Cenozoic tectonostratigraphy and structural styles in the Nangong Sag, Bohai Bay Basin, eastern China: Implications for the generation of oil-gas traps

Several hydrocarbon signatures were discovered in the Nangong Sag during early drilling, indicating the significant exploration potential of this region. However, the structural geometry and kinematics, tectonic evolution, genesis of structural traps, and hydrocarbon distributions in the sag are still poorly understood. Here, we studied the two-dimensional seismic sections, drilling data, logging data, cores, and field outcrops of the Nangong Sag to elucidate its geological architecture, structural style, spatiotemporal differences in structural deformation, and tectonic evolution and predicted traps that are prone to hydrocarbon accumulation. We identified two regional angular unconformities at the base of the Palaeogene and the Neogene. Three tectonostratigraphic units were delineated: the basement unit, the Palaeogene unit characterised by a considerable change in bed thickness and a larger number of faults, and the Neogene-Quaternary unit characterised by a slight change in bed thickness and few faults. There are three tectonic deformation sequences, namely extensional, strike-slip, and inversion deformation sequences, and 10 structural styles were classified. The intensity of deformation varied spatiotemporally. Overall, the extension at the centre of the Nangong Sag was the strongest, followed by that in the north, while the extension in the south was the weakest in the Palaeogene. The northern extension was the strongest in the depositional period of the Kongdian Formation and Member 4 of the Shahejie Formation, the central extension was the strongest in the depositional periods of Members 3, 2, and 1 of the Shahejie Formation, and the overall extension was weak during the Neogene to Quaternary. The strike-slip activity was relatively strong in the depositional periods of Members 3 and 2 of the Shahejie Formation. Strike-slip structures and extensional structures are often symbiotic and jointly controlled and affected the development of the sag. The Cenozoic tectonic evolution was divided into three stages: the syn-rift stage during the Palaeocene to early Oligocene, inversion stage in the late Oligocene, and post-rift stage during the Neogene to Quaternary. The syn-rift stage can be subdivided into the initial, middle, and late syn-rift stages. The formation of structural styles promotes the formation of structural traps, which are transformed and preserved by tectonic evolution. The results suggest that fault block traps, rollover anticline traps, and inversion fold traps, developed in Member 3 of the Shahejie Formation, mainly in the centre of the Nangong Sag, are the potential structural traps in the area.