Yi Haisheng(Oil Shale)THE EARLY JURASSIC OIL SHALES IN THE QIANGTANG BASIN, NORTHERN TIBET: BIOMARKERS AND TOARCIAN OCEANIC ANOXIC EVENTS
日期:2013年09月05日 01:48:00
 

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

作者:Yi, Haisheng; Chen, Lan; Jenkyns, Hugh C.; Da, Xuejuan; Xia, Minquan; Xu, Guiwen; Ji, Changjun

期刊:OIL SHALE,2013,30(3),441-455

WOS000323548500005

摘要: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 remarkable, 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 discussions. 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 C-16 or C-17, nC(17)/nC(31) ratio values between 9.4 and 17.8, and low Pr/Ph ratios. Furthermore, a series of C-27 and C-29-C-35 hopanes with minor amounts of gammacerane are present in all samples, as indicated by gammacerane/C-30-17 alpha-hopane values from 0.06 to 0.12 and the steranes C-27/C-29 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 with a higher level of thermal maturity. Widespread anoxia led to the deposition of organic-rich sediments that removed isotopically light carbon from the oceans and drove carbon-isotope ratios to higher values with a positive excursion close to 2.17 parts per thousand. From biomarker distributions, the differences in delta C-13(kerogen) are related to the difference in delta C-13 of CO2 in the photic zone rather than the organic matter compositions.