The Characteristics And Formation Mechanism Of Gas Chimneys In The Weiyuan Structure

The gas reservoir of the Sinian Dengying Formation in the Weiyuan Structure?the Weiyuan Gas Field?,as the cradle of Chinese natural gas industry,its exploration has made the important contribution to the development of modern gas industry in the early stages of the People's Republic of China.Although the Weiyuan Structure is the biggest surface structure in the Sichuan Basin,the abundance of Dengying Formation gas reservoir in the Weiyuan Structure is relatively low.The height of gas column is 244 m and the integrated abundance is only 26.3%.Among the reasons for it,few related researches these on and far from reaching agreement.Over nearly 40 years of exploration,the Gaoshi-1 Well in the adjacent area of the Gaoshiti-Moxi Structure yielded natural gas with high production in 2011;the following year of August,the Moxi-8 Well in the Cambrian also yielded gas flow which made another important exploration success after the Sinian Dengying Formation gas 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 subsurface data,as well as comprehensive geological-geophysical-geochemical research methods,the paper made a reasonable explanation on the low abundance of gas reservoir in the Weiyuan structure.?1?Nowadays,the Weiyuan Structure is the apex of the Dengying Fm in the Sichuan Basin?except steep structural belt in East Sichuan?,The Guang'an,Longnvsi,Gaoshiti-Moxi,and Weiyuan Structure are the series traps of the Dengying Formation with gradually uplifting spill and closure points during the regional uplifting of the Himalayan period.The natural gas of the Dengying Formation in the Sichuan Basin exist differential accumulation with wide range and long distance.?2?Approximately 40 Ma,the Weiyuan area had uplifted and formed the present structure,and it is the only outcropped area with the Triassic Jialingjiang Formation and Leikoupo Formation in 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 "skylight" of nature gas escaping.The evidences of gas chimney include:?1?the component percentage of non-hydrocarbon gas: which have shown decreased tendency from the bottom to the top;?2?the pressure coefficient: the gas reservoir from the Upper Sinian to Lower Permian have the normal pressure coefficient commonly?average 1.0?;?3?isotope geochemistry: the argon which have represented abiogenic characteristics of deep source mostly,its Ar⁴⁰/Ar³⁶ ratio is still as high as 2 855⁵ 222 in the upper Permian.All of these show sufficient evidence of gas chimney effect.The characteristics of the low abundance in the Weiyuan Structure can be a typical example of studying the later reconstruction of the deep oil and gas reservoirs in superimposed basins in western China.