Thermal maturity evaluation from inertinites by Raman spectroscopy: The ‘RaMM’ technique

•New thermal maturity evaluation tool developed using Raman spectra of inertinite.•Method can also use vitrinite also but does not require delineation from inertinite.•A reasonable estimate of maturity can be estimated from a single inertinite grain.•Method appears to be able to solve problem of ‘suppressed vitrinite reflectance’.

A new thermal maturity evaluation tool, RaMM (Raman Maturity Method), enables determination of equivalent vitrinite reflectance from Raman spectra of inertinite. Except for the avoidance of liptinites, the method does not require identification of maceral type; measured assemblages ranging from all inertinite to all vitrinite are suitable. The method uses five parameters derived from the first order vibrational spectrum of carbon. Data from about a dozen humic grains are averaged although a reasonable estimate of thermal maturity can be obtained from a single inertinite grain. RaMM yields equivalent vitrinite reflectance (‘RaMM EqVR’) values based on a multilinear regression equation derived from data on 163 macerals from ten reference coals having vitrinite reflectances ranging from ~ 0.4 to ~ 1.2%. Cross-calibration on the reference coals, and application to a suite of 14 other coals spanning this rank range agree well with measured VR values. Limited data on isometamorphic coals suggest that RaMM EqVR can solve the problems caused by vitrinites with high H/C atomic ratios and consequent suppressed vitrinite reflectance; this is accomplished by deriving ‘normal’ reflectance values in a similar way as inertinites with low H/C are referred back to their equivalent vitrinite reflectance values. The RaMM method is affected by post-sampling maceral oxidation but to what degree is uncertain at present. The methodology and calibration have been established and future work will demonstrate applications to dispersed organic matter in samples from petroleum exploration wells, especially where vitrinite is absent or rare.