Deposition Of The Organic Matter-rich Fine-grained Sedimentary Rocks In The Cenozoic Lake Basins,Central Tibetan Plateau

With the rapid economic development of China,demands on the domestic energy supply are now becoming increasingly serious;over 60%of the oil consumption is imported now.Tibet is the largest under-explorated area of China and has the greatest potential to achieve the breakthrough of oil exploration.The Cenozoic lacustrine strata of the Tibetan Plateau have attracted increasing attentions from the petroleum geologists considering their promising hydrocarbon potential.Moreover,the successive Cenozoic sediments in these basins record the tectonic and climatic evolution of the Tibetan Plateau providing us a unique opportunity to investigate the formation and expansion of the plateau and its interactions with global and regional paleoclimate.In this study we choose the organic matter-rich fine-grained sediments in the Lunpola Basin and the Namling-Oiyug Basin as representatives.Methods of sedimentology,cyclostratigraphy,and organic and elemental geochemistry were conducted to investigate their sedimentary environments,provenance,geochronology,regional paleoclimate,and organic and elemental properties and to further reconstruct their depositional history and mechanisms.Our results indicate that the organic matter-rich fine grained sedimentary rocks of the Dingqinghu Formation,Lunpola Basin were mainly composed of Type I kerogen and deposited between 25.93-16.64 Ma;however,the TOC and HI values decrease rapidly at the upper part of the section which likely resulted from the regional uplift induced dry event at 23.53 Ma.Provenance analysis demonstrates that the source area of the samples is quite stable and seems mostly like the Baingoin batholiths consisting primarily of Cretaceous granodiorite and adamellite.Catchment reconstruction confirms that the drainage system contains a main river originating from the south,an upstream Baingoin Lake,and a downstream Lunpola Lake.The chemical indexes of alteration(CIA)values signify that in this drainage system,paleoclimate clearly restricted the development of rivers around the lakes.Although terrestrial organic matter(higher plants)dominated the drainage area,the main river originating from the south could only bring a certain amount of them to the lakes.Moreover,considering the filtration of the upstream ancient Baingoin Lake,minor terrestrial organic matter could be carried and deposited in the ancient Lunpola Lake.Therefore,lacustrine algae could proliferate as the main organic source and preserve in the stratified hypersaline lake to form the largest oil shale resource in Tibet.For the fine grained sedimentary rocks of the Mangxiang Formation(Oligocene-Miocene),Namling-Oiyug Basin,two periods could be divided during the lake basin evolution history:the balanced-fill and overfilled stages.In the balanced-fill basin stage,oil-prone laminated shale was deposited in a relatively deep lake environment,with total organic carbon(TOC)contents around 5 wt.%,and HI values up to 316 mg HC/g TOC,indicating hydrogen-poor Type II kerogen with transition to Type Ⅲ kerogen.Comparison between maceral composition and geochemical properties suggests that the relative low HI values are results of thermal maturity.The high value of the gammacerane index indicates water stratification of the lake.In this stage,water and sediment input approximately equaled potential accommodation,the lake was relatively stable and hosted large amounts of lacustrine algae,and accumulated OM with low terrestrial input in an anoxic environment.After the onset of the thrusting,the potential accommodation was decreased,large volumes of clastic sediments also drained into the lake from the surrounding area,resulting in an overfilled lake basin.During this stage,peat was deposited in freshwater environment,with>30 wt.%TOC,low HI,and Type Ⅲ OM.The high Pr/Ph(>3)implies oxic conditions.