New Age Constraints of the Bilong Co Oil Shale in the Qiangtang Basin, Northern Tibet: Evidence from In Situ U–Pb Dating and Palaeontology

Author:

Zhang Haowei12,Wang Jian123,Mansour Ahmed124ORCID,Zhang Jianyong5,Wei Hengye12,Fu Xiugen123,Shen Lijun12,Xiong Shaoyun5,Ahmed Mohamed S.6ORCID,Gentzis Thomas7ORCID

Affiliation:

1. School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China

2. Qiangtang Institute of Sedimentary Basin, Southwest Petroleum University, Chengdu 610500, China

3. National Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610050, China

4. Geology Department, Faculty of Science, Minia University, Minia 61519, Egypt

5. Key Laboratory of Carbonate Reservoirs, CNPC, Hangzhou 310023, China

6. Geology and Geophysics Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia

7. Core Laboratories, 6316 Windfern Road, Houston, TX 77040, USA

Abstract

The Bilong Co oil shale is one of the most significant source rocks in the Mesozoic Qiangtang Basin (Northern Tibet); however, its absolute chronology remains controversial. In this study, in situ carbonate U–Pb isotope dating analysis was carried out for the first time. Detailed field geological investigations yielded some age-diagnostic ammonites, enabling a re-evaluation of the stratigraphic age of the Bilong Co oil shale. A total of 61 spots of U–Pb isotope dating from the middle part of the Bilong Co oil shale section suggests an average age of 181 ± 13 Ma. Elemental geochemistry and diagenetic analysis indicate that the proposed age represents the early deposition of the calcite, and the oil shale was deposited during the Early Jurassic time. This estimated age is further supported by the newly discovered ammonite assemblage of Hildoceratidae–Tiltoniceras sp. at the top part of the oil shale section, which confirms the deposition of the oil shale during the Toarcian age of the late Early Jurassic. Consequently, the Bilong Co oil shale can be assigned to the Quse Formation, which is attributed to the Lower Jurassic rather than the Middle Jurassic. The re-assessment of the stratigraphic age of the Bilong Co oil shale is of great significance for regional evaluation and exploration activities of hydrocarbon source rock layers in the Qiangtang Basin as well as for global stratigraphic correlation of the late Early Jurassic Toarcian oceanic anoxic event.

Funder

National Natural Science Foundation of China

Second Tibetan Plateau Scientific Expedition and Research Program

Researchers Supporting Project

14th Five-Year Prospective and Basic Research Project of Petro China

Publisher

MDPI AG

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