HYDROCARBON GENERATION IN THE NORTHERN PERTH BASIN

Abstract

All known commercial hydrocarbon accumulatios in the Perth Basin, Western Australia, occur within the Dandaragan Trough or along its flanks. Land plant-rich source rocks are widely distributed throughout the Permian, Triassic and Jurassic sections of the basin. Hydrocarbon accumulations are mainly dry gas and gas/condensate, although secondary occurrences of light, waxy oil are also of economic significance. The Lower Jurassic Cattamarra Coal Measures provide both source and reservoir for gas/condensate accumulations in the central Dandaragan Trough (Walyering, Gingin). Gas at Dongara, Mondarra, Yardarino and the more recent discovery, Woodada, may have been generated from both the Lower Triassic and Permian, although there is some evidence that the Permian is the principal source. The associated thin oil legs encountered in parts of these fields and at Mt Horner is attributed to the oil-prone basal Kockatea Shale (Lower Triassic). Regional studies indicate a Neocomian uplift of the western flank of the Dandaragan Trough, centred on the Beagle Ridge. Vitrinite reflectance data suggest that the uplift and erosion of the Beagle Ridge was accompanied by higher geothermal gradients, up to 7.5°C/100 m in the Neocomian. Modern gradients of up to 5.0°C/100 m have been measured on the Beagle Ridge and possibly represent this waning geothermal anomaly. In contrast, low geothermal gradients are found in the Dandaragan Trough (around 2.5°C/100 m), and hydrocarbon generation presently occurs at great depths where sandstone reservoir properties are often inadequate for commercial production.