The tectono-sedimentary evolution characteristics and regional reservoir-forming model of Nan’an Basin in the Cenozoic

Nan’an Basin is one of the important large oil-gas basins in the southern South China Sea, and the in-depth analysis of its tectono-sedimentary evolution characteristics is of great significance for oil and gas exploration. Based on the existing research results, the 2D seismic data of the basin was interpreted, and five main second-order sequence interfaces and four sets of seismic sequences were identified in the basin in the Cenozoic. Based on the sequence stratigraphic framework, five types of seismic facies such as wedge-shaped and S-shaped (mound), seven seismic lithofacies types (such as sand-coated mudstone and interbedded sand-mudstone), and eight major sedimentary facies (such as fan delta and river delta) were identified. The tectonic evolution of the Cenozoic south basin was divided into four stages: initial rift stage, main rift stage, fault-depression transformation stage, and depression thermal subsidence stage. Before the Mid-Miocene was the main rift stage, during which terrigenous clastic deposits formed from early lacustrine deposition to later offshore deposition. After the Middle Miocene was the depression stage, during which terrigenous clastic deposits and carbonate deposits developed. The study of hydrocarbon accumulation model showed that, the source rocks in Nan’an Basin are well developed and of good quality, which favored the reservoir facies belt enlargement in a large longitudinal thickness and a wide lateral range. The regional cap rocks covered the whole basin, and two types of hydrocarbon accumulation systems were formed: self-generation and self-storage in the early stage, and lower-generation and upper-reservoir in the late stage.