Lacustrine carbonates in the southwest margin of the Qikou Sag, Huanghua Depression, Bohai Bay Basin

The southwest margin of the Qikou Sag is a flexural gentle slope formed by the deformation of a drape structure due to its underlying basement lifting. The sedimentary period of the Lower Member 1 of Shahejie Formation is the maximum extensional time of the Paleogene lacustrine basin. Debris from source areas is in shortage. In the broad littoral shallow lake, hundreds of square kilometers of lacustrine carbonates are deposited and distributed stably. The main lithologies of the sediments are biolithite and dolomite (micritic dolomite and dolomitic limestone), and the dolomites are mainly middle late buried dolomite and penecontemporaneous dolomite. Their distribution area is more than 1 650 m2, the cumulative thickness is between 5 and 25 meters, and the average thickness is about 10 meters. Intercrystal pores, intergranular pores, corrosion pores and frequent fissures are developed in the medium porosity and low permeability dolomite reservoirs. The dolomites alternate with oil shale and dark mudstone, favorable for the charge of low mature hydrocarbons and the formation of large scale accumulations. Drilling exploration practices show that the lacustrine dolomite reservoirs are characterized by medium burial depth and regional petroleum accumulation and are a new exploration area for Paleogene oil and gas.