Geological Conditions Of Shale Gas Reservoir Formation Of Wufeng Formation-Longmaxi Formation In Enshi Area,Western Hubei Province

With the development of the economy,shale gas exploration and development has received more and more attention.As of 2018,according to relevant agencies,China's shale gas production is the third largest in the world,and the reserves of shale gas can be the first in the world.This shows that China's shale gas resources are abundant.However,there are few studies on the shale gas in the Wufeng Formation-Longmaxi Formation in the Enshi area of western Hubei Province,and the research is not enough.Therefore,based on the previous studies,this paper combines the drilling conditions and core sample analysis of the Wells of Laidi 2,ZY1 and Heye 1 in the study area,and the shale sedimentary facies and organic-rich shale in the Wufeng Formation-Longmaxi Formation of the study area.The thickness,shale geochemical characteristics,shale reservoir physical characteristics and shale preservation conditions were analyzed to evaluate the geological conditions of shale gas rich and integrated reservoirs in Enshi area of western Hubei Province.The main findings are as follows:(1)The Enshi area in western Hubei Province is located in the middle Yangtze area.The sedimentary facies of the Wufeng Formation-Longmaxi Formation are mainly composed of deep-water shelf-sediment-shallow-water shelf deposition.The Wufeng period-Longmaxi period is a deep-water continental shelf sedimentation.The lithology is mainly black-grey-black carbonaceous shale and argillaceous siltstone.The Longmaxi period is shallow-water shed deposition,and the lithology is mainly grey black-grey mudstone and argillaceous siltstone.(2)From north to south of the study area,the thickness of the organic-rich shale in the study area is stable and about 25m thick.According to the analysis of various organic geochemical parameters,the TOC content of organic-rich shale interval in the study area is high,and the content of most samples is more than 2%.The carbon isotope analysis of rock kerogen and the analysis of organic matter microscopic composition indicate that the organic matter type is mainly type ?1;the maturity of organic matter is high,and the Ro value is distributed from 1.84%to 2.87%,reaching a stage of high maturity-over-maturation.These characteristics are similar to those of typical shale gas wells in Jiaoshiba area and its surrounding areas in southern Sichuan.(3)The shale mineral composition analysis shows that the shale mineral composition in the Enshi area of western Hubei is mainly composed of brittle minerals and clay minerals.The vertical upwards show that the brittle minerals increase gradually and the clay minerals gradually decrease.Shale pore permeability analysis also shows that the Wufeng Formation-Longmaxi Formation shale in the study area is characterized by low porosity and low permeability,which is similar to the pore permeability characteristics of typical shale gas basins in the United States.Scanning electron microscopy and shale specific surface area analysis and isothermal adsorption experiments of shale indicate that the shale in the study area has rich void type,large specific surface area and strong gas-bearing characteristics.Compared with the geological parameters of typical shale gas basins in the United States,they have similar or better parameters and show good shale gas reservoir chara^cteristics.(4)Through the lithology observation of the wells of Laidi 2,ZY1 and Heye 1,the Enshi area in western Hubei Province has good preservation conditions.The conditions of floor and roof preserved in Wufeng Formation-Longmaxi Formation shale are compact limestone in the lower part,tumorous limestone in the upper part and thick siltstone,sandstone and mudstone up to 200 m thick in the upper part,which have good sealing effect on shale gas escape.At the same time,due to the influence of excessive period of tectonic movement in the study area,some areas are severely denuded.In the complex anticline,due to the erosion of the overlying strata,the preservation conditions are worse.The complex oblique belt is opposite to the complex anticline.With a small amount of uplift and denudation,the deformation strength is weak,so it has better shale gas preservation conditions.Therefore,it is speculated that the cap rock development area of the Lichuan syncline and the Huaguoping syncline area is a favorable area for shale gas accumulation.