Sedimentary Characteristics And Tectonic Control Of Dark Mudstone Of Shahejie Formation In Cangdong Sag

The Xichang Basin is located on the western margin of the Upper Yangtze Plate.It has a long geological history before the Late Mesozoic.As part of the Upper Yangtze sedimentary area,it has similar tectonic-sedimentary evolution characteristics to the Sichuan Basin.Therefore,high-quality Longma are developed in the Xichang Basin and adjacent areas Xisha black shale.As a part of the Xichang Basin,the research area has the characteristics of multi-stage and multi-cycle tectonic activities.In order to ascertain the basic conditions required for the preservation of shale gas,through field geological survey,sampling and analysis,combined with previous research results,the purpose The analysis and summary of the shale lithology,thickness,organic carbon abundance and organic maturity of the Silurian Longmaxi Formation shale in the lower layer,and the structural evolution characteristics and fluid geochemical characteristics of the study area were analyzed to clarify the basics of shale gas preservation Conditions,specific results are as follows:The overall lithology of the Longmaxi Formation is dominated by black carbonaceous shale,silty mudstone,calcareous mudstone,calcareous siltstone,and sandy muddy dolomite,with pendant fossils at the bottom,sedimentary structures are developed,and some at the bottom are mainly developed Horizontal bedding,with some parallel bedding and discontinuous limestone bands on the top,indicating that the bottom is mainly deep-water continental shelf deposits and gradually transitions to the top shallow-lime muddy continental shelf deposits.Sedimentary cycle,which provides a good sedimentary environment for shale.The southern area of the Xichang Basin has undergone multi-stage and multi-cycle tectonic superimposition and can be divided into four structural zones.The Muhe fault zone,the Mishi basin wide and gentle fold fault zone,butuowuku strong deformation fold fault zone and cloth Dragging-fiery broad pleated broken belt.Analysis of the characteristics of joints,scratches,folds and faults in field observations,combined with previous research results,for five parameters of shale gas preservation,formation erosion index,altitude index,fault index,dip angle index,formation depth and pressure coefficient,etc.This index,combined with the development conditions of the roof and floor,qualitatively-semi-quantitatively evaluates the preservation conditions of shale gas in the Longmaxi Formation in the southern area of the Xichang Basin,and selects the optimal zones 1 and 2 for structural preservation conditions.The uniform temperature of fluid inclusions in the southern Xichang Basin is concentrated in two temperature zones,which indicates that the study area has experienced two stages of tectonic activity and corresponds to two fluid charges.The strontium isotope test data shows that there are no veins in the basin in the study area,and the 87 Sr / 86 Sr geochemical characteristics of some veins and surrounding rocks are less different.Therefore,it is judged that the fluid is in the same layer and the location is different,indicating that the internal connectivity of the basin is poor The shale gas preservation conditions are good,and the 87 Sr / 86 Sr geochemical characteristics in the areas with strong tectonic activity on the edge of the basin differ greatly.It is judged as the outer fluid,the formation connectivity is good,and the shale gas preservation conditions are poor.The preservation conditions of the southern Xichang Basin were evaluated based on the conditions of fluid geochemistry,structural deformation,uplift and erosion.The overall feature is that the internal structural activity of the basin has little damage to the stratum,and the structural activity at the edge of the basin has strong damage to the stratum.The shale gas preservation conditions in the Mishi Basin in the study area are good,followed by the storage conditions of the Butuohuo broad and gentle fold fault zone,and the Butuowuku strong deformation fold fault zone is the destruction area of the Longmaxi Formation shale gas preservation conditions,then the wood No source rocks are developed in the He fault zone,and shale gas preservation is not considered.