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Two Major Source Rock Units of Bolin Basin, Southwestern Tibet Plateau

Wang, Licheng 1; Wang, Chengshan 1; Zhang, Xiangning 2; Li, Yalin 1
1 China University of Geosciences in Beijing, Beijing, China.
2 Petrochina, Beijing, China.

The Bolin Basin, located on the southwestern margin of Tibetan Plateau, is a Mesozoic marine petroliferous basin covering an area of 30000 km2. The basin has been received little hydrocarbon interest for its severe climate and life conditions, although it situates in the east of Tethyan realm,which is concentrated about 70% of the total world petroleum resources.

Two major source rock units have been identified to better understand the hydrocarbon potential through recent field work:the Upper Jurassic Menkadun Formation and the Lower Cretaceous Gangba Group. Their potential source rock successions, mainly composed of dark shales during Oxfordian-Tithonian and Aptian, respectively, were deposited in outer shelf. Total Organic Carbon (TOC), Maceral analysis, Rock-Eval, and vitrinite reflectance data of the samples from both units were used to map the present-day organic richness, type and thermal maturity. TOC content ranges from 0.67 to 1.62 weight percent with an average value of about 1 weight percent for the Menkadun Formation and 0.7 to 1.08 weight percent with an average value of about 0.86 weight percent for the Gangba Group. All shale samples are composed of a mixed type(oil prone) kerogen, dominated by amorphous organic matter. Rock-Eval Tmax and vitrinite reflectance data suggest that the Gangba Group is at late mature with respect to petroleum generation and the Menkadun Formation is overmature with respect to liquid hydrocarbon generation. These data indicate the shales of the units are good protential source rocks. Moreover, evidences from paleogeography and paleoclimate show that these source rock units deposited in a warm and moist environment with high sea level, and especially the dark shales of Gangba Group deposited in an interval of Ocean Anoxic Events (OAE1a). Therefore, the Bolin Basin has good potential for further exploration.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009