The Methane Content of San Juan Basin Coal: Implications for Coalbed Methane Resource Assessments Available to Purchase

The potential methane resource for the Fruitland coal formation of the San Juan Basin of northwest New Mexico has been analyzed. Gas contents ranging from 1 to 8 cm³/g were found for 33 samples from 14 wells which were at depths from 300 to 1000 m. Gas contents were determined using the "chip desorption" method. This method allows the use of drill cuttings obtained from conventional drilling operations as the coal seam is penetrated. The use of core samples which is required in the Bureau of Mines method is eliminated.

Contrary to past basin studies which used a limited number of data points, no direct correlation between depth and in-place gas content was evident. In order to explain the lack of a depth/gas content relationship, the physical properties of the coal relevant to gas adsorption were studied. Surface area measurements using low-pressure CO₂, CH₄, and neopentane adsorption have been conducted. Differences in adsorption capacity between the various samples is small and cannot explain the order of magnitude variation in gas content. In summary, it appears that local geological factors (i.e., the nature of the over and underburden) play a much larger role in the in-place gas content of coal than previously thought. This implies that a large number of data points is required to accurately characterize the total quantity of coalbed methane in a particular basin.