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This investigation combines traditional and newly available investigative techniques to characterize the hydrocarbon system of the Fruitland Formation coals, both at outcrop and in the subsurface. These analyses indicate that the Fruitland coal hydrocarbon system began with Late Cretaceous–early Tertiary deposition and maturation of the coal source rocks; Late Cretaceous–early Paleocene tilting of the basin; Eocene uplift, exposure, and erosion of the basin margins; Eocene groundwater recharge, which maintained hydrodynamic pressure in the reservoirs; and continued uplift, which caused occlusion of permeability to occur ca. 35 Ma. Present-day erosion is slowly breaching biosome-scale reservoirs and allowing methane to escape to the atmosphere at the outcrop. Oligocene opening of the Rio Grande rift changed the stress regime of the San Juan Basin, allowing fractures to open and fluid to migrate from pre-Cretaceous rocks to the surface.

Outcrop seeps have been ongoing throughout Recent geologic time and probably have been active since the coals were first exposed at the outcrop. Methane production from the coal in deeper parts of the basin has not contributed to methane gas seeps at the outcrop. Our analysis calls into question hydrologic assumptions regarding the flow of water in coalbed aquifers and finds that a reexamination of coalbed aquifers in other basins is also warranted.

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