Pyrolysis of kerogens in the absence and presence of montmorillonite—I. The generation, degradation and isomerization of steranes and triterpanes at 200 and 300°C

Abstract

The sterane and triterpane biomarker distribution obtained from extracted bitumens and kerogen (PB) pyrolysate bitumens of immature Cretaceous black shale (CBS) and Rocky Mountain coal (RMC) at 200 and 300°C, in the presence and absence of montmorillonite, reveal that the generation (release) of biomarkers was more significant than isomerization at 200°C pyrolysis, whereas isomerization and degradation were most important at 300°C pyrolysis. Montmorillonite has a pronounced catalytic effect on these reactions, i.e. generation, isomerization, and degradation. The ratios of C 29 ααα 20 S/S + R steranes and C 31 and C 32 αβ S/S + R hopanes display reversals with heating time at 200°C pyrolysis, irrespective of the presence or absence of montmorillonite, whereas they increased uniformly with thermal stress at 300°C pyrolysis. The extent of isomerization occurring at chiral centers in ring systems and in acyclic systems are different. It has been confirmed that the isomerization at the C 22 and C 20 position of the extended αβ hopanes and the ααα steranes respectively are slower than those of the ββ hopanes and the βαα steranes. In the 300°C pyrolysis, the C 27 steranes increased relative to C 29 steranes with heating time in the pyrolysate of the Rocky Mountain coal kerogen. This phenomenon is more significant in the presence of montmorillonite. The increase may indicate either the formation of secondary C 27 steranes or the early loss of C 29 steranes relative to C 27 steranes. In a sedimentary basin, isomerization probably occurs in the bitumen and kerogen at different rates. These data show that maturation indices may be reversed with depth by using the examples of ααα 20 S/S + R and αβ 22 S/S + R, where the rate of generation of the compounds from kerogen is faster than the rate of isomerization reactions occurring in the bitumen with time and temperature.