Recently, Paul Hackley et al. (2020) pul;ished a great artcile in journal Marine and Petroleum Geology (Marine and Petroleum Geology 114 (2020) 104172). I highly recommend it for those interested.

The authors discuss results of an interlaboratory comparison study conducted by 31 different petrographers on vitrinite/solid bitumen reflectance from selected six marine source rocks that have been lately famed for their unconventinoal HC resource potentials in the US. Moreover, nearly 70 % of petrographers contributed to this study have 10+ years experience. These results are awful probably not because incompetence of the petrographers but because the technique and the nature of samples (marine shales with nearly no vitrinite)

When petroleum explorationists (who would normally assess the unconventional hydrocarbon resource potential of a given mudrock and perhaps decide on drilling) see the results of this study, they will simply be terrified. Moreover, if a trained eye examines their Figure 4 where they plot historgrams of vitrinite/solid bitumen measurements, will simply give up believing in any use of this technique for maturity assessment in marine source (unconventional reservoir) rocks. Hackley et al.'s (2020) great publication clearly shows this.

Hackley et al. also provided guidance for future reearch on this subject. They mention improvement and standardization of sample polish and measurement technique as well as further training of petrographers etc. These are all good measures to be taken. However, I believe the solution is to quit using measurement techniques where there is obvious operator-bias. Measurement of reflectence by photometer sounds like it is quantitative but in fact there are many factors involved that makes this technique semi-quantitative if not qualitative at all. Optical microscopy is great for macreal analyses and vitrinite reflectance measurements for coal samples but when the sample is marine shale or carbonate where vitrinite will be nearly absent and.or present in minute amounts and sizes, then this technique should not be used. For maturity assessmen, instead, I would recommend use of pyrolysis Tmax derived from pyrolys analyses of solvent-extracted (or thermovaporized) samples.