Bedded humic coals generate, store, and expel large amounts of gas when they achieve ranks greater than those approximating a high-volatile A bituminous containing 37.8% volatile matter. Gas volumes generated in excess of temperature- and pressure-dependent storage capacity are expelled into nearby sandstone reservoirs. High-volume rates of generation and/or expulsion lead to associated high formation fluid pressures both in the coals and associated sandstones. Lowering of temperature may decrease the generation rate and increase the coal storage capacity resulting in reabsorption of previously generated gas from adjacent sandstones. Cooling-related reabsorption processes may contribute to the formation of fluid under-pressures. The generation-expulsion and cooling-reabso ption processes may cause pressure cycles within a basin that create and control both overpressured and underpressured "basin bottom" gas accumulations in complex and interrelated coal-bed and sandstone reservoirs. Examples of the phenomena are present in the Mesaverde Group of the Green River and San Juan basins, Wyoming and New Mexico.

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