Coupling Relation Between Tectonic Evolution And Sedimentary Filling In Central Part Of North Margin Of Qaidam Basin During Paleogene-Neogene

Guided by multidisciplinary theories including tectonics,sequence stratigraphy,sedimentology,organic geochemistry,and hydrocarbon accumulation theory,take into account of macroscopic tectonic setting together with microscopic architecture,geology together with geophysics,as well as static analysis together with dynamic evolution,the author carried out basic study on structural modeling,sequence division and evolution of sedimentary filling,then it is feasible to analyze fault activity and its influence on sedimentation,sandbody distribution,reservoir-seal assemblage,coupling relation between tectonic evolution and sedimentary filling,hydrocarbon generation history and reservoir forming in Paleogene.Under influence of NE-SW directed compression between Qilian Block and Qiangtang Block and left-lateral slip of Aerjin Mountain,thrust and nappe structural belt in central part of the north margin of Qaidam basin was mainly controlled by the south Qilian boundary fault and intrabasinal Sainan fault-Lvnan fault and Maxian fault,and exhibited in rhombic shaped tectonic framework during Paleogene-Neogene,which is dominated by NW-SE directed fault steps.Maxian fault and Sainan fault keep moving continuously since late Mesozoic Era with ever increasing intensity,and this controlled sedimentation and erosion in two walls of the fault.This induced big difference in thickness between two walls of the fault.Lvliang low-relief upwarping played no effect on sedimentation during Palaeocene-Miocene.The Lvnan fault started its intensive activity since Pliocene,Pre-Mesozoic Erathem exposed in the upthrown wall,and strata in the downthrown wall folded into Mahai-Dahonggou anticlinal belt.In the rhombic tectonic framework,the three dominant provenances,namely Aolaohe-Lulehe,Yuka,Hongshan-Dahonggou,accumulated along the west flank of Mabei upwarping.During late Himalayan period,boundary fault and intrabasinal fault intensely thrusted,and resulted in explicit sedimentary hiatus and denudation.Since Paleogene,Qaidam basin experienced long term arid environment,which is characterized by multiple cycles of transgression-regression.Influenced by palaeogeomorphology,lake level fluctuation,divergence in supply capacity among different provenances,there well developed complete system tracts and formed 3-element typed third level sequence.There are two group and three kind of sequences including low stand system tract,partly denuded lacustrine transgressive system tract and partly denuded lacustrine regressive system tract.4 kinds sedimentary facies developed such as alluvial fan,braided river,braided river delta,and shore-shallow lacustrine.,Alluvial fan-braided river were dominate facies in early the period,braided river-braided river delta-shore shallow lacustrine were dominant in the middle period,and alluvial fan-braided river were dominant in the late period.Multiple fining upward cycles that formed during Paleogene can provide good reservoir-seal assemblage in transgressive system tract.Overlapped structure and draped structure on top of Mabei-Lvliang low-relief upwarping is ready to form big scaled stratigraphic trap,structural trap,as well as lithologic trap.coupling relation between tectonic-sedimentation controlled lateral distribution of depositional system tract,and combination of system tract controlled sedimentary scale and reservoir-seal assemblage.Persistent subsided Gaxi sag with huge layers from Paleogene to Neogene accelerated hydrocarbon generation in lacustrine oil shale and dark colored limnetic argillaceous rock in Jurassic.Positive inverted structure such as Yuka sag has very little residual Paleogene-Neogene due to denudation,and hydrocarbon generation process in Jurassic was almost suspended.Mabei-Lvliang upwarping is located closely to Gaxi sag with superior condition of oil source and trap area,and it is recommended to lay exploration emphasis in Mabei-Lvliang upwarping.