Molecular fossils in coastal bitumens from southern Australia: signatures of precursor biota and source rock environments

Weathered waxy crude oil and black asphaltite (tar) are regularly washed ashore along the coastlines of South Australia and western Victoria. These bitumens generally strand on medium to high-energy, sandy beaches in west or southwest facing bays. Some very weathered bitumens have been discovered in Holocene dune deposits up to 200 m from the present-day coastline. Differences in the sulphur content and carbon, hydrogen and sulphur isotopic compositions of the coastal bitumens suggest that they belong to at least four oil families, one of which may be emanating from submarine seeps along Australia's highly faulted southern continental margin. The remainder appear to be flotsam carried south from S.E. Asia, possibly Sumatra. The waxy bitumens (Families 1-3) range from paraffinic to aromatic-intermediate in bulk composition and contain up to 2.6% S. Their density (13-38° API) dictates that they are near-surface drifters. They have low pristane/phytane ratios (pr/ph ≤ 2), abundant 4α-methyl steranes (C₃₀ with 24-ethyl substitution) and, in most cases, high concentrations of botryococcane. This biomarker assemblage indicates accumulation of freshwater algal source material (including the remains of Botryococcus sp. and dinoflagellates) under anoxic to suboxic conditions. The additional presence of 28,30-bisnorhopane, dinosterane and 24-n-propylcholestane among the molecular fossils in Family 3 bitumens suggests derivation from a lacustrine organic facies subject to marine incursions; or, alternatively, mixing in the reservoir of two end-member (lacustrine and marine) oil types. Trace amounts of oleanane, and isomeric bicadinanes in the waxy bitumens constrain the age of their source rocks (Late Cretaceous or younger) and also preclude their local derivation; an Indonesian origin is considered likely, implying long-distance transport by surface ocean currents. The Family 4 bitumens (∼ 5° API) are bottom drifters. These sulphur-rich (3-6% S), aromatic-asphaltic crudes have molecular signatures that include: pr/ph ∼ 1; C_{27 > C29 > C28 > C30} desmethyl steranes; dinosterane dominant over 4α-methyl-24-ethylcholestane; an enhanced concentration of 28,30-bisnorhopane relative to hopane; and the absence of 2- or 3-methylhopanes. A distal, anoxic marine facies of the Late Cretaceous Belfast Mudstone in the eastern Otway Basin, or its lithostratigraphic equivalent in the Duntroon Basin, are possible local sources of the asphaltic bitumens.