Fast and accurate shale maturity determination by Raman spectroscopy measurement with minimal sample preparation

Abstract

This study presents a robust correlation between Raman spectroscopy signal (expressed as the Raman band separation) and thermal maturity obtained by the vitrinite reflectance technique. The organic-rich mudstones used to build this correlation originate from a variety of paleo-marine sedimentary basins. The resulting correlation enables thermal maturity of kerogen expressed as vitrinite reflectance equivalent to be estimated in unknown formation samples using Raman spectroscopy. Raman spectroscopy can thus be used for determination of maturation windows (immature, oil, wet gas, or dry gas). Moreover, different to other Raman measurements that are performed on isolated kerogen or on polished surfaces of drill core and cutting fragments, the technique here is executed directly on fragments with minimal preparation, making it potentially applicable for wellsite maturity estimations. It is further shown that differences in the Raman analysis of kerogen seen among different published studies can be ascribed in part to the use of different Raman laser wavelengths. Taking wavelength dependence into account, the maturity determination of organic-rich mudstones by Raman spectroscopy may be developed into a generalized method, independent of the instrumental setup.