藏北羌塘盆地晚侏罗世海相油页岩生物标志物特征、沉积环境分析及意义

胜利河油页岩位于青藏高原腹地的羌塘盆地,具单层厚度薄、区域延伸长的特点,为羌塘盆地新发现的又一处海相油页岩.采用GC-MS(IRMS)分析方法对该油页岩进行了研究,结果表明,胜利河油页岩(11层以及13层)含有丰富的正烷烃、类异戊二烯烃、萜类化合物和甾类化合物.正构烷烃呈前高后低的单峰型分布,nC15、nC16为主峰碳,轻烃组分占有绝对优势,OEP值0.96～0.97,接近平衡值1.00,Pr/Ph为0.57～0.75,显示弱的植烷优势;萜烷丰度顺序为五环三萜烷》三环萜烷》四环萜烷;规则甾烷呈不对称的"V"字型分布,表现为C27》C29》C28的分布特征.这些特征与剖面中其他岩性的生物标志物特征存在一定的差异,9层泥晶灰岩C21-/C21 值为0.86,轻烃优势并不明显;14层泥灰岩C21-/C21 值为0.41,具有明显的重碳优势;9层泥晶灰岩以及12层泥灰岩规则甾烷呈C29》C27》C28的特征也与油页岩层存在显著的差异.这些差异不仅反映了油页岩层与其他岩性间生物母源输入的差异,也反映了他们之间沉积环境的不同.胜利河地区油页岩的形成是综合因素控制的结果,古地理变化、海平面升降、生物群差异以及气候的变化均对该区油页岩的形成有一定的影响.

Biomarkers and sedimentary environment of Late Jurassic marine oil shale in Qiangtang basin, northern Xizang and its geological significance

Shenglihe oil shale, located in Qiangtang basin, northern Qinghai- Xizang (Tibet) plateau, with a length of more than 2.5 km along the east- west direction and a width of 9 m along the north- south direction, is important to assess oil and gas resources in Qiangtang basin. Abundant biomarkers, including n- alkanes, isoprenoid, steranes and terpanes, are detected by GC- MS from the oil shale section. The n- alkanes of marine oil shale samples show a dominance of low carbon molecular components with highest peaks of nC15 or nC16. OEP values of the n- alkanes show no odd- even carbon dominance. Pr/Ph ratios indicates oil shale possess weak phytane dominance. The terpanes distribution patterns of oil shale samples are pentacyclic terpanes > tricyclic terpanes > tetracyclic terpanes. The steranes consist mainly of C27- steranes, C28- steranes and C29- steranes. According to abundance, C27- steranes > C29- steranes > C28- steranes, pregn- steranes and 4- methyl- steranes are also identified. All these characteristics indicate organic matters in marine oil shales are of algae origin. Various features are observed from the marl strata including samples XP- 12 and XP- 14 and micritic limestone strata (sample XP- 9) from this oil shale section. ratios show a slightly and strong dominance of higher carbon molecular for sample XP- 9 or XP- 14, respectively. The characteristics of C29- steranes > C27- steranes > C28- steranes are also detected from samples XP- 9 and XP- 12. The above differences are in close relation with organic matter import and sedimentary environment. The deposition of the Shenglihe marine oil shale was controlled by many factors, such as lithofacies palaeogeography, sea level, biota, climate and so on.