Origin and accumulation models of ultra-low permeability-tight sandstone (gravel) gas in Bohai Bay Basin, China

Ultra-low permeability–tight sandstone (gravel) gas reservoir is an important exploration target for natural gas both domestically and internationally. As a crucial oil and gas exploration basin in China, the Bohai Bay Basin has discovered tight sandstone (gravel) gas in every depression. However, the distribution, origin, and accumulation model of tight sandstone (gravel) gas reservoirs were not systematically studied by previous researchers, who instead mainly concentrated on depressions from the basin-wide perspective. The research conclusively demonstrates that ultra-low permeability–tight sandstone (gravel) gas reservoirs are widely distributed in the Bohai Bay Basin, covering various depressions and multiple strata of Paleozoic, Cenozoic, and Upper Paleozoic coal-formed gas as well as Paleogene oil-type gas. The Upper Paleozoic ultra-low permeability–tight sandstone gas reservoirs are mainly distributed in residual Upper Paleozoic strata of the Huanghua, Linqing, and Jiyang depressions. Most of those reservoirs, which are located in uplifted sag zones, are structural gas reservoirs, and because the strata were buried deeply in the past, most of the reservoirs have worse physical properties. This natural gas is typical coal-formed gas originating in Carboniferous and Permian, and others are Paleogene oil-type gas. Paleogene ultra-low permeability–tight gas reservoirs are widely distributed in all depressions, including the multiple strata in the Kongdian Formation and the second, third. and fourth members of the Shahejie Formation (Es₂, Es_3, and Es₄); The gas reservoirs are located in the uplift, slope, and steep slope zones of the sags, including a variety of trap types, such as lithology, lithology-structure, structure reservoirs, and others. Sandstone reservoirs often found in the Es₂ and Es₃ are distributed in gentle slopes, whereas sandstone and glutenite reservoirs can be found in the Es₄ and Kongdian Formation (Ek), with the glutenites mainly distributed in steep slope area. Since the Paleogene is a continuous deposit, burial depth appears to be a controlling factor in the reservoirs physical properties. Densification threshold depths for distinct depressions range from 3200 m to 4000 m. The majority of the natural gas is gas formed from Paleogene oil-type, while some of it is gas formed from Upper Paleozoic coal. Four different hydrocarbon accumulation models have been developed in this area in accordance with the relationship between gas sources and reservoirs. These models include migration along fault with old source rock and new reservoir, near-source accumulation with new source rock and new reservoir, distant-source accumulation with old source rock and old reservoir, and migration along fault with new source rock and old reservoir, of which the first two are the main hydrocarbon accumulation models. Two sets of Carboniferous–Permian and Paleogene strata are developed, and the remaining natural gas resources have great potential in the Bohai Bay Basin. The Carboniferous–Permian beneficial zone of secondary hydrocarbon generation, the structures adjacent to the margin of the Paleogene gas generation center, and the sweet spot in lithology reservoirs would all be promising areas for further exploration. The research is of instructive significance for tight sandstone (gravel) gas exploration in the Bohai Bay Basin.