Within bituminite particles, high reflectance oxidation rims (HROR) were observed in close proximity to framboidal pyrites. Through examining a maceral with the framboidal pyrite physically removed, a high number of reflectance measurements were made covering the entire surface of the maceral. Additional measurements were taken in the form of transects across the maceral, allowing an examination of the relationship between reflectance and distance from the bacterial sulfate reduction (BSR) zone. A baseline reflectance value of approximately 0.50%R-o was measured, with peak values of approximately 0.75%R-o measured adjacent to the pyrite. This large variability in bituminite reflectance (BRo) was not induced via thermal catagenesis; however, it could be accounted for by a diagenetic process. This process involves labile organic matter (OM) entering a BSR zone. In this zone, anaerobic bacteria reduces dissolved sulfate to oxidize OM. This process results in the formation of bacterially-derived framboidal pyrite in close proximity to HROR in bituminite macerals. Although there has been a previous report of elevated BR. associated with biogenic gas in the regional scale, there was no microscopic evidence of such occurrence reported. This study examines low reflectance bituminite particles with embedded framboidal pyrite and associated HROR. Examination of the measured transects yielded a very rapid drop in reflectance a few micrometers away from the BSR zone. This study demonstrates a bacterial process that causes elevation of BR,, on a micrometer scale and leads to a positively skewed, highly variable BRo population. We also demonstrate the magnitude of variation that can be expected when measuring BRo which could result in significant overestimation of thermal maturity. (C) 2016 Elsevier B.V. All rights reserved.