New geological insights and exploration breakthroughs of buried-hill hydrocarbon reservoirs in the Yingen-Ejinaqi Basin

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.