The Yingen-Ejinaqi Basin (also referred to as the Yin'e Basin) represents a Meso-Cenozoic faulted lacustrine basin occurring on the pre-Mesozoic metamorphic folded basement. In 2025, high-yield oil flow with a daily oil equivalent exceeding 400 m³ was obtained from well G18 drilled in the hinterland of the basin. In this study, based on the latest exploration outcomes from wells G6 and G18, we investigate the factors governing the reservoir formation, as well as the hydrocarbon properties and sources, of hydrocarbon reservoirs in the metamorphic folded buried hills within the Yin'e Basin. Accordingly, the major factors controlling the hydrocarbon enrichment and high productivity of the buried-hill hydrocarbon reservoirs are analyzed, and the hydrocarbon accumulation pattern of these reservoirs is established. Field geological explorations and laboratory tests indicate that dolomite marbles represent the dominant lithofacies for reservoir formation in bedrock buried hills. These marbles contain three-stage fractures due to the modification by multi-stage tectonic activities. Reticular storage complexes are formed by the coupling of pores, fractures, and vugs, with the distribution of favorable reservoirs primarily controlled by the coupling of regional dynamic metamorphism and deep-seated faults. The test and analytical results of trace elements reveal that the 1st member of the Bayin Gobi Formation (also referred to as the Ba 1 Member) was deposited in a weakly reducing, brackish-water environment. Such a sedimentary setting created favorable conditions for the development and preservation of organic matter, leading to the formation of lamellar shales with organic matter dominated by type Ⅱ₁ kerogen. These shales are characterized by high total organic carbon (TOC) content, early-stage oil generation, and late-stage gas generation. Controlled by the depocenters of sub-sags, these shales primarily occur in the middle and lower parts of slope zones. The hydrocarbon enrichment and high productivity of hydrocarbon reservoirs in metamorphic folded buried hills in the Yin'e Basin are attributed to three key factors. First, multi-stage faulting, along with weathering and leaching, plays a key role in the reservoir formation of dolomite marbles. Second, the distribution of high-quality source rocks in the middle and lower parts of slope zones determines the hydrocarbon generation, migration, and enrichment of the buried-hill hydrocarbon reservoirs. Third, sustained and intense proximal hydrocarbon charging is identified as the key factor contributing to the high productivity of these reservoirs. The key reason for exploration breakthroughs in the buried-hill hydrocarbon reservoirs is the integrated application of multi-source information fusion technologies.

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.