Novel C15 sesquiterpanes and their origin in different crude oils from the Pearl River Mouth Basin of China

In the saturate fractions of crude oils from the Zhu1 and Zhu2 depressions of the Pearl River Mouth Basin,two novel C_(15) sesquiterpanes have been detected except for drimanes and rearranged drimanes.By comparison with spectra and retention times in previous studies,they have been identified as 2,2,4α,7,8-pentamethyl-trans-decalin and 2,2,4α,7,8-pentamethyl-cis-decalin,respectively,which are related to the D/Eringsof18α(H)-and18β(H)-oleanane.ResultsshowthatthenovelC15sesquiterpanesarerelativelyabundantincondensateoilsthathavehighPr/PhratiosandarerichinterrigenoustriterpanessuchasoleananeandbicadinanesfromtheZhu2depression,butevidentlylowinlacustrineoilsthathavelowPr/Phratiosandarerichin4-methylsteranesfromtheZhu1depression.ThissuggeststhatthenovelC15sesquiterpanescouldberelatedtoterrigenousorganicmatterinoriginandcanhelpdistinguishtheoriginandsourceofcrudeoilsinthestudyarea.Inaddition,thereexistcompletelydifferentcorrelationsbetweentherelativeabundancesofdifferentC15sesquiterpanesthathavedifferentsourcesandthePr/Phratio,whichindicatesredoxconditionsinthecrudeoils.Forexample,thenovelC15sesquiterpanesandcadinanethatindicateterrigenousinputarepositivelycorrelatedwiththePr/Phratio,butdrimanesandrearrangeddrimaneshavingmicrobialoriginarenegativelycorrelatedwiththePr/Phratio.ThisimpliesthattheformationofthenovelC15sesquiterpanesrequiresrelativelyoxicconditions.ThenovelC15sesquiterpanesaremuchmoreabundantincarbonaceousmudstone(Pr/Ph=7.39)thanindarkmudstone(Pr/Ph=2.19)fromtheEnpingformation,althoughtheirorganicmatterishumic.However,theyarenotcommoninsapropeliclacustrinesourcerocks(Pr/Ph=2.29)fromtheWenchangFormation.Thissuggeststhatterrigenousorganicmatterisonlyasufficientcondition,butanoxicenvironmentduringdepositionanddiagenesismaybeaprerequisitefortheformationofnovelC15sesquiterpanes.