Molecular characterization of kerogen and its implications for determining hydrocarbon potential, organic matter sources and thermal maturity in Marcellus Shale

•Kerogens from oil window, wet gas and dry gas window were analyzed by 13C NMR.•Controls of sources of OM and maturation on kerogen structure was determined.•Previous models for HC potential and thermal maturity not valid for mature shales.•Structural parameters used to determine kerogen type where traditional methods fail.