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. IMG SRC=IMAGE/10310044.JPG HEIGHT=11 WIDTH=40 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.