Abstract:

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.

Key words: Subei Basin, Gaoyou Sag, deep layer of Dainan Formation, third submember of the first member of Dainan Formation, main controlling factors, oil and gas accumulation characteristics, subtle oil and gas reservoirs