Basement Reservoirs in China: Distribution and Factors Controlling Hydrocarbon Accumulation

The oil reserves of global basement reservoirs are 248 x 108 t and natural gas reserves are 2681 x 108 m3; they are crucial links in the future oil and natural gas exploration field and play an irreplaceable role in increasing oil and natural gas reserves and production. Based on research on the definition and classification of basement reservoirs, this study dissected three major basement reservoirs in China (i.e., the Dongping region located in the Qaidam Basin, the Bozhong 19-6 gas field located the Bohai Bay Basin, and the Central Uplift area of the Songliao Basin). The geological conditions and controlling factors of oil and natural gas accumulation in basement reservoirs in China are summarized. The results of this study are as follows: (1) Basement reservoirs can be classified into three distinct types, namely, the weathered carapace type, weathered inner type, and weathered composite type. They are characterized by a large burial depth, strong concealment, and huge reserves and are mostly distributed at the margins of continental plates and in zones with stratum intensive tectonic activity; (2) Basement reservoirs in different basins have different controlling factors. The basement reservoir in the Dongping region, located in the Qaidam Basin, has favorable geological conditions with laterally connected sources and reservoirs. In this reservoir, oil and natural gas have transferred along faults and unconformities to accumulate in uplifted areas, forming a weathered carapace-type basement reservoir controlled by structures. The Bozhong 19-6 gas field, which is situated in the Bohai Bay Basin, has favorable multiple hydrocarbon supplies of source rocks. Under the communication of faults and cracks, oil resources form a weathered inner type basement reservoir. In the Central Uplift area of the Songliao Basin, the basement reservoir exhibits a dual-sided hydrocarbon supply condition from the uplift. In this reservoir, oil and natural gas have transferred to traps through the fault and inner fracture system and have been properly preserved thanks to the extensive overlying cap rocks. It can be concluded that, after being attenuated by millions of years of weathering and leaching, basement rocks can form large-scale and medium-scale basement reservoirs with reserves of more than 100 million barrels in the presence of favorable geological conditions, such as a multi-directional hydrocarbon supply, a high brittle mineral content in the reservoirs, diverse reservoir spaces, and high-quality cap rocks.