PETROLEUM OCCURRENCE IN THE GIPPSLAND BASIN AND ITS RELATIONSHIP TO RANK AND ORGANIC MATTER TYPE

Abstract

Coal rank, sediment age and downhole temperature data indicate that the rates of burial and palaeothermal gradients in the Gippsland Basin have varied both areally and with time over the Late Cretaceous to Recent period. The generation and occurrence of petroleum are controlled mainly by the burial metamorphic history. The inshore areas are gas prone because the Late Cainozoic burial meta-morphism is moderate and overprints an earlier phase of substantial burial metamorphism in the Late Cretaceous-Early Tertiary. The areas offshore in the Central Deep are oil prone because the earlier burial metamorphism was minor and the burial metamorphism during the last 20 Ma has been rapid and substantial.

Vitrinite reflectance values (R̅vmax) vary from about 0.2 per cent at near-surface depths to over 1.2 per cent in the Upper Cretaceous sediments at depths of about 4 km and more. Exinite reflectance values (R̅emax) are about 0.05 per cent at near-surface depths increasing gradually to only 0.15 per cent at 3 km. Significant exinite metamorphism is evident at depths between 3 and 4 km, with major exinite metamorphism at 4-5 km and more at the base of the Upper Cretaceous sequence.

The proportion of organic matter and its specific generative capacity increases up through the Latrobe Group. The Late Cretaceous to Early Eocene organic matter consists of orthohydrous vitrinite and diverse inertinite and is distinct from the Middle to Late Eocene coaly matter which consists of perhydrous vitrinite and minor amounts of inertinite. The Oligocene to Miocene organic matter is dominated by perhydrous vitrinites and is inertinite-poor. The overall proportion of exinite is roughly constant up through the Upper Cretaceous to Miocene terrestrial sequences although some forms of alginite are more common in the Eocene to Miocene sediments. Petrographic and geologic evidence suggests that much of the petroleum probably is generated from vitrinite in addition to exinite at low coal ranks (R̅vmax 0.4-0.8 per cent) and low burial depths (2-4 km).