New exploration fields and resource potential of tight oil and gas in Beibu Gulf Basin

Beibu Gulf Basin is an important oil and gas exploration and production area in the western South China Sea. In recent years, the medium-deep tight oil and gas reservoirs have become new exploration targets. To determine the hydrocarbon accumulation and regional enrichment patterns of tight oil and gas and further guide subsequent exploration and development, the paper studies the hydrocarbon source conditions, reservoir characteristics, and accumulation models of tight oil and gas in Beibu Gulf Basin, and summarizes the accumulation rules. The study finds that multiple phases of tectonic movements play a role in controlling the structure and sedimentary filling of the basin. The oil shales and medium-deep lacustrine mudstones in Member 2 of the Eocene Liushagang Formation in Paleogene Period are the most important source rocks in the basin. Additionally, multiple sets of source rocks develop in Member 3 of Liushagang Formation. Tight oil and gas reservoirs are mainly distributed in Member 2 and 3 of Liushagang Formation in Weixinan sag and Wushi sag. The reservoir types are dominated by underwater distributary channels of fan deltas and braided river deltas, and beach-bar systems. The main reasons for the formation of tight reservoirs include strong compaction and diagenesis under high geothermal gradients due to deep burial. Sweet spots of tight reservoirs are distributed in areas with the development of early overpressure and microfractures adjacent to sags. There are mainly three tight oil and gas accumulation modes:"vertical downward filling" and "laterally continuous filling under source-reservoir contact mode" in zones near the sag, and "three-dimensional stepped vertical and lateral filling" in areas far from the sag. Overall, hydrocarbons are enriched near sags or along nose-like structural belts. The tight oil and gas reservoirs in Member 2 and 3 of Liushagang Formation in the eastern Weixinan sag and Wushi sag represent important new types and fields of exploration in Beibu Gulf Basin, where the resource potential amounts to 2.1×10⁸m³.