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Abstract
The composition of bitumen extracted from two suites of vitrinite‐rich New Zealand coals spanning a rank range associated with the generation and expulsion of oil is reported, together with implications for the factors controlling both the expulsion of oil from coals and the various molecular ratios routinely used as maturity parameters. Solvent‐extracted bitumen accounts for a small proportion of the free hydrocarbons present in the coals, but the distribution of both compound classes and components within each compound class in solvent extracts appears representative of the bulk of free hydrocarbons, upon comparison with thermal extracts. Although no obvious increase was observed in solvent‐extracted bitumen or its constituent saturates and n‐alkanes with increasing maturity, there were changes in the carbon‐number distributions of n‐alkanes indicative of the generation and expulsion of n‐alkanes at Rank(Sr) values >12. The amount of aromatics increased sharply at Rank(Sr) c. 13 in the Tara coals, and bulk compositional changes suggest that paraffinic components were expelled at that rank. The decline in carbon preference index (CPI) over the Rank(Sr) range 11–13 in the Tara coals could be explained by the generation of modest additional amounts of n‐alkanes, if expulsion occurred at the same rate as generation leading to the observed approximately constant n‐alkane concentrations of c. 1 mg/ g Corg recovered by solvent extraction. An alternative mechanism is that the generated n‐alkanes may remain largely within closed pores, inaccessible to solvent extraction, although some leakage into open pores would be required to account for the observed changes in n‐alkane distributions. Generation of aromatic compounds, which exhibit a greater adsorption affinity for coal than saturates, may aid expulsion, which could account for the association of the onset of significant paraffinic oil expulsion with increase in aromatic hydrocarbon concentrations in the Tara coals at Rank(Sr) c. 13. Only after significant amounts of thermally generated aromatics have accumulated and/or displaced pre‐existing aromatics do maturity‐related trends become established. These trends are masked at Rank(Sr) values < 13 by the free aromatic hydrocarbons inherited from diagenesis.