Mechanisms of deep coalbed methane occurrence: Review and perspectives

Abstract

Deep coalbed methane occurs in geological settings characterized by high stress, high temperature, and high pressure, commonly referred to as “three-high” conditions. Owing to differences in genetic types, reservoir properties, and gas phases, its occurrence mechanisms differ substantially from those of shallow coalbed methane. Existing studies have generally reached a consensus on its occurrence boundaries and enrichment patterns. Based on an extensive literature review and representative cases from global basins, this review synthesized the genetic classifications, reservoir property evolution, coupled temperature-pressure-stress effects, hydrodynamic sealing conditions, and enrichment patterns of deep coalbed methane. A comprehensive formula for determining critical depth was proposed by integrating stress transition depth, adsorption capacity inflection point, and multiple geological parameters. This formula can be applied to regional-scale predictions and data-limited conditions. A coupled fracture-seepage–matrix-diffusion equation was established, and a revised productivity prediction model was developed. Existing evidence indicates that deep coalbed methane commonly forms through multistage hydrocarbon generation. It is dominated by thermogenic gas, with minor biogenic contributions. Gas mainly occurs in the adsorbed state, whereas the proportion of free gas increases with burial depth. Its enrichment is governed by structural style, cap rock sealing capacity, and fracture evolution. Critical depth indicators that were previously evaluated independently were integrated into a comprehensive multifactor criterion. The occurrence characteristics and migration behavior of deep coalbed methane were elucidated. The main controls on reservoir productivity were identified. These results provide theoretical support for efficient exploration and targeted exploitation of deep coalbed methane resources.