Gas Content Evaluation For Longmaxi Formation,Lower Silurian Shale In Northwest Hunan

Northwestern Hunan is one of the key exploration zones of China's marine shale gas reservoir,and the Longmaxi Formation of Lower Silurian in Northwestern Hunan has good shale gas accumulation conditions.This paper talks about geological background,geological requirements of shale gas reservoir,gas characteristics of shale reservoir of the area and evaluation of shale gas main controlling factors,which analyzed the ability of holding shale gas in the Longmaxi Formation,and obtained the following conclusions.Northwest Hunan experienced multiple stages of tectonic reforms,the north north east tectonic unit in northwestern area is the most significant structural framework.Early study of the area shows that it is the deep shelf deposit,the later sedimentary environment of water body becomes shallow gradually,evolved to the shallow shelf and tidal flat.The potential shale of Longmaxi Formation is far away from sedimentary center,the main lithology are carbonaceous shale,siliceous shale,containing carbon argillaceous siltstone and arenaceous shale,and the thickness of black shale section can be up to 50m.Be bounded by regional sedimentary facies and structural control,the types of kerogen are mainly type ? and ?1,however,type ?1 is more dominative in the area.TOC content is about 0.55%?7.14%for Longmaxi Formation with the median of 2.06%,the TOC content is moderate.The Ro is about 2.33%?3.35%for Longmanxi Formation with the median of 2.85%and the maturity is high,which presents the feature of a little bit over maturity.Longmaxi Formation shows good shale gas forming geochemical characteristics.Be bounded by regional sedimentary facies and structural control,the conclusion can be drawn that the closer to the sedimentary center,the higher content of clay mineral and lower content of detrital mineral.The fragile mineral content of Longmaxi Formation black shale is up to more than 40%,of which mainly is quartz,clay mineral content is less than 50%,among which goeschwitzite and andreattite are the main mineral compositions.The porosity is mainly distributed between 1.8%and 15.3%,the permeability of most black shale are less than 0.01mD,representing the good reservoir conditions.of Longmaxi Formation.The gas volume of Longmaxi Formation can be calculated using well site desorption,logging analysis and isothermal adsorption simulate test.The main controlling factors of shale gas accumulation of Lower Silurian Longmaxi Formation in Northwestern Hunan are the total organic carbon content,quartz content,mineral content in clay,pyrite content,BET surface area and BJH total pore volume.Quantitative analysis of the main control factors and establish the gas content geological model.Longmaxi Formation longitudinal,of which shale gas concentration section is mainly located at the base of Longmaxi formation;In the plane,shale gas concentration area is located in baojing,yongshun,sangzhi,longshan area,and the gas content mainly distribute between 0.5m3/t and 2.5m3/t.Lower Silurian Longmaxi Formation in Northwestern Hunan has high potential of cumulating shale gas resource and has favorable exploitive value for gas industry.The research of gas content improved and perfected the prediction and calculation methods of gas content,enrich the research system of gas content.Established gas content geological model which suitable for assessment and selection for shale gas exploration and development work of the Lower Silurian Longmaxi Formation in Northwestern Hunan and peripheral regions which have similar geological conditions.