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

In the Sichuan Basin there is abundant evidence in the lowerCambrian Qiongzhusi Formation and Ediacaran (upper Sinian)Dengying Formation for the existence of the oldest petroleumsystem in China. Here, we discuss the essential elements of thepetroleum system-source rock in the Qiongzhusi Formationand reservoir rock in the Dengying Formation. The source rockin the Qiongzhusi Formation was deposited in a shelf-likemarine environment. More than 100 m (328 ft) of black shalewas deposited in the north-south-striking Mianyang-Changningintracratonic sag. The reservoir rock is composed of dolomiticalgal mound-facies in the Dengying Formation. In the outcrop,the presence of dissolution features (vugs and caverns) and geo-chemical results indicates freshwater karstification during earlydiagenesis with subsequent hydrocarbon charge. Generatedhydrocarbons underwent two distinct phases of evolution fromoil generation and migration during the Permian to thermalcracking to gas beginning in the Late Triassic. Thus, the oldestpetroleum system in the Sichuan Basin is characterized bypaleo-oil and paleogasfields and present-day gasfields, all closelycontrolled by the tectonic evolution of the Sichuan Basin. Thedevelopment of the intracratonic sag in the basin center con-trolled deposition and preservation of the high-quality sourcerock and highly efficient hydrocarbon transformation and expul-sion. The development of the Ordovician-Jurassic Leshan-Longnvsi paleouplift in the basin center focused hydrocarbonmigration to charge the paleo-oil and paleogasfields. Lateral andvertical migration of hydrocarbons was facilitated along unconformities and faults in upper Sinian and lower Cambrianstrata across the Sichuan Basin-in particular, the Weiyuangasfield.