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    25 June 2006, Volume 27 Issue 3
    New progresses in research of China's typical superimposed basins and reservoiring of hydrocarbons(Part Ⅱ):taking Tarim basin as an example
    Jin Zhijun
    2006, 27(3):  281-288.  doi:10.11743/ogg20060301
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    Research of reservoiring in typical superimposed basins in China has made great progresses in recent years.In respect of basin formation:(1) Composite geophysical profiles reveal that the coupling of shallow and deep layers in Tarim basin and Tianshan orogenic belt is characterized by segmentation along strike.It appears as near E-W trending oblique extensional features to the east of Yangbulake,and near S-N trending extensional features vertical to the strike of strata.(2) The evolution of Tarim basin is characterized by tilting superimposition,i.e.from Cambrian to Quaternary,the depocenter migrated from the east to the west and finally to the periphery area.In respect of hydrocarbon generation:(1) Environmental indexes and basin types for development of good marine carbonate source rocks have been established.Source rocks with high abundance of organic matter have only been developed in rift valleys of passive continental margin,intra-cratonic down-warped basins,and frontal slopes and flexured foreland depressions in active continental margin.(2) The lower limit of organic matter abundance for effective carbonate source rocks has been determined to be 0.25%-0.30%.In respect of reservoiring:(1) Hydrocarbon inclusions in the Lower Paleozoic carbonate reservoirs in Lunnan area have been formed in Late Silurian-Early Devonian,Late Cretaceous and Neogene,respectively,and they are related with regional charging events of hydrocarbons.(2) Oil-source correlation shows that the oil in Lunnan and Tahe might have been sourced from the Middle-Upper Ordovician source rocks.(3) Isotope tracer study and temperature measurement of inclusions show that the fluids in Tazhong have deep source characteristics.Moreover,fluorite replaces limestone would have greatly improved the poroperm characteristics of the reservoirs. The discoveries in new series of strata and new type discoveries obtained in exploration of carbonates in Paleozoic craton area,as well as the breakthroughs got in exploration of Silurian and Devonian clastic rocks in Paleozoic craton area,show that Tarim basin has large exploration potentials.

    Understanding exploration of marine carbonate reservoirs in South China through Sichuan basin
    Ran Longhui, Xie Yaoxiang, Wang Lansheng
    2006, 27(3):  289-294.  doi:10.11743/ogg20060302
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    Marine carbonate reservoirs in Sichuan basin are characterized by 5 distinct geological features,i.e.there is more gas than oil,the reservoirs are laterally and vertically superimposed,the gas pools have different sizes,there are plenty of good reservoirs though they are relatively tight as a whole,and there are various trap types but dominated by anticlinal traps. Five key problems influencing exploration of marine carbonate reservoirs that should be highlighted are presented in this paper,i.e.the development zone of marine carbonates with the features of superimposed basin,the formation and evolution of palaeohighs in the process of deposition in the craton;accurate identification of the distribution of marine carbonate reservoirs and the subsurface geometry of local structures in the formations of interest,the significant technical principles of insisting on targets but not solely on the targets,and focusing on discovery of oil and gas;as well as development of practical and advanced exploration technologies.

    Constraint of Meso-Cenozoic tectonic evolution of Yangtze massif on formation of marine reservoirs
    Guo Xusheng, Mei Lianfu, Tang Jiguang, Shen Chuanbo
    2006, 27(3):  295-304,325.  doi:10.11743/ogg20060303
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    Mesozoic and Paleozoic marine reservoirs are controlled by the difference between Meso-Cenozoic macroscopic tectonic evolution and dynamics of Yangtze massif.Analyses of the tectonic evolution of Yangtze massif since Indosinian epoch and the typical oil and gas reservoirs in different segments of Yangtze massif show that differential tectonic evolution has resulted in the different major sources of hydrocarbon fluids,preservation conditions and reservoiring evolution in different segments of Yangtze massif.The sources of hydrocarbon fluids in the Upper Yangtze massif are mainly of cracking gas cracked from oil and kerogen.Completely enclosed environment is developed in marine formations.The reservoiring evolution series include the formation of fossil oil reservoir,early gas reservoir and late gas reservoir,and the reservoiring pattern is early reservoir formation-later transformation-late gas accumulation.The sources of hydrocarbon fluids in Middle and Lower Yangtze massifs are mainly of secondary hydrocarbon generation and early primary oil and gas sources in marine strata.Enclosed systems occur in both continental and marine strata.The reservoiring evolution process is exactly the process of repeated reservoiring,reformation,reservoiring and destruction.The reseroviring patterns in the reversed area of fault depression include reservoirs occurring above the source rocks,residual primary reservoirs and secondary hydrocarbon generation,while that in reversed fault block areas include residual primary reservoirs and late secondary reservoirs.

