New insights into marine hydrocarbon geological conditions in the South Yellow Sea Basin: evidence from borehole CSDP-2

Abstract

The South Yellow Sea Basin (SYSB) has multiple sets of proven source rocks and good hydrocarbon prospects, but no industrial oil and gas has been explored at present. To solve this puzzle for petroleum geologists, we systematically investigated the marine hydrocarbon geological conditions based on cores and testing data from borehole CSDP-2, the first exploration well with continuous coring in SYSB. The qualities of source rocks are evaluated in detail according to organic matter abundance, type, and maturity. The reservoir characterization mainly includes porosity, permeability, and reservoir space. Displacement pressure test and stratum thickness are the main foundations for defining the caprocks. Then, the oil-source rock correlation in the Permian and stratum model are analyzed to determine the favorable source-reservoir-caprock assemblages. The results show that three sets of effective source rocks (the Lower Triassic, Upper Permian, and Lower Permian), two sets of tight sandstone reservoirs (the Upper Permian and Lower Silurian-Upper Devonian), and two sets of caprocks (the Lower Triassic and Carboniferous) combine to constitute the hydrocarbon reservoir-forming assemblages of “lower-generation and upper-accumulation” and “self-generation and self-accumulation”, thus laying a solid foundation for promising petroleum prospects. The three sets of marine source rocks are characterized by successive generation and expulsion stages, which guarantees multistage hydrocarbon accumulation. Another three sets of continental source rocks distributed across the Middle Jurassic, Upper Cretaceous, and Paleogene depression areas, especially in the Northern Depression, may supplement some hydrocarbons for the Central Uplift through faults and the Indosinian unconformity. The favorable Permian exploration strata have been identified in the Central Uplift of SYSB. First, the Lower Permian and Upper Permian source rocks with high organic matter abundance and high thermal maturity supply sufficient hydrocarbons. Secondly, the interbedding relationship between the source rocks and sandstones in the Upper Permian strata ensures that hydrocarbons have been migrated into the nearby Upper Permian sandstones, reflecting near-source hydrocarbon accumulation. Finally, the good sealing property of the Lower Triassic Qinglong Formation caprocks plays an indispensable role in hydrocarbon preservation of the Permian reservoirs. This conclusion is supported by direct oil shows, gas logging anomalous layers, and hydrocarbon-bearing fluid inclusions.