Reply to comment by Oxtoby on "Assessing the maturity of oil trapped in fluid inclusions using molecular geochemistry data and visually-determined fluorescence colours"

The authors acknowledge the criticisms of Oxtoby (2002) regarding the petrological procedures used. The authors point out that there is considerable interlaboratory variation in microscope set-up, particularly regarding barrier filters, and that the system used is fairly typical. The authors believe that the measurements give a reproducible result for the purpose of supporting the key point of the paper, which is that there is danger in applying subjective visual description of fluorescence colours for determining maturity of included oil, unless careful calibration of the basin in question is carried out. The reason that George et al. (2001) did not have a detailed discussion of the causes of the wide maturity range for blue fluorescing oil inclusions is because no strong evidence was available to provide an explanation, and this is still the case. Fluorescence is a highly complex phenomenon and its relationship to chemical composition cannot be fully described using gross measures such as aliphatic/aromatic hydrocarbon ratio or % polar compounds. Tiny amounts of some chemicals may have the ability to induce or quench fluorescence, and intramolecular interactions are also undoubtedly of great importance (e.g. Ralston et al., 1996). The authors suspect that source variation, biodegradation (e.g. Barwise and Hay, 1996) and water washing of oils are processes that may in some instances have an effect on fluorescence properties of oil inclusions. The data and the data of others support one of the conclusions in George et al. (2001) that samples with mainly yellow and orange-fluorescing oil inclusions tend to have maturities in the lower hall of the oil window. However, the authors strongly caution against the widespread application of the paradigm that blue fluorescent oil inclusions equals high maturity oil. Furthermore, the detailed molecular data and that of others (e.g. Burruss, 199l) show that samples containing dominantly blue fluorescing oil inclusions can have considerable variability in chemical composition caused by source, biodegradation and maturity. Thus just as petrographers should not assume that oil inclusions with different fluorescence colours represent multiple oil charges (George et al., 2001), equally well they should not assume that oil inclusions with the same fluorescence colours are co-genetic. An integrated combination of careful petrographic observation and detailed molecular geochemistry remains the most powerful way of understanding fill histories of reservoirs using oil inclusions.