Formation mechanism and evolution model of buried hill reservoir in BZ19-6 structural belt, Bozhong Sag, Bohai Bay, East China

The BZ19-6 structural belt is the largest buried hill oil-gas-bearing area recently discovered in Bohai Bay Basin. The formation and evolution of the buried hill reservoirs are complex. Presently, the development law and controlling factors of this reservoir is unclear. Based on 3D seismic, drilling data, thin section, and regional tectonic background, we explored the controlling factors and development law of the high-quality buried hill reservoir. The results show that the buried hill reservoir can be vertically divided into a weathered glutenite zone, weathered fracture zone, internal fracture zone, and tight zone. The development of reservoir is mainly controlled by lithotypes, tectonic movement, and weathering. The rocks in the study area with low compressive strength, provided the material basis for the formation of fractures. The strongly reformed multi-stage tectonic movements provides the external dynamics for the formation of fractures. Weathering further improved the physical properties of the reservoir. The buried hill reservoir is mainly distributed in the weathered fracture zone and internal fracture zone. Currently, natural gas mainly found in nearly E-Wtrending and N-S-trending fractures. The nearly E-W-trending fractures are developed in the thrust nappe structure formed by N-S-trending extrusion in the Indosinian. The hinge zone of the structure is the main development area of fractures. The nearly N-S-trending fractures are associated with strike-slip faults formed by sinistral strike-slip stress in the Yanshanian. Under the background of NNE-SSW direction extensional stress during the Himalayan, early fractures were reactivated, opened and expanded, forming the reservoir space for later natural gas charging.