Table of Content

25 October 2007, Volume 28 Issue 5

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Views on exploration success or failure

Qian Ji

2007, 28(5):  552-556.  doi:10.11743/ogg20070502

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Different people have different views on petroleum exploration success or failure.But during the exploration process,geologists couldn't help feeling blue.Their pessimistic moods often occur in the period after achieving a major breakthrough and before getting on a fast track of discovery in an oil region.There are three reasons for this mood:incomprehensive understanding of petroleum distributions and inappropriate choices of exploration technologies;weak mastery of petroleum exploration patterns apart from the geological rules;and geologists'fixed thinking modes always preventing them from cutting down deeper into the researches because of an inherent weakness of overestimating their knowledge.

Correct strategies and decisions being critical for the sustainable development of oil/gas exploration in China

Wang Tingbin

2007, 28(5):  557-563.  doi:10.11743/ogg20070503

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People's correct thinking plays an important role in oil/gas exploration.Correct exploration decisions and strategies are the basis for discovering large oil/gas fields.To ensure a sustainable development of oil/gas exploration in China,the same set of standards should be used to eliminate factors that might affect the right decisions,and exploration decisions and strategies should be continuously adjusted and changed based on the latest geological data in the whole process of exploration.

Correct understanding of geology and suitable application of exploration technologies jointly leading to discoveries—an example from the western Luliang Uplift in the Junggar Basin

Zhang Yijie, Dong Dazhong

2007, 28(5):  564-569.  doi:10.11743/ogg20070504

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Oil/gas exploration is a systematic engineering that applies available knowledge and technologies to the exploration of unknown areas for oil and gas,and a dialectical process that follows the route of practice,cognition,re-practice,and re-cognition.Oil/gas discoveries are the ultimate pursuit for people who are engaged in exploration and a result of correct geological understanding and appropriate selection and application of exploration technologies.Correct cognitive process generally starts from comprehensive study of the whole subject and then focuses on one point to make a breakthrough so as to achieve an understanding.After that,the understan-ding shall be tested by applying it to the whole subject.A correct understanding of geological conditions and oil/gas accumulation pattern serves as the foundation of exploration successes.Since recognition is an endless process,exploration for oil and gas guided by new recognition shall also be sustainable.Suitable exploration technologies are the bridge between the correct understanding of geological information and oil/gas discovery.Right selection of exploration technologies is more important than simply pursuing the most advanced technologies.Understanding of geological information and advancement of exploration technologies makes it possible for the expansion of oil and gas exploration domains and more discoveries of oil and gas fields.

Review and revelation of oil/gas discoveries in the Paleozoic marine strata of China

Kang Yuzhu

2007, 28(5):  570-575.  doi:10.11743/ogg20070505

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In 1984,the Shacan-2 well,positioned in Tarim basin,flowed for the first time with large amount of oil and gas,marking the discovery of giant Tahe oilfield.It was not only the first significant breakthrough in exploration of the Paleozoic strata,but also an important milestone in Chinese oil/gas exploration history.Since then,several other Paleozoic oilfields were discovered and the theory of hydrocarbon reservoiring in the strata in China was established.It is a period characterized by independent innovations in exploration of the Paleozoic marine strata.Based on a systematic summaries and review of the time,the article concludes that more large oil/gas discoveries depend on many factors,among which deeper basic researches are the precondition,innovations and creativity are the key,technology advancements are the necessary conditions,and continuous explorations and practices are the guarantee.

Potential areas and exploration direction in the central uplift belt of the Tarim Basin

Li Pilong

2007, 28(5):  576-583,589.  doi:10.11743/ogg20070506

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Central uplift belt in Tarim basin is a typical large petroleum accumulation zone characterized by multiple sets of source rocks,various types of reservoir rocks,many kinds of traps and oil/gas pools,multistage hydrocarbon charging,and various plays.According to the available drilling and seismic data and comprehensive geological research,it is believed that the Ordovician reef-bank facies,Caledonian palaeokarst,Silurian and Devonian "unconformity triangle zone",and Cambrian pre-salt dolomite are the potential targets of exploration in the central uplift belt of Tarim basin.While stressing on exploration in these key areas,researches shall be focused on the matching technologies for exploration,so as to develop effective and integrated exploration technologies.

