Abstract

The extreme variation in the natural endowment of petroleum resources between regions has been a key geo-political driver in the last century and may well remain so in the decades ahead. Most of the world's oil is located in a latitudinal belt lying predominantly north of the equator, running from the Gulf of Mexico and Venezuela, to North Africa, through the Middle East, the Caspian and Central Asia and down to Indonesia. Klemme and Ulmishek (1991) calculated that this Tethyan Petroleum Province contained 68% of global original petroleum reserves. Its vast petroleum resources were derived largely from the organic rich marine rocks deposited in low latitude in restricted basins and on shallow carbonate shelves flanking the various Palaeozoic, Mesozoic and Cainozoic incarnations of the east-west orientated Tethys Ocean.

A Boreal Petroleum Province was also recognised by Klemme and Ulmishek (1991) located in high northerly latitudes and containing about 23% of the global resource in such oil rich regions as the North Slope of Alaska, North Sea and West Siberia. The Boreal belt demonstrates that not all major oil provinces require source rocks deposited in equatorial or tropical environments. The key source rocks in these basins are Late Jurassic and Cretaceous clastic marine facies deposited at high palaeo-latitudes.

The Klemme and Ulmishek (1991) analysis indicated that less than ten percent of the world's petroleum had been found outside the Tethyan and Boreal provinces. The major new deep-water oil provinces of the 1990's, Brazil and West Africa, point to the increasing importance of the southern hemisphere in the distribution of remaining global petroleum resources. The petroliferous basins of offshore Brazil and West Africa are the product of Atlantic rifting which re-made the world in the late Mesozoic to be dominated by longitudinally oriented oceans, with the Mediterranean left as the last remnant of Tethys.

Beyond the current day focus of the deepwater "Golden Triangle" of the Gulf of Mexico, Brazil and West Africa, recent discoveries are starting to build the pattern of a southerly belt of oil occurrences to match the Boreal province. As with major oil reserves found in the far north, the key source rocks are clastic marine Cretaceous facies deposited in basins then located in high palaeolatitudes. There is established production from the Magallanes/Austral Basin in Chile and Argentina at the southern tip of South America; and there is now oil production from the Oribi field in the Bredasdorp Basin, offshore the southern coast of South Africa, east of Cape Town. In both cases Early Cretaceous marine claystones are the source facies.

However, in south-eastern Australia, Late Cretaceous fluvio-deltaic coaly facies are the source for the billion barrel oil fields in the world-class offshore Gippsland Basin. Source rocks of similar age and depositional facies have produced the petroleum accumulations of the Taranaki Basin in New Zealand. Where are the oil accumulations derived from high latitude marine Cretaceous sources that are so important in South America and South Africa, and in the corresponding Boreal belt?

Current drilling in the Great Australian Bight may answer this question and clearly establish the pattern of a belt of petroleum basins in the far south related to marine deposition in the greenhouse world of the Cretaceous. Evidence from palaeogeographic reconstructions, limited offshore drilling, oil strandings, remote sensing and seismic data all point to the occurrence of Cretaceous clastic marine source facies in the deepwater basins along the southern margin of Australia.

Presented at: 2005 South East Asia Petroleum Exploration Society (SEAPEX) Conference, Singapore, 2003