Organic Geochemical Characteristics of Mudstone and Marl from Western Hoh Xil Basin of Tibet

Abstract

The mudstone and marl from western Hoh Xil basin, located in Tibet of the west of China, were deposited in Tertiary lacustrine environment. Investigation of organic geochemistry, sedimentary characteristics, and 13C in kerogen was conducted to analyze the sedimentary environment, biomarkers, paleoclimate, and source of organic matter during deposition. The Cenozoic sedimentary facies of the basin included upper lacustrine facies and lower alluvial fan facies, which belong to Miocene Wudaoliang Formation and Oligocene Yaxicuo Group, respectively. The Miocene marl-sandstone-mudstone from Wudaoliang Formation was analyzed. Maceral composition was dominated by amorphous organic matter. $T_{\max }$ values indicated that the mudstones were thermally immature-low maturity with mainly type II and III organic matter, while organic matter in marlite belongs mainly to type I-II1 with low maturity-maturity stage. The biomarkers showed the characteristics of odd-over-even predominance of long-chain n-alkanes, higher proportion of C27 sterane in most of the samples, heavy δ13Corg composition, low Pr/Ph ratios (0.11-0.36), and so on. Organic geochemistry indicated that the organic matter originated from bacteria, algae, and higher plants. The rocks were formed in reducing environments with stratified water column and high productivity. The paleoclimate became more humid during depositional stage in the western Hoh Xil basin.