Main factors controlling the formation of interior reservoirs in the metamorphic palaeo-buried hills of the Liaohe Depression

Meng Weigong, Li Xiaoguang, Liu Baohong

2007, 28(5):  584-589.  doi:10.11743/ogg20070507

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By focusing on Xinglongtai buried hills in the western sag of Liaohe depression,this article summarizes the distribution characteristics of multileveled interior reservoirs in metamorphic buried hills,analyzes the controlling factors,such as lithologic characteristics,tectonic movements,source rocks,oil charging "window" and sealing system,upon the formation of interior reservoirs in buried hills,and establishes hydrocarbon accumulation pattern of interior reservoirs in metamorphic buried hills.It concludes that the development of fractures is jointly controlled by tectonic movements and dominant lithology;the existence of various lithologic combinations in me-tamorphic buried hills makes it possible for an alternate distribution of reservoirs and barriers;and the lateral charging "window" for oil migration is the key to the formation of interior reservoirs and its extent determinates the vertical range of oil/gas pools.These understandings effectively guide the exploration in deep buried hills in Liaohe depression.

Natural gas exploration in deep volcanic rocks in the northern Songliao Basin

Qi Jingshun

2007, 28(5):  590-596.  doi:10.11743/ogg20070508

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Geological reserves of Xushen gas field discovered in Daqing exploration area exceed 100 bcm,the biggest among the volcanic gas reservoirs in China so far.The discovery of Xushen gas field had experienced several stages,including initial exploration,discovery of small structural gas reservoirs,and discovery of large lithologic gas reservoirs.Xujiaweizi fault depression has ideal source rocks and well-developed,volcanic reservoirs,which together with regional seals form vertically excellent source-reservoir-cap rock combinations,thus is quite favorable for the formation of large gas fields.

Discoveries and their implications in the stratigraphic-lithologic reservoir exploration of Huabei oilfield

Zhao Xianzheng, Lu Xuejun, Wang Quan

2007, 28(5):  597-604.  doi:10.11743/ogg20070509

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Huabei Oilfield Company has obtained many important achievements on the study and exploration of stratigraphic reservoirs in recent years.In the Bayindulan Sag of the Erlian Basin,guided by new exploration concept of searching for stratigraphic-lithologic reservoirs,the Baolige oilfield mainly composed of lithologic re-servoirs was finally discovered through establishing isochronous stratigraphic framework based on sequence stratigraphic study,determining potential exploration areas by comprehensive study of structure and sedimentation,establishing the pooling pattern of updip pinchout lithologic reservoir of fan delta front sand based on reviewing old well data.While in the Jizhong Depression,new breakthrough was made by deepening the study of reservoiring conditions in Changyangdian buried hills and establishing a new reservoiring pattern of "new-generating,old-accumulating and old-sealing".These two successful cases shed light on the future exploration practices in similar areas.

A case study of hydrocarbon exploration in the southern steep slope zone of the Biyang Depression

Qiu Ronghua, Fu Daiguo, Wan Li

2007, 28(5):  605-609.  doi:10.11743/ogg20070510

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Further study of the geology in the steep slope zone of Biyang depression reveals favorable reservoiring conditions and great exploration potential.In view of the large surface relief,inaccurate positioning of the major boundary faults through seismic processing,and low ascertainment degree of traps,prestack depth migration processing and subtle trap identification are carried out by using the re-acquired 3D seismic data,leading to the discovery of several oil-bearing traps with controlled geological reserves of 17.4 millions of tons in Liyuan area of southern steep slope zone.

Exploration prospects of low-mature oil/gas in the shallow layers of the A'nan Sag,the Erlian Basin

Dong Yong, Ma Liqiao, Sun Fengxia

2007, 28(5):  610-614,633.  doi:10.11743/ogg20070511

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Exploration prospect of low-mature oil/gas in shallow of A'nan sag of Erlian basin is discussed based on sedimentary environments,distribution of source rocks,and shallow fault systems.It is believed that thick low-mature source rocks are developed in the upper 1^st member and 2^nd member of Tengge'er Formation of the Lower Cretaceous and have certain potential for hydrocarbon generation;the shallow fault systems might act as the major vertical pathways for hydrocarbon migration;and the shallow large anticlines are favorable for large-scale hydrocarbon accumulation.Therefore,it is proposed to carry out a systematic study of exploration in the shallow 2nd member of Tengge'er Formation.

Discovery of BMY oilfield in the Subei Basin and its significance

Qiu Xuming, Yan Yuanfeng, Tangyan

2007, 28(5):  615-620.  doi:10.11743/ogg20070512

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Through reviewing the discovery history of the Bianminyang(BMY)oilfield in the Subei Basin,this paper analyzes the changes of petroleum geology understandings,such as hydrocarbon generation,expulsion,migration,structure formation,and accumulation,after discovery of the oilfield,and summarizes the application results of new technologies including 3D seismic survey and deviated well-drilling.The discovery of BMY oilfield bears great significances and has a positive impact upon subsequent assessment of the hydrocarbon resources in the basin,understanding of reservoiring patterns,shifting of exploration concepts,and application of new technologies.

