Major exploration breakthroughs and main controlling factors of hydrocarbon accumulation and enrichment in the northern steep slope zone of the Haizhong Sag, Beibuwan Basin

Despite over 40 years of hydrocarbon exploration in the Haizhong Sag of the Beibuwan Basin, no substantial breakthroughs had been achieved until the successful drilling of two exploration wells: inclined well Hai 3 and well Hai 301. To determine the major factors controlling hydrocarbon accumulation and enrichment in the northern steep slope zone of the Haizhong sag (also referred to as the Haizhong steep slope zone), we thoroughly investigate the relationships of hydrocarbon accumulation and enrichment with source rocks, reservoirs, hydrocarbon transport, and overpressure using the latest available data. The results indicate that high-quality source rocks of deep lacustrine facies are widely developed in the Liushagang Formation, Haizhong Sag, with hydrocarbon resources estimated at 2.5×10⁸ t of oil equivalent, demonstrating great potential for sustained exploration. Large-scale, high-quality beach-bar sandstone reservoirs occur within the Haizhong steep slope zone, where beach-bar sand bodies exhibit vertical superimposition and lateral contiguous distribution. Among these, beach-bar sandstone reservoirs with relatively coarse grains show low matrix content and strong dissolution and have undergone fracturing modification, thereby featuring medium porosity and medium-low permeability. The slope zone contains an efficient three-dimensional fault-fracture transport system consisting of the No.3 fault and its associated faults, as well as fractures. The major active period of these faults aligns with the peak hydrocarbon expulsion of source rocks, ensuring efficient hydrocarbon charging. The extensive development of high-quality source rocks provides a resource foundation for hydrocarbon accumulation in the Haizhong steep slope zone. Regarding the hydrocarbon enrichment in this zone, the occurrence of large-scale high-quality beachbar sandstone reservoirs serves as a prerequisite, while the presence of an efficient overpressured fault-fracture transport system is identified as a key factor. Additionally, hydrocarbon charging under strong driving forces in an overpressure environment creates favorable conditions for hydrocarbon accumulation and enrichment.