Characteristics and implication of biomarker in source rocks of the Upper Ganchaigou Formation of Neogene in northwestern Qaidam Basin
Paleogene-Neogene source rocks in northwestern Qaidam Basin are classic saline lacustrine facies source rocks in West China.Taken such rocks of the Neogene Miocene Upper Ganchaigou Formation in the study area as examples,the geological implica-tion of biomarker in these rocks for organic-matter origin,sedimentary environment and thermal evolution was discussed using both gas chromatography and mass spectrometry technologies.Results show that(ⅰ)the source rocks exhibit a lower light-to-heavy ratio of n-alkanes,a higher C24TeT/C26TT value of terpane,and rich content of both C27 and C29 in regular sterane,presenting an asymmet-ric V-shaped extension.Combined with discrimination charts,their organic matter are believed to have originated from hybrid input of lower hydrobiont and higher terrigenous plants;(ⅱ)among some environmental proxies,there exists a lower ratio of pristane to phytane(Pr/Ph)as well as a higher gammacerane index(Ga/C30H)and the other parameter of β-carotene/Cmax.According to the data of arene,the content of dibenzothiophen in trifluorene series is high,and the DBT/Phen value is low.Coupling with the charts,it is considered that the source rocks are deposited in a transitional setting from reduction to strong reduction;and(ⅲ)the n-alkanes are observed with no evident predominance in both carbon and odd-even,and exhibit a high Ts/Tm ratio for trisnorhopane,and both C29-αββ/(αββ+ααα)and C29-ααα20S/(20S+20R)for sterane maturity are averaged at 0.52 and 0.46,respectively,suggesting that for the rocks,the thermal evolution almost reaches a mature stage.In conclusion,this study discusses some parameters of regular biomarker and their implication,which may provide references for further probing into the source rocks and is conducive to both hydrocarbon exploration and development in Qaidam Basin.
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