Division of fluvial sequence stratigraphy—an example from the Lower Cretaceous Fuyu oil-bearing layer,the Songliao Basin

Deng Hongwen, Wu Haibo, Wang Ning, Timothy A. Cross

2007, 28(5):  621-627.  doi:10.11743/ogg20070513

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Taking Fuyu oil-bearing layer of the Lower Cretaceous Quantou Formation in the Songliao Basin as an example,this article discusses the analysis methods of sequence stratigraphy of fluvial sedimentary system.It starts with the identification of the genetic types of fluvial system and facies tracts(micro-facies),including meander channels,crevasse channels,crevasse fans,alluvial plains,and shallow lakes,and recognition of the autocyclicity caused by fluvial overflow and the allocyclicity(base-level cycle) caused by base level changes.Then it analyzes the relation between the formation of different facies tracts and base-level change and identifies the sedimentologic and stratigraphic responses of turning points of base-level changes(i.e.,the base level turning from ascending to descending or vise versa) by applying the fluvial sedimentology theory.Finally,it establishes a framework for isochronous stratigraphic correlation of fluvial sequences.

Coupling of sequence stratigraphy and hydrocarbon accumulation dynamics and its significance in petroleum exploration

Liu Jingqiang, Tian Shicheng, Jiao Yong, Zhang Ruxiang

2007, 28(5):  628-633.  doi:10.11743/ogg20070514

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Both sequence stratigraphic frameworks and hydrocarbon reservoiring dynamic systems employ the multi-cyclicity of stratigraphy sedimentation as the basis of sequence or subsystem division.This article discusses the coupling of sequence stratigraphic frameworks and hydrocarbon reservoiring dynamic systems and points out that the maximum flooding surface not only serves as the important boundary in sequence stratigraphic frameworks,but also acts as the boundary for dividing subsystems in the hydrocarbon reservoiring dynamic systems.It analyzes the relationship between the sequence stratigraphy system tracts and hydrocarbon reservoiring dynamic subsystems in terms of hydrocarbon accumulation conditions,which include source kitchens,lateral carrier systems,sealings and subtle traps.It also stresses the importance of research on oil and gas pooling processes,such as hydrocarbon generation,migration and accumulation in subtle reservoir exploration by using sequence stratigraphy theories.Moreover,it emphasizes that abiding by the concept of "hydrocarbon from source rocks to traps" is essential to the application of sequence stratigraphy technologies in the exploration of subtle reservoirs.

Structural styles of extensional basins and their main controlling factors of dynamics

Qi Jiafu, Yang Qiao

2007, 28(5):  634-640.  doi:10.11743/ogg20070515

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The extensional deformation of lithosphere can occur by different patterns and results in different structures of extensional basins.According to the deformational characteristics on the cross-section of extensional basins,they can be divided into the following four types:"sag basin","faulted-sag basin","fault-downthrown basin",and "sag fault-downthrown basin".The extensional deformation mechanisms of lithosphere include thermal action of the upper mantle,ductile extension of the lower lithosphere,and normal faulting of the upper lithosphere(perhaps it is just shallow crust).The "sag basin" is formed by homogeneous extensional deformation at crust scale;while the "fault-downthrow basin" is shaped by faulting of nonhomogeneous extensional deformation at crust scale.Both "faulted-sag basin" and "sag fault-downthrow basin" are the result of superposition of homogenous flexural deformation and non-homogeneous faulting deformation at crust scale,and their subsidence is controlled partly by extensional faulting and partly by flexural deformation of the extending crust.Although all of the mechanisms work simultaneously,the contribution of each dynamic mechanism varies in different parts of basin and at different periods,causing differences in extensional basin structures at different periods and in diffe-rent parts of the basin at the same period.

