Table of Content

25 December 1997, Volume 18 Issue 4

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FAULT STRUCTURES AND ACCUMULATING DYNAMIC SYSTEMS

Zhang Shulin, Tian Shicheng, Chen Jianyu

1997, 18(4):  261-266.  doi:10.11743/ogg19970401

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According to abnormal pressure fluid compartments or expulsion fluid assemblages,a basin can be divided into many accumulating dynamic systems.There are 6 accumulating dynamic systems in Bohai Bay Basin by expulsion expulsion fluid assemblages and 3 accumulating dynamic systems in Yinggehai Basin by abnormal pressured compartrment.The classification of accumulating systems is various according to different criteria,that is:normal pressure,overpressure and low pressure by distribution of fluid pressure systems;external source,internal source and compound source systems by oil source;and open,closed and semi closed systems by sealing condition.Hydrocarbon migrates mainly within particular dynamic systems,and the basic migration model of cross systems is periodic migration along large faults and fluid diapirs.The structural zones associated with large faults and fluid diapirs are favorable to form complex oil and gas zones of multiple accumulating dynamic systems.

RELATIONSHIP BETWEEN PALEO TECTONIC STRESS FIELDS AND MAJOR GAS FIELDS IN ORDOS BASIN

Wu Chonglong, Zhou Jiangyu, Wang Genfa, Zhou Hui, Zhang Qiongyan

1997, 18(4):  267-275.  doi:10.11743/ogg19970402

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Residual trend surface analysis on elevation of stratigraphic top surface of prototype basins and the analysis on the geometry of corresponding sedimentary basements indicate that a large flat uplift close to the Caledonian erosional surface exists in the central Ordos Basin.Combining the analysis of syndepositional tectonic deformation in the basements with that of regional tectonic history,ways and directions of the tectonic stress in early and late Yenshan stages are determined.Then the tectonic stress field and strain field of the two stages are simulated by using finite element method,and their genetic relationships to the above mentioned large flat uplift and big gas fields in the central Ordos Basin are studied.It is considered that vertical expansion of Early Paleozoic mudstone and gypsum salt rock layers in compressional stress field might promote the formation of large flat structure trap close to the Caledonian erosional surface where the gas fields are situated.The superimposition of low compressional stress and other factors resulted in negative pressure environment,the central part of the basin became the accumulation area and container for natural gas.

DEFINITION OF MESOZOIC LISHUI NANLING PULL APART BASIN IN LOWER YANGTZE REGION

Lin Heming, Xia Bangdong

1997, 18(4):  276-281.  doi:10.11743/ogg19970403

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Lishui Nanling Basin is a Mesozoic pull apart basin which possesses two sedimentary depression.The boundary faults of the basin's two sides are left step sinistral en echelon strike slip faults with NNE strike.The basin has similar origin to that of Ningwu pull apart basin. They constitute a pull apart basin set.The basin has following features:(1)The stratigraphic thickness is up to 6 000 m,and the sedimentation rate is more than 690 m/Ma.(2)The sedimentary facies show evident asymmetry in basin's plane and cross sections.(3)The palaeocurrent directions of the basin margins point to intra basin,those in the axial region of the basin are accord with the basin axis.Along the palaeocurrent directions,the content and the size of the gravel decreases.(4)Strike slip movement of the boundary faults led to the fan/source displacement and the continuous change of the palaeocurrent directions.(5)Volcanic rocks changed from calc alkaline into alkaline into alkaline in the basin,indicating the change of it's tectonic background from compression to extension.

A NEW MODEL OF SANDSTONE DIAGENESIS IN COAL MEASURES:A CASE OF JURASSIC IN BEISHAN BASIN

Zhou Jingsong, Zhao Chenglin, Han Chunyuan

1997, 18(4):  282-287.  doi:10.11743/ogg19970404

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Strong compaction is the most typical diagenesis as well as the principal factor of reduing porosity.Coarse clastic rocks far from coalbed are characterized by extensive cementation and replacement of calcite,while fine sandstones and siltstones interlayered by coalbeds or carbon rich shales show strong compaction.This is related to original sedimental enverinment structural position,coal measure evolution and internal circulating fluid under compaction.

HYDROCARBON ACCUMULATION AND DISTRIBUTION IN TAZHONG LOW UPLIFT OF TARIM BASIN

L? Xiuxiang, Hu Xuan

1997, 18(4):  288-293,308.  doi:10.11743/ogg19970405

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Tazhong low-uplift with abundant resources in Tarim Basin is a remnant Paleozoic uplift formed during late Caledonian and early Hercynian orogenies.The uplift possesses several multi sets of source beds and oil bearing horizons,several types of traps,multi stage of hydrocarbon formation,and are characterized by complex petroleum system.There are 4 kinds of oil and gas accumulation models:buried hill and anticlinal types of Lower Ordovician carbonate rock,overlapped stratigraphic type of the Carboniferous Donghe sandstone,faulted anticlinal and anticlinal types of the Carboniferous,anticlinal and lithological types of Silurian sandstone.Ordovician oil and gas bearing horizons are generally related to weathering crust,Silurian oil and gas accumulations are controlled by both structure and lithology,Carboniferous oil and gas accumulations are normally related to faults and unconformity.It is only by improving the technique of desert seismic exploration,the success ratio of oil and gas exploration could be increased,and giant oil and gas fields might be found in the pitching position of the uplift.

