Quantitative evaluation of the sealing capacity and identification of the preferred petroleum migration path in the Qintong subbasin, Subei basin

Abstract：In the Qintong subbasin, the fault system is well-developed, and the faults controlling petroleum accumulation include faults F1, Ⅲ, F2 and F3 and their branch faults, which divide the fault steps into the Zhuzhuang wrench fault belt in the north and Caoshe wrench fault belt in the south. Based on the quantitative evaluation of the vertical and lateral sealing capacities of the faults, combined with the history of hydrocarbon generation and expulsion, distribution of oil sources, formation stage of petroleum accumulations and tracing analysis of pyrrolic nitrogen compounds, there is appreciable difference in sealing capacities of faults Ⅲ, F2 and F3 in different study areas. In the Zhuzhuang fault belt, the vertical sealing capacity of fault Ⅲ is good and cannot act as conduits of petroleum migration; so in the Jiaodunzi area, the oil was mainly derived from source rocks of the fourth member of the Funing Formation, but there are no contributions from the second member of the Funing Formation and the Taizhou Formation. The activity of fault F3 is short-continued and its lateral sealing capacity is poor; so this fault is not a conduit of petroleum migration either. By contract, fault F2 is long-active and its vertical sealing capacity is poor while its lateral sealing capacity is good; so this fault is a main conduit of petroleum migration in the Zhuzhuang area and has a good match with the stage of formation of petroleum accumulations. The distribution of fluid inclusions and tracing analysis of pyrrolic nitrogen compounds also support this. Study also indicates that fault 2 in the Caoshe fault belt is short-active and has a good vertical sealing capacity but a poor lateral sealing capacity; so this fault is not a conduit of petroleum migration. Fault F3 is long-active and has a weak vertical sealing capacity; so it is a preferred conduit of petroleum migration. In the late stage, petroleum migrated and was accumulated mainly through sand bodies and unconformities, which also matches well with the history of hydrocarbon generation and expulsion, the formation stage of accumulation and the analysis of pyrrolic nitrogen compounds.