Abstract

The Dandaragan trough and its surrounding elements contain the known commercial hydrocarbon accumulations in the Perth basin, Western Australia. Organically rich sediments are widely distributed throughout the Permian, Triassic, and Jurassic section of the basin and the organic type is mainly humic as a result of the proximity of the Precambrian shield on the east and the predominance of continental sedimentation in the basin. Hydrocarbon accumulations are therefore mainly gas/condensate, although a secondary occurrence of light, paraffinic oil is of economic significance. Regional studies indicate a Neocomian uplift of the western flank of the Dandaragan trough centered on the Beagle ridge. The extent and magnitude of truncation appear to be reflected in systematic variations in sandstone porosity trends. Vitrinite reflectance data suggest that the uplift and erosion of the Beagle ridge were accompanied by higher geothermal gradients than are measured today in exploratory boreholes. Modern gradients of up to 5°C/100 m have been measured on the Beagle ridge and possibly represent this waning geothermal anomaly. In contrast, low geothermal gradients in the axis of the Dandaragan trough (<2.0°C/100 m) mean that generative temperatures are reached in this area at relatively great depths. Producible accumulations commonly depend on a delicate balance between generative depth and the level at which porosity declines to unacceptable levels for gas production.

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