Characteristics, genesis and development pattern of lacustrine carbonate rocks from first member of Paleogene Shahejie formation of Shanan sag in Bohai sea——by taking BZ13 oilfield as an example

By taking BZ13 oilfield of Shanan Sag in Bohai sea as an example, with the data of casting thin sections and scanning electron microscopy(SEM) of core, sidewall core and cuttings, in combination with geochemical indicators of cathodoluminescence and carbon and oxygen isotope, it is believed that there are two types of rocks in the bottom of the first member of Shahejie formation, which are peperite and bioclastic dolomite, with the corresponding of shoreland mixed deposition of beach and shallow lacustrine bioclastic beach. Two typical sedimentary sequence types consist of "inside gentle slope type" biological beach-half/deep lake mudstone and "lateral gentle slope type" mixed product beach-biological beach-half/deep lacustrine mudstone. The shallow deposits of the bottom of the first member of Shahejie formation is retrogradation-overlap sedimentary under the background of continued expansion of lacustrine water body. Sedimentary evolution experienced formation of weathering residue of highland rim and retrogradation-overlap of "inside gentle slope type" biological beach in the early stages of expansion of lacustrine water body and "lateral gentle slope type" mixed product beach, biological beach last retrogradation overlap in the late stages of expansion of lacustrine water body.