Sequence-palaeogeography And Coal-accumulation Of The Middle Jurassic Yan’an Formation In Ordos Basin

The Ordos Basin is an important coal basin in northern China, with the Middle Jurassic Yan’an Formation to be one of its major coal-bearing series. The Yan’an Formation is mainly composed of conglomerates, sandstones, siltstones, mudstones and coals. A total of28lithofacies and5lithofacies associations were identified in this formation. The braided fluvial, meandering fluvial, delta and lacustrine depositional systems were recognized and4kinds of typical coal-bearing models were established.The sequence stratigraphic framework is further reconstructed according to the developments of key surfaces. The sequence boundaries were mainly characterized by regional unconformity, and base of incided channel sandstones, represented by the bases of Baotanshan, Peizhuang, Zhenwudong, Zhiluo sandstones. A total of3third-order sequences were subdivided, with each of them being compoed of lowstand normal regression systems tract, lake transgressive systems tract, highstand normal regression system tracts, and forced regression systems tract. The thick coals were mainly developed in the middle and late periods of the lake transgressive systems tracts and highstand normal regression system tracts and the coal accumulation models controlled by accommodation space were established. On the basis of the contour maps of the stratal thickness, mudstone thickness, sandstone thickness percentage, mudstone thickness and the total coal thickness, the lithofacies paleogeography maps of each systems tract combination (LNR&TST, HNR&FR) have been reconstructed. The rich coal centers were mainly developed in flood plain swamp, delta plain swamp, coastal plain swamp of lake. This paper analyzed of the main controlling factors of coal accumulation, and analyzed the genesis of the super-thick coal seam from the aspective of accommodation space creation rates balanced with peat/coal accumulation rates.