Optimization And Evaluation Of Favorable Areas Of Shale Gas In Wufeng-Longmaxi Formation In Yongshan Area,Southwest Sichuan

Wufeng Formation of the upper Ordovician and Longmaxi formation of the Lower Silurian in Sichuan Basin are important strata for the development of black shale,and are favorable shale gas exploration strata in the upper Yangtze region.In this paper,field outcrops profile observation and sampling are carried out in Yongshan area,Southwest Sichuan.Collect existing profile data and drilling data in the area and adjacent areas.The sedimentary characteristics,geochemical characteristics,reservoir pore development and adsorbed gas volume of Wufeng-Longmaxi formation shale in Yongshan area,Southwest Sichuan are studied and discussed.The favorable target area of shale gas is selected and integrated Evaluation.The organic rich shale of Wufeng-Longmaxi formation in Yongshan area of Southwest Sichuan mainly develops four lithofacies:black carbonaceous shale,gray black calcareous carbonaceous shale,gray black silty shale with globular sand body and gray black carbonaceous argillaceous siltstone.The whole study area is shelf facies,including deep-water shelf subfacies and shallow water shelf subfacies and six kinds of sedimentary microfacies:from the plane distribution of the sedimentary facies,from the southwest to the northwest to the northeast.It is found that the type of organic matter in Wufeng-Longmaxi formation in Yongshan area of Southwest Sichuan Basin is mainly type I(sapropel type);the maturity of organic matter is high,and the content of Ro is between 2.52%and 3.42%,which belongs to over mature stage and dry gas generation stage;the content of organic matter is high,which is the highest in Leibo-Yongshan area,up to 4.0%,and the TOC content is increasing from southwest to northeast.The TOC content of the Northeast sections of Yongshan Sutian Village(YS01),Yongshan Wujiawan(YS02),Yongshan Yunqiao reservoir(YS03)and Leibo Shanlenggang(LB01)is higher,the average value is more than 2%.The organic rich shale of Wufeng-Longmaxi formation is composed of brittle minerals,clay minerals and organic matter,in which the brittle minerals are mainly quartz and carbonate minerals,with a small amount of feldspar,anhydrite and pyrite;the clay minerals are mainly illite,followed by chlorite,with a small amount of Iran Mongolia mixed layer.The content of brittle minerals in shale is as high as 53%～82%,and the average value is 68.7%,which is favorable for shale gas development.The shale of Wufeng-Longmaxi formation in the study area is characterized by high porosity and low permeability,with many micro pores,mainly intergranular pores,intragranular pores and micro fractures;the specific surface area is 8.4876～22.2863m~2/g,with an average of 14.3918m~2/g.The maximum adsorbed gas volume of organic rich shale is 0.7～2.0m~3/t,and the peak value of the maximum adsorbed gas volume is between 8～10Mpa,and the adsorbed gas volume is positively correlated with TOC content.According to the division standard of shale gas favorable target areas,the effective thickness,organic carbon content,organic matter maturity,shale outcropping and burial depth and favorable structure are comprehensively analyzed.Three favorable target areas are delineated in Yongshan area,including Yongshan in the middle,Daguan in the north and Muchuan-Shuifu in the North West.The total shale gas resources in the favorable target area is 1.1798×1012 m~3,which provides a basis for shale gas exploration and development in this area in the future.