POROSITY OF WILCOX GROUP COAL IN RELATION TO THE DISTRIBUTION OF BIOGENIC GAS, U.S. GULF COAST REGION

Pore characteristics of subbituminous coal samples from the Paleocene-Eocene Wilcox Group of Ouachita and Caldwell Parishes, Louisiana, and Zavala County, Texas, are examined in relation to desorbed gas content and other characteristics of the coal. Published studies suggested that Wilcox Group coalbed gas was primarily biogenic in origin. Desorbed gas contents of the samples in this study range (on as-received basis) from relatively high levels (up to 5.19 cm³/g) for the Caldwell Parish samples, medium levels (0.58 to 1.28 cm³/g) for the Ouachita Parish samples; and low levels (0.002 to 0.06 cm³/g) for the Zavala County samples. Twenty-seven samples from the three sites were analyzed for micropore (0-2 nm) and mesopore (2-50 nm) characteristics using a Micromeritics ASAP-2020 porosimeter and statistically evaluated.

For all of the Louisiana coal samples, desorbed gas content is positively correlated with micropore surface area (r = 0.48) and micropore volume (r = 0.58), indicating that micropore surface area and volume are critical factors in terms of gas-holding capacity. Micropore surface area and micropore volume also are positively correlated with the natural log of the carbon/ash ratio (r = 0.97 and 0.96, respectively), suggesting that mineral matter has a lower surface area than does organic matter and that its presence reduces the sorption capacity of coal. These results agree with published studies showing that mineral matter in coal reduces gas holding capacity. For all of the Louisiana samples, mesopore size decreases with depth (r = -0.82), probably as a result of increasing hydrostatic pressure and coal rank. Whereas the relatively shallow Ouachita Parish core samples (405.08 - 481.34 m depth) have an average mesopore size that is positively correlated with desorbed gas content (r = 0.80), the mesopore size of the deeper Caldwell Parish samples (513.65 - 820.61 m depth) is negatively correlated (r = -0.37) with desorbed gas. Overall, the low gas Zavala County samples are statistically different from the Louisiana samples, and no correlation is found between micropore surface area and desorbed gas content. This difference may be related to an increased depth of burial during foreland basin subsidence in south Texas or possibly environmental conditions that were unfavorable to microbial methane generation.