Study On The Oil And Gas Accumulation Conditions Of The Permian Qixia Formation In Northwestern Sichuan

The Permian system in the Northwest Sichuan basin has been explored for more than 40 years,but the Qixia Formation has only been used as a probing layer for a long time and no substantial progress has been made in exploration.In recent years,the exploration intensity of the Permian Qixia Formation in the area has been increased,and the multi-well drilling in the porous dolomite reservoirs has obtained high-yield gas flow,which shows a good prospect for exploration.But for the Qixia area,the origin and structural characteristics of natural gas,distribution of dolomite reservoirs and reservoir-forming combinations are relatively weak,which restricts the next exploration direction and favorable area selection.This paper is guided by theories of organic geochemistry,natural gas geology,and seismic geology.Based on previous research results,the paper combines field outcrops,drilling,logging,seismic and analytical laboratory data to evaluate the quality and scale of source rocks,Analyze the geochemical characteristics,origins and sources of natural gas,establish tectonic models,clarify trap conditions,predict the distribution of dolomite reservoirs,point out favorable hydrocarbon accumulation combinations,and optimize favorable zones.The natural gas of the Permian Qixia formation in northwestern Sichuan is mainly derived from the Permian and Cambrian source rocks,partly from the Silurian source rocks.The abundance of organic matter in source rocks is generally high,and the organic matter is dominated by the sapropel type,with higher maturity,larger thickness,wider distribution,and large hydrocarbon potential.The natural gas of Qixia group is mainly composed of hydrocarbon gas,with high CH4 content,low C2H6 content and large drying coefficient,which is the cracking gas of aggregated type crude oil.The tectonic deformation in northwest of Sichuan is a typical double slipping medium deformation,which is a complex overthrust system formed by the combined action of deep sliding and central soft slip surface.The Permian Qixia formation has geological conditions for the formation of large complex traps,and the local tectonic traps are developed.The reservoir rocks of the Permian Qixia Formation in northwestern Sichuan Province are diverse,and the reservoir properties of the medium-coarse crystalline dolomite are good.The dolomite reservoir is widely distributed and has large thickness,and its development is controlled by the high energy sedimentary facies belt and the sedimentary paleogomorphic highland such as bank of platform and intra-platform.The northwest region of sichuan has experienced three stages of early tectonic paleooil accumulation,ancient crude oil cracking and natural gas adjustment and accumulation.The Permian Qixia Formation has the characteristics of "near source,multiplex transmission,large area".There are three sets of source-reservoir combinations and multiple transmissions,and the preservation conditions are good.The coupling between ancient and modern structures and the peak period of hydrocarbon production is good,which has the advantage of forming large and medium-sized gas zones.