    Composite basin-and-range system and reformation of basin prototype in Indosinian-Yanshanian in South China
    Wu Chonglong, Du Yuansheng, Mei Lianfu, Zhou Jiangyu, Kong Chunfang
    2006, 27(3):  305-315.  doi:10.11743/ogg20060304
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    Several basin-and-range systems of foreland-type were developed in Indosinian-Yanshanian in South China,and they became "a composite basin-and-range system" through mutual influence,interference and superimposition.Based on detailed analysis of the Upper Yangtze composite basin-and-range system,it is believed that the composite basin-and-range system has been formed through the closing of Paleotethys archipelagic ocean,the convergence and collision of multiple blocks and orogeny.The researches reveal that in the long-term evolution of basin-and-range coupling and composite evolution since Indosinian,Longmen Mountain,Daba Mountain,Jiangnan uplifted tectonic zone and their foreland basins have not only led to the formation of several composite basin-and-range systems but have also led to the final formation of the huge composite basin-and-range system with "three mountains and one basin",i.e.Longmen-Daba-Jiangnan uplift composite basin-and-range system,also called the Upper Yangtze composite basin-and-range system for short.After Indosinian-Early Yanshanian,the Upper Yangtze composite basin-and-range system,together with the other composite basin-and-range systems in South China,have entirely been reformed and finally resulted in the present complicated basin-and-range structures and appearances.

    Petroleum reservoiring characteristics and exploration direction in marine strata in southern China
    Xiao Kaihua, Wo Yujin, Zhou Yan, Tian Haiqin
    2006, 27(3):  316-325.  doi:10.11743/ogg20060305
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    In the long geological history,the marine strata in southern China have experienced polycyclic tectonic movement and intensive late reformation,and the hydrocarbon generation,migration,accumulation,preservation and destruction in Mesozoic and Paleozoic have experienced multi-stage phasic development and evolution.Areas with relatively stable tectonic settings and well preservation conditions,such as Sichuan basin,are characterized by multiple sources,early accumulation,oil-gas conversion,and late finalization;the Middle and Lower Yangtze areas overlain by Mesozoic and Cenozoic strata are characterized by secondary hydrocarbon generation and late reservoiring; while the areas with outcropping carbonates,such as western Hunan and Hubei and Yunnan-Guangxi-Guizhou regions,are characterized by early hydrocarbon generation and late destruction and reformation.According to early or late hydrocarbon generation and whether the original sealing systems have been broken in the course of primary hydrocarbon accumulation through to the finalization of the oil and gas pools,the reservoiring models of petroleum in marine strata in southern China can be divided into three types: including primary,secondary and regeneration types.Based on the reservoiring conditions and characterstics in different areas,exploration of marine strata in southern China should be carried out in three different levels: key exploration should be carried out in Sichuan basin,try by all measures to get breakthroughs in northwestern Guizhou,southern Jianghan plain in Middle Yangtze area and Jurong-Hai'an regions in Lower Yangtze area,and regional reconnaissance should be carried out in southern Guizhou,Yunnan-Guangxi and western Hunan and Hubei regions.

