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Framework and Background 

The Morum Sub-basin (area = 40,000 km²) lies beyond the shelf edge off the southeastern coast of South Australia at the northern end of the Otway Basin. Not a single well has been drilled into the prospective part of this 6 second-TWT-deep, Albian to Late Cretaceous depocenter.  

Geochemical analysis undertaken on an oil show within Upper Albian–Cenomanian(?) rocks from the Crayfish-A1 well (Figure 1), which is close to the edge of this depocenter, suggest its origin is an anoxic marine source rock, and aromatic hydrocarbon analysis strongly suggests that it is migrated oil. The oil show is also consistent with the modeled development of a significant oil-prone source pod in the Morum Sub-basin north of the Discovery Bay High, which possibly correlates with the prolific world-wide Albian ocean anoxic event (OAE).  

Seismic data are extremely sparse in the deep-water Otway Basin. Nevertheless, trend analysis of potential field data (Figure 1) and calculation of extension rates by Palmowski (2004) of two key, deep seismic lines across the basin (Figure 2) have shown that the Morum Sub-basin is tectonically quite distinct from the Nelson Sub-basin, which lies to the south of the Discovery Bay High. Palmowski (2004) concluded that extension rates in the Nelson Sub-basin peaked during the Turonian and tailed off from there onward— behaviour which is typical of passive margins around the world. However, for the Morum Sub-basin he concluded that extension rates peaked earlier than the Turonian. Given the greater thickness of sediments in the Morum Sub-basin, we estimate that at least some of this was deposited prior to the Turonian. Due to the existence of known Albian marine rocks in the next basin to the northwest of this area and the knowledge that the southern margin of Australia ‘unzipped’ itself from Antarctica in a west to east direction, we postulate that the Upper Albian sediments of the Morum Sub-basin are possibly marine and may contain source rocks that are age equivalent to the prolific world-wide Albian OAE.   

Thus the potential distribution of Albian marine source rocks (Figure 1), which could be the source of the analyzed oil in Crayfish A1, may be limited to the Morum Sub-basin and absent from Nelson Sub-basin (where the recently abandoned Amrit-1 well was drilled) and the rest of the Otway Basin because of the well-documented Late Albian– Cenomanian unconformity there (Krassay et al., 2004).

Migration Considerations 

The limited extent of the interpreted marine Albian source rocks is also consistent with the geographic distribution of common heavy (4–9° API) asphaltite beach strandings, which contain mid-Cretaceous marine biomarkers (Figure 3) (Edwards et al., 1998). Such strandings, sometimes in great abundance, have been reported since first settlement on the South Australian coast south of Kingston and on Kangaroo Island, but strandings are rare between Backstairs Passage and Kingston. Sprigg (1963) collected ‘almost half a ton’ of asphaltite strandings from one location south of Kingston after a severe storm in May, 1961.  

The most significant attribute of these asphaltites is that they are heavier than seawater and are thus moved along the sea floor by saltation, unlike the lighter bitumen, which is also found on these beaches. The latter has been linked back to Indonesian source rocks and is assumed to have drifted around the west coast of Australia and across the Great Australian Bight on the Leeuwin Current (Smart, 1999; Summons et al., 2001).

The following scenario may explain the local distribution of asphalities along the coast. In winter the oceanic thermocline, which separates cold from relatively warm water, intersects the sea floor at the shelf edge. The deep cold water, known as the Flinders Current, moves northward (Middleton and Platov, 2003) along the continental slope while the warmer Leeuwin Current moves from west to east at the surface. The Leeuwin Current is relatively strong in winter being driven by prevailing westerlies and ocean swell and upper level mixing is caused by common storms. In the summer, as the prevailing westerly winds subside, the influence of the Leeuwin Current wanes and the thermocline migrates onto the shelf as the Flinders Current becomes more dominant (Figure 3). An upwelling event occurs when a persistent high-pressure cell sits over the Great Australian Bight causing strong southeasterly winds to blow along the SE coast of South Australia and wind shear to be sufficient to move the warmer water offshore. This can be detected by satellite as a plume of cold water parallel to the shelf break where the shelf is relatively narrow (Figure 3). The upwelling appears to be focus upward 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 (Figures 4 and 5). Please note that overpressure is known to occur at the edge of the Morum Sub-basin in Breaksea Reef-1, and it is possible that overpressure is providing the slip mechanism for this large-scale shelf collapse.  

Near the base of the slope, in the vicinity of the large-scale shelf collapse, one canyon cuts as deeply as 1.6 km through the stratigraphic succession into the toe-thrust inversion structure that may contain the interpreted Upper Albian marine source rocks. Numerous sea-surface anomalies have been detected over this canyon using Synthetic Aperture Radar (SAR) images (Figure 3). Potential gas chimneys (Figure 6), diapric structures, and amplitude anomalies are also interpreted on a regional, deep seismic line that transects the canyon. We postulate that the maturation of hydrocarbons is contributing to the generation of overpressure, migrating upward along faults to the distal canyon floor and nearby slope where they form asphaltite mats. Meanwhile lighter hydrocarbons escape to the sea surface and this may explain the occurrence of SAR anomalies. The asphaltite mats are then periodically dislodged from the seabed and swept up the canyon by offshoots of the Flinders Current, which are driven by the summer upwelling. Tar balls entrained in the upwelling water are then spread across the shelf and are later moved ashore as beach strandings by winter storms that come in from the west. Tar balls make it to shore as large asphaltite strandings where the continental shelf is narrow, but where the shelf is wide, such as between Backstairs Passage and Kingston, tar balls are destroyed by abrasion on the sea floor during saltation and thus rarely make it to the beach.

References 

Edwards D., McKirdy, D.M., and Summons, R.E., 1998, Enigmatic asphaltites from the southern Australian margin: molecular and carbon isotopic composition: Petroleum Exploration Society Australia Journal, v. 26, p. 106–129.

Krassay, A.A, Cathro, D.L., and Ryan, D.J., 2004, A regional tectonotratigraphic framework for the Otway Basin, in Boult, P.J., Johns, D.R. and Lang S.C., eds., Proceedings of Petroleum Exploration Society of Australia, Eastern Australasian Basins Symposium II, p. 97–116.

Middleton, J.F., and Platov, G., 2003, The mean summertime circulation along Australia’s southern shelves: a numerical study: Journal of Physical Oceanography, v. 33, p. 2270– 2287.

Palmowski, D., Hill, K.C., and Hoffman, N., 2004, Structural-stratigraphic styles and evolution of the offshore Otway Basin – a structural seismic analysis, in Boult, P.J., Johns, D.R. and Lang S.C., eds., Proceedings of Petroleum Exploration Society of Australia, Eastern Australasian Basins Symposium II, p. 75–96.

Smart, S. M., 1999, Asphaltites from the southern Australian Margin: Submarine oil seeps or maritime artefacts?: Unpublished Honours thesis, The National Centre for Petroleum Geology and Geophysics at the University of Adelaide, South Australia.

Sprigg, R.C., and Woolley, J.B., 1963, Coastal bitumen in southern Australia, with special reference to observations at Geltwood Beach, southeast South Australia: Transactions of the Royal Society Australia, v. 88, p. 67–103.

Summons, R.E., Logan, G.A., Edwards, D.S., Boreham, C.J., Bradshaw, M.T., Blevin, J.E., Totterdell, J.M., and Zumberge, J.E., 2001, Geochemical analogues for Australian coastal asphaltites - search for the source rock (Abstract): AAPG Bulletin, v. 85 (no. 13 – Supplement).

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