THE TECTONO-STRATIGRAPHY AND PETROLEUM POTENTIAL OF THE NORTHERN ABROLHOS SUB BASIN, WESTERN AUSTRALIA

The Abrolhos Sub-basin lies offshore in Western Australia to the west of Geraldton and has geological affinities with the northern Perth Basin and the southern Carnarvon Basin. Both of these basins contain commercial petroleum accumulations, whereas the Abrolhos Sub-basin is a frontier area which is largely unexplored. A moderate seismic coverage of the sub-basin now exists but only two wells have been drilled, both of which were dry. Four main tectono-stratigraphic sequences are recognisable above Precambrian basement:Lower Palaeozoic Pre Rift SequenceCarboniferous-Permian Synrift/Rift Sequence S Triassic-Jurassic Rift Sequence 4 Cretaceous to Recent Drift Sequence. The Lower Palaeozoic is only known on the eastern basin margins where it mainly consists of Silurian fluvial and alluvial fan red beds. The Carboniferous-Permian marine and coal measure volcanogenic synrift and rift sequences are characterised by north-south, mainly east-dipping extensional faulting, followed by widespread erosion. The Triassic sequence is about 2 km thick and comprises a basal marine Kockatea Shale, overlain by the marginal marine Woodada Formation and the Lesueur Formation red bed sequence. Subsidence during the Triassic was rapid but controlled by large NNW-SSE trending, high angle west-dipping, planar normal faults with minor rotation and extension. The Jurassic is poorly known, being confined to structurally deep blocks along the Mesozoic basin axis to the south and west. A renewed phase of NNW-SSE west-dipping extensional faulting began during the Jurassic and resulted in the development of rollover anticlines. Considerable erosion and non-deposition occurred forming a regional Neocomian unconformity. The postrift or drift sequence consists of transgressive marine shelf carbonates dipping basinward without further significant structuring. The main prospect types in the sub-basin include base Triassic transgressive sandstones or top Permian sandstones sealed by the Kockatea Shale in tilted fault blocks, and Triassic-Jurassic sandstones within rollover anticlines sealed by intraformational shales or the middle Jurassic Cadda Formation. The main source rocks include the Woodada and Kockatea formations which are within the oil generative zone over much of the sub-basin. However, identification of areas with the required coincidence of source, reservoir, seal and structural timing appears elusive.