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.