DRAINAGE PATTERNS CONTROL ON SOURCE ROCK DEVELOPMENT IN CENTRAL TIBET (LATE OLIGOCENE)

Cenozoic lake basins are developed along the Bangong-Nujiang suture zone (BNSZ), central Tibet and Oligocene source rocks are distributed in them. These years, much attention has been paid to these source rocks considering their important roles in studying petroleum geology as well as tectonic and climatic evolution of the Tibetan Plateau. Over all, BNSZ source rocks are immature to marginal mature; however, their hydrocarbon potentials are quite different and only one basin (the Lunpola Basin) contains dominantly good to excellent source rocks with Type I kerogen. To decipher reasons resulting in this differentiation, we conduct sedimentological and geochemical works on source rocks collected from four different lake basins. Our results indicate that tectonic induced drainage patterns rather than climatic conditions are the primary control. A long-lived hierarchical drainage system is developed in the Lunpola area while other basins were covered by single lake systems. The hierarchical pattern could minimize clastic dilution effects during tectonic active and/or relatively wet period and prevent establishment of hypersaline environments during dry period. So the drainage center in this kind of hierarchical system has lower sedimentation accumulation rate and could receive much more aquatic organic matter (OM) at any climatic conditions. Thus, compared with single lakes developed in other basins, a long-lived hierarchical drainage system ensured the deposition of OM with high quantity and quality in the Lunpola Basin.