the connotation and position of hydrocarbon accumulation geology is helpful to the development of petroleum geology.

Advances in geochronology of hydrocarbon accumulation

Chen Honghan

2007, 28(2):  143-150.  doi:10.11743/ogg20070203

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Through summarizing the dating techniques and methods commonly employed in geochronology of hydrocarbon accumulation and evaluating their advantages and disadvantages,it is suggested that the indirect dating of hydrocarbon accumulation through geological methods be replaced by direct determination of hydrocarbon charging ages through radioactive isotope approaches.By using highly sensitive mass spectrometer equipped with electron multiplier and UV laser probe system,an integrated method of fluid inclusion with Ar/Ar dating of authigenic K-feldspar overgrowth has been successfully applied in precise dating of multiple-phase hydrocarbon charging of small amount in small areas,bearing significance for the kinetic modeling of hydrocarbon migration and accumulation and the exploration of secondary reservoirs in superimposed basins.It is also worthy of note that the geochronology study of hydrocarbon accumulation needs not only the intersection and interpenetration of multiple disciplines,but also the development of more representative diagenetic mineral dating methods that could be validated by the results of many other dating techniques,thus providing reliable bases for hydrocarbon accumulation dating in different sedimentary basins.

A discussion on hydrocarbon accumulation dating determined by homogenization temperature and burial history of fluid inclusions——An example from the Fushan depression,Beibuwan basin

Li Meijun, Wang Tieguan

2007, 28(2):  151-158.  doi:10.11743/ogg20070204

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The reconstruction of burial history and thermal history of the Fushan depression,Beibuwan basin,by the Basin Mod-1D program,in combination with the homogenization temperature of fluid inclusions,reveals that the Eocene Liushagang Formation and the Oligocene Weizhuo Formation are reservoirs with only one episode of hydrocarbon charge and are formed during the late Middle Miocene-the early Pliocene.The main charging timing of the reservoirs matches well with the hydrocarbon generation and expulsion periods,which is favorable for hydrocarbon accumulation in the area.

Analysis and calculation of hydrocarbon accumulation history and paleocurrent potential by studying fluid inclusions——An example from the Upper Paleozoic in Yulin,Ordos basin

Shan Xiuqin, Li Jian, Hu Guoyi, Chen Honghan, Luo Xia, Gao Jiayu

2007, 28(2):  159-165.  doi:10.11743/ogg20070205

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Research on fluid inclusion temperature,fluorescent spectroscopy,current potential analysis,and regional geological data,indicates that the Upper Paleozoic pools are characterized by multistage but continuous charging in Yulin,Ordos basin.From the middle Late Triassic to the end of the Early Cretaceous,hydrocarbons were continuously inputting into the reservoirs and their maturity evolved from low to high along with increasing burial depth.The charging is actually a continuous one-stage process,though it can be divided into 6 stages,of which the middle Early Jurassic-end of Middle Jurassic and the end of Middle Jurassic-end of Early Jurassic are two large scale gas charging stages.The calculated fluid inclusion potentials with PVT simulation software of VIFLINC indicate that paleocurrent potential is low in well Yu32 and high in well Yu43-8 and Yu25.

K-Ar isotopic dating of authigenic illite and its application to the investigation of hydrocarbon accumulation history of the Silurian bituminous sandstone reservoirs in the Tazhong uplift,Tarim basin

Zhang Youyu, Horst Zwingmann, Liu Keyu, Luo Xiuquan

2007, 28(2):  166-174.  doi:10.11743/ogg20070206

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K-Ar isotopic dating method of authigenic illites is used to estimate the age of the Silurian bituminous sandstone reservoirs in Tazhong uplift of Tarim basin and the results are compareied with that of Kongquehe area.The age of authigenic illites in the Silurian bituminous sandstones in Tazhong uplift and Kongquehe area ranges from 383.45 to 203.96 Ma,indicating that the Silurian ancient oil pools were formed during the late Caledonian-early Hercynian and late Hercynian.For Qiao-1,Kongque-1,Longkou-1 and Yingnen-2 wells,located in the outer part of the basin,the age of authigenic illites is in the range from 383.45 to 271.20 Ma,indicating a late Caledonian-early Hercynian reservoiring;while for TZ-37,TZ-67,TZ-12,and TZ-32,wells located in the central area of the basin,it is about 203.96-235.17Ma,indicating a pooling in the late Hercynian.However,for some of the wells in the central area of the basin,such as TZ-23 and TZ-30,formed 293 Ma and 296 Ma respectively,indicating a pooling in the early Hercynian.The good correlation between the authigenic illite ages and the thickness of the bituminous sandstones suggests that the palaeotectonic framework be one of the main controlling factors and the ancient oil pools located in and around the depocenter,such as Qiao-1 and Kongque-1,be formed relatively early.The isotopic dating results are basically consistent with the results of conventional hydrocarbon accumulation history study and further highlight the differences in accumulation times between oil pools in different parts of the basin.

