Geochemical analysis techniques and geological applications of oil-bearing fluid inclusions, with some Australian case studies

Abstract

Reliable geochemical information of similar quality to conventional analyses of crude oils and source rocks can be obtained from oil-bearing fluid inclusions (FI). Carefully controlled analytical procedures including sample clean-up, procedural blanks and attention to detail are essential for the successful analysis of inclusion oils. The procedures are technically challenging, but if they are carefully followed, successfully analysed samples can include not only those with high abundances of oil inclusions, such as in current or palaeo-oil reservoirs, but also samples with low amounts of oil inclusions, such as those from oil migration pathways or from Proterozoic or even older rocks. A full range of hydrocarbons can be measured from inclusions, including low molecular weight hydrocarbons, n-alkanes, aliphatic biomarkers such as isoprenoids, hopanes and steranes, and aromatic hydrocarbons. There are many geological applications of the analysis of FI oils, which contribute to reducing regional exploration risk. This paper uses Australian case histories to illustrate the main applications of FI oil analysis. These include better constraining oil charge histories of reservoirs and identifying active source rocks previously unknown in a particular basin. The effects of oil-alteration by biodegradation and/or water-washing in the reservoir can be removed, mixing episodes in reservoirs can be deconvoluted, and the effects of drilling mud additives or other contaminants can be eliminated. Furthermore, the hydrocarbon composition and diversity of Earth's early biosphere can be constrained, and secondary migration pathways can be mapped across prospects or basins.