Formation and petroleum exploration significance of large-scale subsurface thrust belt in the southwestern Bohai Sea

The southwestern Bohai Sea in North China features relatively complete buried-hill strata and superior hydrocarbon source conditions, and is adjacent to the Qikou Sag and the southwestern subsag of the Bozhong Sag. However, exploration in the shallow buried hills in the Cretaceous, Jurassic, and Upper Paleozoic strata is not satisfactory, while the Lower Paleozoic carbonate rocks hosting relatively well-developed reservoirs, remain largely unexplored due to the coverage by thick Mesozoic and Upper Paleozoic strata. To further clarify the distribution patterns of the Lower Paleozoic strata in the study area, a large-scale concealed thrust belt was recognized and characterized based on recent 3D seismic data and structural evolution analysis. The thrust belt exhibits a unique buried hill stratigraphic structure with an absence of Upper Paleozoic strata. The formation of the thrust belt was controlled by the coupling of multiple factors, including persistent Indosinian compressive stress, gravitational effects, and the presence of plastic mudstone at the base of the Cambrian. The structural high background formed by the thrust belt laid the foundation for trap development, provided a window for the Lower Paleozoic carbonates to undergo Indosinian weathering and leaching, and facilitated the development of fractured reservoirs. The discovery of the thrust belt has challenged the traditional view that reservoir formation in the Lower Paleozoic of the study area is difficult. It provides strong guidance for hydrocarbon exploration in the buried hills of the Shanan Sag, which has been confirmed by drilling, and offers valuable insights for buried hill exploration in the western Bohai Sea.