GEOCHEMISTRY AND SIGNIFICANCE OF COASTAL BITUMEN FROM SOUTHERN AND NORTHERN AUSTRALIA

Abstract

Detailed geochemical analysis of eleven Australian coastal bitumens - eight from ocean beaches in South Australia and western Victoria, and three from stranding sites on Bathurst Island and Cobourg Peninsula in the Northern Territory - shows that they are inspissated waxy crudes of low API gravity and low sulphur content.

Only one bitumen appears not to be a natural crude, its composition is that of a weathered fuel oil. The rest may be divided into two types, as follows:

Bitumens from Ocean Beach, Geltwood Beach and Sleaford Bay in South Australia are paraffinic to naphthenic in composition. Those from the Northern Territory, Victoria and Cape Northumberland, SA, together with a second sample from Geltwood Beach, are highly paraffinic to paraffinic. Both types of bitumen strand at Geltwood Beach where each contains appreciably more nickel (85-105 ppm) than bitumens of either type found elsewhere (3-30 ppm).

The effects of exposure are more evident in some bitumens than in others. Weathering has involved evaporative loss of volatile components, bacterial degradation of n-alkanes up to C25, and an increase in asphaltenes at the expense of saturated hydrocarbons.

A high proportion of long-chain (C23+) n-alkanes in the stranded crudes indicates that they are products of the diagenesis of mostly continental vegetation. However, their low pristane/phytane ratios (<2) suggest that the source organic matter underwent little thermal alteration. They have δCPBD13 values (−23.8 to −26.1 per mil) which are typical of oils of Cretaceous orTertiary age. Their n-alkane profiles distinguish them from fuel oils or crude oil sludge discharged at sea during tanker washing operations.

The geochemistry of the bitumens supports the belief that they represent natural submarine seepages. The amount of weathering sustained by the bitumens in each suite correlates with their exposure time. Their degree of alteration, in conjunction with a knowledge of local surface currents, can be used to establish their provenance. Cretaceous freshwater and paralic sediments are present at depth offshore in both the western Otway Basin and the Money Shoal Basin. These sediments constitute the most likely ultimate source for the immature oils that gave rise to the coastal bitumens now found scattered along the southern and northern coastlines of Australia.