Hydrocarbon generation reactions occur within organic-rich shales as a response to thermal maturation. Here, we report observations on samples from the organic-rich Mississippian Barnett shale gas system (Fort Worth Basin, Texas, USA) at varying stages of thermal maturation. A multiscale characterization was achieved using a combination of compositional organic geochemistry and spectromicroscopy techniques, including synchrotron-based scanning transmission X-ray microscopy (STXM) and transmission electron microscopy (TEM). We document the chemical evolution of the macromolecular structure of Barnett Shale kerogen with increasing maturity, from an immature kerogen containing a significant aliphatic component and an important concentration of oxygen and sulfur-containing functional groups to an overmature kerogen dominated by poorly condensed aromatic structures. In addition to the presence of bitumen in samples of oil window maturity, very likely genetically derived from thermally degraded kerogen, the formation of nanoporous pyrobitumen has been inferred for samples of gas window maturity, likely resulting from the formation of gaseous hydrocarbons by secondary cracking of bitumen compounds. (C) 2012 Elsevier B.V. All rights reserved.