Biodegradative production and destruction of norhopanes - An example from residual oil in a Paleogene paleomigration conduit on the Utsira High, Norwegian North Sea

Biodegradation of the more resistant hopanoids was examined using residual stains in a Paleogene paleomigration conduit on the Utsira High, Norwegian North Sea, together with related less degraded oils. Under currently anoxic conditions, the most severely biodegraded samples contain 25-nor, 28-nor and 25,28-dinorhopanes, in addition to regular hopanes, neohopanes, diahopanes and didiahopanes [9,15-dimethyl-25,27-dinorhopanes]. The C ₂₇ member of the 25,28-dinorhopanes comprises closely eluting 17α,21β and 17β,21α isomers. A pair of C ₃₁ compounds eluting just after the 17α-homohopanes were tentatively identified as the 22S/R isomers of homo-18α-neohopane. The 28,30-dinorhopane peak seems to comprise 17α,21β and 17β,21α isomers, with relatively more of the less thermodynamically stable 17α,21β contributing under the most severe biodegradation. The approximate order of increasing susceptibility to biodegradation was: gammacerane ≈ triaromatic 23,24-dimethylcholesteroids ≈ triaromatic dinosteroids < C ₂₁–C ₂₂ C-ring-monoaromatic steroids ≈ C ₂₁–C ₂₂ steranes < C ₂₁–C ₂₂ diasteranes < C ₃₁ methylhopanes < 22,29,30-trinorneohopane (27Ts) < C ₃₀ TPPs ≈ diahopane ≈ didiahopane ≈ 22,29,30-trinorhopane (27Tm) < 30-norneohopane (29Ts) ≈ C ₂₈₊ 28-norhopanes/moretanes < C ₂₉₊ hopanes ≈ C ₂₇₊ diasteranes < C ₂₇₊ steranes. This information can inform the selection of correlation parameters for heavily biodegraded oils, although the sequence should not be considered universally applicable. Regular hopanes declined steadily with increasing biodegradation. The 25-norhopane homologues may be initially produced, but are then also degraded, although the C ₂₆ component appears stable. There is a relative increase in 25,28-dinorhopane abundance with increasing biodegradation, which is likely to result from demethylation of 25- and 28-norhopanes; if so, there must also be some shortening of the E-ring side-chain. Concentration estimates suggest that there is an overall loss of the hopanoid skeleton with increasing severity of biodegradation.