Characteristics And Evaluation Of Shale Reservoirs In Shanxi Formation In Yan'an Area

Ordos Basin is a large oil-bearing basin,which has developed several shale strata of Ordovician Marine facies,Carboniferous-Permian marine-continental transitional facies and Triassic Yanchang Formation marine facies,and has the basic geological conditions for shale gas enrichment.Shanxi Formation in Yan'an area is rich in shale gas resources,which has become another important exploration and development target of Yanchang Petroleum(Group)Co.,Ltd after the continental shale gas of Yanchang Formation.In this paper,the organic rich shale of Shanxi Formation in Yan'an area is taken as the research object,and the argon ion polishing scanning electron microscope analysis,CO2 adsorption experiment,N2 adsorption experiment,high-pressure mercury injection experiment as the main analysis means.Combined with core observation,thin section identification,organic geochemical analysis,X-ray diffraction experiment,the shale pore types and reservoir characteristics are studied,and shale reservoir evaluation is carried out at domestic and abroad.On the basis of the standard,the evaluation standard of shale reservoir in Shanxi formation is established,which provides favorable geological support for ‘sweet spots' and favorable area prediction.This paper mainly obtains the following research results and understandings :(1)the thickness of Shanxi Formation shale is stable,with an average thickness of 20-80 m,and the thickness of the first member of Shanxi Formation is relatively thick,with good horizontal continuity,mainly dark grey or(grey)black mudstone and silty(fine)sandy shale;(2)The shale mineral composition is mainly composed of quartz and clay minerals,which the average content of quartz is 42% and the average content of clay minerals is over 57%.The clay minerals are mainly illite,kaolinite and mixed layer of andreattite.(3)There are many pores and fractures in shale,such as intragranular pores,intergranular pores,organic matter pores and fractures.Fractures are the main reservoir space of shale gas.The pore volume of shale reservoir is dominated by mesopores and macropores,which account for about 33% of the pore volume,while mesopores and micropores account for about 65% of the pore volume.The surface area is mainly provided by micropores and mesopores,and their contribution is basically the same.(4)The shale reservoirs in the study area are divided into three types,I and II of which are mainly distributed in the East and North of Yanchang County,and also in the East from Wuqi to Dingbian.