Hydrocarbon accumulation differences and exploration directions among sub-sags of the Taibei Sag, Tuha Basin, China

The Taibei Sag of the Tuha Basin contains three major hydrocarbon-generating sub-sags—Shengbei, Qiudong, and Xiaocaohu—in the Shuixigou Group. Although these sub-sags share similar tectonic–sequence–sedimentary evolutionary backgrounds, they exhibit distinct petroleum geological characteristics and hydrocarbon accumulation patterns as a result of differential uplift between the southern and northern orogenic belts. Based on a comprehensive analysis of structural evolution, source rocks, sedimentary reservoirs, and accumulation condition differences among the three sub-sags are identified mainly in four aspects. (1) During the Early to Middle Jurassic, the Taibei Sag maintained a relatively unified tectonic–sedimentary framework, internally segmented by local uplifts, with the Xiaocaohu sub-sag in the east serving as the primary depocenter. By the Late Jurassic, eastern uplift shifted the depositional focus to the Shengbei sub-sag. (2) During the main hydrocarbon accumulation stage of the Xiaocaohu sub-sag, source rocks had already reached a highly mature stage. Subsequently, as the Shengbei sub-sag deepened, both sags reached a mature to highly mature stage. Overall, the source rocks of the Shuixigou Group in the Taibei Sag are generally characterized by mature to highly mature hydrocarbon evolutionary stage. (3) The Shengbei sub-sag developed three provenance systems, among which the northwestern long-axis provenance system transported well-sorted sediments over long distances. In contrast, the Qiudong and Xiaocaohu sub-sags are dominated by bidirectional NS braided river delta systems. The southern provenance systems across all three sub-sags contain rigid clasts with strong compressive resistance, which is favorable for high-quality reservoir formation. (4) The Shuixigou Group experienced at least three stages of hydrocarbon accumulation. The Shengbei and Xiaocaohu sub-sags underwent slightly earlier hydrocarbon charging compared to the Qiudong sub-sag. Favorable exploration areas were assessed on the basis of accumulation patterns in the three sub-sags. Three key exploration frontiers are identified: tight sandstone reservoirs in sub-sags, lithologic–stratigraphic reservoirs in slope zones in the south, and structural reservoirs in piedmont buried zones in the north. These areas represent prioritized directions for near-term hydrocarbon exploration in the Taibei Sag.