New Zealand Journal of Geology and Geophysics abstracts
Maturity-related variations in the bitumen compositions of coals from Tara-1
and Toko-1 wells
STEVE KILLOPS
Institute of Geological & Nuclear Sciences
P.O. Box 30 368
Lower Hutt, New Zealand
PAUL WALKER
DAVE WAVREK
Humble Geochemical Services
P.O. Box 789
Humble, TX 77347, USA
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.
Keywords aromatic hydrocarbons; bitumen; coal; maturity;
n-alkanes; rank
New Zealand Journal of Geology & Geophysics, 2001, Vol. 44:
157-169
0028-8306/01/4401-0157 $7.00/0 (c) The Royal Society of New Zealand
2001
PDF file of entire paper: medium quality (1111K); (scanned from paper original: notes about this process)
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