| Using the basic method of sedimentology, and all kinds of geologic documents, this paper has studied particularly on the sedimentary characteristics and the partition of sedimentary system of Yanan Formation in Ningdong area. Study shows that it has two depositional systems in Yanan Formation:river depositional system and lake-delta depositional system, and it develops meandering river facies in river depositional system while delta plain facies in lake-delta depositional system. By utilizing the principle of the high-resolution sequence stratigraphy, this paper divides Yanan Formation to be one super-long sequence cycle (SLSC1), and four long sequence cycle (LSC1-LSC4), eleven middle sequence cycle(MSC1-MSC11), as well as many short sequence cycle. By analyzing we can know that the Yanan Formation is mainly durain, with bright coal taking the second place. Under the control of paleoclimate, paleogeography, paleobotany and paleostructure, the advantageous coal-forming environment of the research area are river and lake-delta depositional systems, and we also summarize the corresponding coal-forming mode. Coal-accumulating processes occurs in most of the long base level sequence cycle, and we summarize the law of coal accumulation based on the analysis of distribution characteristics of the small thickness and stably distributed coal that developed in each long sequence cycle. The development and distribution of coal are controlled by the changing of the base level. That is to say it's the best period of becoming coal when A/C ratio that is caused by the flux changes of sediment compensation is close to 1.
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