Abstract:

In order to research the coal bearing features of Yan’an Formation in Ordos basin, the isochronous stratigraphic framework is established, then the controlling effect of accommodation space changes to coal formation can be analyzed. By analyzing the outcrops, boreholes, logging curves, the typical sequence stratigraphy boundaries were recognized. The Yan’an Formation are divided into 3 third-order sequences, and each of them are composed of lowstand normal regression systems tract, lake transgressive systems tract, highstand normal regression system tracts, and forced regression systems tract. The thick coals mainly developed in the middle and late periods of the lake transgressive systems tract and highstand normal regression system tract. In north part of the basin, the mid-thick and stable coal seams developed on the flat landform. And in south part of the basin, thick-mega and unstable coal seams developed. From the lakeshore plain to delta plain to flood plain of river, the thick coal seams located in the bottom to middleupper part of the fourth base level uprising cycle gradually. It can be concluded that, in different sedimentary area, position of the balance between accommodation space changes rate and peat accumulation rate was different, and the thick coal beds mainly developed at the balance position. The development of thick coal beds migrated following the sedimentary changes.

Key words: stratigraphy, forced regression, accommodation space, coal formation, Yan’an Formation, Ordos basin