The Qiongzhusi-Dengying petroleum system in the Sichuan Basin, China

In the Sichuan Basin there is abundant evidence in the lower Cambrian Qiongzhusi Formation and Ediacaran (upper Sinian) Dengying Formation for the existence of the oldest petroleum system in China. Here, we discuss the essential elements of the petroleum system—source rock in the Qiongzhusi Formation and reservoir rock in the Dengying Formation. The source rock in the Qiongzhusi Formation was deposited in a shelf-like marine environment. More than 100 m (328 ft) of black shale was deposited in the north-south–striking Mianyang-Changning intracratonic sag. The reservoir rock is composed of dolomitic algal mound-facies in the Dengying Formation. In the outcrop, the presence of dissolution features (vugs and caverns) and geochemical results indicates freshwater karstification during early diagenesis with subsequent hydrocarbon charge. Generated hydrocarbons underwent two distinct phases of evolution from oil generation and migration during the Permian to thermal cracking to gas beginning in the Late Triassic. Thus, the oldest petroleum system in the Sichuan Basin is characterized by paleo-oil and paleogas fields and present-day gas fields, all closely controlled by the tectonic evolution of the Sichuan Basin. The development of the intracratonic sag in the basin center controlled deposition and preservation of the high-quality source rock and highly efficient hydrocarbon transformation and expulsion. The development of the Ordovician–Jurassic Leshan-Longnvsi paleouplift in the basin center focused hydrocarbon migration to charge the paleo-oil and paleogas fields. Lateral and vertical migration of hydrocarbons was facilitated along unconformities and faults in upper Sinian and lower Cambrian strata across the Sichuan Basin—in particular, the Weiyuan gas field.