Reservoir Characteristics and Forming Mechanisms of Lacustrine Mixed Sedimentary Rock of First and Second Members of Shahejie Formation in BZ27-A Structure, Huanghekou Sag

There is a set of lacustrine mixed sedimentary rocks in the first and second members of Shahejie Formation(Es_1-2) in BZ27-A structure in Huanghekou sag, Bohai Sea area. At present, the research on the characteristics and genesis of this reservoir is still weak. In this study, based on the core observation, casting thin section and mercury porosimetry, the characteristics of petrology, reservoir space type, physical property of mixed sedimentary rock reservoir are systematically described firstly, and then the controlling factors of the reservoir are put forward. The results show that the mixed sedimentary rock consists of terrigenous clastic, clay minerals, carbonate minerals and biodetritus. The lithology mainly includes calcareous sandstone, dolomitic sandstone, terrigenous clastic micrite dolomite, micrite oolitic dolomite and bioclastic dolomite, which are interbeded or thin interbeded in the vertical section. Most of the mixed sedimentary rock reservoirs are tight due to low porosity and permeability, with few relatively better reservoirs in thin intervals. The main factors controlling the mixed sedimentary rock reservoirs are paleogeographic background, provenance, diagenesis and fluid. Massive carbonate cementation in the diagenetic stage is the main cause of tight reservoir, however, the contributions of bioclastic, oil and gas emplacement in the later stage of hydrocarbon accumulation are important for improving the physical properties of reservoirs, and have the potential to form relatively high-quality reservoirs.