Integrated Fluid History Investigations in the Sydney and Gunnedah Basins, Australia

P. Joe Hamilton, Peter J. Eadington, G. P. Bai, Julian r

The history of fluid flow in the Permian and Triassic sediments in the Sydney and Gunnedah Basins of eastern Australia has been investigated through integrated petrographic, isotopic, and fluid inclusion analyses of diagenetic cements. This investigation has revealed a number of features common to each basin that relate to fluid flow events.

These features include 1) highly ¹⁸O-depleted compositions of early diagenetic cements; 2) precipitation of a wide range of carbonate minerals from the earliest to the latest stages of diagenesis; 3) the final diagenetic product, dawsonite: an hydrated sodium-aluminum carbonate; 4) hydrogen isotope compositions of diagenetic clays indicating post-crystallization isotopic exchange with an introduced meteoric water that was more D-enriched than the pore waters from which these clays originally formed; and 5) apatite fission track data and fluid inclusions that indicate past temperatures as much as 60° C higher than present-day formation temperatures.

These features are the results of fluid-flow processes that prevailed over much of the eastern Australian region and reflect the influence at different times of meteroic flushing subsequent to late Cretaceous, early Tertiary uplift and erosion, local heating associated with igneous activity, and CO₂ flushing. These events had a profound effect on reservoir quality and preservation potential for migrated oil.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995