Table of Content

25 March 1985, Volume 6 Issue 1

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A GEOLOGICAL REVIEW OF THE EAST CHINA SEA AND ITS PETROLEUM POTENTIAL

Zhou Zhiwu, Yin Peiling

1985, 6(1):  1-14.  doi:10.11743/ogg19850101

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Geological surveys of the East China Sea began in 1974.Since then,the Bureau of Marine Geology,Ministry of Geology and Mineral Resources is in charge of the extensive geological and geophysical investigations on this sea area of 770,000 sq km.Up to now,plenty of useful information have been obtained.According to seismic spectrum and well logging data,the stratigraphic sec-tions of the East China Sea can be divided in descending order as follows:the Donghai Group(Q),the Shantan Formation(N₂),the Liulang Formation(N₁³),the Yuquan Formation(N₁²),the Longjing Formation(N₁¹),the Huagang Formation(E₃),the Pinghu Formation(E₂),the Paleocene and Upper Cre-taceous,etc.These sediments,which have a total thickness of 15,000m,consist predominantly of dark gray-greenish gray and light gray mudstones and sand-stones interbedded with conglomerates,carbonates and coal.The East China Sea includes following six tectonic elements(1st order):Zhemin uplift area,the East China Sea shelf basin,Diaoyudao uplift-foldedbelt,the East China Sea shelf foredeep,Okinawa trough basin and Ryukyu uplift-folded belt.The shelf basin can be divided into Fujiang,Zhedong,Taibei,Taixi depressions.The later three depressoins are subdivided into 14 sags(e.g.Xihu,Oujiang,Minjiang,Jilong)and arches(e.g.Hupijiao).Up to now,13 structural belts and more than 100 local structures(e.g.Longjing)have been located.The closed area of the local structure is generally more than 50 sq km,the largest one is up to 1,000 sq km.The East China Sea shelf basin,which has an area of 284,000 sq km,contains thick Meso-Cenozoic sediments,mostly deposited in marine,swamp and lacustrine environments,where there are abundant organic matters,many source rocks,shallower oil threshold depthes,good trap conditions and ade-quate timing.Commerial oil and gas have been tested from three horizons of Eocene,Oligocene and Miocene ages respectively in Pinghu and Longjing structures.The authors suggest that pressed anticlinal and buried-hill drapping anticlinal hydrocarbon pools are predominante in the basin,and other pools of stratigraphic,lithologic,reef and combination types could be also disco-vered there.

GEOLOGICAL FEATURES AND PETROLEUM POTENTIAL OF SHAYAR UPLIFT, NORTHERN TARIM BASIN

Kang Yuzhu, Jia Runxi, Jiang Bingnan, Huang Youyuan

1985, 6(1):  15-23.  doi:10.11743/ogg19850103

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The north part of Tarim Basin,including Kuche depression,Shayar uplift and Awati-Manjiar depression,has an area of more than 100,000 sq km.On Sept.22,1984,more than 1,000m³ oil and 2,000,000m³ natural gas per day have been tested from well Shacan-2 on Yakela structure of Shayar uplift.Between Kuche and Awati-Manjiar depression,there is east-west extending Shayar uplift which is 460km long and 10-50km wide.It existed as a suc-cesional uplift during early Paleozoic to Mesozoic time.In the south of the uplift,Awati-Manjiar depression contains the Cambrian-Ordovician,Carboniferous-Permian and Triassic-Jurassic source rocks,among which the thick Cambrian-Ordo-vician and Triassic-Jurassic source rocks are widespread with better oil-generationpotential.North of the uplift,the Kuche depression contains mainly Triassic-Ju-rassic source rocks.The Shayar uplift,with many structural,stratigraphic,lithologic and combination traps,is in close proximity to those source rocks and in the direction of oil and gas migration.Therefore,it is favourable for searching gaint oil and gas fields.The oil and gas in well Shacan-2 are produced from the weathering zone of the Cambrian-Ordovician dolomites.The oil has high vanadium content(5.88 ppm),high nickel content(0.76 ppm)and high V/Ni ratio(17.76).The GC analysis of the saturated hydrocarbon in the crude oil indicates thatthe oil has even carbon predominance of C₂₈,C₃₀ and C₃₂ and its CPI is 0.988.It is considered that hydrocarbons from the well Shacan-2 were probablyderived from the Cambrian-Ordovician marine source rock.However,the Triassic sporo-pollen fossils have been found in the crude oil and many Acri-tarch fossils have been examined in the Triassic sediments of the well Yoacan-1on Shayar uplift,therefore,marine intercalations might exist in Triassic,hydrocarbons from the well Shacan-2 were also probably derived from the Triassic-Jurassic source rock.From above,the authors consider that the Yakela pool is of multi-source origin.

