Organic matter (OM) in petroleum source rocks is a mixture of organic macerals that follow their own specific evolutionary pathways during thermal maturation. Understanding the transformation of each maceral into oil and gas with increasing thermal maturity is critical for both source rock evaluation and unconventional shale oil/gas reservoir characterization. In this study, organic petrology was used to document the reflectance, abundance, color, and fluorescence properties of primary organic macerals and solid bitumen (SB) in 14 Upper Devonian New Albany Shale samples (kerogen type II sequence) from early mature (vitrinite reflectance [VR_o] of 0.55%) to post-mature (VR_o 1.42%). Micro-Fourier transform infrared (micro-FTIR) spectroscopy analyses were conducted on these samples to derive information on the evolution of the chemical structure of organic macerals and SB with increasing thermal maturity.

Primary OM (amorphous organic matter, alginite, vitrinite, and inertinite) and secondary organic matter (SB) were identified in early mature samples. Amorphous organic matter (AOM) was the dominant organic component in early mature samples and was observed up to the maturity equivalent to VR_o 0.79% but could not be identified at VR_o 0.80%. An organic network composed of AOM and SB was observed from VR_o 0.55 to 0.79%, which, together with the decrease in AOM content being accompanied by an increase in SB content, suggests that with the onset of petroleum generation, SB gradually replaced the original AOM. Alginite, represented by Tasmanites cysts, started to transform to pre-oil bitumen at a maturity corresponding to VR_o 0.80%. It shows weak orange-yellow fluorescence at this maturity, a change from strong greenish-yellow fluorescence in early mature samples. Alginite could not be identified at VR_o 0.89%, and generated bitumen remained in place or migrated over short distances. Petrographic observations and micro-FTIR study of alginite indicate that substantial hydrocarbon generation from alginite does not start until alginite is completely transformed to pre-oil bitumen. In contrast to AOM and alginite, vitrinite and inertinite derived from terrestrial woody materials occur as dispersed particles and do not change significantly during thermal maturation.

A linear relationship between vitrinite and SB reflectance exists for the studied samples. The reflectance of vitrinite is higher than that of SB until VR_o 0.99%, and at higher maturities, SB reflectance exceeds vitrinite reflectance. The inclusion of pre-oil SB converted from alginite in reflectance measurements could result in a lower average SB reflectance and, therefore, caution should be applied when using SB reflectance as an indicator of thermal maturity.