Control of facies-potential coupling on hydrocarbon accumulation in continental faulted basins and its basic geological models

Pang Xiongqi, Li Pilong, Zhang Shanwen, Chen Dongxia, Song Guoqi, Wang Yongshi, Zhang Jun

2007, 28(5):  641-652,663.  doi:10.11743/ogg20070516

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Hydrocarbon migration and accumulation in continental faulted basins feature in "fault-slope controlling sand,multiple hydrocarbon transporting system,sedimentary facies and fluid potentials jointly controlling hydrocarbon accumulation".Taking the Dongying Sag in the Bohaiwan Basin as an example,this article expounds on the characteristics of facies-potential controlled reservoiring and the basic geological model of "dominant facies-low potential coupling controlled reservoiring".The research shows that the controlling effects of sedimentary facies on hydrocarbon accumulation can be divided into four categories: structural facies,sedimentary facies,lithologic facies,and petrophysical facies.They reflect hydrocarbon distribution and accumulation model from macro to micro perspectives.The basic geological mode of facies controlled hydrocarbon accumulation is "high poro-sity and dominant facies".While fluid potential control over oil and gas accumulation can be divided into the following four types,namely geopotential energy,pressure energy,interfacial energy,and hydrodynamic energy.These four types of potentials control the formation and distribution of different reservoirs with a basic geological model of "low potential controlled hydrocarbon accumulation".The coupling of dominant facies and low potential control more than 90% of the discovered hydrocarbon accumulations in the the Dongying Sag and determine the distributions and potential energy differences of different types of oil/gas reservoirs with different burial depth,and helps to explain the decreasing trend of the threshold porosity of effective reservoirs with the increasing bu-rial depth and to illustrate the mechanism that hydrocarbons can accumulate only when the internal potential of a trap being less than a half of the external potential.The reservoiring controlling model of "near source kitchen,dominant facies and low potential" can be applied in predicting the distributions of subtle oil and gas reservoirs in continental faulted basins.

Multicycle tectonic evolution and composite petroleum systems in the Tarim Basin

Zhang Guangya, Zhao Wenzhi, Wang Hongjun, Li Honghui, Liu Lei

2007, 28(5):  653-663.  doi:10.11743/ogg20070517

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The multicycle tectonic evolution of Tarim craton controls the formation of superimposed the Tarim Basin,petroleum-geology conditions as well as the features of composite petroleum system,thus determines hydrocarbon distribution and resources potential in the basin.Dominated by the evolution of Paleo-Asian tectonic domain and Tethyan tectonic domain,the basin experienced three extensional-compressional cycles,which can be divided into six phases,with various prototype basins.Affected by 6 large-scale tectonic movements,including Caledonian,early Hercynian,latest Hercynian,Indosinian,Yanshan,and Himalayan,these prototype basins were reshaped by uplifting,denudation,folding,and faulting.The Tarim Basin have the following geologic characteristics:many sets of source rocks,several hydrocarbon kitchens,various types of reservoirs,multiply of reservoir-cap rock combinations,and multistage hydrocarbon generation and accumulation,as well as multiphase adjustment,reforming,and destruction.According to the distribution of the hydrocarbon kitchens and their relation with re-servoirs containing hydrocarbon generated by them,10 composite petroleum systems can be recognized in the basin.The Palaeozoic petroleum system has the following three critical periods:Late Caledonian-early Hercynian,late Hercynian and Himalayan.The hydrocarbon distribution of the petroleum system in each critical period is controlled by the relationships between the hydrocarbon kitchens and paleohighs,and finalized by the superposition of multi-stage petroleum systems.

Geochemical characteristics of natural gas from different structural units of the Kuqa Depression,the Tarim Basin

Bao Jianping, Zhu Cuishan, Zhang Qiucha, Li Mei, Lu Yuhong

2007, 28(5):  664-668,674.  doi:10.11743/ogg20070518

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Analysis of the geochemical characteristics of natural gas from the main structural units of the Kuqa Depression in Tarim Basin shows that the natural gas from Dabei-Kelasu structural belt is dominated by methane and has a dryness coefficient [C₁/(C₁-C₅)] near 1.00,indicating typical dry gas;while that from the frontal uplift zone has a relatively low content of methane and a dryness coefficient less than 0.95,indicating typical wet gas.In terms of carbon isotope compositions,the gas from the Dabei-Kelasu structural belt contains significantly heavier methane and ethane isotopes(>-31‰ and-20‰,respectively),while that from the frontal uplift zone has obviously lighter methane and ethane isotopes(<-33‰ and-20‰,respectively).Maturity calculation reveals that the gas from the Dabei-Kelasu structural belt belongs to over-mature gases with a vitrinite reflectance ranging between 1.65% and 2.25%,while that from the frontal uplift zone belongs to highly-mature gas with a vitrinite reflectance ranging from 1.00% to 1.35%.All these indicate that the natural gases in different structu-ral units in the Kuqa Depression were generated from the organic matters in the source rocks at different thermal evolution stages.

