Analysis on depositional system of 1st member of Yan'an formation in Northern Ordos Basin

Yan'an formation is a set of coal-bearing stratum in Erdos Basin, which can be divided into 5 member according to the 5 coal. As to the depositional systems, they are the incised valley fulfilling system in LST (1st member), lacustrine-delta depositional system in EST (2nd, 3rd, 4th member) and river alluvial system (5th member). The Suhaitu program mainly focus on the 1st member of Yan'an formation which is braided river sediments, and interlayer-oxidation belt is correlated to its uranic metallogenesis. Based on the sequence stratigraphy theory, The thesis adopts the idea from the 'point' to 'line' to 'area', which are firstly single drill's vertical sequence diagram, secondly regionally stratigraphic contrast, thirdly sandbody dispersed system map, fourthly depositional profile and the target stratum's depositional system tract map. Finally the thesis ascertain two type braided river, one is the steep slope gravelly braided river developed in the middle of northern work area, whose genetic facies include gravelly bars and channel; the gentle slope braided river is mainly composed of channel sediments, sand bar, flood plain and flood fan. The main difference between the channel sediment and sand bar is the top fine mudstone and fine stone deposit-flooding falling sediments that is developed in sand bar. Generally speaking, its channel margin deposit doesn't develop well in braided river sediment. (author)