Superimposed Pattern of the Southern Sichuan Basin Revealed by Seismic Refl ection Profi les Across Lushan–Chishui, China

The Sichuan Basin is a typical intracraton superimposed basin. It is rich in oil and gas resources in the different sets of sedimentary sequences. It underwent multistage tectonic evolution, which resulted in different types of prototype basins. However, there are still many different opinions on the types and superimposed patterns of the Sichuan Basin in different geologic periods, which largely affect the understanding of the mechanism of effective oil and gas accumulation and preservation. This paper aims to re-recognize several prototype types of the Sichuan Basin by discussing the prototype basins and their superimposed models to deepen the significance of superimposed basin evolution for hydrocarbon accumulation. The regional geological and drilling data are used for a detailed interpretation of seismic reflection profiles across Lushan-Chishui. Then, five regional unconformities are identified with the equilibrium profiles technique which is used to flatten the formation interface in different geologic periods. Based on the unconformities, the southern Sichuan Basin is divided into six tectonic layers, each of which is regarded as a prototype basin: a pre-Sinian crystalline basement (AnZ), a marine rift cratonic basin (Z-S), a marine intracratonic sag basin (P(2)l-T(2)l), a marine-continental downfaulted basin (T(3)x(1)-T(3)x(3)), a continental depressed basin (T(3)x(4)-J), and a foreland basin (K-Q). The different prototype basins are vertically superimposed to form a "layered block" geologic structure of the multicycle basins. Affected by the late-stage tectonic transformation, the geologic structure of vertical stratification underwent a strong transformation, which had a profound impact on oil and gas accumulation with the characteristics of early accumulation and late adjustment.