ON THE GEOCHEMISTRY AND ORIGIN OF SUPERHIGHWAX CRUDE OIL AND SOLUBLE ORGANIC MATTERS IN SOURCE ROCK

Li Jinchao, Huang Difan, Zhang Dajiang

1985, 6(1):  25-37.  doi:10.11743/ogg19850106

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Geochemistry of superhigh-wax crude oils and soluble organic matters in source rocks of Nanyang and Damintun downwarps are studied.The content of wax and saturated hydrocarbon in oii is about 50%,80% respectively.From this,we could see that terrestrial lipoid has a great contribution to the wax-bearing petroleum,and the origin of the later might be related to the parent matters rich in phyto-wax.The similarity in distribution of normalalkanes from pyrolytic kerogen,from extraction of source rocks as well as from crude oil has proved that the super high-wax crude oil of Nanyang depressionare resulted from the sapropelic source rocks near the threshold depth of mature stage.Both Nanyang and Damintun downwarps were fault-confined lakes in Tertiary developed nearby with coniferous trees.This kind of forest had certainly con-tributed a great deal of materials rich in phytogenic-wax which give rise to the foundation for formation of superhigh-wax crude oil in these basins.The meta-akaline hydrological environment was favourable to the accumulation of terrestrial lipoid and phytogenic-wax in such a downwarp like Nanyang.

BIOMARKERS OF TERTIARY SOURCE ROCKS IN DONGTAI DEPRESSION AND THEIR GEOLOGICAL-GEOCHEMICAL SIGNIFICANCE

Lu Songnian, Li Weimin, Gu Huimin, Gao Pinwen

1985, 6(1):  38-48.  doi:10.11743/ogg19850108

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GCMS are used to analyse saturated hydrocarbon fractions of 17 argillite rock extracts of Dongtai depression so as to study the sedimentary environment,the type of organic matters and the maturity of early Tertiary source rocks with the aid of biomarkers such as sterane and terpane.Both OEP and isopre-noid distrt bution have proved that Ef² and Ef⁴ are deposits under strong reducing environment in which gamma ceranes have emerged.Based on the ratio of sterane of different carbon number and that between sterane and terpane,sapropelicorganic matter(type Ⅰ and Ⅱ of kerogen)is found to distribute in regions where C₂₇ sterane exceeds 30% and where the ratio between C₂₇ sterane and C₃₀ hopaneplus mortane exceeds 0.5.On the basis of the epimerization of sterane and terpane,it can be deduced that the depth of the threshold of oil generationis about 2,100m.

ON THE PETROLOGY AND SEDIMENT GRAVITY FLOWS OF THE TANGWANGLING FORMATION

Hong Qinyu

1985, 6(1):  49-59.  doi:10.11743/ogg19850109

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Stratigraphically,the Tangwangling Formation,which can be divided into four members,is about 1100.41m in Huangpigou,Liquan County,Shaanxi Province.The sediments are plenty and varied.From shallow to deeper water,there exist in turn:1.light colour carbonate tongue developed in the front margin of platform(79.26m,making up 7.2% of the section thickness);2.continental slope sediments(130.94m,11.9%);3.contour current sedi-ments(7.09m,0.6%);4.mudflow sediments(635.57m,57.8%);5.debris flow sediments(215.32m,19.6%);6.grain-flow sediments(8.01m,0.7%);7.turbidity current sediments(24.22m,2.2%).Continental slope sedimentsappears to be mainly in the first member of the Tangwangling Formation,while the light colour carbonate tongue only found in the second member of the formation.Sediments in the third and fourth member were mainly generated by sediment gravity flows,which were developed in the background of effectiveregression.So it can be seen clearly that these sediments of the Tangwangling Formation were not formed by glacier as were recognised by many people before.

