Shallow water delta depositional model of the lower segment of the Xingouzui Formation in eastern Qianjiang depression

Abstract：The lower segment of the Xingouzui Formation in eastern Qianjiang depression is an important horizon for finding lithologic hydrocarbon reservoir in the future. Based on a synthetic analysis of core description as well as thin section and logging data in combination with sedimentary marks of lithology，paleoclimate，sedimentary structure，and the depositional background of weak tectonic movement and flattened palaeotopography，the authors hold that the lower segment of the Xingouzui Formation is of a shallow water delta depositional system. The shallow water delta has the following characteristics：① there exist well-developed underwater distributary channel microfacies，underdeveloped mouth bar，extensive sand sheet，and frequently alternate pre-delta and front deposition；② various facies belts are smoothly connected with each other without apparent slope gradient；③ depositional model has no three-layer structures like “Gilber delta”. During the depositional evolution，the shallow water delta deposition was affected by such factors as palaeoclimate，density contrast of stream and salt lake，fluviatile action and lake surface fluctuation.，and the shape and the development extent of the underwater distributary channel were especially apparently controlled by these factors，forming strong or weak reformed front sand. Favorable allocation for generating reservoirs and cap rocks in the lower segment of the Xingouzui Formation was favorable for forming lithologic hydrocarbon reservoirs.