Identification and occurrence of 25-norbenzohopanes in biodegraded bitumen from Palaeozoic carbonates in northern Alberta

In addition to the previously reported 25-norhopanes and 25-norhopanoic acids, for the first time we report the identification of 25-norbenzohopanes. The hydrocarbon composition of the bitumen from Palaeozoic carbonates in northern Alberta displays molecular evidence for severe levels of biodegradation characterised by the removal of C30–C35 hopanes. Biodegradation is also indicated by the removal of C32 and C33 benzohopanes. The appearance of C31 and C32 25-norbenzohopanes corresponds to the decrease in C32 and C33 benzohopanes, suggesting that 25-norbenzohopanes originate by demethylation of benzohopane counterparts. Demethylation at C-10 in the hopanoids affects a broader class of compounds that so far includes the hopanes and hopanoic acids, as well as the benzylated hopanoid species.