Overpressure origin and its effects on petroleum accumulation in the conglomerate oil province in Mahu Sag, Junggar Basin, NW China

The origin of overpressure and its effect on petroleum accumulation in the large Permian/Triassic conglomerate oil province in the Mahu Sag, Junggar Basin have been investigated based on comprehensive analysis of log curve combinations, loading-unloading curves, sonic velocity-density cross-plot, and porosity comparison data. The study results show that there are two kinds of normal compaction models in the study area, namely, two-stage linear model and exponent model; overpressure in the large conglomerate reservoirs including Lower Triassic Baikouquan Formation and Permian Upper and Lower Wu’erhe Formations is the result of pressure transfer, and the source of overpressure is the overpressure caused by hydrocarbon generation of Permian Fengcheng Formation major source rock. The petroleum migrated through faults under the driving of hydrocarbon generation overpressure into the reservoirs to accumulate, forming the Permian and Triassic overpressure oil and gas reservoirs. The occurrence and distribution of overpressure are controlled by the source rock maturity and strike-slip faults connecting the source rock and conglomerate reservoirs formed from Indosinian Movement to Himalayan Movement. As overpressure is the driving force for petroleum migration in the large Mahu oil province, the formation and distribution of petroleum reservoirs above the source rock in this area may have a close relationship with the occurrence of overpressure.