Changing patterns of hydrocarbon migration pathway in a up-scaling percolation model

Zhou Bo, Jin Zhijun, Luo Xiaorongi, Wang Yi, Zhang Faqiang

2007, 28(2):  175-180.  doi:10.11743/ogg20070207

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Under the hypothesis that hydrocarbon migration conforms to the description of percolation theory,the relationship between the saturation of invading fluid(the percentage of the migration pathway to the whole lattice) and the width,length,and dimension of the simulation lattice is studied by means of up-scaling and mathematical simulation.The result indicates that,instead of exerting influence upon the numerically simulated secondary migration pathways of hydrocarbon,the size of the simulation lattice has an impact on the saturation of invading phase,which is found to have an exponential relation with the width of the lattice but no connection with the length of the lattice.The discovery of the rule provides sound theoretical basis for the application of the percolation theory in the study of basins.

Quantitative evaluation of fault sealing property with fault connectivity probabilistic method

Zhang Likuan, Luo Xiaorong, Liao Qianjin, Yuan Shuqin, Su Junqing, Xiao Dunqing Wang Zhaoming, Yu Changhua

2007, 28(2):  181-190.  doi:10.11743/ogg20070208

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A new quantitative method for evaluating fault sealing properties based on fault connectivity probability is proposed by taking the occurrence of oil and gas in footwall and hangingwall reservoirs of hydrocarbon controlling faults in Chengbei falut step zone of Dagang oilfield as the index of identifying the sealing properties of faults and by taking into consideration many factors that have influence over the properties.Three parameters,i.e.fluid pressure of mudstone,normal stress on fault plane,and clay smear,are regarded as the major factors that influence the sealing properties of faults,and a dimensionless sealing coefficient is defined according to the mechanisms of their influence over the sealing properties of faults.Then typical reservoir profiles crossing faults are selected to calculate sealing coefficients on different parts of the profile and the sealing properties of faults are determined according to the occurrence of oil in the footwall and hanging wall.Furthermore,statistical analysis of the relationship between the sealing coefficients and fault connectivity probability shows that when the coefficient is less than 1.0,the connectivity probability is naught;when the coefficient ranges from 1.0 to 3.5,the relationship between them can be expressed by a quadratic equation;and when the coefficient is more than 3.5,the connectivity probability equals 100%.The connectivity probability contour on a strike projection reveals the sealing properties of different parts on the fault plane.

Simulation of hydrocarbon migration dynamics in Shahejie Formation of Chengbei fault step zone

Luo Xiaorong, Zhang Likuan, Liao Qianjing, Su Junqing, Yuan Shuqin, Song Haiming, Zhou Bo, Hou Ping

2007, 28(2):  191-197,215.  doi:10.11743/ogg20070209

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Based on the understanding of geological conditions of petroleum systems in Shahejie Formation of Chengbei fault step zone,Bohaiwan basin,the evolution of the basin,hydrocarbon maturity,and dynamic conditions for hydrocarbon migration are successfully reconstructed through basin modeling,some physical properties such as the porosity and permeability of different pathway systems in the area are studied,and the connectivity of faults and sandstone carriers are quantified by the opening probability and net-to-gross ratio respectively.Hydrocarbon migration and accumulation modeling methods based on the percolation theory are used to realize the coupling of migration dynamics and pathways in the two-dimensional carrier framework model built on real data.The results reveal the hydrocarbon migration path and its relative flow rates in the target intervals of the Shahejie Formation.These study results,in combination with other geological conditions,are successfully used in identifying the prospective target zones,including eastern Zhangdong structural belt,the Zhangdong structural belt,and area around the Well Zhuanghai-12 and Well Zhuanghai-9x3.

