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.