Permian sedimentary evolution and hydrocarbon accumulation effects in the central-southern South Yellow Sea Basin

The South Yellow Sea Basin (SYSB) is an extension of the Lower Yangtze Block to the sea. The basin has undergone a complex tectonic-sedimentary evolution, and there has been no breakthrough in oil and gas exploration. Based on newly collected and processed well drilling data, cores, and seismic data of the central-southern SYSB and adjacent areas, the understudy area is set in the Lower Yangtze Region (LYR) to compare the overall sequence division and sedimentary facies. The Permian isochronous stratigraphic framework is methodically established in the central-southern SYSB, and the corresponding sedimentary facies and their distribution are examined in some detail. Finally, together with the previous investigations of tectonic evolution, the potential Permian oil-gas accumulation modes are proposed. The achieved results reveal that the second-order Permian sequence developed in the central-southern SYSB is consistent with the LYR. There exists a relatively complete second-order sequence I developed in the middle-bottom section and the transgressive systems tract of sequence II in the upper section. The Permian sedimentary system in the central-southern SYSB can be compared to the LYR and is generally distributed in a northeast direction. The transgressive systems tract of sequence I is broadly developed with the restricted platform and open platform as the dominant facies. The highstand systems tract of sequence I has been only developed in the northwest of the understudy area, with the shelf as the dominant facies. The transgressive systems tract of sequence II is mostly dominated by delta facies and swamp-lagoon facies with thick coal-measure layers, which provide a material basis for the formation of large-scale oil and gas fields. The thermal evolution of Permian source rocks was controlled by a combination of tectonic evolution and paleothermal gradient. Permian source rocks may have experienced secondary hydrocarbon generation in the western part of the southern Depression, whereas source rocks elsewhere experience only one hydrocarbon generation in the Early Triassic. The obtained results reveal that there are significant differences in the accumulation mode of different structural units.