Structural geological characteristics and hydrocarbon enrichment of the Muglad Basin

Zhang Yamin, Qi Jiafu

2007, 28(5):  669-674.  doi:10.11743/ogg20070519

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Muglad Basin is an intracontinental rift basin in Africa,and its tectonic evolution process can be divided into three stages,including initial rifting in early of the Early Cretaceous inherited rifting in late of the Early Cretaceous to the Late Cretaceous,and depression in the Cenozoic.At the end of the Late Cretaceous,the Muglad Basin was strongly modified by the strike-slip movement of the Central African Fault,superimposing strike slipping deformation on the rifting structures of the Tullus Sag and the Bagarra Sag,and also causing an obvious tectonic inversion in south part of the basin.At the initial rifting stage,the basin was filled with semi-deep or deep lacustrine mudstones which were good source rocks;at the inherited rifting stage,the basin developed va-rious types of sandstone as reservoirs and thick mudstone as regional seals.The favorable source-reservoir-cap rocks combination gives the basin large exploration potential.Later modification of the basin has been an important geological issue for evaluation of oil and gas exploration.

Tectonic evolution characteristics of the foreland basin on the southern margin of the Dabie-Sulu orogenic belt

He Mingxi, Du Jianbo, Cao Jiankang, Guo Shuangting, Xie Qifeng, Ma Rongfang

2007, 28(5):  675-681.  doi:10.11743/ogg20070520

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Controlled by the conversion of tectonic mechanism from continental margin collision orogenesis to intra-continental orogenesis in the eastern segment of the central orogenic belt since the Middle Triassic,the foreland basin on the southern margin of the Dabie-Sulu orogenic belt experienced a whole process of evolution from initiation to flourish and finally to extinction and is featured in continuous transition from continental margin to intra-continent and from marine to continental in terms of tectonic setting.According to the tectonic mechanism conversions and tectonic evolution stages reflected by the structural sequences of the foreland basin,the evolution of the foreland basin can be divided into four stages:1)pre-orogenic passive margin(P₃—T₁—T₂¹),consisting of three sub-stages(partial faulted depression-continuous subsidence-fading residual basin);2)peripheral foreland basin during the main orogenic period(T₂²—T₃),a stage characterized by continuous tectonic evolution of marine-continental transitional(T₂²)to continental(T₃)foreland basin;3)intracontinental foreland basin during intra-continental orogeny(J_1-2),featured in coal-bearing clastic rock deposited in intracontinental foreland area;4)post-orogenic extensional-extensional faulted depression basin(J₃-E),a period when the foreland basin dies out.

Application of seismic frequency-division technology in the study of fan-delta evolution

Fan Hongjun, Li Jun, Xiao Yuxiang, Hu Qingsong

2007, 28(5):  682-686,692.  doi:10.11743/ogg20070521

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Seismic frequency-division is to transform seismic data into different frequency domains via the Discrete Fourier Transform(DFT)or the Maximum Entropy Method(MEM),and is generally used in the predication of thin reservoirs.Based on the thickness and distribution of reservoirs,seismic primary frequency can be extracted through selecting appropriate parameters of time window.Plane changes of seismic primary frequency can then be used to detect edges of sedimentary bodies,thus providing reliable evidences for the study of fan-delta evolutionary process.The relationship of seismic primary frequency and reservoir thickness can also be employed to calculate thicknesses of thin reservoirs.By applying the technology in the analysis of the Jiufetang-Shahai Formations in J2 wellblock of the TLG Oilfield,where deposits of fan-delta facies are developed,the horizontal distribution and vertical evolution pattern of the fan-delta in the research area is defined and the changing characteristics of the source near the southern boundary faults during different times are determined.This field application shows that the technology eliminates the uncertainty of conventional methods for predicting the boundary of sedimentary facies and greatly enhances the accuracy of seismic facies recognition.

A study on the miscible conditions of CO₂ injection in low-permeability sandstone reservoirs

Guo Ping, Li Miao

2007, 28(5):  687-692.  doi:10.11743/ogg20070522

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This article illustrates the impacts of slim-tube experimental conditions on test results of minimum miscibility pressure(MMP).The numerical simulation results show that the MMP is smaller when the slim-tube's length is longer,diameter is smaller,displacement speed is greater,and the porosity is smaller.In addition,it discusses the influences of reservoir temperature,injected gas compositions and crude oil compositions and properties on the miscible pressure.Through comparing the condition of CO₂ miscible flooding reservoirs and fluid flow in foreign and domestic fields,it explains the reasons behind the difficult application of CO₂ miscible flooding in China's fields are high formation temperature and high oil viscosity.It is suggested that a research on technologies of reducing minimum miscibility pressure and preventing gas breakthrough be carried out to address the issue of low sweep efficiency of injected gas and to enhance oil recovery.