A quantitative study on source rocks in the western Leidong depression, northern South China Sea

Abstract

The Leidong depression is one of the secondary tectonic units and the least explored part of the Beibu Gulf Basin, northern South China Sea. Exploration activities in the basin reveal that the Middle Eocene deposition was controlled by NE-SW-striking faults and the second member of the Eocene Liushagang Formation (LⅡ) consists of high-quality source rocks. The nature and origin of this depression has remained enigmatic due to the complex fault structures as compared to those in other basins. It is important to identify whether LⅡ occurs in the Leidong depression in the context of exploration. Here we analyze available geological, seismic, and drilling data of the depression and the controlling factors among faults, deposition, lithology, and hydrocarbon distribution in the basin. Measures such as stress fracture analysis, fault growth index analysis, and sedimentary strata characterization were used to construct an Eocene sedimentary model controlled by NW-SE striking faults. The model with integrated seismic profile characteristics comparison and velocity analysis results was then used to determine whether the Eocene strata in the west of the depression contain the high-quality source rocks that are widely distributed elsewhere in the basin. Finally, calculations were performed to quantify the intensity of hydrocarbon generation and amount of hydrocarbon generated in the depression based on available data of the source rocks, including their distribution and thickness. The results confirmed the assumption that the Middle Eocene strata in the Leidong depression were controlled by NW-SE strike-slip faults and contain high-quality LⅡ source rocks, thus providing an important foundation for future exploration.