Hydrocarbon accumulation in the northwest Chaluhe Fault Depression, Yitong Basin, North China

Abstract

The Yitong Basin is a strike-slip fault basin, which tectonically belongs to the northern section of the Tanlu Fault Zone. Exploration has demonstrated that the northwest Yitong Basin is a hydrocarbon enrichment zone. However, the complex geological conditions and a poor understanding of the accumulation models and hydrocarbon enrichment factors have impeded petroleum exploration in the basin. This paper focuses on the hydrocarbon accumulation processes in the strike-slip fault zone in the northwest Yitong Basin. We proposed the reservoir critical porosity concept for reservoir dynamic evaluation. A mathematical model for sandstone porosity evolution in the northwest Chaluhe Fault Depression is established to reconstruct the sandstone porosity over geological time. A fault critical burial depth is proposed to evaluate the present fault-sealing property. The critical condition for oil charge into sandstone was also simulated using an oil-charge simulation experiment. The results indicate that the reservoir critical porosity of the Shuangyang Formation in the northwest Chaluhe Fault Depression was 11% during the accumulation. The present tight sandstone reservoirs in the northwest Yitong Basin were conventional reservoirs during the hydrocarbon accumulation with relatively high porosity and permeability, beneficial for hydrocarbon accumulation at the time. The critical burial depth of faults in the middle–deep layers is 2050 m in the northwest Chaluhe Fault Depression. The hydrocarbons can be effectively sealed when the burial depth of the fault is greater than the critical burial depth. The critical hydrocarbon accumulation threshold for a sandstone reservoir depends on the coupling of reservoir critical porosity and critical injection pressure during hydrocarbon accumulation. We put forward three hydrocarbon accumulation plays for the northwest Chaluhe Fault Depression: (1) extensional fault block play, which is close to the source rock and experienced two stages of hydrocarbon accumulations; (2) flower structure play, which is far away from the source rock and is characterised by a late hydrocarbon charge and accumulation; and (3) fault-lithological play, which is far away from the source rock and experienced a late hydrocarbon accumulation. For the Yitong Basin, reservoir properties and faulting appear to be the main controlling factors for hydrocarbon accumulations.