Abstract: Modern and Tertiary Temperate Carbonates From Australia and New Zealand; Their Significance in the Exploration for Oil and Gas

Rao, C. Prasada - University of Tasmania

Modern temperate shelf carbonates are moderate to well-sorted sands and cover an area over 100,000 kM² off southern Australia and New Zealand ( 1; Rao, 1996). Temperate carbonates are composed mainly of skeletal hashes. Temperate latitudes are located in high-energy, swell-dominated environments that are often referred as the ?Roaring Forties.? Typically sand grains form by fragmentation, abrasion and rounding of bryozoans. Sand grains are mobile on the seafloor, due to high energy, and fines are winnowed from shelf slope to deep sea. The sands carpet the entire shelf area. Bryozoan sands have excellent intragranular porosity of up to 80%.

In Tasmania, the amount of calcite relative to aragonite increases with increasing water depth due to decreasing water temperatures. Relative amounts of bryozoa, foraminifera and mollusca are related to grain-size of the sediment. Bryozoans increase with increasing water depth, bivalves are high in shallow-depths and foraminifera are high in mid-depth. Facies variation in temperate carbonates is dissimilar to tropical ones, because of dominance of aphotic organisms, growth of fauna in cooler waters, and changing water temperatures. In very cold (<50°C) seawater, saturation relative to calcite and aragonite is low. This leads to dissolution and enhances the porosity.

As Australia moved from polar latitudes to temperate latitudes during the Tertiary, extensive temperate (and polar) carbonates formed in Australia and New Zealand (Fig. 2; Rao, 1997). These are similar to modern temperate carbonates and mainly composed of well-sorted sands with a very high depositional and diagenetic porosity and thus are ideal reservoir rocks. Some Tertiary limestones of New Zealand were deeply buried and strongly affected by burial diagenesis and increased the maturation of organic matter. Tertiary temperate carbonates also occur offshore. Splitting of Tasmania and Australia from Antarctica has increased volcanic activity and shallow burial temperatures, which must have converted organic matter to oil at shallow depths and gas at deeper depths. Extensive stratigraphic traps of oil and gas can occur in temperate carbonates. Recognition of ancient temperate carbonates is essential in oil and gas exploration.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil