Release of kerogen-bound hopanoids by hydropyrolysis

Hydropyrolysis was used to determine the concentrations of kerogen-bound ho panoid hydrocarbons. The samples characterized were a Recent sediment (Prie st 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 proce dure does not overly obscure the original isomer compositions of the sample s 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 th e source rock kerogens, the overall level of hopane isomerization within th e pyrolyzate is significantly lower than in the corresponding extracts. Thi s is particularly the case for the Draupne sample, the pyrolyzate of which contains 17 beta,21 beta-hopanes though these are absent in the bitumen. Di fferences in isomeric composition of the pyrolyzates are in line with the i somer contents of the bitumen extracts. The abundance of kerogen-derived ho panes is highly variable, both in terms of absolute concentration and also relative to the concentration of hopanes in the bitumen. This is in part du e ro maturity differences, but depositional environment and diagenetic cond itions may also exert a control on kerogen-bound hopane concentration. Thes e results show that hydropyrolysis provides valuable data on the concentrat ion of kerogen-bound hopanes, whilst largely preserving their original isom eric composition. (C) 1998 Elsevier Science Ltd. All rights reserved.