New insights and exploration breakthroughs in hydrocarbon exploration in sub-sag zones of the Dongpu Sag, Bohai Bay Basin

Over more than four decades of hydrocarbon exploration in the Dongpu Sag within the Bohai Bay Basin, the sub-sag zones, critical targets for hydrocarbon exploration, have accounted for 49 % of the total exploration area. However, their proven geological reserves constitute only 1 % of the total, highlighting an exploration dilemma characterized by abundant residual hydrocarbon resources, low proven geological reserves, limited oil production despite tested oil flow, and difficulties in expanding the development of discovered hydrocarbon resources. Based on the increasingly intensive investigation of hydrocarbon enrichment patterns in recent years, we systematically explore the source rocks, source rock-reservoir configurations, and hydrocarbon accumulation sequences in different types of sub-sags within the Dongpu Sag while integrating various data from core observation, logging, analyses, and laboratory tests. The results indicate that: ① effective source rocks with relatively small thicknesses and high organic matter abundance (TOC content > 1 %) serve as the major source rocks in the sub-sag zones. This finding challenges the traditional understanding that source rocks are widely developed in sub-sags, facilitating the delineation of exploration targets; ② High-frequency lake-level fluctuations have resulted in a unique sedimentary rhythm of alternating sandstone and mudstone layers. The vertical superimposition of effective source rocks and sand bodies, along with the contiguous distribution of reservoirs near sub-sags, overturns the traditional view that sub-sags contain well-developed mudstones while lacking well-developed reservoirs; ③ Hydrocarbon charging and fluid overpressure are identified as the dominant factors governing the formation of reservoirs near sub-sags, significantly extending the lower depth limit of effective reservoirs. This challenges the long-standing perception that the so-called “death boundary” for the formation of effective reservoirs in continental clastic rocks is 3500 m; ④ Additionally, hydrocarbon reservoirs in the sub-sag zones feature sequential hydrocarbon accumulation, contradicting the traditional belief that no hydrocarbon reservoirs occur below aquifers. Based on these insights, exploration breakthroughs have been achieved, including high-yield hydrocarbon flow exceeding 100 m³ oil-gas valent weight on a daily basis in the Gegangji Sub-sag, kilometer-thick oil-bearing intervals in the southwestern sub-sag zone, and the shale/tight oil resources at the hundred-million-ton level in the Qianliyuan Sub-sag. These achievements have driven the strategic shift in hydrocarbon exploration from tectonic zones to sub-sag zones in the Dongpu Sag, facilitating the delineation of exploration targets for large-scale reserve growth. The results of this study provide valuable guidance for hydrocarbon exploration in sub-sag zones within the Bohai Bay Basin.