Application of AVO techniques to reservoirs prediction of oolitic beach facies in Puguang gas field

Guo Xusheng, Fan Rui

2007, 28(2):  198-202.  doi:10.11743/ogg20070210

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For Puguang gas field,a giant structural-lithologic gas field with dolomite reservoirs of reef flat facies in Changxing-Feixianguan formations as the main zones of interest,the comprehensive reservoir prediction technology is considered one of the key solutions for gas exploration.Through AVO attribute processing of pre-stack time migration data,the problem of migration of AVO abnormality is well solved,the AVO responses are enhanced,and the quality of AVO data analysis is improved.Based on an integrated analysis of logging and geologic data,recognition patterns are established for the AVO abnormality of the Feixianguan Formation gas reservoirs of oolitic beach facies in Puguang gas field through forward modeling,and AVO attribute analysis is also performed.AVO techniques can be effectively applied to oil & gas exploration in northeastern Sichuan.

Significance of excess differential pressure in highly efficient gas accumulation in over-pressured basins

Liu Guangdi, Sun Mingliang

2007, 28(2):  203-208.  doi:10.11743/ogg20070211

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Hydrocarbon accumulation dynamics can be described with excess differential pressure and buoyancy.In normal-pressure basins,buoyancy is the major force for hydrocarbon accumulation.While in over-pressured basins,the resultant force of excess pressure differential and buoyancy is the force for oil migration and the minimum force for gas accumulation.An analysis of the contributions of excess differential pressure and buoyancy to the total accumulation dynamics in different pooling processes indicates that the excess differential pressure dominates both the vertical and lateral gas migration and accumulation thus it is the major force for gas accumulation.Dissections of 60 or so gas reservoirs in China also point out that the excess differential pressure controls the efficiency of gas accumulation.Mid-or highly-efficient gas pools will not form in places where low excess differential pressure exists;and high excess differential pressure is a necessary condition for highly efficient gas accumulations.

Dynamics of sand lens reservoir

Li Mingcheng, Shan Xiuqin, Ma Chenghua, Hu Guoyi

2007, 28(2):  209-215.  doi:10.11743/ogg20070212

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The dynamic mechanism of petroleum accumulation in sand lens remains poorly understood.Therefore,an explanation is still needed to address the key issue concerning how hydrocarbons charged into and expulsed from the closed or semi-closed hydraulic system made of sandstones encircled by source mudstones.By comparing the characteristics of lithology and the reservoiring features of structural traps,a comprehensive study and dynamic analysis was carried out on the driving forces of the primary hydrocarbon migration and the phase transformation and buoyancy effect during the secondary migration in mudstone.The results show that the predominant charging forces include abnormally high reservoir pressure,capillary pressure difference,molecular diffusion,and osmotic pressure,and that the buoyancy and relevant additional pressure contribute greatly to hydrocarbon accumulation and water displacement in sandbodies.These dynamics vary with different evolution stages of source mudstones.The compaction and seepage water dominated the charging during low maturity stage;while in mature stage,two phase flow(hydrocarbon and water seepage),capillary pressure difference,and molecular diffusion,controlled the charging process,and hydrocarbons entered into the lens by the action of buoyancy.In the highly matured stage,the charging process continued and was further accompanied by natural gas diffusion from the top of sandbodies,resulting in a final oil accumulation in the lens.Factors that affect the sand lens reservoirs include the quality and thickness of source mudstone,and the physical properties and occurrence of the sandbodies.In summary,hydrocarbon accumulation in sand lens is much more difficult than that in structural traps.

Mechanism and basic modes of petroleum accumulation dynamics in subtle sand lens reservoir

Pang Xiongqi, Chen Dongxia, Jiang Zhenxue, Zhang Jun, Li Sumei

2007, 28(2):  216-228.  doi:10.11743/ogg20070213

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The major driving forces for hydrocarbon accumulation in subtle sand lens reservoirs includes differential capillary pressure on sand-shale boundary,diffusive force triggered by differential hydrocarbon concentration between source beds and sandbodies,and volume expansion caused by kerogen generating hydrocarbons;while the dominant forces that prevent hydrocarbons from pooling include capillary pressure the hydrocarbons faced when entering sandstones and resistance occurred when displacing water in sandstones.For hydrocarbons to accumulate in sandstones,the driving forces must be stronger than the resistances.Apart form that,the porosity and permeability of sandbodies and the hydrocarbon saturation of sourrouding rocks also control the accumulation of hydrocarbons in sandstones.The critical geologic condition for the hydrocarbons generated by the surrounding source rocks entering sandbodies is that hydrocarbon saturation of the surrounding source rocks exceeds 5%-10%,and while that for the hydrocarbons to accumulate in the sandbodies is that the porosity and permeability of sandstones are higher than 5%-10% and 1×10^-3-2×10^-3μm² respectively.The accumulation of hydrocarbons in sand lens reservoirs can be divided into three stages.The first is preparation stage when no large quantity of hydrocarbons are generated by the source rocks and the strong capillary pressure in the micro pores of these rocks prevents the migration of dispersed oil drops.The second is migration and pooling stage when large amount of hydrocarbons are generated and accumulate in the reservoirs with the differential capillary pressure between the surrounding rocks and sandstones as the main driving force.The third is preservation stage when the hydrocarbon no longer generated in large amount and the differential capillary pressure between the surrounding rocks and sandstones is weaker than the resistant forces.The reservoiring dynamics and controls on hydrocarbon accumulation can be used to predict the hydrocarbon abundance and changing patterns.

