Influence of in-situ bitumen on the generation of gas and oil in New Zealand coals

New Zealand coals of peat to high volatile bituminous rank have been studied in order to assess the effect of in-situ bitumen (soluble in a methanol:acetone:chloroform azeotrope), on petroleum potential during progressive maturation. Both non-extracted and extracted coals, as well as the extracted bitumen itself, were analyzed using Rock-Eval and pyrolysis gas chromatography. The difference in yields between non-extracted and extracted coals, attributable to the influence of bitumen, were compared with direct measurements made on the bitumen itself. Major compositional changes in pyrolysis products were seen when the bitumen was finely dispersed throughout the kerogen matrix. Notably, second-order reactions between kerogen and bitumen during pyrolysis reduce primary gas yield and enhance both oil yield and potential secondary gas yield. The stabilization of free radicals, and prevention of recombination/repolymerization processes have been attributed to the presence of the dispersed in-situ bitumen. These results demonstrate that petroleum formation in coals likely proceeds via intermolecular interactions and not simply first order cracking. The common procedure of employing solvent extraction prior to pyrolysis is clearly not appropriate when assessing the petroleum potential of coal.