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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.
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 were 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, thereexistcompletelydifferentcorrelationsbetweentherelativeabundancesofdifferentC15sesquiterpanes thathavedifferentsourcesandthePr / 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. This discovery has terrigenous organic matterisonlyasufficientcondition, butanoxicenvironmentduringdepositionanddiagenesismaybeaprerequisitefortheformationofnovelC15sesquiterpanes.