Hydrocarbon Generation in Gippsland Basin, Australia--Comparison with Cooper Basin, Australia

Abstract

The Gippsland basin, with its oil and gas resources, provides an excellent area for organic geochemical and petrologic research on the generation, migration, and alteration of hydrocarbons. The main source rocks for the known oil and gas deposits appear to be at depths greater than those reached by any of the exploration wells. The type of organic material originally present in the rocks now at depths greater than 4,000 m (and now at temperatures greater than 130°C) is unknown but, at least within the Latrobe Group, it appears to have had a high exinite content similar to that observed in the upper part of the group. At such temperatures, thermal cracking of exinite would yield a considerable amount of oil, whereas the products from vitrinite would be mainly gas and olid residue. Migration to reservoirs below the unconformity at the top of the Latrobe Group would follow. Both generation and migration are believed to be occurring at the present time, as immature carbonaceous material is being exposed to higher temperatures by deeper burial. Chromatographic analysis of Gippsland crude oils suggests that the oils originate from solid organic matter derived from algae and land plants, the latter contributing to the high wax content. Gas generation in the Cooper basin provides an interesting comparison with the Gippsland basin in relation to type of organic material and geothermal history.