Characteristics and formation mechanisms of buried hill carbonate reservoirs in Bonan Low Uplift, Bohai Bay

The carbonate rock buried hills are important exploration and development targets in the Bonan Low Uplift of the Bohai Bay. Based on drilling, seismic, core, thin section and well logging data, the characteristics and formation mechanisms of the buried hill carbonate reservoirs were studied. They were divided into two reservoir systems, including the weathering crust karst reservoir and the inner dissolution reservoir. Dolomites were dominant, while limestones took second place. Reservoir space mainly included intercrystalline porosity, fractures and dissolved pores. The results of physical characteristic analysis showed that inequality grain dolomites, powder crystal-aplitic dolomites and silt crystal dolomites were better than limestones. Sedimentary facies, karstification and tectonic disruption were recognized as the main formation mechanisms of the buried hill carbonate reservoirs in the Bonan Low Uplift. (1) Sedimentary facies were the basis of reservoir formation. Favorable reservoirs mainly distributed in the dolomites and limestones of restricted sea, intertidal flat and tidal flat facies. (2) Karstification was the key to the formation of buried hill carbonate reservoirs. Weathering crust karst reservoirs and inner dissolution reservoirs developed within 0-250 and 0-150 meters under the top of the unconformity. Favorable karst reservoirs developed in the karst highland and karst slope of karst palaeogeomorphology. (3) Tectonic disruption was the connection of the formation of buried hill carbonate reservoir. Semi-filled or unfilled structural fractures and dissolution fractures provided important reservoir porosity and migration channels. When combined with dissolution pores, they formed fracture-pore and pore-fracture reservoirs, which improved reservoir performance.