藏北羌塘上侏罗统—下白垩统胜利河油页岩单体烃碳同位素研究

Compound-Specific Carbon Isotopic Compositions of the Upper Jurassic-Lower Cretaceous Shenglihe Oil Shales in the Qiangtang Basin,Northern Xizang

上侏罗统—下白垩统胜利河油页岩位于北羌塘盆地与中央隆起之间的过渡带上,呈北西西—南东东向展布,长约30km。油页岩含有丰富的正烷烃、类异戊二烯烃、萜类化合物、甾类化合物和芳香烃化合物。正构烷烃呈前高后低的单峰型分布,nC17、nC_(19)为主峰碳,∑C21-/∑C22+比值范围为0.78~3.43,均值为1.57,轻烃组分占有绝对优势,OEP值介于0.77~1.12之间,平均值0.95,接近平衡值1.00;Pr/Ph值介于0.33~1.28之间,平均值0.71,绝大多数样品Pr/Ph值小于1.0,显示弱的植烷优势;萜烷丰度顺序为五环三萜烷>三环萜烷;规则甾烷呈不对称的"V"字形分布,表现为C27>C29>C28的分布特征。油页岩正构烷烃碳同位素δ13 C值介于-29.89‰^-23.73‰之间(平均-26.99‰),为略为倾斜的平直型分布,其不同碳数单体烃同位素及生物标志物特征均显示有机质主要来自低等生物菌藻类。类异茂二烯烷烃姥鲛烷和植烷碳同位素δ13 C值分别介于-30.72‰^-24.57‰(平均-28.39‰)和-29.87‰^-27.24‰(平均-28.52‰)之间,两者δ13 C值非常相似。与通常情况不同的是,姥鲛烷和植烷碳同位素明显比正构烷烃轻,姥鲛烷平均轻1.59‰(范围0.02‰~3.66‰),植烷平均轻1.84‰(范围0.37‰~3.69‰);分析认为水体中沟鞭藻或群体绿藻—丛绿藻Botryococus的繁盛可能是导致此碳同位素异常的原因。镜检显示有机质类型为腐殖腐泥型(Ⅱ1)和腐泥腐殖型(Ⅱ2),镜质体反射率(Ro)值介于0.37~0.9%之间,均值为0.55%,有机质处于未成熟—低成熟阶段。热解峰温Tmax值、孢粉颜色、饱和烃生物标志化合物成熟度参数和芳香烃甲基菲成熟度指数也证实有机质处于未成熟—低成熟阶段。

The Upper Jurassic—Lower Cretaceous Shenglihe oil shales are located in about 30-kilometer-long NWW-SEE-trending the transitional belt between north Qiangtang basin and central uplift belt.Abundant biomarkers,including n-alkanes,isoprenoid,steranes,terpanes and aromatics,have been detected by GCMS from the oil shales.The n-alkanes of marine oil shale samples show a dominance of low carbon molecular components（∑C-21/∑C＋22ranges from 0.78 to 3.43 with an average of 1.57）with highest peaks of nC17 or nC（19）.OEP values of the n-alkanes show no odd-even carbon dominance of 0.77-1.12（averaging 0.95）.Pr/Ph ratios range from 0.33 to 1.28（averaging 0.71）with most Pr/Ph less than 1.0,showing a weak a weak phytane dominance.The terpanes distribution abundances of oil shale samples are pentacyclic terpanestricyclic terpanestetracyclic terpanes.he steranes presents an asymmetric Vshaped distribution with C27-steranes C28-steranes C29-steranes.All these characteristics indicate organic matters in marine oil shale are of algae and bacterial origin.The carbon isotopicδ13C compositions of n-alkanes range from-29.89‰ to-23.73‰ with an average of-26.99‰ and show an horizontal distribution with slight decline.Theδ13C of pristine and phytane of isoprenoid hydrocarbon range from-30.72‰to-24.57‰ with an average of-28.39‰ and range from-29.87‰ to-27.24‰ with an average of-28.52‰ respectively.The n-alkanes are enriched inδ13C by 0.02‰-3.66‰ and 0.37‰-3.69‰related to pristine and phytane respectively.The unconventional abnormally positiveδ13C of nalkanes maybe caused by blooming of dinofragellates or colonial green algae（Botryococus）during that period.Microscopic observation reveals two types of organic matters：type II1（humic mud）and type II2（sapropelic-humic）.Vitrinite reflectances of kerogen range from 0.37% to 0.9% with an average of0.55%,indicating that organic matter of oil shale underwent a stage of immature to low maturation.Thermolysis peak temperature,sporo-pollen colour index,maturity parameter of n-alkanes biomarker and alkylphenanthrene parameter of aromatics also confirmed that the organic matter of oil shale was at the stage of immature to low maturation.