Organic-rich shales are a vital component of the US energy sector. Kerogen, a high molecular weight macromolecule is the largest reservoir of organic carbon on earth and serves as the starting material for the oil and gas generation in these shales. Despite its importance, kerogen structure and its evolution on maturation are still not well understood, especially for mature shales (VRo > 1). Moreover, most of the models built to determine hydrocarbon (HC) potential and thermal maturity of the source rocks have used the structural parameters of kerogen extracted from immature shales (VRo < 1). Therefore, these models might not yield accurate results for mature and over-mature shales like Marcellus. In this study, we determine the structural parameters of kerogen extracted from three Marcellus Shale cores using C-13 solid-state Nuclear magnetic resonance (NMR). Samples were acquired from the upper and lower Marcellus Shale Formation from a dry gas well WV-6 (VRo > 2.5), a wet gas well WV-7 (VRo similar to 1.4) and an oil window well BG-1 (VRo similar to 0.81) in Monongalia, Wetzel and Brooke County, West Virginia, respectively. Our results indicate that the percentage of carbon chains such as mobile (freely rotating) and immobile alkyl without heteroatoms (with restricted rotation), and alkyl-substituted aromatic carbons decrease with increasing maturity indicating that these chains are more prone to thermal degradation and might have higher HC generating potential. However, carbon chains such as O-substituted alkyl (ether), O-2 substituted alkyl (dioxy alkyl), amine, protonated aromatic carbons, O-substituted aromatic (phenol) and bridgehead aromatic carbons does not decrease directly with thermal maturity suggesting that these groups are either more refractory in nature or their carbon fraction is influenced by changing sources of OM. Our results also indicate that the previous models based on the structural parameters of kerogen derived from immature shales overestimate the HC generation potential and underestimate the thermal maturity in mature shales from the Marcellus. In addition, H, O, C ratios derived from structural parameters of kerogen can be utilized to determine the kerogen type in these mature shales where traditional pyrolysis analysis fails to characterize the kerogen.