OIL&GAS GEOLOGY-1996, Volume 17 Issue 1

THE THOUGHT AND METHOD OF SEQUENCE STRATIGRAPHIC ANALYSIS IN EPICONTINENTALCOAL-BEARING BASIN

Li Zengxue, Li Shouchun, Wei Jiuehuan

1996, 17(1): 1-7. doi:10.11743/ogg19960101

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The sequence stratigraphic analysis of epicontinental coal-bearing basin differs from thatof continental margin basin. In the vast epicontinental coal basin of the Late Paleozoic inNorth China, the sea level changed suddenly and frequently, the paralic coal measuresformed in this condition is thinner than the sequence of marginal sea basin. Based on thebasin-filling characteristics, the control of sea level change over the types and spatial distribution of depositional systems, the basic thought and method of sequence stratigraphic analysis of epicontinental basin have been proposed as fallows:the classification of third order sequence and the intrasequence structural units could be taken at the same time; the recognition of maximun marine flooding surface,sequence interfaces,subsequence interface and depositional system tract could be taken at the same time

ON IDENTIFICATION OF WRENCH STRUCTURE

Yan Junjun, Wang Xiepei

1996, 17(1): 8-14. doi:10.11743/ogg19960102

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The subsurface markers for identifying wrench structure could be concluded as follows:(1)echelon structure 3(2) flower-form structure;(3)braid structure;(4).narrow deformationzone;(5) deep narrow half-graben;(6) narrow thick coarse deposit zone;(7) unmatchablestrata across a fault;(8)swaying and variable fault dip;(9)clutter seismic reflection.However,the trend of overstating the effect ofwrenching may occur due to inaccurate identification in actual application, especially in identifying echelon structure and flower-formstructure.The key problems are(1) tenso-echelon does not always indicate wrench fracture;(2) as for the shallow-rooted structures in a basin,"basement-involving"could not be theessential identificative criterion of wrench fault I(3)the back--thrust blocks that widely developed in oil provinces in West China are mostly not flower-form structures.

COMMENTS ON GLOBAL CYCLOSTRATIGRAPHY

Wu Zhiyong, Jiang Yanwen

1996, 17(1): 15-21. doi:10.11743/ogg19960103

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Cyclic deposition on a global scale is the product of the changes in sea-level,tectonics,climate, geography and depositional features of the global or the combination of these. Globalcyclostratigraphy is defined as the study of cyclic depositional patterns in the geologic recordproduced by the interaction of climate and tectonic processes. It is a stratigraphic branchwhich is developed based on the sequence stratigraphy. Stratigraphic changes through estimating sediment types and depositional environment in a basin can be predicted by means ofthe global cyclostratigraphy. This paper introduces systematically the fundament of theglobal cyclostratigraphy and application to predicting the stratigraphic thickness,lithofaciesdistribution and growing patterns of carbonate.

ANALYSIS HYDROCARBON PROSPECTS OF IN TIBET

Lei Qingliang, Dai Guohan, Fu Xiaoyue

1996, 17(1): 22-26. doi:10.11743/ogg19960104

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There are two domains of petroleum exploration in Tibet-the Mesozoic marine domainand Cenozoic continental domain.The marine domain can be further divided into Qiangtang,Lasa and Jiangzi blocks. Among them,Qiangtang block has relatively favourable prospectsbecause the deep tectonic movement and the deformation of sedimentary cover in it are lessserious than the other two blocks. Continental domain mainly includes two kinds of basinswith different properties:compresso-depressed basins and strike-slip faulted basins. LunpolaBasin,a typical strike-slip faulted basin, is the focal target for oil and gas exploration in Tibet recently since commercial oil flow has been drilled in this basin.Various kinds of residual plateau basins developed in the region since Himalaya movement. They are a special kind of basins in orogenic zone. Wether a complex of geomorphicstructural-sedimental basin could form is an important factor to determine hydrocarbon potential of the plateau basins; the depth of oil oxidation surface in the basin is also a key factor in evaluation of preservation condition.

DISTRIBUTION OF OPEN FRACTURES IN ABNORMAL OVERPRESSURE MUDSTONE AND PRIMARY MIGRATION OF HYDROCARBON

Zhang Jingong, Wang Dingyi, Di Shixiang, Li Pilong

1996, 17(1): 27-31. doi:10.11743/ogg19960105

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The distribution of open-fracture in Eogene abnormal overpressure mudstone and shaleof Changdong-Nanpi Sag in Huanghua Depression suggests that the mudstone and shale withopen fractures centralized in certain depth interval in vertical, but laterally the overpressurezone is regionally distributed. The porosity,the content of ascanite, the residual total hydrocarbons/total organic carbon (TH/TOC) and ααα20S/20S+20R C₂₉ of the mudstone and shale decrease more than that of the upper and lower beds. It thus proved that so long as the openfractures form,a large scale of continuous fluid expulsion would start and the fractures wouldnot close until the expulsion potential exhaust. According to the distribution of the open fractures,the main zone of oil and gas migration could be determined.

