The Study On Relationship Between Faults, Unconformities And Hydrocarbon Accumulation Of Sanghe Subsag In Bayan Qagan Depression

This paper applies the modern hydrocarbon reservoir theory on the reservoir formation of the Bayan Qagan Depression. The research on the formation and evolvement history of Bayan Qagan Depression reveals that active period of major faults in Sanghe Subsag was Aershan-Duhongmu stage and the activity of faults controls the migration of subsidence centers. The faults evolvement can be apparently classified into early stage, middle stage, and late stage. Faults in the early stage trap oil to form reservoirs, middle stage faults make oil migrate vertically but not laterally, late stage faults were not beneficial to hydrocarbon accumulation, and faults were main pathways for the vertical migration of oil. The sealing properties of regional faults were good enough to obstruct oil migration. But the sealing properties of sub faults are different in active periods and stable periods when they can be the passages for hydrocarbon migration, showing periodic features.The unconformities in Sanghe area can be divided into truncation unconformity, onlap unconformity, fault-fold unconformity and parallel unconformity. Multiple truncation unconformities developed near the steep slope of north area, forming unconformity sealing traps. Onlap unconformities distributed on southern slope of the sag, forming onlap and lithologic traps. Fault-fold unconformities occurred along the northern great faults. Faults and unconformities constituted good migaration pathways, and always generated fault-nose and fault-anticline traps.Oil-source correlation shows that the oil in Sanghe Subsag was from Cretaceous Du1 and A2 Formation. There were there sets of reservoirs: Aershan Formation, Tenggeer Formation and Du1 Formation, corresponding to Du2, Saihantala and Saihantala post depositional stage respectively.In Du1-Saihantala depositional stage, hydrocarbon from Du1 and A2 Formation migarated to the northern fault belt, along fault-fold unconformities and sand bodies. Moreover, the faults became the main vertical migaration pathways, forming self-source and self-reservoir structural reservoirs. During late Duhongmu stage, hydrocarbon from A2 Formation magirated along delta front sand bodies and onlap unconformities to the Xilinhaolai, sealed by faults and mudstones, and generated lithologic reservoirs. During late Tenggeer and Duhongmu stage, hydrocarbons were expelled from A2 and Lower Tenggeer formation, accumulated in Daerqi area to form structural-lithologic pools.