Abstract: The authors studied the organic geochemical characteristics of the oil shale of the Middle Jurassic Shimengou Formation in the Tuanyushan area of the Northern Qaidam basin. The hydrocarbon generation potential, origin of organic matters, depositional environment and metallogenic differences were discussed based on Leco, rock pyrolysis, GC and GC-MS analysis. The results show that three layers of oil shales are developed in the shale member of the Shimengou Formation, and the content of organic carbon is high with an average of 8.16%; the organic matter contains type Ⅰ and type Ⅱ₁ kerogens, with better hydrocarbon generation potential in the first and second layers. Abundant biomarkers, including n-alkanes, isoprenoid, sterane and terpenoid were detected in the samples. The n-alkanes have the features of unimodal distributions with nC₂₃ or nC₂₇ as mean peaks, Pr/Ph from 0.20 to 1.08, C₂₇/C₂₉ sterane ratios, aryl isoprenoids, hopanes and hopenes of the oil shale in the first and second layers are significantly higher than those in the third layer, revealing that the oil shale in the first and second layers were formed under strong reduction conditions, and the organic matters are mainly lower aquatic organisms, whereas the oil shale in the third layer was formed in a weak reduction to reduction environment, and the organic matters are characterized by dual origins of lower aquatic organisms and higher plants. Combined with the differences in the characteristics of oil shale and paleo-lake conditions, it is suggested that deeper lake water is not necessary for the formation of high class oil shale; while the biological input of high class organic matter such as algae and bacteria and strong reduction conditions of bottom water are more conducive to the formation of high class oil shales.

Key words: Qaidam basin, Shimengou Formation, oil shale, organic geochemistry, origin of organic matters, depositional environment