    A discussion on Kaijiang-Liangping ocean trough
    Ma Yongsheng, Mou Chuanlong, Tan Qinyin, Yu Qian
    2006, 27(3):  326-331.  doi:10.11743/ogg20060306
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    It is previously believed that Kaijiang-Liangping area was an ocean trough during Changxing age of Late Permian-Early Indo-Chinese epoch in Early Triassic,and was called "Kaijiang-Liangping ocean trough".The understanding and definition of the sedimentary framework in this area are directly related with exploration deployment in the northeastern part of Sichuan basin.Based on sedimentological study and the new understan-(dings) obtained from drilling and seismic data,it is demonstrated,in respects of tectonic setting and the concept and sedimentation of ocean trough,that Kaijiang-Liangping area was not an ocean trough during Changxing age of Late Permian-early Early Triassic,but a platform-shelf with relatively deep water within a carbonate platform,and the sediment was dominated by micrite with some high energy shoal deposits.

    Prediction of reef and shoal facies reservoirs in Changxing-Feixianuan Formations in northeastern Sichuan basin
    Liu Shu, Tang Jianming, Ma Yongsheng, Zhao Shuang
    2006, 27(3):  332-339,347.  doi:10.11743/ogg20060307
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    Puguang gas field has typical slope facies on platform margin.Large reef and shoal facies reservoirs have been deposited on the slope with relatively obvious chimney effect,i.e.chaotic and disrupted wave groups with push-down shape.The reservoir prediction method has been developed on the basis of analyzing the known gas fields.It is then used to make a comprehensive prediction of the reservoirs in northeastern Sichuan basin.It is found that the Malubei structure in Tongnanba structural zone and Jiulongshan structure in Cangxi County show the characteristics of typical sequence stratigraphy of platform margin facies.The predicted Cangxi reef and shoal facies show the characteristics of typical progradational sequence stratigraphy with reliable push-down. It is predicted that there would possibly be a super large reef and shaol facies gas field in Changxing-Feixianguan Formations in Cangxi area.The sequence stratigraphic characteristics of the Malubei reef and shoal facies are basically the same with that in Puguang gas field,i.e.a slope zone where large reef and shoal facies reservoirs would have been developed.It is,therefore,predicted that there would be a large gas reservoir.

    Geochemical behaviors and origin of reservoir bitumen in Puguang gas pool
    Cai Xunyu, Zhu Yangming, Huang Renchun
    2006, 27(3):  340-347.  doi:10.11743/ogg20060308
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    Systematic geochemical analyses show that the reservoir bitumen in Feixianguan and Changxing Formations in Puguang gas pool are residuals of the original oil in fossil oil pools after they have been thermally cracked into gas.The organic elementary compositions of the bitumen are characterized by poor hydrogen and rich oxygen and sulfur,suggesting that it is of pyrobitumen as a result of high maturity and TSR.The bitumen has δ13C values ranging from-25.60‰ to-27.78‰ and fits isotopically with kerogen in the Upper Permian source rocks(-26.5‰ to-29.2‰),indicating a genetic relationship between them.The n-alkanes with a complete range of carbon number are observed in the saturated hydrocarbons of the bitumen and no evidences of severe biodegradation occur.They exhibit a phytane predominance with Pr/Ph ratios less than 1.0; predominance of C27 compounds occur in the steranes with detectable C30 and C26 steranes;and the terpanes are characterized by abundant tricyclic terpanes and gammacerane.All these indicate that aquatic organism-dominated marine source rocks have been deposited in a reducing environment.As a result of aromatization of organic matter during the thermal cracking of original bitumen into gas,the aromatic hydrocarbons in the bitumen are characterized by abnormally abundant tetra-and penta-cyclic compounds,and predominated by parent aromatic compounds without alkyl chain,such as fluoranthene,pyrene,chrysene, benzofluoranthene etc.The distribution pattern and MPI index of aromatic hydrocarbons reveal a high thermal evolution for the bitumen with a corresponding Ro value of over 2.0%.

