RaMM (Raman maturity method) study of samples used in an interlaboratory exercise on a standard test method for determination of vitrinite reflectance on dispersed organic matter in rocks

RaMM (Raman maturity method) is a thermal maturity tool for humic macerals in coals or as dispersed organic matter (DOM), expressed in equivalent vitrinite reflectance (EqVR). The EqVR values are based on a recently established calibration according to multi-linear regressions of selected parameters from Raman spectra of vitrinite and inertinite in a suite of Australian coals with vitrinite reflectance (VR) in the range 0.4-2.5%. The study contributes to the application of Raman spectroscopy for thermal maturity determinations as applicable to the petroleum exploration industry and researched by many groups worldwide. A suite of samples to apply the technique was made available from an interlaboratory round robin exercise on application of ASTM D7708, a method for determination of VR of DOM in sedimentary rocks. This exercise involved the distribution of two sets of six unknown rock samples to all participating laboratories. Collated VR results from some samples of the first set, having high maturities and low organic matter (OM) contents, showed a disturbingly high degree of scatter. Although general agreement exists between RaMM determinations and the average of 'mean random' VR values determined at the various laboratories, some important differences emerged for some samples in the range EqVR = 1.2-1.8%. On RaMM diagrams, which involve plots of EqVRs derived from the two regression equations defining the method, the pattern of distribution of vitrinite and inertinite changes with thermal maturity for 'normal' coals and with OM composition in subhydrous versus perhydrous coals. Most of the round robin samples contained solid bitumens having reflectances close to those of the associated vitrinite. Measurements taken from solid bitumen were avoided as much as possible, but because the RaMM study was done in air medium, some contamination of the humic maceral data was inevitable. Many of the solid bitumens and other potentially complicating components such as Tasmanites-related alginite can be identified on the RaMM diagram and eliminated from RaMM results for lower maturity samples (EqVR < 1.4%) whereas solid bitumen and humic macerals appear to provide similar EqVR for higher maturity samples (EqVR > 1.8%). The humic DOM of marine samples having EqVR > 1.0% has spectral properties that differ from those of humic macerals of the same maturity in terrestrially deposited rocks; this could possibly explain any discrepancies between RaMM EqVR and VR data for the marine rocks. The presence of contaminating solid bitumen with reflectance similar to that of vitrinite, is the main complicating factor for thermal maturity evaluation using either VR or RaMM analyses of gas shales and tight oil reservoir rocks. To enable the best results from RaMM studies of these types of rocks, a preliminary study of the solid bitumen component should be carried out to determine its field on the RaMM diagram with the object of eliminating contaminating solid bitumen data from EqVR calculations.