Sedimentary features of C-P coal bearing strata controlled by variation of accommodation spaces in east margin of Ordos Basin

The east margin of Ordos Basin is an important area for unconventional natural gases exploration and devel- opment in coal bearing strata,with extended study areas and complex sedimentary environment. The field outcrops, cores observation,and the cross sections of single and multiple wells were used to describe the tectonic movement, transgression and regressions,multiple direction sedimentary sources and their influence on the sequence stratigraphy and sedimentation evolution. The results show that the strata of Taiyuan Formation can be classified into four sequences and the Shanxi Formation of three sequences,based on the uniform of marked layers in different areas. The roof of the main coal seam in Taiyuan Formation is the tectonic transformation surface of the study areas,with the north and south areas being relatively leveled,and the subsidence continues in the middle areas consequently. The source rocks from the north influenced at the beginning of the Taiyuan Stage,while the obvious source rock supply was seen until Shanxi Stage. The 38° tectonic belt served as the fulcrum of the tectonic movement,also controlled the thickness and lithology variations in the coal forming strata,with a control on thick coal seams deposition. During the Taiyuan Stage,the water center of the transgression showed a transition from south to north,which varied from Yanchuannan area in Sequence (Sq1) to Daning-Jixian area in Sq2,and then to Liulin and Shilou areas in Sq4,and the Sq3 was relatively of the lar- gest transgression. The results would be benefitial for a clear understanding of the coal development rules in the study areas and also lay a foundation for unconventional natural gases exploration.