Organic chemical structure relationships to maturity and stability in shales

The systematic understanding of organic matter chemical structure-thermal stability relationships is central to resource outcomes in sedimentary basins. This study integrates molecular spectroscopy (solid-state C-13 NMR) and step-wise isothermal analysis (isoTGA) in elucidating key structure-maturity-stability relationships in shales containing type I and II kerogens (Green River and Eagle Ford Formations, respectively) that were subjected to simulated maturation via hydrous pyrolysis. Hydrous pyrolysis temperatures ranged from 250 to 400 degrees C - the window for immature to oil generation in the catagenic process (corresponding to Easy %R-o 0.36 to 1.07). Specifically, mole fraction aromatic C (f(a)) revealed strong correlations with pyrolysis temperature across both kerogen types, increasing 14.8 to 44.9 mol% and 19.7 to 75.2 mol% in the Green River and Eagle Ford samples, respectively. Aromatic bridgehead C (f(a)(B)), a measure of aromatic ring condensation, showed limited predictive variation with maturity. The increase in aromatization/condensation was accompanied by increased pyrolytic cracking of alkyl C (f(a1)), which decreased from 66.2 to 42.9 mol% in the Green River and 56.1 to 12.2 mol% in the Eagle Ford kerogen. Our C-13 NMR analyses of the kerogens revealed empirical relationships that can be used for predicting pyrolysis temperature and maturity (Easy %R-o) based on the major organic structural properties of kerogen. Step-wise, isothermal thermogravimetric analyses allowed for the estimation of kinetic parameters associated with catagenesis: activation energy (E-a) and frequency factor (A). Thermal decomposition of kerogen was revealed to be a two-stage process, and increasing maturity generally resulted in an increase of thermal stability/activation energy. First-stage activation energies (E-a1) ranged 64.0 to 44.7 kJ mol(-1), while second-stage activation energies (E-a2) ranged 151.4 to 198.9 kJ mol(-1) with increasing maturity in the Green River samples. Similarly, E-a1 and E-a2 of the Eagle Ford ranged 13.1 to 46.0 kJ mol(-1) and 208.0 to 286.2 kJ mol(-1), respectively. The quantitative determination of covariation in organic structure and thermal stability of kerogen with increasing maturity can be useful in further parameterizing Basin and Petroleum System Models (BPSMs). Furthermore, these results can support the development of computational models of petroleum generation and further facilitate the experimentally-driven understanding of the interplay between organic structure and thermal stability in petroleum systems.