Abstract:The 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) Nowadays,theWeiyuan 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