Stratigraphic architecture and vertical evolution of various types of structural slope breaks in Paleogene Qikou sag, Bohai Bay Basin, Northeastern China

Multivariate geological interactions cause distinctive sequence stratigraphic patterns in various tectonic positions. Syndepositional faulting and slope break belts play important roles in controlling the development and dispersion of depositional systems. According to the genesis of marginal fault, section pattern, and plane combination, three types of structural slope breaks are distinguished in different tectonic units of Qikou Sag. Firstly, the fault controlled steep slope region, which is controlled by growth fault, usually develops near the boundary fault of basin or marginal fault of sub-sag. Secondly, the lower part of the flexure slope break presents onlap and thickening of strata, as the upper part is gentle gradient, whose thickness is relatively thinner. Thirdly, the multistage fault terrace zone that is located at the intersection of several contemporaneous faults present as step-shaped on profile and in parallel or penicillate combination on plane. In the same structural slope break unit, sequence stratigraphic patterns evolve longitudinally according to the vertical evolution through time, which has experienced a process from intense differential fault rifting to integral subsidence in Paleogene. Meanwhile, the sequence stratigraphic patterns are diverse in various types of structural slope break units in transverse space according to the distinct structural and sedimentary architectures. The associated combination of particular tectonic units, geomorphic units, and sequence patterns, as well as their effective allocation with qualitative source rocks, provide advantageous conditions for hydrocarbon accumulation. The tectonic-sequence-sedimentary composite patterns provide the basis for prospecting of structural-lithologic and lithologic-hydrocarbon reservoirs in particular tectonic units.