Formation and characteristics of large-medium buried-hill hydrocarbon reservoirs in Bohai Sea

There had been a long history of the buried-hill hydrocarbon reservoirs exploration operation in Bohai Sea. Between 1970s and 1990s, based on the onshore exploration experiences in eastern China, many boreholes were drilled in order to detect large-scale carbonate buried-hill hydrocarbon reservoirs in Bohai Sea, but no prominent discoveries was achieved. In-depth re-evaluation and examination were conducted upon these exploration failures, a new understanding that Bohai Sea had unique geological characteristics of buried hills was concluded. Bedrocks on the uplifts of Bohai oil province were mainly granites and migmatitic granites of Archean, Proterozoic and Mesozoic, as well as minor Lower Paleozoic carbonates. Proterozoic algae dolomite was most favorable for buried-hill hydrocarbon reservoirs however which were not developed in Bohai Sea. A large number of faults in Bohai oil province were developed and faults activities were intense in the late period. Thus, conventionally, reservoirs and preservation conditions of the buried hills were believed to be poor in Bohai Sea, and were not favourable for formation of large-scale buried-hill hydrocarbon reservoirs. Through the long-term practices and deep analyses, the coupling between granites, migmatitic granites and intense bedrock faults was favorable for buried-hill reservoirs with high test production and relatively low core recovery rate. Geophysical data could be used for accurate predictions of bedrock lithology and faults. The monadnock buried hills on the low uplifts adjacent to hydrocarbon-rich sags were covered by shallow to semi-deep lacustrine shale, which was favorable for buried-hill hydrocarbon reservoirs. Under the guidance of a series of innovative understandings, the re-exploration of buried hills in Bohai Sea achieved two largest granite buried-hill oil and gas fields with 100 million tons of reserves. The understanding and practice processes might also inspire exploration of other areas.