Complex fault interpretation of buried hills: A case study of Zhuanghai region in Jiyang Depression, Bohai Bay Basin

The Zhuanghai oil field is located in the north of the Jiyang Depression, Bohai Bay Basin. Drilling data illustrated that the pre-Tertiary formations of the Zhuanghai area are rich in oil. However, complicated structures developed in the pre-Tertiary formations with unclear fault directions and distributions, which made the exploration of the pre-Tertiary structures very difficult. The nature of faults was determined by considering the mechanism of fault evolution. The key techniques for the interpretation of faults of different natures were studied to accurately describe the fault system and deduce fault development patterns to define the structural features. The Zhuanghai area has experienced several stages of deformation, including regional uplift, stable structure, extrusion, stable structure, tension, tense-shearing and compresso-shearing. The tectonic compressions during the Indosinian and the late Yanshanian periods formed thrust tectonics, which laid the foundation for buried hills. The extensional strike-slips in the Himalayan period made the buried hills more complicated. A fault interpretation was made, mainly focusing on reversed faults. The derivative of stratigraphic thickness change rate and dip can be calculated to interpret reversed faults. Time slices and fault planes were combined to interpret strike-slip faults. Coherent cube, ant-colony and 3D visualization were combined to interpret normal faults. The Paleozoic in the Zhuanghai area developed NNE-trending reverse faults and EW-trending extensional faults, with thrust, negative reversal and strike-slip fault types, showing a complex pattern of "barriers and depressions".