An experimental study on oil migration and accumulation in low-permeability sandstone

Zhu Zhiqiang, Zeng Jianhui, Wu Heyong, Feng Zihui, Zhang Shun, Liang Xiaodong, Sheng Xiaofeng

2007, 28(2):  229-234.  doi:10.11743/ogg20070214

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An experimental study of oil migration and accumulation in low-permeability sandstone is performed through injecting fluid into core samples from low-permeability sandstone under computer-controlled pressures.The preliminary results indicate that(1) oil migration in low permeability beds presents the features of non-Darcy flow and most of the flow curves take the concave upward shape and some of them are modified Darcy flow curves;(2) the core permeability has a strong impact upon the locations of flow curves,curvatures and variation range of the non-linear parts,and the intercepts of the linear portions on the pressure gradient axis;(3) the oil saturation in low-permeability cores increases in three stages,including rapid,slow and stable growth,with the final oil saturation mainly ranging from 35% to 60%;and(4) the relationship between oil saturation and porosity or permeability in low permeability cores is not simple linear relationship,but a positive correlation exists between the injection pressure and the oil saturation.

Factors controlling the Es₃ Lithologic Oil Reservoir in Minfeng sag,Dongying depression

Zhang Jing, Wang Weifeng, Rong Qihong, Zhao Mifu

2007, 28(2):  235-239,249.  doi:10.11743/ogg20070215

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A study on the characteristics,patterns,and controlling factors of hydrocarbon pooling in Minfeng sag reveals that the sag occupies an advantageous place for oil pooling in the northeastern margin of Dongying depression.The Paleogene lithologic oil reservoirs are mainly sand lens reservoirs,updip pinchout sandbody,and sandbody reservoirs cut through by faults.Major controlling factors of the lithologic oil reservoirs include the properties and distribution of delta front sandbodies and fluxoturbidite sandbodies,the favorable structural location within the distributions of sandbodies,and the prospective sedimentary reservoir facies belts.It is suggested that the exploration be focused on the delta front and fluxoturbidite reservoir sandbodies of different ages formed in difference structural positions.

Characteristics of relative early reservoiring and episodic charging of lithologic oil pools in Erlian basin

Liu Zhen, Liu Jun-bang, Gao Xianzhi, Xin Haiyan, Xiao Wei

2007, 28(2):  240-249.  doi:10.11743/ogg20070216

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The analyses of reservoiring conditions,and burial and thermal history of the three major depressions in Erlian basin,combined with the study on the characteristics and the quantitative analysis of homogenization temperature of its diagenetic fluid inclusions,lead to the conclusion that the lithologic pools in the basin feature in episodic hydrocarbon charging.A comparison with the reservoiring periods of the structural pools at the same horizon in this depression suggests that the lithologic oil pools were formed relatively early.The early formation of lithologic traps was favorable for the early capture of hydrocarbons,and the multistage formation of the lithologic traps and episodic multistage charging of hydrocarbons was conducive for the formation of large-scale lithologic pools.

Study of hydrocarbon migration and accumulation in the Mosuowan-Mobei area,Junggar Basin

Gao Xiaokang, Hu Wenxuan, Cao Jian, Zhang Yueqian, Tang Yong, Tao Guoliang

2007, 28(2):  250-256.  doi:10.11743/ogg20070217

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The inorganic geochemical element tracing approach is adopted in the analysis of changing trends of the MnO content in the calcite cements of reservoirs and the systematic study of hydrocarbon migration direction in the Mosuowan-Mobei area of the Junggar Basin.The results show that the source rocks in the Well Pen-1 west sag,Changji sag,and Ddhzb sag,all have contributed to the pools in the study area,but they vary in the composition of oil,charging ranges,and intensities.Three plays are recognized in the study area,including Well Pen-1 west sag-east loop belt,Changji sag-Mosuowan uplift,and northern slope of Ddhzb sag.The southeastern flank of Musuowan uplift and the northwestern slope zone of Ddhzb sag are prospective targets for exploration.

