[目的]车排子凸起中生界稠油资源丰富,但原油生物降解严重,制约了勘探进程.[方法]为明确车排子凸起东翼中生界原油的生物降解程度,对车排子凸起东翼侏罗纪和白垩纪储层样品抽提出的饱和烃和芳香烃进行了气相色谱—质谱(GC-MS)分析,采用分子地球化学指标,对其进行定性与定量评价.[结果]通过聚类分析,将原油分为Ⅰ、Ⅱ、Ⅲ、Ⅳ四类族群.PM(Peters and Moldowan)法分析结果表明Ⅰ1类和Ⅱ1类原油的生物降解程度达PM7,Ⅰ2类和Ⅳ类原油的生物降解程度为PM8;Ⅱ2类和Ⅲ1类原油的生物降解程度为PM9,Ⅲ2类原油生物降解程度为PM9+.基于优化的Manco法求得的MN1值介于19693～215623,Ⅰ类原油的MN2值介于769～919,Ⅱ类原油的MN2值介于768～907,Ⅲ类原油的MN2值介于906～954,Ⅳ类原油的MN2值介于774～817.同一原油族群的Manco数与原油密度、黏度、非烃和沥青质含量均呈现良好的正相关性,与总烃呈现良好的负相关性.优化的Manco法成功厘定了生物降解程度在PM7-PM9+的储层样品,与PM等级评价具有一致性.[结论]与PM法相比,优化的Manco法对原油生物降解的识别分辨率更高.通过PM法定性评价和优化Manco法定量评价相结合,可以很好地揭示原油物性的变化,对稠油区的油气勘探有重要的指导作用.
Evaluation of the Biodegradation of Mesozoic Crude Oils in the Eastern Chepaizi Uplift
[Objective]Biodegraded crude oil is commonly found in poriferous basins globally.Biodegradation will have a profound impact on the physical properties and group components of crude oil,resulting in an increase in the oil density and viscosity,an enrichment in the non-hydrocarbon,asphaltene,sulfur metallic ion content and acid val-ue.In recent years,the Mesozoic(Jurassic and Cretaceous)oil-bearing sandstone reservoirs in the east margin of the Chepaizi uplift have attracted much more attention due to the discovery of large quantities of crude oil,but their se-vere biodegradation restricts the exploration process.In order to clarify the biodegradation level of Mesozoic crude oil in the east margin of the Chepaizi uplift,the saturated hydrocarbon and the aromatic hydrocarbon fractions extracted from the reservoir samples from 10 wells in the Mesozoic east margin of Chepaezi uplift were analyzed by gas chroma-tography-mass spectrometry(GC-MS),and molecular biomarkers strongly resistant to biodegradation were investigat-ed.[Methods]Cluster analysis,a statistical method,is an effective tool to classify studied samples into different groups by their similarity distances which were calculated from the different variables investigated.Allowing for the severe oil alteration,the following nine biomarker parameters related to tricyclic terpenes(TT),tetracyclic terpenes(Tet)and triaromatic steranes(TAS)with strong biodegradation resistance were selected:(C19TT+C20TT)/(C23TT+C24TT),C19TT/C23TT,C23TT/C21TT,C22TT/C21TT,C24TT/C23TT,ETR,C24 Tet/C26TT,C27 TAS/C28TAS(20R),C26TAS/C28TAS(20S).Cluster analysis(HCA)was performed to classify the Mesozoic crude oil populations.On this basis,the biodegradation level of Mesozoic crude oil was qualitatively evaluated according to the PM(Peters and Moldowan)method and descriptive terms.Then,following the academic theory of Manco scale,eight compounds with different sensitivities to microbial degradation,which characterize progressive bioresistance,were selected.Variables was assigned for them to optimize the quantitative evaluation scale of Manco method.The parametric vec-tors of the eight compounds generally covered the whole degradation level(PM0～PM10),including(1)n-alkanes(m/z 85);(2)naphthalene and alkyl naphthalene(m/z 128,142,156,170,184,198);(3)alkyl dibenzothio-phene(m/z 198,212,226);(4)C0-3 phenanthrene(m/z 178,192,206,220);(5)pentacyclic triterpenes(m/z 191,177,205);(6)sterane(m/z 217,218,259);(7)tricyclic terpenoids(m/z 191,177);and(8)triaromatic steroids(m/z 231,245).MN1(Manco Number 1)and MN2(Manco Number 2)were calculated by the formulas,and the correlation between them and crude oil properties was analyzed.