The Sedimentary Environment And Reservoir Characteristics Of Middle Permian Shale In The Lower Yangtze Area Of Southern Anhui

The Gufeng Formation of Permian in southern Anhui area of Lower Yangtze is a set of well-developed marine sediments,which has great potential for shale gas.Through field geological survey,drilling core observations,element tests,organic geochemical tests,and reservoir tests,the Gufeng Formation shale sedimentary environment and reservoir characteristics have been studied.The main results and understandings are as follows:(1)Observation of drilling cores and thin-section analysis are used to obtain the lithological change characteristics of the Gufeng Formation,and to define the sedimentary facies according to the vertical changes of lithology,and analyze the lithofacies distribution laterally.The main lithology deposited in the Gufeng Formation is black shale,gray-black siliceous shale,and gray-black silty shale.There is a small amount of carbonaceous bioclastic mudstone in the bottom,siliceous shale in the middle,and silty shale in the top.On the plane,the lithofacies are distributed in the basin facies in the northwest,deep-water shelf facies in the middle,and open platform facies in the southeast.The terrigenous clastics mainly come from the south-east direction.(2)Using the data of major and trace elements to analyze the sedimentary environment,it is believed that the Gufeng Formation shale sedimentation period was mainly in a warm environment.When the water body is shallower,it is more affected by terrigenous clastics.When the water body is deeper,it is half-salinity and The stratification of the water body is strong,and the nutrients brought by the upwelling make the organisms more developed.At the same time,siliceous rocks of biological and hydrothermal origin are deposited.The reduction environment at the bottom is conducive to the preservation of organic matter,which is the development mode of organic-rich shale.(3)Geochemical analysis shows that the Gufeng Formation shale has good hydrocarbon generation potential.The content of organic matter varies greatly in different regions.The TOC of samples in most areas is between 5%and 20%,and a few areas have TOC less than 1%.The organic matter content in Xuancheng and Wuxi is relatively high;the organic matter type is mainly type I,and a small amount is II₁ Type,derived from biology;shale organic matter maturity is in the over-mature-mature stage.(4)Through X-ray diffraction,argon ion profiling scanning electron microscopy,nitrogen isothermal adsorption,carbon dioxide isothermal adsorption,porosity analysis,etc.,it is analyzed that the Gufeng Formation shale formation is brittle,and the content of brittle minerals and clay minerals is appropriate.It meets the conditions of high-quality shale reservoirs.The pores developed in shale can be divided into intragranular pores,intergranular pores,organic pores and microcracks.Shale pores are mainly micropores and mesopores.Macropores are relatively rare.Micropores and mesopores smaller than 10nm contribute the most to the total pore volume and specific surface area,providing a large number of places for shale gas.The effective porosity and permeability of shale are generally low.The collated data show that the burial depth of the Gufeng Formation is above 2500 meters,and the residual thickness of shale is relatively high in the Chaohu-Xuancheng-Guangde area.Based on the characteristics of organic matter and reservoir development,Xuancheng-Chaohu area is considered to be a better shale gas exploration area.