Will Australia Fulfill the Promise of the Paleozoic?

Marita T. Bradshaw
Geoscience Australia, Canberra, Australia

The recent success of the shale gas industry based on the Paleozoic sequences of North America has prompted a serious reconsideration of the petroleum potential of Australia’s onshore basins. From 2010 to 2012 there was a near doubling of the onshore basin area under exploration license and several major international companies have joined local explorers in their endeavors to unlock the potential of Australian Lower Paleozoic and Proterozoic petroleum systems.

Australia’s geological history has created a great diversity of petroleum systems (Bradshaw et al. 2012) including a number of Lower Paleozoic and Proterozoic systems. Some of the most oil-prone source rocks are Cambrian and Ordovician marine shales deposited in warm, shallow seas that traversed Australia when it was located in equatorial latitudes within Gondwana. Since 1984, oil has been produced from the Ordovician section of the Amadeus Basin in central Australia, and oil shows have been reported from the Ordovician of the Canning and Arafura basins and the Cambrian of the Georgina and Officer basins. The Canning Basin also has proven oil source rocks of Late Devonian age deposited in tropical marine environments. But despite wide spread excellent source rocks, less than one per cent of Australia’s crude oil has been produced from Lower Paleozoic systems.

Hydrocarbon charge is considered to be the element of the petroleum system most liable to fail. Some areas of Lower Paleozoic basins were never buried deeply enough to have generated hydrocarbons; and where hydrocarbons were expelled preservation of accumulations remains a significant uncertainty due to the adverse impact of the Late Paleozoic Alice Springs Orogeny. Reservoir quality is also an issue for those conventional accumulations that have been preserved in tact. Technological advances such as horizontal drilling and hydraulic fracturing now offer the opportunity to develop hydrocarbon resources in poor quality reservoirs or those still retained within source rocks.

When discussing Australia’s potential petroleum provinces of the 21^st Century, Longley et al. (2001) focused on the offshore Mesozoic rifted margins and only included a short section entitled “the promise of the Paleozoic”. History has shown that extensive areas of organic-rich rocks have encouraged exploration in Lower Paleozoic onshore basins over several decades but with little reward. Maturation history and preservation are seen as the major risks. However, the unconventional revolution has prompted a revision of this thinking. Evidence will be presented suggesting that the “promise of the Paleozoic” in Australia may still yet be fulfilled.

References
Bradshaw, M.T., Borissova, I., Edwards, D.S., Gibson, G.M., Hashimoto, T., Nelson, G.T., Rollet, N. and Totterdell, 2012. Out of Gondwana. In: Shaping a Nation: A Geology of Australia, Blewett RS (ed.), Geoscience Australia and ANU E Press, Canberra, 332–379. http://epress.anu.edu.au?p=194981
Longley, I.M., Bradshaw, M.T. & Hebberger, J., 2001. Australian Petroleum Provinces of the 21st Century. In Downey, M., Threet. J., and Morgan, W.(eds.), Petroleum Provinces of the 21st Century, American Association of Petroleum Geologists Memoir 74, 287- 318

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013