The Development Characteristics Of Longtan Formation Shale In Eastern Sichuan

The achievements of marine shale gas exploration in Sichuan Basin are remarkable,and the shale gas breakthrough has been obtained in the Cambrian Qiongzhusi Formation and the Silurian Longmaxi Formation and other strata in Paleozoic.Coke dam shale gas field has been built,as well as Zhaotong and other shale gas demonstration areas,indicating that Paleozoic marine shale gas has great potential in exploration and development in Sichuan Basin.Results of previous research shows the shale gas accumulation conditions of upper Paleozoic Permian Longtan Formation/Wujiaping Formation in Eastern Sichuan are good and gas potential is considerable,but there is also an objective fact that the overall exploration degree is insufficient.Through a comprehensive analysis of drilling and logging data,seismic data,field profile data and previous studies,and using sequence stratigraphy,sedimentology,lithofacies palaeogeography and whole rock analysis,SEM analysis,X-ray diffraction analysis and so on a variety test and analysis methods,the development characteristics of Longtan Formation shale in research area have been studied in detail,main findings are as follows:?1?On the basis of field profile research,interpretation of logging data and indoor microsection identification,Longtan Formation in Eastern Sichuan was divided into three 3^rd sequences-SQ1,SQ2,SQ3,and its stratigraphic sequence framework was established.?2?From the southwest to the northeast,the study area developed 4 sedimentary facies-shoreland,tidal flat,mixed shelf and inter-platform basin,the sedimentary evolution can be divided into two stages:stage SQ1LST,shoreland-tidal flat face was the main sedimentary face in study area;stage SQ1TST-SQ3,from the southwest to northeast,study area developed fluvial face—shoreland-tidal flat face—shallow mixed shelf face—deep mixed shelf face—inter-platform basin face.?3?The main shale rock types are carbonaceous shale,sandy shale and calcareous mudstone of Longtan Formation shale in the study area.The shale has a high organic content,and organic matter is in the mature stage.?4?Within the isochronal stratigraphic framework,according to criteria for effective thickness of shale,Longtan Formation shale in study area was divided into 3sections-SQ1,SQ2 and SQ3.Generally,the main rock assemblage types of SQ1,SQ2,SQ3 are mud–pack-sand,mud-pack-ash and pure mud in Longtan Formation.And in the southwest of the study area,the depositional enviroment was shoreland-tidal flat,the shale thickness was large and the distribution was wide,and the main rock assemblage was mud-pack-sand.In the northeast of the study area,its depositional enviroment was mixed shelf,the shale thickness was small and the distribution was limited,and the main rock assemblage was mud-pack-ash.The pure mud assemblage in these two depositional environments had varying degrees of development.?5?The shale is mainly composed of quartz and clay mineral,the content of clay mineral is higher.From southwest to northeast,the content of brittle mineral gradually increases,and the content of clay mineral gradually decreases.?6?Based on the evaluation criteria of shale gas in domestic,it is considered that the southwest is a favorable area for shale gas exploration in the Longtan Formation.