CONTROL OF ORDOVICIAN PALEOKARST ZONES IN SOUTH ORDOS ON GAS RESERVOIRS

Wu Xichun, Li Peihua, Jin Xiangfu, Yuan Weiguo

1997, 18(4):  294-299.  doi:10.11743/ogg19970406

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The paleokarst zone of Caledonian weathering crust at the top of Ordovician in south Ordos is different from the one in Middle Ordos'but the gypsum cast corroded pore reservoirs of the 5th Member of the Lower Ordovician Majiagou Formation in the northeast part of south Ordos is similar to those in Middle Ordos.Different formations are located in different positions of the weathering crust of south Ordos.Reservoirs of the 3rd and 5th Members of the Majiagou Formation,undergone multi stage interstratum karst corrosion in geohistory,can be induced into four major stages and develop into favourable gypsum cast resorvoirs.The target area of oil and gas exploration should be set northward of Huangling Huanglong area;that of the platform marginal reef and backreef beach of Middle Ordovician should be set between Yongshou-Chunhua-Xunyi areas.

FAN DELTA IN YING'ER DEPRESSION,JIUDONG BASIN

Li Wenhou, Zhou Lifa, Zhao Wenzhi, Xue Liangqing

1997, 18(4):  300-304.  doi:10.11743/ogg19970407

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Mesozoic fan delta facies in Ying'er Depression,Jiudong Basin developed well.It's sedimentation is brisk and fluid types are abundant.Sediments are mainly sheet grain flow deposit associated with sheet tractional deposit.The fan delta deposits in the region are distributed close to active boundary faults to the north and east of the basin.It's sedimentary facies belt,including fan delta plain,fan delta front and pre fan delta subfacies,developed completely.In vertical direction,the fan delta is a set of retrograding ingression lacustrine sequence with coarse sediments in the lower part and fine ones in the upper part,suggesting that subsiding velocity of lake basin surpassed sedimentary compensation velocity.The sandstones and conglomerates of fan delta facies and dark mudstone of deeper lake subfacies are paragenous in the region.In addition,sedimentary sequence of mudstone and dolostone of lake facies is continuous in Upper Mesozoic and its thickness is big.They constitute a set of complete source reservoir cap rock assemblages.

GRADATION OF RESERVES AND RESOURCES EXTENTS OF COALBED GAS IN CHINA

Sun Wanlu, Ying Wenmin, Fan Mingzhu, Wang Shuhua

1997, 18(4):  305-308.  doi:10.11743/ogg19970408

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The grades of coalbed gas reserves and resources extents are mainly determined by exploration stage and degree of a wildcat area.According to the numbers of development wells,the numbers of assessment parameters of reserves and resource extents,China's coalbed gas could be classified into two kinds of resources extents and four types of geological reserves.Based on incomplete statistics,the total coalbed gas resources buried shallower than 2 000 m is about 326 366×10⁸m³,within which the resources buried between 1 000～2 000 m is about 215 666×10⁸m³,the resources buried shallower than 1 000 m is 1107× 10⁸m³.

PROPERTIES OF VOLCANIC ROCKS AND STRUCTURAL ATTRIBUTES IN ZHANGQIANG DEPRESSION AND ITS NEIGHBOURING AREA

Chen Quanmao, Zhang Guangya, Liang Xiaodong, Chen Zhaonian, Zhou Shaoqiang, Wang Zhanzhong

1997, 18(4):  309-313.  doi:10.11743/ogg19970409

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The geochemical properties of volcanic rocks bear close relations with structural background.It has been proved by analysis result that volcanic rocks in Zhangqiang and its neighbouning Dawa Changtu Depressions have following characteristics:(1)the volcanic rocks are neutral acid,and most of them are alkaline basalt;(2)the volcanic rocks are sodium alkaline basalts in Dawa Changtu Depression,but sodium and potassium alkaline basalts as well as calc alkali and continental tholeiitic basalts in Zhangqiang Depression;(3)the standard rare earth element distribution curves of chondrite in the volcanic rocks tilted rightwards and light rare earth element are rich in the rocks.(4)Zr-Ce and Zr-La diagrams show distinct differences between the two depressions.It is suggested that the volcanic rocks in the two depressions are all resulted from continental rift,the magma sources are different and should belong to different tectonic units.

GEOCHEMICAL CHARACTERISTICS OF EARLY CARBONIFEROUS BASALTS IN SANTANGHU REGION,XINGJIANG AND THEIR SIGNIFICANCE IN TECTONIC SETTINGS

Gong Wenping, Lin Kexiang, Li Yibin

1997, 18(4):  314-318.  doi:10.11743/ogg19970410

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Volcanicity was violent during the Early Carboniferous in Santanghu region.The volcanic rocks consist of basalts,andesites and dacite liparite,with andesites playing the main part.Geochemical analysis indicates that the basalts are characterized by high content of Al₂O₃,low contents of FeO+0.9Fe₂O₃ and TiO₂,rich macroion lithophile element(LIL)and LREE.The spider diagram of trace element ratio and the REE distribution patterns show the characteristics of the island arc calc alkaline series.In addition,basalt discrimination diagrams,such as Nb-Zr-Y and Cr-Ce/Sr,suggest that Santanghu area was in an island arc envirnment during the Early Carboniferous.

