Evaluating thermal maturity using transmitted light techniques: Color changes in structureless organic matter and palynomorphs

Abstract

Progressive color changes were observed in immature to postmature shale samples from various ages (Silurian to Tertiary) and geographic locations. The thermal alteration index, assessed based on the color of structureless organic matter, along with the spore color index determined on palynomorphs, were compared with vitrinite reflectance values obtained from more than 200 samples. While some correspondence occurs, the resolution of the thermal alteration index and the spore color index is not as precise as changes in vitrinite reflectance, as was expected. The 138 photomicrographs of structureless organic matter and palynomorphs included in this paper can serve as a color reference for the various stages of maturity.