Study On Shale Gas Formation Condition And Control Factors In Typical Area In The North Qaidam Basin

The shale of Middle Jurassic Shimengou formation in the northern margin of Qaidam Basin is characterized by high TOC and large thickness,which is a favorable target for the study of formation conditions of continental shale gas.Guided by the theories of sedimentology,geochemistry and petroleum geology,154 samples were collected from Tuanyushan,Yuka,Dameigou and other typical areas to test the total organic carbon content,rock pyrolysis,X-ray diffraction,major and trace elements,isotherm adsorption,scanning electron microscopy,porosity and permeability,and to carry out shale sedimentary environment,formation conditions and reservoir.The results show that:The shale of Shimengou formation in the study area is mainly formed in the fresh and reducing water body of semi deep lake to deep lake facies under the warm and humid paleoclimate conditions,which has the characteristics of large single layer thickness and stable development in the whole area on the plane;while the average TOC content in the shale section is 4.8%,and the organic matter type is mainly type ?-?,which generally has a good material basis for shale gas formation.At the same time,shale minerals are mainly composed of clay minerals,quartz and siderite,the average content of clay minerals is 40%-50%,the average content of quartz is about 30%,and the average content of siderite is about 20%.The pore types are mainly microcracks,intergranular pores,dissolution pores and intergranular pores.The organic pores are not well developed and the reservoir performance is relatively good.Due to the low maturity of organic matter,although the shale in the study area has good abnormal gas detection of total hydrocarbon,the gas content is generally low.It is speculated that towards the center of the basin,the gas content will increase with the increase of maturity and the increase of burial depth.Based on the study of shale sedimentary facies,shale thickness,burial depth,paleoenvironment and paleoproductivity,TOC,organic matter type,Ro,brittle minerals and clay minerals,reservoir performance and other evaluation parameters,combined with other geological features of the study area,it is considered that the controlling factors of the formation of shale gas in the Middle Jurassic Shimengou formation in the northern margin of Qaidam Basin are ranked in order of importance:organic matter mature(Ro),total organic carbon content(TOC),shale thickness,preservation conditions and reservoir performance.This understanding can provide some guidance for shale gas research and exploration in this region in the future.