    Study on capability of hydrocarbon generation and expulsion from marine carbonate source rocks
    Qin Jianzhong, Liu Baoquan, Zheng Lunju, Zhang Qu
    2006, 27(3):  348-355.  doi:10.11743/ogg20060309
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    Based on selection and comparison of 5000 marine source rock samples of various types,34 samples acquired from Jurassic in Qinghai-Tibet plateau,Upper Proterozoic in northern Hebei province and Devonian in Luquan of Yunnan province have been selected for artificially watering,heating and pressurizing simulation experiment.Comprehensive study shows that the marine carbonate rock has relatively higher accelerating effect on hydrocarbon generation from kerogen and expulsion in comparison with marine mudstone and shale,lowers the temperature of peaks of total oil generation and expulsion,and easily forms immature-low-mature heavy oil,resulting in increasing oil expulsion efficiency,and enlarging yields of total gas,CO2,H2,total oil,oil carried by gas,residual oil,hydrocarbon gas and total hydrocarbons.All these are related to the catalytic reaction of α-carbon atom forming free radical,the relatively high content of planktonic algae in kerogens of carbonate rocks,and the weak adsorption of carbonate rock to organic matter and hydrocarbons.However,calcareous mudstone and shale,with carbonate content in the range of 5%-25%,relatively have the highest yields of hydrocarbons including total oil.They are mainly formed in closed and strong reduction environments and have good kerogen type and high abundance of organic matter.

    Problems of oil-source correlation for marine reservoirs in Paleozoic craton area in Tarim basin and future direction of research
    Ma Anlai, Jin Zhijun, Wang Yi
    2006, 27(3):  356-362.  doi:10.11743/ogg20060310
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    Understanding of the major source rocks of oil in marine reservoirs in the Paleozoic craton area is always a controversial issue because of the high mature to overmature Cambrian source rocks and the limited distribution of the encountered Upper Ordovician Lianglitage Formation source rocks.Based on study of the geochemical behaviors of oils,it is believed that the oil in marine reservoirs is mainly of oil generated during oil generating peak to late generating stage.There is an obvious difference between the oil and the maturity of Cambrian source rocks,the basin is characterized by multistage reservoiring of oils generated by multiple sets of source rocks,and the oils have experienced secondary alteration,therefore the use of biomarkers for oil-source correlation is restricted in the Paleozoic craton area.Future study should be focused on the evolutionary pattern of specific biomarkers of the Upper Ordovician Lianglitage Formation source rocks that are now in the range of "oil-window" stage and the thermal evolution of Lower Paleozoic source rocks.

    Geochemical research of hydrocarbon reservoiring in Lower Triassic Jialingjiang Formation in Luzhou palaeohigh
    Li Yanjun, Li Qirong, Wang Tingdong, Hu Xiong, Liang Yan, Yang Jian, Yuan Xuzu
    2006, 27(3):  363-369.  doi:10.11743/ogg20060311
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    Luzhou palaeohigh can be divided into 3 parts,including core,slope and periphery.The core of the palaeohigh is dominated by gas reservoirs in the first member of Jialingjiang Formation,the slope on the northern edge is dominated by gas reservoirs in the second member of Jialingjiang Formation,and the slope on the southern edge is dominated by multilayered gas reservoirs in the third member of Jialingjiang Formation.The properties of gas in Jialingjiang Formation in Luzhou palaeohigh are complex,i.e.there are both the wet gas with condensate oil as that in the core of the palaeohigh and the dry gas without condensate oil as that in the slope zones of the palaeohigh. Based on geochemical analysis of total hydrocarbon by using gas composition and biomarkers,gas in the core of the palaeohigh has mainly been sourced from the Jurassic source rocks with some from the Upper Permian Longtan Fm coal-measure source rocks,while that in the slope zone has mainly been sourced from the Lower Permian carbonate source rocks with some from the Upper Permian Longtan Fm coal-measure source rocks.Oil and gas reservoirs in Jialingjiang Fm in Luzhou palaeohigh have been formed in 4 stages: formation and destruction of fossil oil reservoirs in the core of the palaeohigh during Indo-Chinese epoch;formation of gas reservoirs with some oil in the core of the palaeohigh during Early-Middle Yanshanian epoch;formation of dry gas reservoirs in the slope zone at the end of Yanshanian epoch;and further enrichment and adjustment of gas reservoirs during Himalayan epoch.The evolution of Luzhou palaeohigh controlled the distribution of the oil and gas reservoirs in Jialingjiang Fm;plenty of multistage and multi-source oil and gas charging was the material base of oil and gas reservoiring;"faults connecting the source rocks" controlled the level of oil and gas enrichment;later development and enlargement of dissolution pores determined the reservoiring capability of gas reservoirs;the solid bitumen and asphaltene in the core of the palaeohigh provided the sealing conditions,as well as ensured oil and gas enrichement.

