Quantification of Exhumation from Sonic Velocity Data, Cooper Basin, Australia, and Implications for Hydrocarbon Exploration

Abstract

Exhumation (defined as rock uplift minus surface uplift) in the Cooper Basin of South Australia and Queensland has been quantified using the compaction methodology. The sonic log, which is strongly controlled by the amount of porosity, is an appropriate indicator of compaction, and hence is used for quantifying exhumation from compaction. The traditional way of estimating exhumation based on the degree of overcompaction of a single shale unit has been modified and five units ranging in age from Permian to Triassic have been analysed. The results reveal that exhumation increases eastwards from the South Australia into the Queensland sector of the basin. The results show that exhumation in Late Triassic -Early Jurassic times, after the Cooper Basin deposition, seems to be 200-400 m higher than exhumation in Late Cretaceous -Tertiary times, after the Eromanga Basin deposition. This study has major implications for hydrocarbon exploration. Maturation of source rocks will be greater for any given geothermal history if exhumation is incorporated in maturation modelling. Exhumation values can also be used to improve porosity predictions of reservoir units in undrilled targets.

Review of Exhumation and Implications for Hydrocarbon Exploration in Australia

“…The Cooper Basin, underlying the Eromanga Basin was, of course, subject to the same Late Cretaceous -Tertiary exhumation that is witnessed by the overcompaction of the units analyzed in the Eromanga Basin sequence. However, the Cooper Basin appears, at least locally, to have been subjected to a more major period of exhumation in Late Triassic -Early Jurassic times, prior to the deposition of the Eromanga Basin [3,4]. Given a ±200m error on apparent exhumation values from individual units, the excess of Cooper Basin apparent exhumation over that of the Eromanga Basin is within error limits [4].…”

“…However, the Cooper Basin appears, at least locally, to have been subjected to a more major period of exhumation in Late Triassic -Early Jurassic times, prior to the deposition of the Eromanga Basin [3,4]. Given a ±200m error on apparent exhumation values from individual units, the excess of Cooper Basin apparent exhumation over that of the Eromanga Basin is within error limits [4]. However, there are significant areas where the difference is in excess of 400m, and, perhaps even more importantly, the map of the difference between Cooper and Eromanga Basin apparent exhumation (Fig.…”

“…(7). Map showing the difference between mean apparent exhumation in Cooper Basin and Eromanga Basin [4]. Major fields, well control points and tectonic elements are also shown.…”

“…The sediments of the Cooper Basin were deposited during Late Carboniferous-Triassic times in predominantly fluvial and lacustrine environments [1]. After the deposition of the Cooper Basin, in Late Triassic -Early Jurassic times, an exhumational event took place which resulted in the basin wide Nappamerri unconformity [2][3][4][5][6] (Fig. 2).…”

“…The generation of hydrocarbons and migration occurs not within the present structural framework of the basin, but within a framework modified by subsequent exhumation. Hence, if the magnitude of exhumation is quantified it can be used to better predict source rock maturity and to reconstruct basin morphology at the time of hydrocarbon migration and hence elucidate pre-exhumation migration pathways [3][4][5][6]. Additionally, porosities and seismic velocities in the basin are generally attained at burial-depths greater than that presently observed.…”