STUDIES ON ORGANIC GEOCHEMICAL CHARACTERISTICS OF EOGENE SOURCE ROCK IN GAOYOU DEPRESSION AND ITS OIL GENERATION THRESHOLD

Wang Benshan, Fan Shanfa, Shi Jiyang, Xu Fenfang, Zhang Lijie, Hong Zhiqing, Lu Guoying, Huang Wanping, Gan Yinu, Wang Wenjun

1985, 6(1):  60-70.  doi:10.11743/ogg19850110

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Oil generation threshold(OGT)of the Funing Formation(Ef,Eocene series)in Gaoyou depression of Subei Basin is one of the most important research subjects which arouse reseachers' wide interests.In this study,a suit of samples collected from the same source bed(Ef₄)but buried at different depths due to the staircase faults are investigated.Based upon the comprehensive studies of various organic geochemistry indices,such as vitrinite reflectance(R°%),odd-even predominance(OEP)of n-alkanes,centents of sterane and terpaneand their configuration parameters(e.g.C₂₉-((20S)/(20S+20R))-5α(H),14α(H),17α(H)-sterane,C₂₉-moretane/C₂₉-hopane ratio,C₃₁-((22S)/(22S+22R))-17α(H), 21β(H)-hopane)and the like.The OGT of the Funing Formation has been determined at the depth of ca 2,750m referring to the judging criteria of OGT at home and abroad.In terms of the result,it has been suggested that the Funing Formation in this area only buried at the depth of 2,750m or more is the real source rock of petroleum.Thus provide a prospective evaluation for this oil-bearing basin with effective parameter.

TECTONICS OF THE WEST EDGE FAULT BELT OF ORDOS REGION AND ITS PROSPECT IN PETEOLEUM EXPLORATION

Zhang Kang, Wu Zidian

1985, 6(1):  71-81.  doi:10.11743/ogg19850112

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Along the west edge of Ordos fault block,there is a bunch of zigzag faults which,in fact.comprises part of the great N-S treading tectonic belt running through central China.This belt is inherited from the basement,which control-ling the formation and deformation of its sedimentary covers.During the period when tension stress prevailed,the fault belt took the form of normal fault with its west side seriously subsiding in the Paleozoic stage,but its east side subsided in the Mesozoic.In the period of compression,i.e.at the late stage of its tectonic cycle,it took the from of eastwards thrust-ing with some folds accompanied.The deformation caused by the Caledonian Orogeny was not quite violent,while intensive imbricate thrust faults were formed by the Yanshanian Orogeny during Late Jurassic.Compressed by the Siberian and Indian plates unequally both from the north and south during the Yanshanian Orogeny,Ordos area was subjected to clock-wise rotation enhanced by Alashan fault block squeezing toward it from the west.So this results in the above mentioned tectonic deformation along the west edge of Ordos fault block.It is concluded that this fault belt possesses better prospects for petroleum exploration due to the existence of source rocks on its both sides and good preservation condition along the fault belt and its eastern area.

THE UPPER PERMIAN REEF AND ITS DIAGENESIS IN SICHUAN BASIN

Qiang Zitong, Guo Yihua, Zhang Fan, Yan Chuantai, Zheng Jiafeng

1985, 6(1):  82-90.  doi:10.11743/ogg19850114

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Loalongdong reef,found recently in the Upper Permian Changxing Formation along the southern end of Huayingshan area is 160m thick and tapers out within 2000m.The research of petrography,paleontology and microfacies indicates that this reef belongs to the framewark reef composed mainly of hydrozoan-sponge.The main reef-building organism is calcisponge,Tabulozoa,hydrozoan and bryzoan which make up 20-30% in average,maxium 50%,of the total or-ganism.Other reef-dwelling organism,such as echinoides,crinoids,foraminiforas,gastropods,brachiopods and calcareous algae,can be found quite often.Thereef facies can be distingushed easily from the non-reef ones,the former isgenerally characterized by bright-grey,massive,silica-free,and in situ biolith,i.e.framestone,bafflestone,bindstone,with a updomed morphology,whilethe later is characterized by dark-to-blackgrey and moderatly or thickly bed dedargillaceous rock with biogenic detritus and/or flints.The biogenic and non-biogenic rocks are interfingeringly contacting each other.This reef had went through four diagenetic environments,i.e.the submarinephreatic,the meteoric phreatic and vadose,reginal ground water and deep buried environments.It is obvious that the diagenesis has close relationship to variation in porosity of the oil reservoir.Comparing with the non-biogenicrocks,the biogenic rocks exhibit better space for oil accumulation,but the primary porosities may be diminished to some extent in the diagenetic process,and thus lost their importance.Since the secondary porosities which formed at the same time,including the important dolomitized intercrystal porosity in reefcore facies,the solutionized intergranular and in-granular porosities in reef-flank are favourable for oil accumulation.Sometimes they are infilled with bitumen.

