苏北盆地高邮凹陷深层戴南组一段（以下简称戴一段）三亚段下方为阜四段优质烃源岩，是极具潜力的资源接替层系，但其埋藏深、失利井多、构造储备圈闭少，长期以来形成了“难成藏、没潜力、没效益”的传统认识。为了明确戴一段三亚段的成藏关键和勘探潜力，综合钻井、测井、录井、实验、地震等资料，开展了层序地层、沉积体系、储盖组合、圈闭类型、成藏主控因素等多方面的基础研究。研究表明：戴一段三亚段是高邮凹陷断坳—断陷转换初期沉积的首套地层，特殊的地层发育位置使其具有“近源区盖、北洲南扇、孔缝双储、二元控藏”的油气富集特征。具体表现为6个方面：①戴一段三亚段顶部发育稳定的湖侵域泥岩，是独立的成藏单元；②戴一段三亚段下方为阜宁组四段烃源岩，位于阜宁组四段成熟烃源岩范围内，是戴南组供烃最充足的亚段；③北部缓坡带发育大型三角洲，南部陡坡带发育各类扇体，砂体发育程度呈东高西低的特征；④除构造圈闭外，还发育4种隐蔽圈闭类型，隐蔽圈闭个数多、规模大；⑤发育孔隙和裂缝两类储集空间；⑥侧向封堵和有效储层是成藏主控因素。研究新认识带来了该层系勘探思路的两大转变，指明了勘探方向，部署一批探井获得成功，推动高邮凹陷戴一段三亚段成为苏北盆地重要的资源接替阵地。

The third submember of the first member of Dainan Formation in the deep layers of Gaoyou Sag, Subei Basin, which overlies high-quality source rocks of the fourth member of Funing Formation, serves as a potential replacement for oil and gas resources. However, due to its deep burial depth, numerous failed wells, and limited structural reservoir traps, it has long been perceived as “difficult for accumulation, unpromising, and unprofitable”. To clarify the key factors for oil and gas accumulation and the exploration potential of the third submember of the first member of Dainan Formation, using drilling, well logging, mud logging, experimental data, and seismic data, fundamental studies were conducted on multiple aspects, including sequence stratigraphy, sedimentary systems, reservoir-seal combination, trap types, and main controlling factors of oil and gas accumulation. The results showed that the third submember of the first member of Dainan Formation was the first sedimentary strata at the early stage of the transition from fault sag to fault subsidence in Gaoyou Sag. Its unique stratigraphic development position endowed it with oil and gas accumulation characteristics of “near-source sealing, northern delta and southern fan, dual pore-fracture reservoirs, and dual control on reservoirs”. Specifically, it was manifested in six aspects: (1) The top of the third submember of the first member of Dainan Formation was characterized by stable transgressive lacustrine mudstone, serving as an independent oil and gas accumulation unit. (2) Underlying the third submember of the first member of Dainan Formation, the fourth member of Funing Formation was located within the mature source rock zone, making it the most oil and gas-rich submember in Dainan Formation. (3) A large delta developed in the northern gentle slope and various types of fan bodies in the southern steep slope. The sand body development showed an east-high-west-low distribution pattern. (4) In addition to structural traps, four types of subtle traps developed abundantly in large scale. (5) Two types of reservoir spaces—porosity and fractures—developed. (6) Lateral sealing and effective reservoirs were the main controlling factors for reservoir formation. The findings from the research brought two major shifts in exploration strategies for this stratigraphic sequence, identified exploration directions, and led to successful drilling of multiple exploration wells. This promotes the third submember of the first member of Dainan Formation in Gaoyou Sag to become an important resource replacement area in Subei Basin.