GIPPSLAND TERRESTRIAL OILS — RECOGNITION OF COMPOSITIONAL VARIATIONS DUE TO MATURITY AND BIODEGRADATION EFFECTS

Abstract

The Gippsland oils, though derived from a common terrestrial source, show considerable variation in their chemical compositions. They range from being very waxy and paraffinic to light, almost condensate-like, oils. Much of this variation can be explained as a function of increasing maturity at the time of generation, with the earliest generated oil being characterised by dominant n-alkanes in the C22-23 is range, remnant odd-over-even preference above C25 and general lack of lower molecular weight gasoline and kerosene range hydrocarbons. At peak generation, the oils show a trimodal distribution as increased thermal cracking generates lighter hydrocarbons with a maximum around n-C7-8 and lesser maxima at n-C14 and n-C23. The most mature oils show the destruction of nearly all of the high molecular weight hydrocarbons and exhibit a unimodal composition maximising at n-C9 or less. These light oils are nearly always associated with gas caps, suggesting a link between the two. Secondary alteration by biodegradation and water-washing is common but its severity and the composition of the resulting oil is varied. Reservoir temperature is one of the key controls on biodegradation with no degraded oils observed at temperatures above 75-80°C. This, combined with the known limits of fresh water influx, makes it possible to predict the probable biodegradation effects on reservoired oil prior to drilling.