Fault sealing evaluation and its effect on hydrocarbon accumulation in buried-hill fault-block oilfields, Bohai Bay Basin, China

The buried-hill fault-block oilfield is the primary exploration target in rift basins with complex tectonic setting. The study of fault sealing is of great significance for determining the distribution of hydrocarbons in fault blocks and reducing exploration risks. This study systematically analyses the sealing characteristics of different fault types based on 3D seismic data, logging interpretations and drilling information, and evaluates the sealing conditions of buried-hill fault-block traps. Results suggest that the buried-hill fault-block trap in the Eastern Bohai Bay Basin is composed of Archaean–Mesozoic weathering crust reservoir, Upper Palaeozoic caprock, Mesozoic caprock (or Palaeogene caprock) and a multiple fault system. High-quality reservoirs (porosity > 6%) are mainly concentrated within 100 m of the weathering crust's top. Multi-stage fault activities lead to different source rock-reservoir configuration relationships in buried-hill traps in different areas. The NNE-striking strike-slip faults dominate the strip distribution of buried-hill oilfields. The sealing properties of different parts of a NNE-striking strike-slip fault are different. The sealing ability of the stress-restraining area is significantly stronger than that of stress-releasing area. Not all of EW-striking extensional fault have sealing capacity. When evaluating traps dominated by EW-striking fault, it is necessary to evaluate not only the contact relationship between the two walls of the fault, but also the sealing property of the trap itself. The sealing mechanism in the buried-hill fault-block oilfield is characterised by the contact between the high reservoir position of the upthrown of the reverse normal fault and the Mesozoic–Palaeozoic tight layers. Alternatively, when the throw of the strike-slip fault is significant, the high reservoir position of the downthrown contacts the tight Archaean layer. Both of these scenarios demonstrate effective lateral sealing. The novelty of the research is that the sealing conditions of extensional faults during the Indosinian–Yanshanian period are influenced by their contact relationships. Additionally, it reveals that the sealing capacity varies among different segments of strike-slip faults, surpassing previous research which suggested that faults during this period generally possess sealing capabilities.