Shale Gas Accumulation Model For Longmaxi Formation,Lower Silurian In The Junctional Zone Among Sichuan-chongqing-hunan-hubei Provinces

The junctional zone among Sichuan-Chongqing-Hunan-Hubei provinces is one of the key exploration and development zones of marine shale gas in China,where the Longmaxi Formation of Lower Silurian has favorable geological conditions for shale gas development.This paper talks about geological background of shale‘s development,the sedimentary and structural control on the conformation of shale gas accumulation,and gas characteristics of shale reservoir of the study area,which committed to analyze the accumulation model for shale gas in Longmaxi Formation and find the favorable exploration direction in this area,and obtained the following conclusions.In early Longmaxi period there mainly were two kinds of sedimentary types which were deep continental shelf and shallow continental shelf.From the middle area to the southeast of the study area,the sedimentary environment of Longmaxi formation changed from center of deep continental shelf to the boundary and the shallow continental shelf,and the thickness of rich organic shale became thinner,vertically from the bottom to the top,the content of sand in the shale gradually increased,while the TOC decreased.Controlled by the multiple stages of tectonic reforms,two main tectonic systems which were called east Sichuan battlements-like structure belt and west Hunan & Hubei trough-like structure belt formed,and the structure deformation is gradually enhanced from northwest to southeast of the study area.From the basin inside to the outside,the Longmaxi formation was gradually uplifted,the preservation conditions of shale gas became worse,all above directly determined the level of gas content in the shale,which gradually decreased from the basin inside stability region to the outside destruction region.The shale gas of Longmaxi formation of the study area complied with the law of accumulation that ?the deposit system controlled the generation of gas,the tectonic system controlled the preservation of gas,two elements matched and then the gas accumulated where they were generated?.In the inside basin where the sedimentary environment had been deep continental shelf and stability in tectonic,the accumulation model can be described as the shale has large thickness,high organic carbon content,the shale gas has good preservation conditions,lead to high level of gas content in the shale;while in the boundary or outside basin where the sedimentary environment had been the boundary of deep continental shelf or shallow continental shelf,where the structure deformation was severity,the accumulation model and characteristic is the thickness of shale becomes thinner and thinner,the TOC decreases gradually,the preservation conditions of shale gas becomes worse and the gas reservoir has been damaged,which lead to low level of gas content in the shale.The two models gradually transform each other and the accumulation conditions for shale gas become worse and worse.With evaluating the favorable for development of shale gas in typical areas,the paper conclusioned that there could be 3 types of preservation condition of shale gas which can be divided into well preserved inside the basin,general preserved outside the basin and loss of damaged outside the basin.According to the advantage level of shale gas ?s development,using the multi factor overlay method to divide the study area into favorable zones,potential zones and damaged zones.With the comprehensive quantitative analysis of geological evaluation on five zones in the study area,this paper predicted that the favorable exploration direction of shale gas of Longmaxi formation is around Zhongxian-Shizhu-Fuling in the middle of the study area.