DEDOLOMITIZATION OF CARBONATE ROCKS IN ORDOS AND ITS RELATION TO RESERVOIR PROPERTY

Qin Jianxiong, Yang Zuosheng

1997, 18(4):  319-325.  doi:10.11743/ogg19970411

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The dedolomitization of Lower Ordovician carbonate reservoirs in Ordos Basin could be divided into five types.Dedolonitization related to unconformity is characterized by producing lots of corrosion fabrics, small amounts of dedolomitized calcite with low contents of Sr,Fe,Mn and low homogenization temperature inclusions.Burial environment dedolomitization related to suture lines,gypsum nodules,burial dissolution caves and cracks,structure fissures are mainly resulted from the loss of Mg²⁺,the increase of Ca²⁺,the thermal maturation of organic matter and hydrocarbon migration and accumulation.The dedolomitization is charactorized by rich Mg,Fe,Mn,Sr and low δ ¹⁸O,δ ¹³C,high homogenization temperature inclusions,thus forming large amounts of dedolamitized calcite.The porosity of the dedolomitized dolomite is clearly higher than that of other dolomites and limestones.This could not only improve reservoir permeability,but also change nonpermeable beds into permeable ones.

APPLICATION OF FLUID INCLUSIONS TO PETROLEUM GEOLOGICAL STUDY AND DISCUSSION

Liu Shaobo, Gu Jiayu

1997, 18(4):  326-331,342.  doi:10.11743/ogg19970412

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This paper summerizes the research advances and application methods of fluid inclusion homogenization temperature,especially in the aspects of determining the time of oil reservoir formation,petroleum expulsion,oil migration,history of deposition and uplift and geothermal gradient.Some problems relating to the criteria of homogenization temperature resetting and pressure correction,the application of fluid inclusion homogenization temperature and petroleum geology are discussed.

CHARACTERISTICS OF LIGHT HYDROCARBONS IN TAZHONG AREA AND ITS SIGNIFICANCE

Zhang Chunming, Jin Diwei

1997, 18(4):  332-334.  doi:10.11743/ogg19970413

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The ratio K,reported by Mango(1987),ranges from 1.09 to 1.16 in the oils produced in north slope of Tazhong area shows relative invariant;the K of oils trapped in the east part of Tazhong fault uplift shows not only high value(1.20～1.54),but also displays regular changes in the reservoirs.The index of methylcyclohexane shows similar variance to the ratio K.It indicates that the oils trapped in the north slope are different from the oils of light hydrocarbons in the east part of Tazhong fault uplift,and that some of the oils with high K value are formed by mixing procession.

SEDIMENTARY CHARACTERS AND BIOGAS ACCUMULATION IN POSTGLACIAL STRATA OF QIANTANGJIANG RIVER MOUTH REGION

Lin Chunming, Li Congxian, Jiang Weisan

1997, 18(4):  335-342.  doi:10.11743/ogg19970414

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The bottom boundary of the postglacial strata in Qiantangjiang River Mouth region is recognized as the surface of the regional unconformity formed during the lower sea level period of the last glacial age.The postglacial strata can be divided into four sets corresponding to primary,early,mid and late stages.Channel grails deposited in the primary stage,alluvial flat clay rock and sandy clay rock deposited in the early stage,mudstone deposited in the mide stage,silty clay and limnic clay deposited in the late stage.Sedimentary facies include river bed,alluvial flat,neritic,river mouth and limnetic,littoral,weathered continental drift bed and flood deposit facies.It is suggested that the sandy lenticular bodies of alluvial flat and neritic facies are favourable reservoir zones for ultra shallow biogas exploration.

RESEARCH START POINT AND SCIENTIFIC OBSERVATION

He Ziai, Peng Yu'e, Xu Yangang, Yang Jianchao

1997, 18(4):  343-348.  doi:10.11743/ogg19970415

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Abstract In view of some tendencies in geological study,the thinking and method relating to two basic problems-research start point and scientific observation are discussed in this paper.Research start point is essentially a problem of inheritance.The setting of start point should be based on the scope and depth of scientific and technical document study,and this is the key problem of setting start point.Scientific and technical documents can not replace researches' thinking,but can give them inspiration.Only be good at using the documents can one stand on the top of subject's present level to determine new research thinking,to adopt new method,set new thinking direction and new start point.Scientific observation is the conbination of sensation and thinking.The key of efficient observation includes selective observation and finding problem from minuteness.Sponteneous observation and induced observation are two inseparable links.Unexpected phenomena that result in induced observation usually lead to new discovery and unexpected achievements.So be good at finding out differences is the nucleus of the observation.