    Sequence stratigraphic framework of marine lower assemblage in South China and petroleum exploration
    Chen Hongde, Ni Xinfeng, Tian Jingchun, Wei Dongxiao
    2006, 27(3):  370-377.  doi:10.11743/ogg20060312
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    Based on establishment of regional sequence stratigraphic framework,it is believed that the regional main source rocks and cap rocks would have been formed in the transgressive system tract of the supersequence;while the high-quality regional reservoirs would be consistent with the highstand and lowstand of the supersequence.Vertically,3 source-reservoir-cap combinations occur in the lower assemblage(Lower Paleozoic) marine carbonates in South China,and they can be divided into 5 types,including boundary type,lowstand type,transgressive type, highstand type and composite type.Each type of source-reservoir-cap combination has different features of reservoiring geology.It is suggested that reef flat limestone-dolomitized limestone/dolomite-palaeokarst are the most favorable reservoirs,palaeohigh-late structural traps are the most favorable location;and southeastern Sichuan-northern Guizhou area is the frontier where exploration breakthrough would be expected in the near future.

    Features of condensed section and its relationship with source rocks: taking marine Sinian-Middle Triassic in southern China as examples
    Tian Jingchun, Chen Hongde, Zhang Xiang, Nie Yongsheng, Xia Qingsong, Zhao Qiang
    2006, 27(3):  378-383.  doi:10.11743/ogg20060313
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    In sequence stratigraphy,boundaries that can be used to define sequences include bottom boundaries,initial marine flooding surface and maximum flooding surface,of which the sediments corresponding to the maximum flooding surface constitute the condensed section.Identification of condensed section is not only the key to understanding of sequence structure,but also critical to analysis of global relative sea level changes during geologic history.Taking the marine Sinian-Middle Triassic in southern China as examples,6 rock formations are recognized in condensed section,including silicalite,black shale,argillaceous rock,biogenic fossil bed,micrite and nodular limestone.Although different types of condensed sections have been deposited in different sedimentary environments,they all have been deposited when the global relative sea level reached the highest position.Their formation time,therefore,corresponds to the optimum development stage of source rocks in southern China.

    Geological Characteristics of large Xinchang gas field in western Sichuan depression and key predication technologies
    Ye Jun, Chen Zhaoguo
    2006, 27(3):  384-391.  doi:10.11743/ogg20060314
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    Xinchang gas field in western Sichuan depression is a large superimposed gas field characterized by multiple layers and various reservoir types,with a vertical span of over 4000 m,and is composed of the shallow structural-lithologic gas reservoirs,the middle structural-diagenetic gas reservoirs and the deep structural-fracture gas reservoirs.Vertically,its major gas reservoirs include the shallow layer Upper Jurassic Penglaizhen Formation,the middle layer Middle Jurassic Upper Shaximiao Formation and the deep layer Upper Triassic 2nd member of Xujiahe Formation.The main control factors of Penglaizhen Formation gas reservoirs are the channel sandstone subfacies in the nose-like anticline,the main control factors of the Upper Shaximiao Formation gas reservoirs are the favorable diagenetic facies in the sandstones of superimposed mouth bar and distributary channel microfacies on the palaeohigh,while that of Xujiahe Formation gas reservoirs are the fracture systems developed in the large palaeohigh strucutral zone.The key technology for prediction of prolific zone in the shallow gas reservoirs is bright spot identification of seismic response with "low frequency but strong amplitude";the key technology for prediction of prolific zone in the middle gas reservoirs is multi-disciplinary comprehensive reservoir model building;while the key technology for prediction of the prolific zone in the deep gas reservoirs is the comprehensive prediction technology through geology-seismic-logging integration and gas-bearing property detection technology.

