THE GEOLOGY OF THE OFFSHORE CANNING BASIN, WESTERN AUSTRALIA

The Canning Basin is a major geological province of Western Australia and of the Australian continent. It constitutes a major crustal down-warp between the Precambrian Kimberley and Pilbara Blocks and extends offshore onto the wide Rowley Shelf. A large part of this shelf is held under petroleum exploration permit by a group of companies of which B.O.C. of Australia Limited is operator. To date no wells have been drilled in this offshore Canning Basin but extensive geophysical surveys have allowed an interpretive geological model to be presented. This model is based upon an interpretation of geophysical data by extrapolation from offshore data and well control in the Carnarvon Basin and onshore well, outcrop and geophysical data in the Canning Basin. The major Palaeozoic structural features of the Broome Swell and Fitzroy Graben which characterise the onshore Canning Basin extend offshore beneath Mesozoic and Tertiary cover. Thick Palaeozoic and Permo-Triassic sediments are thus expected offshore in the Fitzroy Graben extension and in a major sub-basin at the offshore margin of the Pilbara Block called the Bedout Sub-basin. This sub-basin is believed to have had continued development during the Mesozoic with the accumulation of large thicknesses of Jurassic-Cretaceous sediments. It is restricted further offshore by the structurally positive Rankin Platform, a feature reflected in a large positive gravity anomaly. Mesozoic deposition was associated with a westerly tilt of the Rowley Shelf and Mesozoic thickness consequently increases westwards. The tilt increased during the Tertiary with the development of a major Tertiary stratigraphic wedge locally reaching 8000-10,000 feet in thickness at the shelf edge. Sedimentary deposition was controlled throughout Phanerozoic time by periodic epeirogenic warping related to uplift of the two major blocks. These movements caused considerable structural fragmentation and initiated periods of exiensive erosion. Complex lithofacies variations are thus expected and multiple intra-sectional unconformities are evident. The area contains many fault induced fold features, some compressional folds and extensive stratigraphic trap potential. Coarse clastic units with high hydrocarbon reservoir potential are expected to occur in close association with postulated marine Mesozoic basinal shale sequences. Similar deposits in the Carnarvon Basin and its offshore extension have been proved to be hydrocarbon bearing and productive (Barrow Island). The hydrocarbon potential of the offshore Canning Basin is thus rated highly.