Constraining Late Cretaceous Exhumation in the Eromanga Basin Using Sonic Velocity Data
Mitchell W. KEANY
Bachelor of Science (Petroleum Geology and Geophysics) 2015
Australian School of Petroleum
The University of Adelaide
Exhumation in sedimentary basins can have significant consequences for their petroleum systems. For example, source rocks may be more mature than their present-day burial depths suggest, increased compaction can result in reduced reservoir porosity, and seal integrity problems are commonly exhibited in exhumed basins. The Eromanga Basin in central Australia experienced an important phase of exhumation during the Late Cretaceous, though the magnitude and spatial distribution of exhumation is poorly constrained. Exhumation magnitudes have been recorded for 100 petroleum wells based on sonic transit time analyses of fine grained shales, siltstones and mudstones within the Cretaceous succession of the Eromanga Basin.
Observed sonic transit times are compared to normal compaction trends (NCTs) determined for suitable stratigraphic units. The Winton Formation and the Bulldog Shale/Wallumbilla Formation were chosen for analysis in this study for their homogenous, fine grained, and laterally extensive properties. Exhumation magnitudes for these stratigraphic units are statistically similar. Results show significant net exhumation in the southern Cooper-Eromanga Basin (<500m [~1640ft]) and higher net exhumation magnitudes (up to 1400m [~3937ft]) being recorded in the north eastern margins of the Cooper-Eromanga Basin. Gross exhumation magnitudes show significant variation across short distances suggesting different tectonic processes acting upon the basin. Independent vitrinite reflectance and Apatite Fission Track Analysis data, available for a subset of wells, give statistically similar exhumation magnitudes that have been calculated through the compaction methodology, giving confidence in these results. The effect on source rock generation is illustrated through 1D basin modelling where exhumation is shown to impact the timing and extent of the hydrocarbons generated. The improved quantification of this exhumation permits a better understanding of both the Late Cretaceous tectonics and palaeogeography of central Australia.