Thermal maturity evaluation using Raman spectroscopy for oil shale samples of USA: comparisons with vitrinite reflectance and pyrolysis methods

Thermal maturity is commonly assessed by various geochemical screening methods (e.g., pyrolysis and organic petrology). In this contribution, we attempt to establish an alternative approach to estimating thermal maturity with Raman spectroscopy, using 24 North American oil shale samples with thermal maturity data generated by vitrinite reflectance (VRo%) and pyrolysis (T-max)-based maturity calculation (VRe%). The representative shale samples are from the Haynesville (East Texas), Woodford (West Texas), Eagle Ford and Pearsall (South Texas) Formations, as well as Gothic, Mancos, and Niobrara Formation shales (all from Colorado). The Raman spectra of disordered carbonaceous matter (D1 and G bands separation) of these samples were directly obtained from the rock chips without prior sample preparation. Using the Gaussian and Lorentzian distribution approach, thermal maturities from VR were correlated with carbon G and D1. We found that the Raman band separation (RBS) displayed a better correlation for equivalent VRe% than vitrinite reflectance VRo%. The RBS (D1-G) distance versus total organic carbon, free hydrocarbons from thermal extraction (S1), and the remaining hydrocarbon generating potential (S2) indicate that the RBS (D1-G) distance is also related to kerogen type. Data presented here from three methods of maturity determination of shale demonstrate that Raman spectroscopy is a quick and valid approach to thermal maturity assessment.