Maturity Trends in Raman Spectra from Kerogen and Coal

The present work explores the potential of Raman spectroscopy to provide maturity information about catagenesis stage kerogens and coals. The first-order Raman spectra of coals and kerogens show a broad amorphous A band between 1310 and 1360 cm-1 and a graphite-like G band near 1580−1600 cm-1. As vitrinite reflectance (R0) increases, the Raman A band becomes narrower and shifts to lower frequencies while the absorption strength of the A band relative to the G band decreases. The area ratio of the A band to G band (R = fA/fG) decreases from 3.2 to 2.4 in going from the least mature sample up to R0 = 2.0%. Similar trends were observed for maturation suites of Type II and other Type III kerogens. Laboratory thermolysis produced a range of samples with well-defined laboratory R0. The changes in the Raman A and G bands from the laboratory matured samples directionally paralled the natural samples. These results demonstrate that the Raman spectra of catagenesis stage samples vary in regular ways with sample maturity. The underlying chemical and structural changes associated with changes in the Raman A and G bands are discussed.