Oil generation capacity of coals as a function of coal age and aliphatic structure

Abstract

The relative proportions of aliphatic CH 2, CH 3, aliphatic hydrogen (H ali) together with the aliphatic chain length in a series of Carboniferous, Permian, Jurassic and Cenozoic humic coals have been analysed using FTIR and ruthenium tetroxide catalysed oxidation (RTCO) followed by GC–MS analysis of the n-monocarboxylic acids (esterified) to assess their ability to generate oil. Most of the coals contain 2–11 vol% liptinite. Only coals with almost similar Hydrogen Indices (HIs) were compared as they were considered to possess comparable hydrocarbon generation potentials. All contain isolated aromatic hydrogen (H aro) in the coal structure and a considerable proportion of aliphatic chains in the range C 16–18. HI values cannot alone explain the proportion of aliphatic CH 2, implying that for coals, H aro contributes to the measured HI. Cenozoic coals contain the highest proportion of aliphatic CH 2 and H ali, and the content of longer chain aliphatics (>C 18) increases considerably from Palaeozoic to Cenozoic coals. Aliphatic chains >C 18 are absent from,or very restricted in, the Carboniferous coals and most of the Permian coals indicate no or a negligible ability to generate oil. Jurassic coals show a slightly better potential, whereas Cenozoic coals contain aliphatic chains up to at least C 35, demonstrating the greatest capacity to generate oil in addition to significant gas potential. Multivariate data analysis suggests that the aliphatic chains >C 24 are related to the collodetrinite + liptodetrinite matrix, which is a prominent constituent of Cenozoic coals. With regard to oil potential, the coals from the major coal-forming periods may be ranked: Carboniferous ⩽ Permian < Jurassic ≪ Cenozoic. This suggests an overall floral control on oil generation potential, linked to evolution from the relatively primitive Carboniferous plants towards the complex angiosperm Cenozoic floras. These results agree with the distribution of known oil and gas accumulations sourced from coals of different age.