Geological Study On Shale Gas Reservoirs Of Upper Permian In South Anhui Province Of Lower Yangtze Region

In this paper,the South Anhui Province of Lower Yangtze Region is taken as the main study area,and the Upper Permian formation is the main study horizons.The geological conditions of Upper Permian shale gas in South Anhui Province of Lower Yangtze Region were studied by means of field profile measurement,drilling data collection,X-ray diffraction analysis,organic geochemical analysis,argon ion polishing scanning electron microscope,isothermal adsorption and other means.Furthermore,the favorable areas of shale gas are preliminarily predicted in the study area.The main achievements and understandings are as follows:(1)The Upper Permian in the South Anhui Province of Lower Yangtze Region has a mixture source,and the parent rocks are mainly felsic volcanic rocks and granite,accompanied by the mixing of andesite and shale.The tectonic background is passive continental margin and continental island arc.(2)The source of the upper Permian shale in South Anhui Province of Lower Yangtze Region mainly develops transitional facies shale.The sedimentary facies include delta front,delta plain,swamp and shallow shelf facies.The delta facies are distributed along the southeast northwest direction.The sedimentary facies of Dalong Formation include open platform facies,deep-water shelf facies and deep-water basin facies.The sedimentary facies belt is controlled by the structure and is distributed in NE-SW direction.(3)The organic matter type is mainly type III,and a small amount is type II₂.The abundance of organic matter in Dalong Formation is obviously higher than that in Longtan Formation.Most Longtan Formation is in high mature stage,and a few areas are in over mature stage.The Dalong Formation is mainly from mature to high maturity stage.(4)The Upper Permian shale in the study area mainly develops interparticle pores,intraparticle pores,organic-matter pores and fractures;The pore shape is mainly wedge-shaped.Mesopores and micropores in the Upper Permian shale of well Jingdi 1 contributes to most of the pore volume.The content of quartz and other brittle minerals in Dalong Formation is higher,with an average of 42%,while the content of clay minerals in Longtan Formation shale is higher,with an average of 50.4%,and the content of quartz is relatively low,with an average of 41.8%.Illite is the most abundant clay mineral in Longtan and Dalong Formation shale,with an average of44.33%,followed by illite/montmorillonite.The brittleness index of shale in the study area is relatively high,especially the brittleness index of Dalong formation shale is more than 50,which is better than Longtan Formation.(5)The thickest shale of Longtan Formation in the study area is near Ma'anshan area,with a thickness of about 90m,and the thickness decreases to the southwest.The shale thickness of Dalong Formation is larger than that of Longtan Formation,mainly distributed between 40-60m.The distribution is similar to that of Longtan Formation,which is thick in Northeast and thin in southwest.In Wuhu area,the thickness can reach more than 80m.According to the selected parameters,the Upper Permian shale gas favorable areas in South Anhui Province of Lower Yangtze Region can be divided into three types.The type I favorable areas are mainly located in Wuwei basin and Xuanjing basin,and the type II favorable areas are located in Ma'anshan-Jingxian area.