Geological characteristics, favorable accumulation factors, and developmental models of deep coal-rock gas in complex fault-bounded basins: A case study of the Dagang exploration area in the Bohai Bay Basin

In recent years, deep coal-rock gas has gradually become a popular target for the exploration of unconventional hydrocarbons. In China, the deep coal-rock gas exploration is concentrated in the Ordos Basin, where breakthroughs in the exploration and production pilot tests have been achieved. In contrast, the exploration of deep coal-rock gas in the Bohai Bay Basin remains in its initial stage. Using geologic data of coal seams obtained from core observations and well logging, we systematically analyze the characteristics and favorable accumulation factors of the deep Upper Paleozoic coal seams in the Dagang exploration area. Furthermore, the mechanisms underlying the gas generation, storage, and preservation in deep coal seams are explored and the developmental model of the deep Upper Paleozoic coal-rock gas is thereby clarified. The results indicate that the Dagang exploration area contains multiple favorable sweet spot intervals for the deep Upper Paleozoic coal-rock gas occurrence, where the coal seams exhibit high organic matter abundance, moderate thermal maturity, and high gas-generating intensity. In terms of pore structure, these coal seams manifest multi-scale pores and fractures, with the large-scale, intensively distributed microfractures providing effective storage spaces. The roofs and floors of the coal seams are composed predominately of mudstones, creating favorable preservation conditions. The coal seams experienced two large-scale gas-generating processes under the influence of multistage tectonic movements. Against the backdrop of complex fault-bounded depressions, the coal seams exhibit weathering-degradation zones, saturated adsorption zones,and structural hydrocarbon accumulation zones, with its deep parts characterized by supersaturated gas-bearing properties and coexistence of adsorbed and free gas. Based on these findings, we establish a developmental model for the complex accumulation of deep coal-rock gas in a complex fault-bounded basin. The theoretical research and exploration practices of the deep Upper Paleozoic coal-rock gas in the Dagang exploration area serve as a significant guide and reference for the exploration and development of deep coal-rock gas in the Bohai Bay Basin.