Shales spanning a range of thermal maturity (similar to 0.6 to >4.5%R-e) were examined by high resolution transmission electron microscopy (HRTEM) in order to characterize the occurrence of turbostratic carbon nanostructures. Such structures, termed Basic Structural Units (BSU), were found in both demineralized and whole-rock samples. Highly aromatic turbostratic nanostructures found in a pre-oil window shale were verified by electron energy loss spectroscopy (EELS). Turbostratic nanostructures increase in abundance with increasing thermal maturity with their greatest frequency occurring in high maturity (>3.6%R), natural and artificially matured organic rich shales that exhibit anomalously high electrical conductivity in well logs. We hypothesize that these turbostratic carbon nanostructures initially form upon oil generation, increase in frequency with increasing thermal maturation, and eventually become numerous enough to establish mutual contact, either directly or through graphene-like sheets, so that the shale becomes conductive on a macroscopic scale. (C) 2013 Published by Elsevier B.V.