The relationship between micro-Raman spectral parameters and reflectance of solid bitumen

• Bitumens with different maturities were obtained by pyrolysis of a low mature bitumen.
• The D and G bands of their Raman spectra show regular changes with maturation.
• Raman parameters RBS and ID/IG show the best correlation with BRo.
• Raman data of natural bitumens follow the models of the artificial bitumens.

Solid bitumen occurs widely in the Early Paleozoic and Precambrian strata and its reflectance is a generally-accepted indicator for thermal maturity. Even though some recent papers have been published concerning the Raman characteristics and spectral parameters of solid bitumen, a systematic investigation on the relationship between the Raman spectral parameters and thermal maturity of solid bitumen is still lacking. In this study, a low maturity solid bitumen sample was pyrolysed under laboratory-controlled conditions to obtain a suite of artificial bitumen samples with different maturities (BRo = 1.1–4.81%), which are used to investigate the relationships between Raman spectral parameters and reflectance of solid bitumen. The Raman spectral parameters of the artificial bitumens, including band position (WD and WG), band separation (RBS), full width at half maximum (FWHM-D and FWHM-G), and band intensity ratio (ID/IG) are all related to the bitumen reflectance, but with considerably different correlations, constrained apparently by thermal maturity. Linear regressions were performed between these parameters and bitumen reflectance, and two parameters with higher correlation were selected. They are RBS (within 1.5–3.5% of BRo) and ID/IG (within 3.0–5.0% of BRo), with a correlation coefficient as high as 0.97. It is believed that the two Raman spectral parameters of solid bitumen will be of significant practical use for the maturity assessment of the Early Paleozoic and Precambrian strata when standard measurement and curve fitting procedures are utilized.