Paleofloral dependence of coal methane sorption capacity

Abstract

The aim of this study is to reveal key physiochemical factors controlling gas sorption in coals during periods of dramatic coal deposition in Earth's history – the Upper Carboniferous (326–299 Ma) and Permian (299–252 Ma). As a starting point we developed a database of about 1000 entries, including coal-specific parameters such as proximate and ultimate analysis, sample origin, and specific surface area. Our study proposes an innovative approach that links emerging, quantitative organic molecular analyses with more established bulk (bio)geochemical techniques as a context for (1) describing gas–coal (i.e., coalbed methane) formations, (2) sweet-spot identification in terms of methane sorption potential. We found that the main controlling parameter is coal age; Permian coals generally adsorb significantly more methane than Carboniferous coals at comparable maturity. This difference is reflected in micropore surface area and oxygen content of the two sets, with Permian having higher values than the corresponding Carboniferous coals. We attribute this difference in Langmuir volume or surface area to the original plant material, expressed through differences in the molecular structures and surface chemistries of the sorption sites which is also clearly reflected in a lower aromaticity for Permian coals. This leads to a more detailed view on the evolution of original plant material from the Carboniferous to Permian age, coinciding with an evolution in plant material, specifically lignite, changes in climatic conditions towards more arid environments, possible changes in water compositions and bacterial activities.