Tectono-sequence evolution of an extensional strike-slip composite basin, a case study of the Miaoxi'nan sub-sag in the eastern Bohai Bay Basin

The significance of tectonics in shaping sequence stratigraphy within continental rift basins, especially those with extensional strike-slip composite characteristics, cannot be overstated. These basins, often repositories of rich oil and gas reserves, are defined by their intricate fault systems and dynamic tectonic interplay. Despite their economic importance, the tectono-sequence characteristics of these basins remain underexplored, affecting the understanding and prediction of petroleum systems. This study presents a comprehensive tectono-sequence analysis of the Miaoxi'nan sub-sag in the eastern Bohai Bay Basin, a continental lacustrine sag intersected by the eastern branch of the Tan-Lu fault zone. By integrating extensive 3D seismic data, wireline logs, and previous research, we establish a detailed tectono-sequence framework and unravel the complex interplay of geometric and kinematic characteristics within the extensional strike-slip fault system that governs the evolution of the basin. Our investigation reveals a multi-tiered sequence stratigraphic structure comprising two first-order, 4 s-order, and seven third-order sequences. The west sub-branch of the East branch of the Tan-Lu fault zone (EB-TLFZ) is identified as a strike-slip fault with a predominantly south-north strike, while the east sub-branch exhibits characteristics of a persistent tension-shear strike-slip fault zone. The study also shows a transitional fault strike of the rifting system from a multi-directional orientation at the base to a northeast-northeast-east direction, corresponding with different tectonic stages that have directed fault activities and sedimentary deformation. Additionally, both the extensional and strike-slip fault systems of the sag have experienced segmental growth evolution, influencing the westward and northward migration patterns of the basin's depocenter and subsiding center. Overall, the sequence development within the Miaoxi'nan sub-sag is predominantly regulated by the first- and second-order extensional faults, with the EB-TLFZ sub-branch faults providing further adjustment. This study provides vital insights into the tectono-sequence dynamics of extensional strike-slip composite basins, with implications for future hydrocarbon exploration and exploitation.