THE EARLY JURASSIC OIL SHALES IN THE QIANGTANG BASIN, NORTHERN TIBET: BIOMARKERS AND TOARCIAN OCEANIC ANOXIC EVENTS

Abstract

In the Qiangtang Basin, northern Tibet, the most complete and extensive marine sedimentary strata outcropped in the Shuanghu-Sewa-Amdo area during the Jurassic, especially the Early Jurassic. The organic-rich marine sediments - commonly referred to as black shales - were deposited in the Early Jurassic, therefore, many petroleum geologists have been focusing on them for many years. Although achievements in geological investigations and petroleum resource assessments during recent years have been remark- able, the environmental conditions, mechanics, and process that resulted in the deposition of high-organic sediments during the Early Toarcian (183- 176.5 Ma, Early Jurassic) Oceanic Anoxic Event are still a matter of dis- cussions. In this paper, we deal with the biomarker distributions of Lower Jurassic oil shales in the Biluo Co section, Shuanghu area of northern Tibet. The oil shales are characterized by a marked predominance of short chain n- alkanes with a maxiumun at C16 or C17, nC17/nC31 ratio values between 9.4 and 17.8, and low Pr/Ph ratios. Furthermore, a series of C27 and C29-C35 hopanes with minor amounts of gammacerane are present in all samples, as indicated by gammacerane/C30-17α-hopane values from 0.06 to 0.12 and the steranes C27/C29 ratios higher than 1 in the three samples. The above- mentioned parameters indicate that the organic matter source is attributed to an algal/bacterial contribution. According to maturity parameters, all the homohopane 22S/(22S+22R) values in this study are > 0.58 and the sterane 20S/(20S+20R) values are all between 0.48 and 0.59, which is consistent