[Results and Discussions]The result shows that the Mesozoic crude oil is a typical heavy oil with high density and high viscosity,especially the viscosity of crude oil shows an order of magnitude change,which essentially reflects the alteration effect of microbial degradation.The oil density showed roughly positive correlation with the viscosity and the NSO(resin+asphaltene)fraction content,and negative correlation with the burial depth.Progressive biodegradation of crude oils may be responsible.The results of cluster analysis shows that the Mesozoic crude oil in the eastern Chepaizi uplift can be divided into four oil popula-tions,i.e.,Ⅰ,Ⅱ,Ⅲ and Ⅳ.According to the difference of crude oil degradation level,different crude oil groups can be subdivided into different families.For example,group Ⅰ crude oil is subdivided into family Ⅰ1 crude oil(well P1,781.5 m and 721.5 m)with a biodegradation level of PM7(heavy)and family Ⅰ2 crude oil(wells P607 and P624)with a biodegradation level of PM8(very heavy).Group Ⅱ is subdivided into family Ⅱ1 crude oil(well P1,748.5 m and 751.2 m)with a biodegradation level of PM7(heavy)and family Ⅱ2 crude oil(wells P60 and P604)with a bio-degradation level of PM9(very heavy).Group Ⅲ is subdivided into family Ⅲ1 crude oil(wells P629,P68,and P606)with a biodegradation level of PM9(very heavy)and family Ⅲ2 crude oil(well P646)with a biodegradation level of PM9+(severe).Group Ⅳ includes well P1(756.8 m,757 m,761.6 m,and 766.5 m)and well P609,and the biodegradation level of group Ⅳ is PM8(very heavy).Qualitative evaluation result by the PM method shows that the degradation level of Mesozoic crude oil can reach PM7-PM9+.The MN1 value obtained by the optimized Manco method ranges from 19693 to 215623;the MN2 value obtained by the optimized Manco method ranges from 768 to 954.For a single oil population,the values of MN1 and MN2 increased with the biodegradation level.In addition,the Manco number shows a good positive correlation with oil density,viscosity,non-hydrocarbon and asphaltene con-tent,and a good negative correlation with total hydrocarbon content.The exception is the oil from well P646,which has a higher level of biodegradation than well P606 but shows a lower viscosity.Factually,there is not a simple rela-tionship between the oil viscosity and the Manco number.Processes other than biodegradation,i.e.,secondary oil charge,water washing,mixing of multiple maturity oil charges,and loss of light ends from heavy oils could produce variations in oil viscosity and density.The two episodes of oil charging,early biodegraded oils mixed with the later re-migration of preexisting oils due to the structural adjustment,yet the same oil origin in the Chepaizi uplift maybe re-sponsible for this case.In addition,differences in preservation conditions due to changes in burial depth can be one of the factors affecting biodegradation.[Conclusions]The optimized Manco method successfully distinguishes the oil-bearing reservoir samples with biodegradation level in PM7-PM9+,which is consistent with the PM result,that is,the biodegradation level of family Ⅰ1,family Ⅱ1,family Ⅰ2,group Ⅳ,family Ⅱ2,family Ⅲ1,family Ⅲ2 succes-sively increases.In addition,the optimized Manco method provides a more detailed classification than the PM meth-od,because it can clearly distinguish crude oil with the same PM level but different crude oil physical properties.This way,the differences in the biodegradation of oil-bearing reservoir samples can be clearly shown.The combina-tion of qualitative evaluation by the PM method and quantitative evaluation by the optimized Manco method can reveal the difference in crude oil physical properties,which plays a guiding role for oil and gas exploration in this area.
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