Petroleum system for the continuous oil play in the lacustrine Lower Triassic, Junggar Basin, China

Continuous petroleum accumulations are becoming important in petroleum exploration. This paper reports a new giant oil play within a lacustrine stratum, the Lower Triassic Baikouquan Formation of the northwestern Junggar Basin, northwestern China. This giant oil play is typical of a continuous petroleum accumulation, with low reservoir porosity and permeability (averages of 7.52% and 0.97 md, respectively), continuous hydrocarbon distribution over a large area (= 1000 km2), stratigraphic traps, and anomalously high formation pressure (pressure coefficient >1.1). Detailed molecular as well as isotope geochemistry of hydrocarbons reveal that crude oil and natural gas in the Baikouquan oil play were derived predominantly from the alkaline lacustrine source rock of the lower Permian Fengcheng Formation. Dolomitic source rock is the main contributor to the crude oil and was deposited under anoxic, saline to hypersaline conditions. The analysis of spatial difference of oil maturity in combination with stratigraphic burial-thermal evolution indicates multiple charging events and vertical migration as the dominant process for oil charging in the Baikouquan play. As such, this giant Baikouquan oil play is interpreted to differ from classic continuous petroleum accumulation because of long-distance (1500-2500 m) vertical hydrocarbon migration from deeper Permian sources and is thus referred to as a somewhat atypical continuous oil play in this study. A strike-slip fault network provides migration pathways for the vertical oil charging, and a fan-delta-front conglomerate facies provides high-quality reservoir rocks. The discovery of the Baikouquan oil play exemplifies the successful exploration of a large-area continuous oil accumulation at a long distance (kilometers) from the pod of active source rock, and highlights new exploration opportunities worldwide within continuous oil accumulations, where the slope and central zones of a sag might represent viable petroleum reservoir rock despite their distance from a mature source rock.