Sedimentary facies of the Shanxi Formation in Daniudi gas field,northeastern Ordos area

Based on interpretation and correlation of logging data over 300 wells and in combination with rock assemblage,sedimentary structure,profile structure,electrofacies combinations,and the regional geological settings of the Late Carboniferous-Early Permian carton basin in North China,this paper analyzes the lithologic association and distribution characteristics of the Shanxi Formation by using the sedimentary facies analysis technique that unifies macroscopic and microscopic geological evidence.The first member of Shanxi Formation was deposited in an epicontinental environment as evidenced by marine fossils.The study area experienced marine transgression,leading to the deposition of shore delta systems.The second member of Shanxi Formation was deposited during the transition from epicontinental sea to intracontinental depression,when the subduction of Manchuria and Mongolia oceanic crust to the North China plate intensified and the tilting of the northern part of the plate continued,leading to the almost complete regression of sea water from the study area and the deposition of offshore delta system.The first member of Shanxi Formation is dominated by epicontinental sea delta front deposits and its sedimentary facies mainly include underwater distributary channel,bay between underwater distributary channels and peat swamps.In contrast,the second member is dominated by delta plain deposits and its sedimentary facies mainly include distributary channel and bay between distributary channels.The underwater distributary channel and the lower distributary channel are favorable reservoir facies.