C19-49 Extended Tricyclic Terpanes and Their Geochemical Significance in Crude Oils:A case of crude oils from the Yingxi area in the western Qaidam Basin
[Objective]To determine the maturity and origin of crude oils from the Yingxi area,western Qaidam Ba-sin,[Methods]the distribution and composition of various biomarkers,alkyl phenanthrenes and diamondoid hydro-carbons were analyzed in detail by gas chromatography-mass spectrometry (GC-MS) and gas chromatography-mass spectrometry-mass spectrometry (GC-MS-MS).[Results and Discussion]The analytical results demonstrate that these oils have an even carbon predominance at nC20 and nC22 in the normal alkane series,the (nC20+nC22)/(nC19+nC21) ratios are more than 1.0. In C13-20 regular isoprenoids,phytane is very abundant,the Pr/Ph ratios are less than 0.60,but the Ph/nC18 ratios are higher than 1.30. Moreover,the content of gammacerane indicating the salinity of depositional environment is relatively high,and the gammacerane index are between 0.90 and 1.36. These characte-ristics are consistent with the geological background of saline depositional environment during the Paleogene in the western Qaidam Basin. Based on the distribution and relative abundance of extended tricyclic terpanes and other bio-markers in different crude oils in the study area,they could be classified into two types. One of them from well Shi 62 and Shixi 58-1 has unusually abundant C19-49 tricyclic terpanes,the C19-30T/C27-35H and C23T/C30H ratios are more than 1.60 and 1.40,respectively. In addition,they have higher sterane/hopane (>4.50) and β-carotane/nC37 ratios (>3.20) and lower nC37/nC36 ratios (<1.0),suggesting that algae are their main biological input. The other one from well Shi 57 and Shi 61 has the moderate content of extended C19-30 tricyclic terpanes,the C19-30T/C27-35H and C23T/C30H ratios are less than 0.50 and 0.55,respectively. In this king of crude oils,the sterane/hopane and β-carotane/nC37 ratios are less than 2.0 and 1.50,respectively,but the nC37/nC36 ratios are more than 1.0,demonstrating that the algae biological input is relatively low compared to the first one. In the study crude oils,the sterane maturity parame-ters such as C2920S/(20S+20R) and ββ/(αα+ββ) have reached their equilibrium values and can not be used to eva-luate their maturity. Furthermore,aromatic maturity parameters related to methyl phenanthrene isomers such as MPI1 and MPR are usually used as maturity indicators,but their distribution and composition in the Paleogene saline source rocks in the study area does not change with increasing the maturity of organic matter,suggesting that they may not be effective maturity parameters in saline environment and can not be used to evaluate the maturity of crudeoils. However,the distribution and composition of diamondoid hydrocarbons in source rocks and crude oils are mainly controlled by thermal evolution of organic matter and widely used to evaluate the maturity of organic matter and the cracking extent of crude oils. Based on the maturity parameters MAI and MDI related to diamondoid hydrocarbons,the maturity of these oils from the Yingxi area is very similar,and their Ro values are approximately 1.1％-1.3％,suggesting that they were generated after the hydrocarbon generation peak,but before the condensate stage.[Conclusions]In other words,biological input,not maturity,is the main factor affecting the distribution and abun-dance of extended tricyclic terpanes in different oils from the Yingxi area. Therefore,the two types of crude oils in the study area should have their own sources,and more attention should be given to them in the future.
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