DEEPWATER EXPLORATION: NORTH WESTERN AUSTRALIA COMPARED WITH GULF OF MEXICO AND MAURITANIA

Some of Australia’s deepwater frontiers are opening up for exploration, with existing and new companies taking acreage positions. Despite favourable fiscal terms and political stability, interest levels have not matched those in international hot spots due to key differences in perceived prospectivity.In this paper, Australia’s deepwater plays in the Northern Carnarvon Basin are compared and contrasted with deepwater plays in the Gulf of Mexico and offshore Mauritania. This comparison is largely based on Woodside Energy Ltd’s exploration pursuits in these areas.The Northern Carnarvon Basin deepwater plays are principally an extension of shallower water petroleum fairways, submerged to greater water depths by the absence of the Tertiary progradational, carbonate shelf sequence. Trap types and reservoir-seal pairs in the deepwater prospects are similar to their shallow water counterparts, but extensive deepwater areas carry an increased exploration risk due to the absence of this shelf overburden to load the Jurassic source rocks into the oil expulsion window. Hydrocarbons generated typically comprise dry gas from deeper Triassic source rocks, often trapped in sub-commercial quantities. Although the basin lacks a world class, widespread, oil-generating source rock, recent deepwater commercial oil discoveries in the Exmouth Sub-basin indicate the existence of a localised sweet spot associated with a Late Jurassic depocentre, similar to the proven Barrow-Dampier Subbasins located in shallower waters.In contrast, Woodside’s deepwater Gulf of Mexico and offshore Mauritania plays combine deepwater depositional systems with present day deepwater. They have reservoir-quality turbidite sandstones, well imaged on excellent quality 3D seismic, sealed by deep marine shales and charged by world class, organic-rich, prolific source rocks. Salt tectonics, shale diapirism and sloperelated slumping and thrusting have generated appealing structural styles, resulting in multiple play types and a density of prospects and leads not seen in Australia’s deepwater frontiers to date.Although elements of these plays are present at some locations in Australia’s deepwater, nowhere yet have all the required exploration ingredients for a major oil province been found juxtaposed as in the proven Gulf of Mexico and the highly prospective offshore Mauritania. Political stability and relatively favourable fiscal terms remain essential in attracting the exploration investment dollar to Australia’s deepwater.

Wanganui and East Coast Basins - Two of New Zealand's Little Explored Sedimentary Basins

Of the 5 largest basins, with over 20,000 km and 4 km plus of sediments, only one (Taranaki) has seen more than reconnaissance exploration. The other basins have barely been scratched, with results that in all cases remain inconclusive. There is a wide potential still untested in New Zealand. Two examples are presented: The Wanganui Basin, where the older part of the sequence has never been drilled. Wells were located on buried hills with older sediments onlapping along flanks. There are extensive updip wedgeouts and potential for structural-stratigraphic trap combinations. Aspects of source rock and flushing, while initially thought of as downgrading prospects, may look more favourable upon proper evaluation. East Coast Basin. With very thick. Cretaceous-Tertiary sediments, multiple deformation, local unconformities and facies changes, this is one of the more exiting basins, measuring 40,000-50,000 km2 on and offshore. Oil and gas generation is widespread, source rocks being of Paleocene and Cretaceous age. Potential reservoirs are Cretaceous and Miocene sandstones and Oligocene and Pliocene limestones, the latter of excellent qualities. Lower Tertiary, undercompacted and gas-charged mudstones may be an additional target for gas production. Shale diapirism has contributed widely to structural trap formation. Since 1960, only 14 wells have been drilled, of which I offshore. This amounts to 0.3 wells per 1,000 km2, or 0.6 m/km2.