    Assessment of petroleum resources in Lower assemblage under Xuefeng thrust nappe
    Li Zhongdong, Luo Zhili, Liu Shugen, Yong Ziquan
    2006, 27(3):  392-398.  doi:10.11743/ogg20060315
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    Xuefeng thrust nappe was located on the continental slope on the southeastern margin of Yangtze fossil plate during Sinian-Silurian,and evolved progressively into Xuefeng ancient land along with the ascending and erosion of "Jiangnan uplift" during Caledonian movement.The pattern of paleofold in Xuefeng thrust nappe was near recumbent fold thrusting toward northwest during Caledonian.The frontal zone of the thrust nappe overrode large area of Paleozoic strata due to the multiple superimposition and thrusting from southeast to northwest during Indosinian-Yanshanian.Based on lithofacies zones and geophysical data,it is predicted that the thrusting distance of Xuefeng thrust nappe would not exceed dozens of kilometers,the depth of Lower Paleozoic overridden by the frontal zone is about 4000 m,and the overridden area is at least 3000 km2.The lower assemblage(Sinian-Silurian) of Xuefeng thrust nappe consists of black carbonaceous mudstone,siliceous rock,phosphate rock and biogenic limestone deposited in a bathyal,stagnant and anaerobic environment with TOC value in the range of 0.01%-4.00% and good oil-generating conditions.Oil and gas pools had been formed in Caledonian,and 9 fossil oil pools and residual oil and gas pools have been discovered along the western margin of Jiangnan uplift.Reservoiring condition analysis of Huzhuang residual oil and gas pools shows that the oil and gas have come from the secondary hydrocarbon generatioin of the marine lower assemblage(Sinian-Silurian) and upper assemblage(Upper Permian-Lower Triassic).Hydrocarbons have experienced long-distance migration and have been accumulated in places having a certain preservation condition.Oil and gas pools have been formed during Late Yanshanian-Himalayan,the same as the finalization of Xuefeng thrust nappe.Therefore,it is believed that the lower assemblage of Xuefeng thrust nappe has relatively large potential of petroleum resources,and the arcuate zone along Zhenyuan-Kaili-Sandu is favorable for exploration.

    Characteristics and variation pattern of acoustic parameters of carbonate reservoir rocks in Tahe oilfield,Tarim basin
    Liu Shugen, Shan Yuming, Huang Sijing, Liu Weiguo, Cao Jun, He Zhenhua, Han Gehua
    2006, 27(3):  399-404.  doi:10.11743/ogg20060316
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    Taking the complex media of carbonates in Tahe area, Tarim basin,as an example,and based on the experimental test results of rocks' acoustic parameters under formation conditions,the following understandings are obtained: 1) P-wave and S-wave velocities increase with increasing confining pressure but decrease with increasing temperature;2) the dynamic elastic modulus increases with increasing confining pressure but decreases with increasing temperature;3) the attenuations of P-waves and S-waves decrease with increasing confining pressure,and increase with increasing temperature.

    Distribution and exploration potential of Ordovician reservoir in Ordos basin
    Xi Shengli, Li Zhenhong, Wang Xin, Zheng Congbin
    2006, 27(3):  405-412.  doi:10.11743/ogg20060317
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    The Ordovician in Ordos basin had been deposited in a "three uplifts-two saddles-two depressions" paleotectonic framework.The three uplifts were horizontally distributed as the letter "L" and together controlled the sedimentary facies of Ordovician and the regional distributions of different types of reservoirs.The main reservoir types in Ordovician include sub-salt reservoirs in the eastern depression,residuum karst reservoir on the slope zone of the palaeohigh,dolomite reservoirs in the 4th member of Majiagou Formation on the saddle of the palaeohigh,Middle Ordovician gravity flow reservoirs in the western part of the palaeohigh,and bioherm or reef reservoirs on the southern slope zone of the palaeohigh.Among these reservoir types,the residuum karst reservoirs are the most favorable,and the large Jingbian gas field has been discovered in this reservoir type.Based on analysis of reservoir characteristics and hydrocarbon migration and accumulation,it is believed that the other 4 types of reservoirs still have relatively large exploration potentials,though no substantial progress has been made in exploration,thus they would be the key frontiers for exploration in the Lower Paleozoic in Ordos basin.

