Genetic Mechanism and Evolution of the Covert Fault Zone and Its Oil-Controlling Mode in Qikou Sag, Eastern China

Covert fault zone is an important type of geological phenomenon that is closely related to hydrocarbon formation and distribution but has often been overlooked because it lacks obvious fault displacement and fault plane. To meet this challenge, a novel cognitive framework is proposed in this study, in which criteria for identifying the existence of covert fault zone are developed based on the regional tectonic backgrounds and geophysical data. The Riedel shear model is then utilized to analyze the genetic mechanism of the covert fault zone. The Mohr-Coulomb theory is also introduced to conduct a structural physical simulation to interpret the evolution process of the covert fault zone. Information about the genetic mechanism and evolution of the covert fault zone is finally combined to determine the oil-controlling mode. The study site is Qikou Sag in Eastern China. It is found that the covert fault zone in Qikou Sag meets four recognition criteria and is generated by the stress transferred from the strike-slip activity of the basement fault. Moreover, it can be concluded that the covert fault zone in Qikou Sag contains five evolution stages and controls the reservoir mainly via three aspects, that is, sedimentary sand, subtle traps and oil accumulation mode.