THE GEOCHEMISTRY OF SOME BIODEGRADED AUSTRALIAN OILS

Abstract

Degradation of petroleum in reservoirs by microorganisms is a process which has affected the chemical composition of many Australian crude oils. Oils from the Carnarvon and Gippsland Basins are used as examples to illustrate how the various classes of hydrocarbons are depleted in sequential fashion during the biodegradation process. Depletion of normal and branched alkanes, which is evident from gas chromatography, indicates that a crude oil has been mildly biodegraded. More advanced levels of biodegradation can be assessed from biomarker data obtained using gas chromatography-mass spectrometry. As the biodegradation process progresses one observes a sequential loss of n-alkanes, branched alkanes, δC-16 bicyclic alkanes, steranes with biological configurations, and finally, hopanes are converted into 25-norhopanes.The chemical compositions of the Eaglehawk, Rankin, and Windalia crude oils from the Carnarvon Basin suggest that they all result from a complex series of episodes of accumulation and biodegradation in the reservoirs. By assessing the proportions of each of the classes of characteristic marker compounds, and using the established relationship between reservoir temperature (depth) and the stages of hydrocarbon alteration during biodegradation, the approximate burial depths of the reservoirs at the times of biodegradation have been inferred. Thus, where multiple accumulation and degradation episodes can be recognised, the accumulation history of the oil in a reservoir can be reconstructed.