A study on molecular structural evolution of type II kerogen in a gold tube thermal system: Insights from solid-state 13C NMR

Lacustrine type II kerogen is widely distributed in China and has important hydrocarbon potential, but the study of its structural evolution is not comprehensive enough. In this study, a closed gold tube system was used to simulate the maturation of kerogen extracted from the Nengjiang Formation of the Songliao Basin to investigate a model of hydrocarbon generation and molecular structure change in lacustrine type II kerogen. The pyrolysis products at two heating rates were collected and quantitatively analyzed, and the Easy%Ro of 20 solid kerogen residue samples (solid pyrolysis products after extraction of methylene chloride and methanol) ranged from 0.69 to 2.49. From elemental analysis and 13C nuclear magnetic resonance (13C NMR), the molecular structure evolution of kerogen was clearly determined, and four average molecular models of kerogen were established. The results show that with increasing maturity, the aliphatic fractions of kerogen decreased, and the aromatic fractions increased. Before the main oil window (Easy%Ro < 0.89), the main products were nonhydrocarbon heteroatomic functional groups (NSOs: asphaltene and resin). The oil peak occurred at an Easy%Ro of approximately 1.1 when hydrocarbon C14+ production reached the maximum and kerogen residue was the lowest. After the oil peak, aromatic groups became dominant in the structure. C14+ began to decrease, and the kerogen residue proportion increased, which was related to the occurrence of secondary coking reactions after the main oil window. The hydrocarbon generation capacity of kerogen was exhausted after the oil peak, and the kerogen likely became the precursor of generated gaseous products. In the gas window (Easy%Ro > 1.53), the large quantity of C1-5 was mainly due to the secondary cracking of hydrocarbons. The number of aromatic structures caused by alkyl bond breaking and condensation was estimated. It was found that before the oil peak, the number of aromatic structures caused by alkyl bond breaking decreased and was higher than that of aromatic structures caused by condensation. After the oil peak, the latter structures were obviously dominant, which further indicates that polymerization is the main structural change mechanism of kerogen evolution.