Sedimentary and organic geochemical investigation of tertiary lacustrine oil shale in the central Tibetan plateau: Palaeolimnological and palaeoclimatic significances

• Published in: International journal of coal geology 2011-05, Vol.86 (2), p.254-265
• Main Authors: Wang, Licheng, Wang, Chengshan, Li, Yalin, Zhu, Lidong, Wei, Yushuai
• Format: Article
• Language: English
• Subjects: Biomarkers; Coal; Earth sciences; Earth, ocean, space; Exact sciences and technology; Organic matter source; Palaeoclimate; Palaeoenvironment; Sedimentary rocks; Stratigraphy; Tertiary Oil shales; Tibet
• ISSN: 0166-5162; 1872-7840
• DOI: 10.1016/j.coal.2011.02.011

The Zhuonai Lake and Lunpori oil shale, located in the central Tibetan Plateau, were deposited in a Tertiary lacustrine environment in the Hoh Xil and Lunpola Basins, respectively. A combined investigation of sedimentary, organic geochemistry, and organic carbon isotope was performed to define source rock potential, the palaeoenvironment, and source of the organic matter. Three types of facies sequences are developed in the Zhuonai Lake section and oil shale mainly occurs in the oil shale-marl sequence. The oil shale-silty claystone and oil shale-marl facies sequences are developed in the Lunpori section. Both oil shales are finely stratified especially for the Lunpori oil shale with thickness of the laminae about 10 μm which contains abundant pyrite crystals and fossilized fish bones. Both the Zhuonai Lake and Lunpori oil shale contain high values of total organic matter (TOC), S 2, and hydrogen index (HI), indicating the oil shales have good oil source rock potential. T max values show an immature stage of organic matter for the Zhuonai Lake oil shale and thermally marginally mature for the Lunpori oil shale because of difference in their burial history. Lunpori oil shale shows the characteristics of amorphous organic matter (AOM) dominance, odd-over-even predominance, maximum n-alkanes peak at nC 25 or nC 23, higher proportion of C 29 sterane, light δ 13C org values (− 30.0‰ to − 27.4‰), high concentrations of phytane (Pr/Ph ratio ranging from 0.02 to 0.18), high values of gammacerane index (up to 25.26), and presence of β-carotane, which are consistent with a reducing, stratified, and hypersaline palaeo-lake with main contribution of algae and bacteria to the organic matter (OM). Oil shale samples from the lower and upper part of the Zhuonai Lake section are characterized by low H/C ratios and high O/C ratios, amorphous humic OM and vitrinite dominance, odd-over-even predominance with long-chain n-alkanes dominance, heavy organic carbon isotope, and C 29 > C 27 sterane distribution, in agreement with a main contribution of land plant. The heavy organic carbon isotope suggests the expanding of C 4 land plant during the early Miocene. While the most samples show characteristics of AOM dominance, maximum peak at nC 29 or nC 31, and high C 27 sterane concentration, consistent with a main contribution from algae to the OM. The Pr/Ph ratio cannot be applied as a measure for redox conditions in the section because of maturity effects. Biomarker analysis indic