Publication Name: PESA's Eastern Australasian Basin Symposium II
Authors: R. Sykes, L.R. Snowdon and P.E. Johansen
Date Published: December 2004
Number of Pages: 39
Reference Type: Book Section
Abstract:
Late Cretaceous-Eocene coal measures have sourced waxy, paraffinic crude oils in the otherwise gas-rich Taranaki Basin, but incomplete understanding of organofacies controls on the generation and expulsion of non-volatile oil has hindered prediction of phase distribution. Multivariate analysis of the bulk chemistry, petrography and pyrolysis-gas chromatography of coals and coaly mudstones within the onshore, marginally mature, Late Cretaceous Rakopi Formation, has shown their paraffinic oil potentials to be controlled primarily by the abundance of leaf-derived liptinites-cutinite and liptodetrinite. Total liptinite content (7 .3-22. 7%) shows strong positive correlations with volatile and non-volatile paraffinic oil yields and the total paraffinic oil fraction, despite all samples being vitrinite-rich (74.1-91.0%). Hydrogen index, atomic H/C and volatile matter can be reliable indicators of paraffinic oil potential for humic coals that are not unduly affected by rank variation, marine influence or resin. Leaf-derived liptinite also influences modelled temperatures for initial oil generation, with high-liptinite samples commencing generation as much as l7?C lower than low-liptinite samples at a uniform heating rate of 3oC/my. Examples of solid bitumen expulsion from cutinite provide corroboratingevidence for early oil generation. The sampled coals seams are thin, typical of the Rakopi Formation, and formed in planar, rheotrophic mires rich in leafy, herbaceous vegetation. Comparison of pyrolysate compositions with those of other Cretaceous-Tertiary New Zealand and Indonesian coals indicates that thin, planar mire facies coals are more likely to produce waxy crude oils than are thick, raised mire facies coals, owing to better preservation of leaf biomass.