Evaluating the gas content of coals and isolated maceral concentrates from the Paleocene Guasare Coalfield,Venezuela

This work presents the results from evaluating the gases sorbed by coal samples extracted from the Paleocene Guasare Coalfield (Marcelina Formation, northwestern Venezuela), as well as by their distinct maceral concentrates. The aim of this work has been to obtain an initial experimental main value of the gas content per unit weight of high volatile bituminous A coal samples from the open-pit Paso Diablo mine. An additional goal was to study differences in the CH₄ storage ability of the distinct maceral groups forming part of the coal matrix. Both the coal samples and the maceral concentrates were studied by thermogravimetric analysis (TGA) in order to determine the temperature to be used in subsequent experiments. On-line analyses of hydrocarbons (C₁, C₂, C₃) and CO₂ yielded gas concentrations, plus δ¹³C values. Thermogenic gas is prevalent in the Guasare coals with vitrinite reflectance (%R_o) values from 0.65% to 0.88%. The amount of gas retained in the coals and maceral concentrates was measured with a special device that allows determination of the volume of gas sorbed by a solid sample subjected to controlled thermal treatment. The average coalbed gas concentration obtained was 0.51 cm³/g. The following list of maceral concentrates shows the relative capacity for the volume of sorbed gas per unit weight: inertinite > low-density vitrinite > liptinite ≈ high-density vitrinite. It is concluded that the gas volumes retained in the distinct maceral concentrates are not controlled by porosity but rather by their microscopic morphology.