Thermal regimes in some Australian sedimentary basins

The intracratonic Cooper/Eromanga Basin is characterised by high geothermal gradients ranging 40°-55°C/km with the neighbouring intracratonic Pedirka/Simpson Desert/Eromanga Basin having slightly lower gradients which range from 35°-45°C/km on average. In contrast, the Perth Basin, located on the western rifted margin of Australia is characterised by generally low gradients in the range 20°-30°C/km with one local positive anomaly (gradients up to 50°C/km) centred on the Beagle Ridge. The Carnarvon Basin to the north, which is part of the same rifted margin, is influenced by higher temperature gradients which average 30°-40°C/km. Temperature gradients in the ancient Amadeus and eastern Officer Basins are low and typically range between 15°-25°C/km, perhaps reflecting a thickened lithosphere beneath these basins. Temporal variation in heatflow is recorded in the maturation of organic matter, which acts as an irreversible maximum recording geothermometer. Maturation levels in the Cooper/Eromanga Basins range to a high of 6.6% vitrinite reflectance in the central, deepest part of the hot Nappamerri Trough. Elsewhere, maturation levels are less extreme but typically reflect the interplay between basin structure and geothermal gradient variation. This implies prolonged exposure to a thermal regime largely similar to that which influences the basin now. This is in accord with evidence for extensionally thinned crust beneath the basin and enhanced levels of radiogenic crustal heat production. The major exception occurs in the central Nappamerri Trough where maturation modelling suggests that even higher temperatures need to be invoked in order to explain observed levels of organic metamorphism. In spite of this early thermal event, the factor having the most profound effect on organic maturation was rapid and deep Cretaceous subsidence. Temperature gradients vary little across the Pedirka/Simpson Desert/Eromanga Basin, which is a relatively shallow (\u3c3300m) saucer-shaped depression. Maturity trends show little variation across the basin in response to both this limited variation and a simple geothermal history. Maximum extent of organic maturation occurs in the basin axis where vitrinite reflectances of about 1.0-1.10% Romax have been observed. Maturation modelling indicates that the observed levels of maturation can be easily achieved using either a static thermal history or an attenuating thermal regime model. Relatively high regional heatflow is believed to relate more to high levels of crustal heat production rather than a late thermal event. The presence of very high rank in parts of the Permian section on the Beagle and Harvey Ridges of the Perth Basin suggests that a Triassic to Early Jurassic thermal event, associated perhaps with the initiation of rifting, is a local controlling factor in rank variation in this basin. Elsewhere, depth to basement is the major control with reflectance surfaces depressed in the cool deep troughs and elevated, and more closely spaced, over basement highs. Regional uplift and truncation also exert significant local control. The overall low temperature gradient is thought to relate to an attenuating post-breakup thermal regime. In the Carnarvon Basin, depth to the various reflectance surfaces in the Barrow-Dampier Sub-basins increases in offshore direction reflecting the regional decline in temperature gradient in this direction and deposition of a thick, prograding carbonate wedge during the Tertiary. Iso-reflectance surfaces are strongly diachronous indicating that most of the observed coalification is related to the Tertiary thermal regime. Well temperatures, at any given maturation level, are on average 40°C higher for the Carnarvon Basin as compared with the Perth Basin. Modelling studies confirm this trend and suggest a strong late thermal event which is geographically restricted to the Barrow-Dampier rift. This thermal event is considered to be related to both rapid Tertiary burial, lateral dewatering of the basin, and convective transfer of heat via the major fault systems. Maturation in the Amadeus Basin, as determined by graptolite and bitumen reflectivities, increases in a northerly direction in keeping with a northward increase in geothermal gradient and a marked northward thickening of the sedimentary section. Maturation modelling shows that deposition of the thick molasse sediments of the Devonian Pertnjara Group had the greatest impact on maturation. Maturation levels are consistent with the concept of a static thermal regime following post-Devonian uplift. Insufficient maturation data are available from the eastern Officer Basin to permit comment on regional rank variation. However, reconstructed burial histories for several wells are adequate to account for the observed levels of maturation under the assumption of a static thermal regime. The conclusion of this study is that the mapping and modelling of rank data (particularly vitrinite reflectance) is one of the most powerful tools available to modern-day basin analysts