Raman Thermal Maturity of Coal and Type II Kerogen Based on Surface-Enhanced Raman Spectroscopy (SERS).

Low-maturity organic samples generate high levels of fluorescence during Raman detection. This fluorescence will obviously affect the Raman signals from organic matter. Our research shows that surface-enhanced Raman spectroscopy (SERS) can significantly enhance the ratio of the Raman signal from organic matter to the fluorescence background without changing the peak positions. This allows us to obtain more accurate Raman parameters for organic matter. In this study, we conducted Raman testing with SERS on coal and type II kerogen from the USA. We found that for both coal and type II kerogen, the exponential correlation between the thermal maturity and the distance between their D and G bands (Δ(G-D)) in the Raman spectra was good, and the R-squared values were 0.968 and 0.988, respectively. However, the Raman thermal maturity evolution curves for the coal and the type II kerogen were different. Compared with the Raman thermal maturity evolution curve of type II kerogen, that of coal was steeper. The two curves crossed each other at a Δ(G-D) value of 223, which corresponds to a calculated vitrinite reflectance value of 0.61%. This study also shows that the Raman thermal maturity evolution model of organic matter is perhaps related mainly to its type.