Abstract: Reservoirs of shallow-water delta facies are the dominant oil and gas reservoirs in the upper Sha-1 member of Shahejie formation in Chaheji structural belt of Baxian sag in Jizhong depression, Bohai Bay basin. Based on the analyses of petrological features, tectono-sedimentary characteristics, grain sizes and fossils, the paper studies the sedimentary characteristics and sedimentary facies distribution of shallow-water delta systematically, summarizes the depositional model of shallow-water delta and describes the favorable accumulation conditions of shallow-water delta. The study results show that in the upper Sha-1 member, the distributary channel of shallow-water delta is well developed, river mouth bar does not occur and large-area sheet sands are extensively distributed. The cyclic variations of lake level result in sandstone interbedded with mud at multi-stages, which is the main controlling factor of sand body distribution. The sedimentary characteristics of shallow-water delta could determine that in this facies there exist pretty good reservoir-caprock assemblages and various traps are then formed due to faulting to sandbodies. The upper Sha-1 member in Chaheji structural belt has good oil and gas accumulation conditions.

Key words: Bohai Bay basin, Baxian sag, Chaheji structural belt, Shahejie formation, shallow-water delta facies, sedimentary characteristic, depositional model