The role of Che-Mo palaeohigh in controlling hydrocarbon accumulation in central Juggar basin

Jia Qingsu, Yi Wei, Chen Fajing, Xiong Zuowei

2007, 28(2):  257-265.  doi:10.11743/ogg20070218

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Che-Mo palaeohigh is a marginal uplift formed when Juggar basin was still a small intracontinental foreland depression in evolution during the Middle-Late Jurassic.Its formation and evolution can be divided into three main stages: growth(J_2x-J₃),burial(K-E),and disappearance(Nt-Q).The spatio-temporal configuration of the uplifts and hydrocarbon kitchens formed in different structural stages controlled the formation and distribution of hydrocarbon accumulations.And among the uplifts,Che-Mo palaeohigh played an important role in dominating the Mesozoic hydrocarbon accumulations in central Juggar basin in that the reservoirs came into being during the late J_2x-J₃,entered critical reservoiring period during the late K and the late E,and started adjustment during the late Nt-Q.It is concluded that the most prospective exploration target in Che-Mo palaeohigh shall be the Yongjin area in the south side of the uplift,while Sawodi and Zhuangdongbei in north part are regarded as the second best targets.

Carrier system and its effects on the hydrocarbon pooling in Tazhong area,Tarim Basin

Wei Fujun, Gao Zhiqian, Fan Tailiang, Li Yao, Zeng Qingbo

2007, 28(2):  266-273.  doi:10.11743/ogg20070219

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Based on seismic,logging,and test data,an analysis was carried out on the carrier systems of Tazhong area,Tarim Basin.The result indicates that faults,fractures,unconformities,karst,and sandbodies are the fundamental components of the hydrocarbon carrier systems in Tazhong area.The plane distributions and vertical evolutions of the carrier systems vary greatly with different structural periods and different sequence boundaries.The effectiveness of the spatial arrangement of carrier system and hydrocarbon kitchen determines the hydrocarbon pooling in the area.Fractures and unconformities are well developed and act as the main migration pathways for hydrocarbon in the area.Tazhong area has the character of multistage hydrocarbon generation,multi-pathways,and multistage hydrocarbon accumulation,with the late Caledonian,the early Hercynian,and the late Hercynian being the main hydrocarbon accumulation periods.

An analysis on pooling history of the Carboniferous thermal methane gas pool,eastern Sichuan basin

Li Yanxia Zhong Ningning

2007, 28(2):  274-279.  doi:10.11743/ogg20070220

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The formation process and charging history of the Carboniferous gas pools in eastern Sichuan basin are reconstructed through the study of hydrocarbon fluid inclusions from the secondary calcite vein in carbonate reservoirs.Besides gaseous hydrocarbons,liquid hydrocarbons,and saline inclusions,large quantity of multiphase inclusions exist in the present gas reservoirs.These fluid inclusions experienced an increase in their homogenization temperatures from 100℃～135℃ to 140℃～180℃,and to 185℃～220℃,and during the process,low temperature inclusions with low CH₄ content evolved into high temperature inclusions with high CH₄ content.The present gas reservoirs are the result of a process through which the ancient hydrocarbon reservoirs went through hydrocarbon generation,accumulation,destruction,and finally thermal cracking into gases.

Determination and its significance of ancient oil pools in Ordovician weathering crust,Ordos basin

