Occurrence of (20R)- and (20S)-Δ8(14) and Δ14 5α(H)-sterenes and the origin of 5α(H),14β(H),17β(H)-steranes in an immature sediment

Series of (20R)- and (20S)-Δ8(14) and Δ145α(H)-sterenes have been identified in an immature marl from a hypersaline deposit (northern Apennines, Italy) by co-injection studies with authentic compounds. These alkenes co-occur with Δ13(17) 5α(H)-spirosterenes and transient (20R)- and (20S)-Δ14 5α(H), 17β(H)-sterenes. The source of these isomeric steroid alkenes is probably Δ7 (and Δ8(14)) 5α(H)sterols via (Δ2,7), Δ2,8(14) and Δ2,14 5α(H)-steradienes. Reduction processes in the sediment, via the Δ14 5α(H),17β(H)-sterenes, can give rise to the C-20 isomeric 5α(H),14β(H),17β(H)-steranes present in “anomalously” high relative abundances from a thermal maturity viewpoint. This diagenetic pathway is supported by laboratory rearrangement/hydrogenation experiments of 5α(H)-cholest-7-ene and from considerations of carbon number distributions of various steroid hydrocarbons in the sediment extract.