Controls of paleo-overpressure, faults and sedimentary facies on the distribution of the high pressure and high production oil pools in the lower Triassic Baikouquan Formation of the Mahu Sag, Junggar Basin, China

Abstract

Large quantities of high-pressure oil pools have been discovered in the glutenite reservoirs with low porosity and low permeability in the Lower Triassic Baikouquan Formation of Mahu sag, Junggar Basin. Overpressure has a close relationship with the generation of high-production oil pools in the Baikouquan Formation. In areas with abnormally high overpressure, the daily oil and gas production of single wells is high. Geological and geophysical analysis, and two-dimensional (2D) basin models have been used to explain the mechanisms of overpressure in the Baikouquan Formation and to establish a hydrocarbon accumulation model of high pressure and high production oil pools. Based on the hydrocarbon accumulation model, the main controlling factors of high pressure and high production oil pools generation have been summarized, and the favorable accumulation zones have been predicted. The results show that the chief reasons for overpressure are disequilibrium compaction and the pressure conducted through faults. In the hydrocarbon accumulation model of high pressure and high production oil pools, the overpressure compels faults that connect source rocks with reservoirs to open and become the valid conductive systems, and the oil-bearing overpressure fluid charges into the Baikouquan Formation along faults and forms high pressure oil pools in the frontier facies glutenite reservoirs near the faults. The main controlling factors of oil accumulation are the paleo-overpressure of Fengcheng Formation source rock during the main hydrocarbon migration stage (the Early Cretaceous, before 146 Ma), the faults on the ancient nose-like belts of structural uplifts and the fan-delta frontier facies glutenite reservoirs in the Baikouquan Formation. According to the accumulation model, the fan-delta frontier facies glutenite reservoirs of the Baikouquan Formation can be divided into three sections based on the favorability of oil accumulation I, II and III, from good to bad.