Quantitative flash pyrolysis of an artificially matured shale kerogen sequence: Deciphering evolution of liquid linear hydrocarbon generation throughout the oil window

Molecular composition of kerogen and its thermal evolution play critical roles in revealing types and potential of hydrocarbon generation. In this study, on-line quantitative flash pyrolysis–gas chromatography (Py–GC) analysis was employed to investigate molecular composition and its evolution throughout the oil window by an artificially matured kerogen sequence. By introducing the Coats-Redfern model and Chen-Nuttall model, degradation kinetics of liquid linear hydrocarbons were calculated. Flash Py–GC of the kerogens yielded a considerable amount of C₈–C₂₈ n-alk-1-enes/n-alkanes that were derived from thermal cleavage of linear aliphatic moieties bonded to the kerogen structure. Generally, the abundance of linear aliphatic moieties shows a decreasing trend with increasing carbon number at a given thermal maturity. For a given chain length of linear aliphatic moieties, their generation also decreases progressively upon thermal maturity. Using the yield of n-alk-1-enes/n-alkanes from the original kerogen as a reference, the linear hydrocarbon generation amount (LHGA) of kerogen upon thermal maturity sequence was estimated. The results show that the calculated LHGA correlates well with the total hydrocarbon generation amount, suggesting that LHGA can be a measure to describe the evolution of liquid hydrocarbons within oil window. Given that thermal decomposition rate of kerogen follows overall first-order kinetics, the activation energies required for the breakdown of C₈C₂₅ linear aliphatic moieties from kerogen structure are in the range of 2745 kJ/mol. Degradation kinetic calculations of the linear aliphatic moieties demonstrate that the longer the chain length is, the easier the linear aliphatic moieties break down during thermal evolution. This has important implications for oil composition prediction and oil mobility due to chain length effects of linear hydrocarbons.