Biomarker binding into kerogens: evidence from hydrous pyrolysis using heavy water (D2O)

Solvent-extracted Kimmeridge Clay Formation kerogen was heated to 315°C for 72 h in the presence of excess water (H2O) or heavy water (D2O). Hydrocarbons generated from the kerogen during the D2O experiments contained variable amounts of deuterium atoms [0 to as high as 12 deuterium atoms per molecule in the compounds we examined (mean of <0.5–4.2 atoms), varying with compound type and pyrolysis conditions]. The observed patterns of deuteration are not a function of simple homogeneous exchange, but must derive from the cleavage process in the presence of excess D2O. The diagnostic mass spectral fragment ions (electron impact ionisation) of selected regular acyclic isoprenoid alkanes, hopanes and steranes generated in the presence of D2O showed highly localised areas of deuteration, at or near the positions or functionality in the original biological precursor molecules. We interpret deuteration to have occurred at, or in close proximity to, the inferred linkages (e.g. ether or sulphur) by which the different biomarker hydrocarbons were formerly attached to kerogen (i.e. the tail of the asymmetric acyclic isoprenoids, the A-ring of steriods, and the side chain of hopanoids).