The Morum Sub-Basin Petroleum System(.), Otway Basin, South Australia

Boult, Peter J.¹, Jane Blevin², Roar Heggeland³, Don R. Vinall⁴, Simon Lang¹, David M. McKirdy¹ (1) University of Adelaide, Adelaide, Australia (2) Australian Geological Survey Organization, Canberra, Australia (3) Statoil, Stavanger, Norway (4) PIRSA, Adelaide, Australia

Seismic data are extremely sparse and not a single well has been drilled into the 6 s TWT deep, primarily Late Cretaceous Morum Sub-basin (250 x 150 km), which lies beyond the shelf edge at the northern end of the Otway Basin. Beach strandings of heavy asphaltite (4­9° API) containing Mesozoic marine biomarkers are common along a section of the nearby coast. Here, the summer Bonney Upwelling is supplied by cold waters of the northward­flowing, deep-water Flinders Current. The upwelling appears to be focussed upwards onto the shelf by canyons incised in the continental slope, particularly those on the southern, upstream side of a slope headland caused by large-scale shelf collapse. Near the base of the slope one canyon cuts as deeply as 1.6 km through the stratigraphic succession into an interpreted toe-thrust inversion structure that may contain potential Late Albian marine source rocks. Numerous sea-surface anomalies have been detected over this canyon using Synthetic Aperture Radar images. Potential gas chimneys, diapric structures and amplitude anomalies are interpreted on a regional, deep seismic line that transects the canyon. We postulate that hydrocarbons are migrating upwards along faults to the distal canyon floor where they form tar mats (asphaltite), while lighter hydrocarbons escape to the sea surface.

    The tar mats are then dislodged from the seabed and swept up the canyon by bottom cur­rents driven by the summer upwelling. Tar balls entrained in the upwelling water are spread across the shelf and eventually moved ashore as beach strandings by winter storms that come in from the west.