The thermal maturity of lower Paleozoic graptolite-bearing marine sediments, which host many hydrocarbon deposits worldwide, has long been difficult to determine due to the absence of wood-derived vitrinite particles for conventional vitrinite reflectance. In 1976, graptolite reflectance was introduced as a new indicator for organic maturity of these deposits and has been used since in many regional studies. The majority of these studies, however, were done on a limited sample set and a limited range of thermal maturity, which resulted in a number of controversial views concerning the usefulness of graptolite reflectance as an alternative paleothermal indicator and its correlation with vitrinite reflectance through various proxies. In this paper, we review previous studies and combine those analyses with new data to assess the physical and chemical characteristics of graptolite periderm with increasing thermal maturity. We conclude that graptolite random reflectance (GR_or) is a better parameter for the thermal maturity assessment than graptolite maximum reflectance (GR_omax) due to the better quality of available data. Combining published data with results of our study of both natural and heat-treated graptolites and vitrinite, we present a new correlation between GR_or and equivalent vitrinite reflectance (EqVR_o), as EqVR_o = 0.99GR_or + 0.08. Chemical composition of graptolite periderm is similar to vitrinite; graptolites are mainly kerogen Type II-III, are gas prone and have a substantial hydrocarbon potential. Lower Paleozoic graptolite-bearing organic-rich sediments are important shale gas source rocks and reservoirs globally and make a significant contribution to worldwide petroleum reserves