Sequence-based lithofacies and palaeogeographic characteristics of Chang 9 oil reservoir set in Longdong area, Ordos Basin

Based on comprehensive analyses of core description, thin section observation, paleontology, source area and well logs, this paper studied the characteristics of sedimentary facies and sequence stratigraphy of Chang 9 oil reservoir set in Longdong area, Ordos Basin. It is determined that Chang 9 oil reservoir set belongs to shallow-water fluvialdominated delta system and it can be divided into one long-term, two middle-term and four short-term sequences. On the basis of the quantitative calculation of paleosalinity and its subarea, the position of palaeo-lakeshore during Chang 9 depositional stage was determined. According to the source area, sedimentary facies and sequence, the rising and falling base level phases of middle-term sequence cycles were selected as the isochronic stratigraphic unit to draw up the sequence-based lithofacies and paleogeography map of Chang 9 oil reservoir set in Longdong area. It is concluded that paleogeography features and sand body distribution are controlled by lake level fluctuation and source direction. Distributary channels and underwater distributary channels during the transgressive phases are favorable for reservoir development, and the sand body direction is parallel to the major source direction. The results canl guide the followingexploration for the Chang 9 reservoirs in Longdong area.