Abstract: The organic petrologic characteristic of coal is one of good assistant indices.The organic petrologic study of Yanchang coal and carbonaceous shale of Ansai oilfield in Ordos basin indicates that the macroscopic petrologic characteristic, material and composite relation of the biogenic rock as well as the occurrence of organic matter can be used to better reveal the marsh paleo-geographic environment and depositional site originated by the coal and carbonaceous shale.Also, the marsh environment is classified into autochthonous terrestrial peat bog, blanket bog, moss bog and allochthonous floating bog.These can be as a significant guide for determination and evolution of paleo-lake strandline at Yanchang series of Triassic and classification of the depositional facies.

Key words: Ordos basin, coal, peat, marsh, depositional environment