Novel saturated hexacyclic C₃₄ and C₃₅ hopanes in lacustrine oils and source rocks

Novel C₃₄ and C₃₅ hexacyclic hopanes having side chains with a pentacyclic or hexacyclic ring at C-22 are abundant in lacustrine oils and source rocks but almost absent from marine oils. The corresponding C₃₄ and C₃₅ hexacyclic hop-17(21)-enes were identified in thermally immature mudstones from Vietnam cored in the 500m deep ENRECA-3 well. All the novel hexacyclic compounds were unambiguously identified by comparison with synthetic standards. Traces of heptacyclic hopanes were detected in a lacustrine oil from Thailand. The C₃₄ and C ₃₅ hexacyclic hopanes have much longer gas chromatography (GC) retention times than the corresponding pentacyclic hopanes. They are isomerized faster at C-22 than the pentacyclic hopanes and their 22S/(22S+22R) ratio values at equilibrium are much higher (0.8 for C₃₄). Precursors could be penta- and hexafunctionalised hopanoids in cyanobacteria and methanotrophs.