Early Triassic paleogeography of the northern Perth Basin, and controls on the distribution of source rock facies

Abstract

The facies distribution of the Lower Triassic Kockatea Shale in the northern Perth Basin is an important factor for assessing the Lower Triassic source rock potential in the basin. Seismic interpretation and well analyses demonstrate that Permian-aged faults are responsible for the morphology of the basin in the Early Triassic and created remnant topography that controls the deposition of Lower Triassic sediments. Cores from petroleum wells and their correlated wireline logs show the lithological variation and lateral distribution of facies across the base of the Kockatea Shale. Darker coloured mudstones were deposited under anoxic conditions in the deeper part of the basin, while lighter coloured mudstones and tidally influenced sediments with bioturbation were deposited under oxic conditions in a shallow marine setting. Microbial mats developed in shallower water depths on structural highs (Beagle Ridge and Turtle Dove Ridge) in the anoxic parts of the basin.The distribution of microbial mats is controlled by two factors during the period of ecological recovery from the end-Permian mass extinction: 1) the presence of topographic highs and, 2) distance from the edge of the basin. The source rock potential of both darker coloured mudstone and microbial mat facies is demonstrated by TOC values between 0.3 and 3.4% and HI values between 39 and 579 mg/gTOC. On the other hand, the lighter coloured mudstones, deposited under more aerobic tidal settings, have poor source rock quality and TOC values between 0.2 and 0.5% and HI values between 59 and 127 mg/gTOC.