Sedimentary characteristics and model of the tight sand of He 8 formation in eastern Ordos Basin

Studies of the petrology, facies assemblages, sedimentary and facies characteristics by typical outcrops and well cores as well as detrital minerals in sandstones and their distribution characteristics indicate that the tight sand of He 8 formation in eastern Ordos Basin is mainly lithic sandstone and subordinately lithic quartz sandstone whose colors are dark gray,gray and grayish white,with the grains being medium sand and coarse sand. There develop three sedimentary systems from north to south which include alluvial fan,braided channel and braided delta. Alluvial fan facies involve chiefly middle-fan and margin-fan subfacies which consist of gravel grit which exhibits massive structure and was derived from alluvial channel. There exists river deposition southward which includes river channel sand dam microfacies and floodplain microfacies, in which the river channel sand dam constitutes the framework made up of grit and gravel grit that exhibit cross stratification and parallel stratification structure. In the south of the study area braided delta plain and braided delta front are developed, in which distributary channel, flood plain, subaquatic distributary channel and interchannel microfacies can be recognized. Among them, the distributary channel and subaquatic distributary channel are skeletons which are analo-gous to river channel sand dam and consist of grit and medium-fine sandstone assuming tabular cross bedding and trough cross stratification structure. The sand bodies are superimposed upon each other vertically and jointed into pieces horizontally, thus serving as favorable reservoirs for He 8 tight sand gas accumulation.