Sedimentary Characteristic Of Cretaceous Gamba-Tingri Basin And Its Implication For Source Rock Potential

The transition of late Cenomanian to early Turonian is characterized by a well known Oceanic Anoxic Events (OAE2) associated with widespread bituminous black shales. The cause of enrichment of organic carbon in the black shales has been under vigorous discussion. The Cenomanian-Turonian black shales, which were regarded as the sediments of OAE2, were found in Gangba, Tingri, and Gyangze areas in southern Tibet. A combined research from 4 well documented sections sedimentary and organic geochemistry in these areas indicates that black shale and darkish calcareous shale of Lengqingre Formation depositioned on shallow shelf at Gamba, black shale of Gangbadongshan Formation depositioned on carbonate ramp at Tingri and black shale of Gyabula Formation depositioned on lower slope at Gyangze provide moderate source rock potential which is characterized by an average TOC more than 0.8%, only some more than 2%, and is originated from marine plankton, bacteria and algea. However, the TOC value of the black shale is apparently lower than those in the western Tethys. The plots of mass accumulation rates of TOC and calcic carbonate present a same trend as a certain section at Gamba, and reach their maximum value at C-T boundary (CTB) ,but show a different trend in Gyangze possibly due to increased carbonate mass accumulation rate has diluted the organic matter content . We calculated the paleoproductivity of sediments during CTB based on the mass accumulation rates, an average values 77.96gC /m2·a and 11gC /m2·a at Gamba and Tingri respectively, which is disagree with the result come out from positive excursion ofδ13 C ,high sedimentation rate and possible upwelling. It certainly be the result of weathering. The paleoproductivity of CTB black shale shows average values of 46.0 gC /m2·a and 17.3 gC /m2·a from Gyabula section and Chuangde section respectively based on a transfer function between carbonate mass accumulation rates and paleoproductivity which is suit for open ocean environments. It also likely resulted in organic-rich black shale deposition during the CTB for its restricted deep-water circulation just like Black Sea. Moreover, the correlation between TOC and sedimentation rate may be used as a paleoenvironmental indicator, indicates that organic carbon-rich sediments should be deposited and the source rock potential of black shale should be much more than expected in southern Tibet. Detailed analysis on paleoproductivity, sedimentation rate, lithology and compositon of organic matter in the different depositional environment indicated that high paleoproductivity, high sedimentation rate and fine-grained sediments is more likely to organic carbon-rich sediments, and kerogen deposited at shelf, slope and abyssal basin has a good oil source rock potential.