Geochemistry Of Alkylbenzenes In The Paleozoic Oils From The Tarim Basin,NW China

Alkylbenzenes are widely distributed in oils and souce rock extracts from the Tarim Basin,but their geochemical significance and application have rarely been investigated..Geochemical study on alkylbenzenes is significant to deeply understand the origin of marine oils and gas condnestaes in cratonic region of the Tarim Basin.In present study,59 oil samples are analyzed quantitatively using gas chromatography–massspectrometry?GC/MS?method.Thedistribution characteristics and influence factors of alkylbenzenes are throgrouly investigated.The relative abundances of alkylbenzenes and their distributions show large variation,which are most likely controlled by both source input during deposition and thermal maturity during the catagenesis stage of petroleum genration.Relatively high abundance of long chain linear substituted alkylbenzenes and alkyltoluenes in the studied samples implies that source rocks of these oils/condensates contain high contents of algal material.The dominance of dodecylbenzene in these n-alkylbenzenes suggests the formation of alkylbenzenes from fatty acids via lactone formation.The 1,2,3,4-tetramethylbenzene?TeMB?,1-methyl-3-phytanylbenzene?1-M-3-PyB?and aryl isoprenoids are mainly derived from carotenoids produced by Chlorobiaceae and their occurrence may indicate a highly reducing depositional environment.Thermal maturity has substantial impact on alkylbenzene distributions and absolute concentrations.With increasing thermal maturity,the prominent trend of demethylation of alkylbenzenes is accompanied by shortening of alkyl side chains.Large variation in the C₄-alkylbenzene isomer distribution is also controlled by thermal maturity.The 1,2,3,5-TeMB and 1,2,4,5-TeMB isomers are more stable than other TeMB isomers and the overall thermodynamic stability of TeMBs is lower than that of dimethylethylbenzenes.Therefore,several maturity parameters can be derived based on alkylbenzene stability.Absolute concentrations of alkylbenzenes generally increase with increasing thermal maturity,but concentrations of aryl isoprenoids decrease significantly at high maturity.Maturity parameters obtained from alkylbenzenes are especially useful for highly mature oils and condensates when most biomarkers are depleted.Biodegradation also has some influence to the alkylbenzenes,some studies indicate that alkylbenzenes are consumed by bacteria slightly later than n-alkanes.But there is little indication that biodegradation has exerted a significant impact on alkylbenzene distributions as multiple charges and mixing masks orginal signatures.Evaporative fractionation are widespread in Tarim basin and has an obviously impact on alkylbenzenes.Although such process will significantly affect the relative abundance of toluene and xylenes in oils/condensates,however,more work is needed for a quantitative assessment.