STUDY OF DIAGENESIS OF RESERVOIR WITH A CO₂ GAS CAP

Du Yunhua

1985, 6(1):  91-95.  doi:10.11743/ogg19850115

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It is usually considered that diagenesis will end by and large as soon as hydrocarbon accumulates in reservoirs.In this paper,a case is presented in which diagenesis still continues after hydrocarbon accumulation.The reservoirs studied are sandstone with a CO₂ gas cap located in the western part of Dongying depression.Diagenesis mainly consists of cementation,replace-ment and solution before oil and gas are accumulated.The cement mineral contains mainly dolomite and calcite,and the latter is a soluble mineral.Cementation of the above dolomite and calcite stops while a large amount of dawsonite associated with kaolinite and primary quartz are produced.Poorly permeable sandstone reservoirs are generated because of the decrease of permea-bility caused by diminished porosity in sandstone filled by dawsonite.The generation of dawsonite is closely related to carbon dioxide.In gas-bearingformation,there is more CO₂,then the amount of dawsonite is large too,asmuch as 35%;in oil-bearing formation,less amount of CO₂ could be found,dawsonite is of less amount too,about 6-8%;in aqueous formation,the amount of CO₂ is the least,the amount of dawsonite is the least too.The poor physic 1 properties of the reservoir caused by diagenesis after the accumulation of oil and gas will not be favourable for the high yielding of oil and gas.However,the effect can be eliminated by acidization and hydraulic fracturing,and production can be increased because dawsonite is the aqueous carbonate.

ON THE CHARACTERISTICS OF NORMAL ALKANE AND ITS PROVENANCE FROM MODERN SEDIMENTS IN EASTERN CHINA

Ding Yaqiu

1985, 6(1):  96-103.  doi:10.11743/ogg19850116

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Comprehensive study is made on saturated hydrocarbon for 80 samples col-lected from modern sediments and organisms in seas,lakes,deltas and rivers in eastern China.The n-alkane from modern sediments of semi-arid climatic zones(from Song-liao Basin to the Yellow Sea)in north China is mainly represented by a single peak with an odd-even predominance and a high-carbon major peak.This distribution pattern is similar to that of the n-alkane of Artemisia from modern organism in Songliao Basin,implying that the organic matters were derived from continental plants.The distribution pattern of n-alkane in modern sediments of the semi-humid climatic zone(along the middle to lower reaches of the Yangtzi River and the East China Sea paralic area)in south China is quite complicated,and represented by single or bimodal peak,partly with even carbon predominance.The modern sediments with normal alkane characterized by even carbon predominance werenot deposited in a strongly reduced environment,and microorganism remoulding might play an important part in the formation of even carbon predominance.

GEOLOGICAL FEATURES OF MESO-CENOZOIC CONTINENTAL BASINS IN ANHUI PROVINCE AND THEIR OIL AND GAS POTENTIAL

He Zhonglin, Xu Yufan

1985, 6(1):  104-111.  doi:10.11743/ogg19850119

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Tectonic structures within Anhui Province can be divided into three tecto-nic units of the first order on the features of geology and geophysical field,i.e.part of the Huabei platform,part of Yangtzi paraplatform and part of Qinling fold belt.A lot of Meso-Cenozoic continental depressions developed there.Their main characteristics are as follows:the formation was controlled by faults,sometimes related to basements.The shape is predominantly half-graben,the formation age ranges from Jurassic to Cretaceous or Cretaceous to Early Tertiary.In these depressions,the sedimental facies are mainly shallow lacustrine and fluvial sediments,though sometimes moderately deeper water to deep water lacustrine sediments might take place near the major fault.It is suggested that,although there are a lot of Meso-Cenozoic sedimentary depressions,and dark argillaceous rocks occasionally found in these depressions are thick enough,most of them are beyond the threshold of oil generation.The most prospective one is Jinhu-Lai'an depressoin,while the others,such as Nanling depression,the east part of Hefei,Guzheng and Zhoukou depressions might also exhibit some oil and gas potential.