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

Novel C-34 and C-35 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 C-34 and C-35 hexacyclic hop-17(21)-enes were identified in thermally immature mudstones from Vietnam cored in the 500 m 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 C-34 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 C-34). Precursors could be penta-and hexafunctionalised hopanoids in cyanobacteria and methanotrophs. (C) 2015 Elsevier Ltd. All rights reserved.