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The analysis of the oldest carbonate gas reservoir in China-the new geological significance of the Dengying gas reservoir in the Weiyuan Structure, Sichuan Basin

Xiao Liang 1,Shugen Liu1,Shubei Wang 2,Bin Deng1 *,Cuilan Zhang 3
1. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China;2. Chuanzhong Division of PetroChina Southwest Oil & Gas Field Company, Suining629001, China;3. Sichuan Geophysical Company of CNPC Chuanqing Drilling Engineering Company Limited, Chengdu 610213, China

AbstractThe Weiyuan Structure is the biggest surface structure in the Sichuan Basin. However, theabundance of the Dengying Formation gas reservoir in the Weiyuan Structure is relatively low. The height of gas column is 244m and the integrated abundance is only 26.4%.Over nearly 40 years, the Gaoshi-1 Well andMoxi-8 Well yieldedgas flow which made another important exploration success afterthe Sinian Dengying Formationgas reservoir in the Weiyuan Structure,Lower-Palaeozoic of the Sichuan Basin in 1964.Combined with the related research instances of oil and gas migration and gas chimney around the world, and based on the surface and subsurfacedata, as well as comprehensive geological-geophysical-geochemical research methods, thispaper made a reasonable explanation on the low abundance of gas reservoir in theWeiyuan structure.Latest research results show that: (1) NowadaystheWeiyuan Structure is the apex of the Dengying Formation in the Middle of the Sichuan Basin. TheGuang’an, Longnvsi, Gaoshiti-Moxi, and Weiyuan Structureare the series traps of the Dengying Formation with gradually uplifting spill and closure points during the regional uplift of the HimalayanPeriod.The natural gas of the Dengying Formation exists differential accumulationwith wide range and long distancein the Sichuan Basin.(2) Approximately 40Ma, the Weiyuan area started to uplift and form the present structure, and it is the only outcropped area withthe Triassic Jialingjiang Formation andLeikoupo Formationin the surface of the Sichuan Basin(except steep structural belt in the East Sichuan).Caused by the uplift and denudation, the core of the Weiyuan Structure has formed escaping "skylight" of nature gas.The evidences of gas chimney include: (1)thecomponent percentage of non-hydrocarbon gas: which have shown decreased tendency from the bottom to the top;(2)pressure coefficient: the gas reservoir from the Upper Sinian to Lower Permian commonly have normal pressure coefficient (on average of 1.0);(3)isotope geochemistry: the argon which mostly representsabiogenic characteristics of deep source.The Ar40/Ar36 ratio is still as high as 2855~5222 in the upper Permian.All of these show sufficient evidences of gas chimney effect.The characteristics of low abundance in the Weiyuan Structurecan be a typical example of studying the late reconstruction of deep oil and gas reservoirs in the superimposed basinsofwestern China.

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Keywords:Sichuan Basin;Weiyuan Structure;Dengying Formation;Late reconstruction;Differentialaccumulation;Gas chimney

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