Main Controlling Factors And Evaluation Of Quaternary Biogenic Shale Gas Enrichment In Sanhu Area

The Sanhu area of the Qaidam Basin has the largest Quaternary biogas field in China.Quaternary mud shale is widely developed in the study area.The mud shale itself has huge residual gas resources,and the main control of biogenic mud shale gas enrichment is carried out.The study of factors is of great significance for guiding the exploration and development of biogas.In this paper,the Quaternary Qigequan Formation mud shale in the Sanhu area is taken as the research object,and the formation conditions of mud shale gas reservoirs are clarified through organic carbon analysis,thin section observation,nitrogen adsorption,high pressure mercury injection,breakthrough pressure and other testing methods.Combined with on-site analysis of shale,recovery of lost gas and methane isothermal adsorption,the gas-bearing characteristics of shale are revealed.By optimizing the logging evaluation parameters,a gas-bearing prediction model is established.Comprehensively analyze the main controlling factors of Quaternary shale gas enrichment,establish shale gas enrichment evaluation criteria,and predict favorable areas for shale gas enrichment.The Quaternary mud shale in the Sanhu area has a high content of soluble organic carbon,the types of organic matter are type III and type II₂,and the overall organic matter is immature.There are three lithofacies developed in the study area:clay shale,mixed shale and felsic shale,and the storage space is intergranular pores,intragranular pores and micro-fractures.The richness of high salinity formation water enhances the sealing ability of shale to gas.The recovered gas content of Quaternary mud shale coring wells is higher than 3m³/t,mainly free gas.The adsorbed gas of shale is most affected by water saturation,and the amount of free gas is mainly restricted by porosity and water saturation.Widely distributed high-quality source rocks are the basis for gas generation.Microscopically,mixed mud shale and felsic mud shale can be vertically superimposed with clayey mud shale to form a favorable lithofacies combination.Macroscopically,the mud shale in the upper depression can be self-sealed and retained to accumulate,and the mud shale gas and sandstone gas in the structural high can coexist in an orderly manner.Based on the parameters of TC,porosity and mud shale thickness,it is predicted that the favorable area for the K7 layer of the Quaternary Qigequan Formation in the Sanhu area is the Sebei area,the favorable area for the K8layer is the Sedong and Sebei areas,and the K9 layer is favorable.The districts are Sedong and Tuodong,and the favorable areas of K10 layer are Tainan and Sedong.