THERMAL HISTORY ANALYSIS OF DONGYING DEPRESSION,SHENGLI OIL REGION: EVIDENCE OFAPATITE FISSION TRACK

Guo Sniping, Shi Xiaobin, Wang Liangshu, Wang Jie, Zhou Guangjia

1996, 17(1): 32-36. doi:10.11743/ogg19960106

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The analysis of apatite fission tracks of 12 samples collected in Dongying Depressionindicates that the depression experienced a continuous cooling thermal history after its formation. Two distinct cooling processes took place during 50～60 Ma and 20～30 Ma respectly,and the cooling during 20～30Ma was resulted from the uplift and erosion of the depressionin the end of Dongying Formation,and the estimated thickness of the uplift and erosionamounts to more than 200 m. The depth of apatite fission tracks annealing zone ranges from 2 000 m to 3 400 m,which is well consistent with the depth of oil generation window.At thesame time,the rapid increase of recent temperature due to burial depth led to the high temperature range of the annealing zone.

RELATIONSHIP OF SPATIAL DISTRIBUTION BETWEEN FAULTS AND OIL-GAS FIELDS IN SICHUAN REGION

Shi Zejin, Luo Zhetan, Peng Dajun, He Zhenhua

1996, 17(1): 37-43. doi:10.11743/ogg19960107

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Selecting pertinent scale, the spatial distribution capacity of oil-gas fields in SichuanBasin can be calculated by fractal dimensions. Meanwhile,self-similarity of fault spatial distribution can be examined more effectively by using multifractal theory. Calculating results ofmultifractal dimensions of different zones in Sichuan Region show that spatial distribution ofoil-gas fields and faults are fractal structures and have quit.6 well self-similarity with correlation coefficients better than 0. 99.Oil-gas fields are closely related to the spatial distributionof faults in these zones.The fractal dimension of capacity, information dimension and correlation dimension of fault spatial distribution are about 1.02,1.03 and 1. 04. These zones aremain oil-gas enriched provinces.Therefore, oil--gas exploration and development should bestrengthened in southeastern and central Sichuan and Mianyang--Leshan Regions and in otherplaces,hydrocarbon exploration should be focused on deep-seated pools

A PRELIMINARY ANALYSIS ON HYDROCARBON PRESERVATION CONDITION OF MARINE BEDS IN MIDDLE YANGZI REGION

Chao Guoxi, Luo Xiaoping

1996, 17(1): 44-47. doi:10.11743/ogg19960108

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Though marine oil provinces in the Middle Yangzi Region underwent serious deformation due to Yenshan movement,and as a whole,the hydrocarbon preservation condition of theregion is poor,some blocks favourable for oil and gas preservation could be found within theregions according to the analysis of their sealing condition and geochemical property of subsurface fluid. It is proposed that Mianyang and Dangyang blocks should be considered as thefirst favorable ones,Lixian block could be taken as secondary favourable one, andChangyang,Qianjiang blocks might be the third ones.

RESERVOIR FRACTURE PROPERTY OF THIRD MEMBER OF SHAHEJIE FORMATION IN HUBUZAI REGION

Ma Xinhua

1996, 17(1): 48-52. doi:10.11743/ogg19960109

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The reservoir rock of the third member of the Shahejie Formation in Hubuzai, DongpuDepression is compacted sandstone with poor petrophysical property, but its reservoiringproperty improved a great deal due to well-developed fractures within the reservoir rock. It isconsidered that the hydraulic fracturing that was resulted from high-pressure fluid of saltinterlayers and the regmagenesis caused by uplift--unloading and stress release are importantorigin of the fractures in the area.

GEOLOGICAL CHARACTERISTICS AND HYDROCARBON POTENTIAL OF MIDDLE YANGZI REGION

Wang Linsheng, Han Dingkun

1996, 17(1): 53-57. doi:10.11743/ogg19960110

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Middle Yangzi Region is an ancient stable latform of continental crust type that is characterized by great.thickness of lithosphere and low heat flow value. Post indo-China structural reformation could not change the whole stable framework of the platform,nor could itessentially damage the Lower Paleozoic oil pools of the region. The protection of Silurian detached formations and the delay of hydrocarbon evolution resulted from cold crust favour theremigration,accumulation and preservation of deep seated oil and gas. The Cenozoic extending basins in which Jianghan Basin is the main body is not located at the continental riftzone,its formation time is new and it is less disturbed. It could be considered as a oil provincewith completely developed structural configuration.

FAVORABLE BLOCKS FOR HYDROCARBON EXPLORATION IN QIANGTANG BASIN

Luo Benjia, Dai Guohan

1996, 17(1): 58-61. doi:10.11743/ogg19960111

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Marine sediments with enormous thickness deposited in Qiangtang Basin, North Tibet inLate Paleozoic-Jurassic. This provides favourable material base for hydrocarbon generation.According to the deformation feature and the framework of the basin's basement,the basincould be divided into three tectonic units as North Qiangtang Depression,GangmacuoXiyargang Uplift and South Qiangtang Depression. The North Qiangtang Depression is considered as the most prosperous one for hydrocarbon exploraton due to its well-developedsource-reservoir-cap assemblage and relatively gentle deformation and transformation. Duogecuorenxi, Qulonghe-Cuoni and Donghu in the depression are the most favourable blocksand could be taken as the first targets for oil exploration.

