Coal-bed Methane in the Raton Basin, Colorado and New Mexico: Thermal Effects of Mafic Intrusions on Coal Maturation and Coal-bed Methane Generation

Jennifer Cooper
University of Missouri-Columbia, Department of Geological Sciences
Columbia, Missouri
[email protected]

The Raton Basin contains significant coal-bed methane resources generated from Cretaceous and Tertiary coals. Previous studies suggest that elevated coal ranks in the central basin cannot be attributed to burial coalification alone, due to the relatively short time and shallow depth of burial, requiring other heat sources such as hydrothermal circulation or igneous activity.

This project will quantify the role of individually small, but ubiquitous, Tertiary mafic sills and dikes in the generation of coal-bed methane. Abundant field evidence demonstrates that interaction between coal and sheet intrusions is extensive. Many sills preferentially intrude coal. Evidence for coal pyrolysis is common, including coal dikes cross-cutting sills. Pyrolysis should coincide with release of methane and other volatiles. One-dimensional thermal calculations indicate that methane generation due to a single intrusion will be extensive within one half-width and significant for approximately one intrusion width away. On a basin-scale, sills may be important due to the volume of coal they affect. Multiple intrusions will produce an additive effect, where the surrounding coal will undergo more extensive maturation than coal surrounding a single intrusion.

To quantify the contribution of intrusions to increased coal rank and methane generation within the basin, coal samples for vitrinite reflectance were collected in detailed profiles across 5 dikes and 11 sills, including 4 outcrops with multiple intrusions. Selected intrusion profiles will be analyzed for δ13C of individual macerals and bulk coal to test the utility of simple mass balance calculations in estimating the quantity of coal-bed methane generated due to intrusions.

AAPG Search and Discovery Article #90033©2004 AAPG Foundation Grants-in-Aid