    Carbonate reservoirs prediction with sequence stratigraphy:application in Shiwandashan area
    Li Guorong, Wang Xin, Zhou Xinhuai, Qiao Zhanfeng, Zeng Yunfu
    2006, 27(3):  413-421.  doi:10.11743/ogg20060318
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    Carbonate reservoirs can be divided into intra-sequence reservoirs and sequence boundary reservoirs based on study of sequence stratigraphy.First of all,the development potentials of the two types of reservoirs mentioned above are defined according to the growth rate of carbonates,then the distribution of intra-sequence reservoirs is predicted through detailed delineation of the distribution of isochronous unit reef flat facies;and the development of sequence boundary reservoirs is predicted on the basis of analysis of the types and natures of sequence unconformity surfaces.The Devonian in Shiwandashan area is a "progradational wedge" platform with low growth rate of carbonates,and is dominated by reservoirs controlled by "type-Ⅰsequence unconformity interface";as a whole,the reservoirs are well developed.The Carboniferous is an "aggradational" platform in epicontinental sea with low growth rate of carbonates,and is dominated by sequence boundary reservoirs under the control of the fourth-order and third-order sequence boundaries;the reservoirs are poorly developed.Both the intra-sequence reservoirs and the sequence boundary reservoirs under the control of third-order sequence boundary in the Lower Permian are poorly developed;only the scattered point reef reservoirs developed to the north of Pingxiang-Dongmen fault and the karstic,fractured and vuggy reservoirs controlled by the sequence boundary formed during Dongwu tectonic movement are worthy to be considered.Neither the intra-sequence re-(servoirs) nor the sequence boundary reservoirs are developed in the Upper Permian carbonates.As for the Lower Triassic,both intra-sequence reservoirs and sequence boundary reservoirs are developed in the northern part of the platform due to the high growth rate of carbonates,but neither of them is developed in the southern part of the platform due to the low growth rate of carbonates.The main reservoirs are closely related with the regression phase of second-order eustatic cycle.

    Qigequan Formation and Quaternary-Neogene boundary in Qaidam basin
    Sun Zhencheng, Qiao Zizhen, Jing Mingchang, Zhang Haiquan, Sun Naida, Lu Yanli, Wang Hongjiang
    2006, 27(3):  422-432.  doi:10.11743/ogg20060319
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    Three gas fields sourced from Pliocene-Quaternary Qigequan Formation have been found in Sanhu depression in southeastern Qaidam basin.It is a misunderstanding to take the base of Qigequan Formation in Sanhu depression as the boundary of Quaternary and Neogene.Based on logging,seismic stratigraphic and magnetobiostratigraphic studies,the age of the base of Qigequan Formation in Sanhu depression is believed to be 2.80 Ma,it is at a depth of 1 747 m in Xinshen 1 well drilled in Sebei area.There are three different opinions on the Quaternary-Neogene boundary in Qaidam basin in literatures.①The end of Olduvai,1.67 Ma,(1.80 Ma); it is at a depth of 710 m in Sezhong 6 well,i.e.19 m above the log-marker K3.②The boundary of Matsuyama and Gauss, 2.48 Ma(2.60 Ma);it is at a depth of 1460 m deep in Seshen 1 well,i.e.37 m below the log-marker K10.③The end of Mammoth,3.05 Ma;it is 2 779 m deep in Dacan 1 well.None of the above three boundaries correlates with the base of Qigequan Formation.The definition of Quaternary-Neogene boundary,with an age of 1.80 Ma,has been adopted by the International Commission on Stratigraphy(ICS) through voting in 1998,and accepted by the International Union of Geological Sciences(IUGS) in 1999.According to this definition,most of the gas source rocks in Sanhu depression were deposited during Late Neogene to Early Quaternary,especially in Pliocene.It is suggested in this paper that the stratigraphic unit name of Qigequan Formation rather than the symbol "Q1+2" should be used in petroleum exploration and development research in Qaidam basin.Correlation of the seismic reflector T0(log-marker K13) and the base of Qigequan Formation with local unconformities at the edge of the basin is very useful for study of tectonic movement,gas generation and reservoiring. As the base of Qigequan Formation,the correlation of seismic reflector T0 is characterized by practicability,isochroneity and operability in central Sanhu depression in Qaidam basin.