CBM a Pathfinder of Petroleum System?

Abstract

The present study aims to understand if there is any relation of commercial methane in CBM producers with oil producing kerogen. This is necessary because methane in coal, by default, is assigned to be sourced by bacterial/thermal actions on biomass during coalification process but critical analysis suggests it cannot be commercial because coal bears a ratio of molar concentrations of Hydrogen to Carbon (H/C) around 0.8 whereas methane require H/C = 4.0. A simple calculation reveals 1Tcf of methane generation may come from around 25 trillion tonnes of coal, suggesting commercial methane cannot be sourced only from coal. Generation of methane can be biogenic or thermogenic, either from coal or related organic biomass or from oil generating kerogen. Technically, we can distinguish biogenic methane from thermogenic but we cannot distinguish same methane either from coal or from oil generating kerogen unless it is evident that one is distinctly biogenic and the other is thermogenic. Further, a review of successful major CBM projects also reveals that most of them are geologically associated with some producing petroliferous basin. This suggests that a close genetic relationship between the occurrence of coal bed methane and petroliferous basin probably exists whereby adsorbed methane in coal bed is possibly sourced from the same oil generating kerogen in the basin. Therefore, discovery of commercial coal bed methane may suggest possible existence of mature source rock in the basin and may act as pathfinder of possible new petroleum system attached to it.