The control of curved faults on structural trap formation: Tiantai slope belt, Xihu Sag, East China Sea

Abstract

Mesoscale (tens of kilometers) curved faults are a common fault geometry in plan view. These types of faults are commonly hydrocarbon traps or sand body controlling faults that play a key role in hydrocarbon exploration. However, restricted by the high-resolution requirements for basic research data, studies of mesoscale curved faults are not as sufficient as those of basin boundary curved faults. Based on high-resolution 3D seismic and well data, through structural interpretation, fault activity analysis, and balanced cross-sections, we investigated the structural characteristics of mesoscale curved normal faults in the Tiantai slope belt of the Xihu Sag, East China Sea, and further revealed the controlling effect on structural trap formation. Our results show a significant association between curved fault evolution and structural trap formation. (a) The curved fault evolution process can be divided into the fault segment stage, hard-linked stage, weak activity stage, fault segment stage Ⅱ, and positive inversion stage, resulting in several east-dipping curved faults. The difference in the fault geometry between different typical curved faults is because they experienced different evolution stages. (b) The formation of curved normal faults is controlled by preexisting sag-controlling faults. In the early Cenozoic, the sag-controlling faults underwent oblique extension, forming segments of curved faults; in the Middle Eocene, the regional extension direction changed to be orthogonal to the curved faults, resulting in the hard linking of fault segments, and the curved normal fault zone formed. This process can be summarized as an accommodation model of mesoscale curved faults. (c) Four types of structural traps have been identified in the curved fault zone. The half-anticlinal trap in the footwall was formed in the hard-linked stage (∼32 Ma) according to the tilting fault block model; the half-anticlinal trap in the hanging wall is a transverse anticline that was formed in the fault segment stage Ⅱ (∼23 Ma) due to the differential subsidence of the strata in the hanging wall; and the faulted anticlinal and the anticlinal traps were controlled by the compression of the Longjing movement and formed at approximately 13 Ma.