INTRUSION GEOMETRY AND COALBED METHANE GENERATION: REGIONAL THERMAL EFFECTS OF VOLUMETRICALLY MINOR SILLS IN THE RATON BASIN, COLORADO – NEW MEXICO

Several previous studies exploring the generation of coal-bed methane (CBM) and elevated coal rank in the Raton Basin have suggested that the timescale and depth of burial may be insufficient for burial alone to account for the high coal rank, particularly in the central part of the basin (around the Purgatoire Valley). Other potential heat sources include circulating groundwater, or magmatic activity, whose thermal effects include both direct (conductive) and hydrothermal (convective) heating.

Previous studies have focused on coal destroyed in the thermal aureole to the Spanish Peaks intrusions and associated radial dike swarm in the northern part of the basin. Roughly east-west trending basalt and lamprophyre dikes and associated sills are present throughout the entire basin. These dikes and sills are typically of the order of 2 m thick, and hence are volumetrically minor, so that their regional significance for thermal maturity of basin sediments has been generally downplayed.

Field observations from this and previous studies show that there is a strong correlation between sills and coal beds. Sills preferentially intrude coals of the Upper Vermejo and Lower Raton Formations throughout the Purgatoire Valley and southern portions of the Raton Basin. Due to their geometry, sills play a much greater role in increasing regional coal rank than dikes, because an entire bed may be thermally affected by a single thin intrusion.

Simple one-dimensional thermal models for cooling igneous sheets, combined with published estimates of the kinetics of methane-generating reactions, indicate that methane production will be extensive within a zone corresponding to one intrusion half-width, and significant out to more than one full intrusion width. Intruded coal beds are typically 1 to 3 m thick, similar to the thickness of intruding sheets, and hence large volumes of CBM were probably generated from these “minor” intrusions. The effects of intrusion on CBM generation are likely to be even greater than indicated by these preliminary calculations, because (i) coal has a lower thermal diffusivity than other sedimentary rocks in the basin (shale and sandstone), and (ii) dikes in particular may act as long-lived conduits. Both of these factors will tend to increase the integrated temperature-time index of coal adjacent to intrusions.