SEQUENCE-BASED LITHOFACIES AND PALAEOGEOGRAPHIC CHARACTERISTICS OF 6TH OIL-BEARING MEMBER OF THE YANCHANG FORMATION OF UPPER TRIASSIC IN THE SHIJIAWAN AREA,ORDOS BASIN

Based on comprehensive analysis of core descriptions,thin section observation,paleontology,source area and well logs,the characteristics of sedimentary facies and sequence stratigraphy of the 6th oil-bearing member of Yanchang Formation in Shijiawan area,Ordos Basin,were studied in detail.It was determined that the fluvial-dominated delta system deposited during the deposition of the 6th oil-bearing member of Yanchang Formation.One long-term,three medium-term and ten short-term sequences were divided accordingly.On the basis of the quantitative calculation of paleosalinity and its sub-area,the position of palaeo-lakeshore during the deposition of 6th oil-bearing member of Yanchang Formation was determined.According to the analytic results of source area,sedimentary facies and sequence,combined with single factor analysis and multiple factors synthesis,the rising and falling base-level phases of medium-term sequence cycles were selected as the iso-chronic stratigraphic unit to compile the sequence-based lithofacies and paleogeography maps.It was concluded that paleogeography and sand body distributions were controlled by lake level fluctuation and source area direction.Distributary channels and underwater distributary channels were the best reservoirs occurred during the transgressive phases,and the direction of the sand body distributions was parallel to the major source direction in the northeast.