Abstract:

         Dongying Sag is one of the major hydrocarbon bearing sags in Bohaibay Basin. After 40-year exploration and development, the upper part of the fourth section of Shahejie Formation (Es₄) as well as those above the sequence have been highly explored. To ensure the sustainable development of Shengli oilfield, it is very urgent to explore favorable prospects. The article analyzed the background for oil and gas accumulation in Es₄ sub-salt reservoir by the tests of geochemistry targets and organic inclusion homogenization temperature, combined with the study of the reservoir capability controls of sedimentary facies, diagenetic facies and high-pressure sealed boxes.It is indicated that in the northern slope belt of Dongying Sag, the lower part of Es₄ develops semi-deep lake facies to intermittent brine lake facies, leading to thick glutenite and dark mudstone deposit in and below evaporate layers, among which the dark mudstone has experienced high-degree organic matter evolution and can yield large amounts of hydrocarbon. Evaporate layers may act as a favorable barrier bed for sub-salt layers to form independent hydrocarbon bearing system which is mainly self-generating and self-storage lithologic hydrocarbon reservoir. The sedimentary facies, diagenesis and high-pressure sealed boxes control the generation of hydrocarbon reservoir,and especially sub-salt high-pressure sealed boxes may slow down the process of hydrocarbon and diagenetic evolution as well as may provide conditions for the preservation of primary pore and fracture, which are in favor of hydrocarbon generation, migration and preservation.

Key words: Es₄, sub-salt, background for oil and gas accumulation, hydrocarbon bearing system, high-pressure sealed box, Dongying Sag