GEOLOGICAL CHARACTERISTICS OF QUATERNARY BIOGAS IN DONGTING BASIN AND ITS PROSPECTS

Huang Zexin, Luo Xiaoping

1996, 17(1): 62-67. doi:10.11743/ogg19960112

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Dongting Basin,covering an area of 1.5×10⁴km²,is a Meso-Cenozoic fault-depressedbasin developed on the basement of the Proterozoic.It subsided several times and received 100～334 m thick Quaternary sediments under the influence of neotectonic movement. Biogasshowing could be seen very often. The accumulation of the biogas is primarily controlled bygas source and reservoir conditions. The organic-rich dark sediments are good source beds;sand beds and gravel beds are major places for biogas accumulation, the clay and sand claybeds of the upper part of the Proterozoic are fine regional cap beds.It is suggested thatXiaojiawan-Yazigang and Sanxianhu-Chapanzhou regions are favourable for Quaternary biogas exploration of the basin.

APPLICATION OF AN ARTIFICIAL NEURAL NETWORK TO EVALUATING CONDITIONS OF HYDROCARBON GENERATION

Sun Shuxia, Wu Zheng, Bei Feng

1996, 17(1): 68-70. doi:10.11743/ogg19960113

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According to the geological model for evaluating condition of hydrocarbon generation,an artifical neural network with three-layer networks,multi-imput and single output isbuilt. The hidden layer consists of four neural cells'This neural network can acquire weightby itself, and modify constantly the connections between the cells. The weight-assigned connections are automatically built by the artifical neural network, thus renewing the old methodof manned weight-assignment. This method is more objective and accurate.The method isused to evaluate the hydrocarbon generation condition of the Ordovician System in OrdosBasin, the result is basically the same as that of the expert system.

GEOLOGICAL FEATURE OF COAL GAS IN HUNAN AND ITS PROSPECTS

Li Qigui, Xie Zengkang, Peng Zhailan

1996, 17(1): 71-76. doi:10.11743/ogg19960114

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Coal measures in Hunan Province are mainly in the Lower Carboniferous Cheshui Formation and Upper Persian Longtan Formation. The coal-bearing areas totalled 5. 5?10⁴km². The coal series wiThin the study area are well developed. The degree of coal evolutionhas reached mature-over mature stage with relatively strong gas-producing capacity and absorbality. The gas content of the coal beds is controlled by the depositional envoironment, thedegree of coal evolution, buried depth, hydrologic condition and geological structure of thecoal-bearing measures, while its porosity and permeability are mainly controlled by coal-rocktype, coalification degree and ground stress. Hydraulic fracturing and cave completion are effective ways to improve poor porosity and permeability. Hongshandian lean coal experimentalzone in Central Hunan and Duolishan coke coal experimental zone are the most favourable regions to develop coal-bed methane.The target bed is the Upper Permian Longtan Formation.

STRUCTURAL FEATURE AND EVOLUTION OFLISHI FRACTURAL ZONE

Bai Yubao, Sun Dongsheng

1996, 17(1): 77-80. doi:10.11743/ogg19960115

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Lishi fractural zone, situated between Ordos Basin and Shanxi Rise,is characterized by.en echelon fault system in its north section and compresso-shear structures in south section.The faulting of the north section started in Late Paleozoic and the system has the property ofsimasphere fault;the compresso-shear started in Middle Triassic and the structures are considered to be sialsphere faults. The fault zone is not in lithosphere but in the earth crust since'it does not separate active zone from stable zone but secondary struetural units within NorthChina Plate

RELATIONSHIP BETWEEN DETACHMENT BED AND DEPOSITIONAL SEQUENCE IN TARIM BASIN

Wu Jincai, Li Cancan, Ye Jianzhong, Wang Guoshou

1996, 17(1): 81-83. doi:10.11743/ogg19960116

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Geological data collected from Tarim Basin indicate that detachment beds are closely related to depositional sequeces.Mudstone-shale detachment is generally corresponding todense section of the depositional sequence; gypsum-salt detachment is roughly equivalent to low-water level system reaml of the sequence. Under certain condition, the two kinds ofdetachment could be used to identify dense section and low-water level system domain and toclassify depositional sequence

DISCUSSION ON THE ARTICLE DESCRIPTION OFRESERVOIR HETEROGENEITY USING WELL-TEST PRESSURE

Tang Xueqing

1996, 17(1): 84-86. doi:10.11743/ogg19960117

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Data of well-test pressure could effectively reflect the vados characteristics of reservoir.But in describing reservoir heterogeneity with well-test data, one must pay attention to thedetermination of fluid phase variation in well-test,the time-correction of the test data, thenoises caused by low accuracy and low resolution of mechanical pressure gauges.

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