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Formation mechanism of coal-rock gas accumulation transition zones
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DOI：10.1016/s1876-3804(26)60717-3.png)
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Abstract
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Research on hydrocarbon accumulation mechanism in coal-measure whole petroleum systems and coal-rock gas exploration has revealed that coal-rock gas occurs along coal-bearing strata and forms different reservoir types with distinct geological characteristics in different structural regions, and that between primary coal-rock gas reservoirs and residual coal-rock gas reservoirs (i.e., traditional shallow coalbed methane reservoirs), the gas–water relationship in the macroscopic pores and fractures of coal reservoirs gradually evolves in space, forming coal-rock gas accumulation transition zones with prominent features. Taking the coal-rock gas of the Carboniferous Benxi Formation in the central-eastern Ordos Basin, China, as an example, this study constructs the dynamic mechanism and mathematical models for the coal-rock gas accumulation transition zone by considering the dual-medium structure, stress sensitivity, and wettability variation of coal reservoirs. On this basis, the dynamic equilibrium among buoyancy, capillary force and hydrocarbon-generation expansion force is clarified. The results show that, under the geological background of uplift and reworking, the cleat fracture system and porosity–permeability properties of coal reservoirs are fundamental for the formation of the transition zone, the hydrocarbon-generation expansion force of coal rocks is an inherent determinant for the depth of the transition zone, and the spatial variation of coal rank and tectonic reworking intensity control the distribution of the transition zone. Numerical simulation results and practical exploration and development have demonstrated that, at the eastern margin of the Ordos Basin, the transition zone for medium- to high-rank coal-rock gas reservoirs in the Daning–Jixian area in the south is mainly buried at 1 300–1 800 m, while the transition zone for low- to medium-rank coal-rock gas reservoirs in the north occurs at a greater depth of 1 500–2 300 m. Through comprehensive investigation, the coal-rock gas accumulation zones at the eastern margin of the Ordos Basin are examined by hydrocarbon accumulation evolution analysis and spatial distribution prediction. Three continuous hydrocarbon accumulation evolution units are identified, i.e. deep coal-rock gas accumulation zone, coal-rock gas accumulation transition zone, and shallow coalbed methane accumulation zone. The research results deepen the understanding of accumulation mechanism and differential enrichment mechanism of coal-rock gas, and provide a theoretical basis for improving coal-rock gas classification and guiding favorable area evaluation and efficient exploration and development.
Keywords:
coal-measure whole petroleum system
coal-rock gas
dynamic mechanism
coal-rock gas accumulation transition zone
Ordos Basin
favorable area evaluation
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