Release of kerogen-bound hopanoids by hydropyrolysis

Hydropyrolysis was used to determine the concentrations of kerogen-bound hopanoid hydrocarbons. The samples characterized were a Recent sediment (Priest Pot, English Lake District) and three source rock kerogens (Green River shale, Draupne shale and Jet Rock shale). The hydropyrolyzate of a hopanoid producing bacterium (Methylobacterium organophilum) showed that this procedure does not overly obscure the original isomer compositions of the samples characterized, although significant side-chain cleavage does occur, which is inevitable with any pyrolysis method. Notable differences are observed between the composition of pyrolyzates and bitumen extracts. For each of the source rock kerogens, the overall level of hopane isomerization within the pyrolyzate is significantly lower than in the corresponding extracts. This is particularly the case for the Draupne sample, the pyrolyzate of which contains 17β,21β-hopanes though these are absent in the bitumen. Differences in isomeric composition of the pyrolyzates are in line with the isomer contents of the bitumen extracts. The abundance of kerogen-derived hopanes is highly variable, both in terms of absolute concentration and also relative to the concentration of hopanes in the bitumen. This is in part due to maturity differences, but depositional environment and diagenetic conditions may also exert a control on kerogen-bound hopane concentration. These results show that hydropyrolysis provides valuable data on the concentration of kerogen-bound hopanes, whilst largely preserving their original isomeric composition.