The Use and abuse of fluorescence colours as maturity indicators of oil in inclusions from Australasian petroleum systems

It is well established that the fluorescence colours of crude oils shift towards the blue end of the spectrum with increased API gravity and implied thermal maturity. Oil inclusions also fluoresce, and it would be useful to correlate these fluorescence colours likewise to thermal maturity, because oil inclusions often contain the only direct evidence of residual oils once present in a reservoir. In order to validate this concept, the geochemical and petrographic properties of fluid inclusion oils in 36 sandstone samples from Australasian oil fields have been compared. A maturity assessment was made for each fluid inclusion oil using 25 molecular maturity ratios. Each fluid inclusion oil was placed in one of four maturity brackets, approximately equivalent to early, mid, peak and post oil generation windows. Samples containing mainly blue-fluorescing oil inclusions have thermal maturities throughout the oil window, including relatively low maturities (Rc <0.65%). Samples with mainly yellow and orange-fluorescing oil inclusions tend to have maturities in the lower half of the oil window. These data strongly suggest that the use of the fluorescence colours of oil inclusions as a qualitative thermal maturity guide is not justified. Fluorescence colour depends primarily on chemical composition, which is controlled not only by maturity but by several other processes and factors, including source, water-washing and trapping fractionation. Care should be taken when interpreting mixed fluorescence colour populations, because the different colour populations may represent a single oil charge, with oil inclusions trapped under slightly different conditions or at slightly different grain surfaces, rather than multiple migration events.