Study On The Middle-Late Permian Paleontology And Stratigraphy Of Northern Tibet And Its Geological Significances

The Middle-Upper Permian in the Tethys tectonic domain contains abundant oil and gas resources,and the Middle-Upper Permian is an important oil and gas producing horizon in the Middle East region.Located in the eastern section of the Tethys domain,the Qinghai-Tibet Plateau is a part of the middle subsection of the Tethys tectonic domain,which has the basic geological characteristics of forming large and medium oil and gas fields.For the Qinghai-Tibet Plateau,it is considered that the favorable strata for oil and gas exploration are mainly concentrated in Mesozoic,and not enough attention has been paid to the exploration of Paleozoic oil and gas resources.In order to scientifically evaluate the northern Qinghai-Tibet plateau region-Permian series of oil and gas prospects in the Lhasa block in the selection of Songyalaiga section,South Qiangtang Land Block Samengxiong section,The section of Qianggong Village in the suture zone of Yarlung Zangbo River,and the Meso-Upper Permian strata in the southwest section of Rejuechaka in the northern Qiangtang block,from the biostratigraphic,palaeogeography,oil and gas exploration and evaluation of three aspects of analysis,The following research progress has been made:Stratigraphy :(1)The Middle-Upper Permian stratigraphic sequence of Lhasa block has been perfected.The Upper Permian Asuo Formation is newly established in Songyalaiga section,Asuo Township,Lhasa Block.The Asuo Formation is mainly composed of limestone,which is a set of strata containing bioclastic limestone and chert nodular limestone.The discovery of Asuo Formation indicates that there are not only the Wenbudangsang Formation of carbonate platform slope facies and the Mujiucuo Formation of evaporite platform facies,but also the normal shallow Marine carbonate platform facies between them in the Upper Permian of Lhasa Block.(2)Complete the Middle-Upper Permian stratigraphic sequence in the South Qiangtang block.A section of chert-bearing nodules limestone and dolomite with few large fossils is identified on the Longge Formation of the Samenxiong section,and the regional contrast is the Wenbudangsang Formation.Compared with the upper Zishisang Formation in the region,coral fossils were found in the lower part of the dolomite,indicating that the lithostratigraphic unit is from the lower end of the Zishisang Formation to the Late Permian.(3)The upper limit of the age of the Middle Permian Langco Formation in the Yarlung Zangbo River strata area has been revised.According to the fossil age of brachiopods,the upper limit of the Langco Formation age has reached the Late Permian.There is an integrated contact between Langco Formation and the overhanging Qiongguo Group of the Lower Triassic.The discovery of Qiangongcun profile completes the continuous Marine sedimentary sequence of the Middle Permian-Lower Triassic in the Yarlung Zangbo River stratigraphic area.In terms of paleogeography: According to the environmental significance indicated by benthic organisms,the Lhasa block and the southern Qiangtang block were located in the low latitude region within30° south latitude during the Middle to Late Permian.In the Lhasa block and south qiangtang block-Permian series with the same stratigraphic sequence,with a similar type of benthic,according to the features of biological combination and the stratigraphic sequence,think in the Lhasa block and south qiangtang block-late Permian itself interconnection together,can be considered as a whole,common within 30° to the south of the equator in low latitude area,has the high Marine productivity.Oil and gas exploration and evaluation: the lower assemblage of the Guge strata is an important oil and gas exploration layer in the Middle to Late Permian in northern Tibet.In terms of sedimentary environment,it is similar to the oil-gas producing areas in the Middle East and Puguang oil and gas field in Sichuan.Carbonate rocks of the Middle Permian Xiala Formation and Longge Formation in northern Tibet are widely distributed in the region.The Upper Permian is a cross zone of shallow Marine carbonate facies,carbonate slope facies and dolomite evaporative facies represented by Asuo Formation,Wenbudangsang Formation and Mujiucuo Formation.Lhasa block in the carbonate platform facies and normal evaporite platform facies transition zone,is with characteristics of hydrocarbon generation,a group of bioclastic limestone and the wood error correction with characteristics of reservoir group of dolomite formed by combination with each other,under the influence of sea-level change role,forming a vertical superimposed and different lithologic pinchout on the lateral,is advantageous to the oil and gas generation and storage nearby,Formation of lithologic reservoirs.