Ning Ning, Chen Mengjin, Sun Fengjin, Xu Huazheng

2007, 28(2):  280-286.  doi:10.11743/ogg20070221

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Core observation and thin-section analysis reveal that the Ordovician weathering crust reservoir within the central uplift belt of Ordos basin is abundant in bitumen,which shows optical features of oil pyrolysis remnant——the direct evidence of the existence of ancient oil pools.The thermal maturity of bitumen is high,resulting in high bitumen reflectivities.For example,the reflectivity of bitumen from Xuntan-1 well ranges from 2.35% to 3.15%(1.85%～2.35% of vitrinite reflectance).Based on the bitumen filling and area coefficient,and well logging interpretation,the thickness of ancient oil polls is estimated to be 10～22 m.Analyses of TOC,organic geochemistry,and petrographic facies suggest that the mudstone in Pingliang Formation gradually thickens from east to west in the western margin depression of the basin,contains rich organic matters of saline organisms of lower form,and can serve as reliable source rocks for the ancient oil pools in the central uplift belt.Pool-forming history indicates that the burial depth and palaeogeotherm of Ordos basin were increased during the Middle Jurassic-Early Cretaceous time,resulting in the pyrolysis of ancient oil pools into ancient gas pools,and that the structural reversal of Ordos basin was caused by Yanshan Movement during the Later Cretaceous-Eogene time,resulting in the formation of the Shanbei slope and the destruction of the ancient oil pools.Part of oil and gas migrated and dissipated towards the shallower layers of Ordos basin,and part headed towards upper part of Shanbei slope,becoming one of the important sources for major gas fields in the central part of the basin.

Pore structure features of lithologic reservoirs in the third member of Shahejie Formation in Huimin sag and their significance in petroleum exploration

Zhang Xiaoli, Zha Ming, Yang Yi

2007, 28(2):  287-292.  doi:10.11743/ogg20070222

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By adopting the method proposed by Edword D.Pittman in reprocessing the mercury injection curves obtained from reservoirs in the third member of Shahejie Formation in Huimin sag,the apex versus pore apertures are determined and their relationship with porosities and permeabilities is established.An analysis of the relationship between the apex versus pore aperture and the hydrocarbon enrichment indicates that reservoirs in the third member of Shahejie Formation in Huiming sag are lithologic traps or structural-lithologic traps with normal pressure,and the minimum apex versus pore aperture depends on the height of the oil column in traps provided that effective oil source presents.In the deep Linnan subsag,the third member of Shahejie Formation is obviously over-pressured and its minimum apex versus pore aperture is determined by the trap height and the formation pressure.The relationship between the apex versus pore apertures of sandstone and its hydrocarbon enrichment reflects the hydrocarbon accumulation dynamics in different basins.With the presence of source rocks and traps,the existence of hydrocarbon accumulation in different reservoiring conditions depends heavily on the apex versus pore apertures.

Geochemical characteristics and genetic types of crude oils

Bao Jianping, Liu Yurui, Zhu Cuishan, Ni Chunhua

2007, 28(2):  293-298.  doi:10.11743/ogg20070223

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Quantitative analyses on hydrocarbon components of typical crude samples from different depressions of Beibuwan basin indicate that the samples from the Weixinan depression in the northern part of the basin have higher concentrations of biomarkers in tricyclic terpanes,hopanes,and steranes,and that from the Fushan depression and Maichen depression in the southern part of the basin have relatively lower concentration of the biomarkers,but higher concentration of aromatic hydrocarbons.It suggests that the depositional environment and geochemical characteristics of source rocks vary dramatically in the northern and southern parts of the basin.The distribution of hopanes,steranes and aromatic hydrocarbons can be used in the establishment of the genetic types of crude oils from different depressions of the basin.The results show that the genetic types of crude oils from the Weixinan depression and from the Fushan and Maichen depressions are different,and that the crude oils from the Wushi depression is a intermediate type with a transitional features.

The impact of saline deposit upon the hydrocarbon accumulation in petroliferous basin

Zhao Zhenyu, Zhou Yaoqi, Ma Xiaoming, Ji Guosheng

2007, 28(2):  299-308.  doi:10.11743/ogg20070224

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Saline-deposits in most basins are formed through evaporation and some are the products of deep thermal brine.Research shows that(1) the formation environments for saline deposits,dominated by evaporate environments in continental lake basins,are favorable for the preservation of organic matters and the generation of hydrocarbon;(2) the upward intrusion of deep thermal fluid enhances transformation ratio of hydrocarbon in source rocks and provides for the formation of inorganic oil and gas accumulations;(3) saline deposits are favorable for the maintenance of high porosity and the formation of secondary pores and fissures in the underlying strata,but sometimes they may cause the plugging of the primary pores,and apart from that,saline deposits themselves serve as excellent hydrocarbon migration pathways or reservoir space.Their presence is regarded as an important condition for the formation of the underlying abnormally-high pressure systems,thus is favorable for the preservation of oil and gas pools;(4) saline deposits have wonderful sealing capacity and make good cap rocks by themselves or in combination with faults;(5) saline deposits have an impact upon the regional structural styles of layers above and under it and controls over migration pathways and accumulation horizons.