Abstract: A Biomarker and Stable Isotope Appraisal of Oil Generation in the Taranaki Basin, New Zealand

S. D. Killops, R. G. Allis, R. H. Funnell

A basin-wide organic geochemical evaluation of source rock characteristics and the genetic relationships of oils has been undertaken. The waxy nature of many oils and high hopane contents suggest that leaf cuticles and bacterial remains are important kerogen components. Among the primarily terrestrial oils, varying contributions from Eocene, Paleocene and Upper Cretaceous source rocks can be recognized, reflecting the rise to dominance of angiosperms over gymnosperms. A low-level, marine influence is also detectable in these oils. With the exception of Maui oils, inferred relative contributions from the main source rock types generally correlates with the relative proportions of suitably thick and mature units near reservoirs, suggesting lateral migration distances are limited. Maui o ls appear to be primarily sourced by Upper Cretaceous coals lying to the east or northeast, whereas in the northern part of the Tarata thrust zone middle-upper Eocene coals seem to be the chief source of oils. Onset of oil expulsion from coals may occur at a maturity level corresponding to a vitrinite reflectance of 0.8% R_o, and may be aided by the evolution of large volumes of supercritical carbon dioxide. The terrestrial influence on Paleogene source rocks diminishes to the north-northwest of the basin, and a marine phytoplankton signature, characterized by high levels of C₃₀ steranes and associated with a upper Paleocene shale, is present in some oils. It is also possible that marginal marine shales interbedded with coals also contribute to oil generation through ut much of the basin.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994