Overpressure reservoirs of the Huangliu Formation of the Dongfang area, Yinggehai Basin, South China Sea

摘要

The relationship between reservoir property and such factors as sedimentary microfacies, clastic component, grain size, and special diagenetic environment of high temperature and overpressure were comprehensively analyzed to study reservoir characteristics and influencing factors in the first member of upper-Miocene Huangliu Formation of the DF13-1 structure in the Dongfang area, Central Diapir Zone of the Yinggehai Basin. The dominant reservoirs in the first member of the Huangliu Formation in the Dongfang area are composed of very fine to fine lithic quartz sandstones that are deposited in submarine fans sourced from the Kuntom Uplift. The reservoirs are characterized by large thickness, broad distribution and good qualities. High-temperature thermal fluids activities and overpressure are two major diagenetic factors contributing to the good properties in the reservoirs. Thermal fluids activities of high tempreture accelerated the organic matter thermal evolution and clay mineral transformation, and in this process organic acids and CO2were generated which dissolved the reservoirs and thus resulted in a secondary pore zone. Overpressure broadened oil generation window, increased corrosion time and strength, making secondary pores developed in deeper formations. Moreover, the overpressure was beneficial to preserve pores. The reservoir economic basement depths of siltstone, very fine sandstone and fine sandstone are about 3 100 m, 3 900 m and 5 000 m respectively. The fine sandstone and very fine sandstone in the first member of the Huangliu Formation could form high-quality gas reservoirs in present depth of 2 600–3 500 m.