Sequence-paleogeography and coal accumulation of the Upper Triassic Wayaopu Formation in Ordos Basin

The sedimentary facies,sequence stratigraphy,lithofacies paleogeography and coal accumulation of the Late Triassic Wayaopu Formation in Ordos Basin were analyzed based on data from borehole cores and logging curves. The Wayaopu Formation was subdivided into five members based on the lithology,of which the first (bottom) and the fifth ( top) members consist of mudstones and siltstones,the second,the third and the fourth members are mainly composed of sandstones in the lower part,and of mudstones and siltstones in the upper part. The sequence boundaries were iden- tified,including the regional unconformity at the top of the Wayaopu Formations,the basal erosional surfaces of the fluvial incised valley fill sand bodies at the bottom of the first,third and fourth members,and abrupt changes in color and lithologies at the boundary between the Wayaopu Formation and its underlying formations. According to these sequence boundaries,the Wayaopu Formation was subdivided into three third-order sequences,namely sequence I,sequence II and sequence III,which were corresponding to the intervals from member 1 to member 2,member 3,and member 4 to member 5,respectively. The lithological data of each third-order sequences were statistically counted,and the contours of a variety of these lithological parameters were drawn,including strata thickness,thickness ratio of sandstone and mudstone,and the thickness of coal seam. According to these contours,the lithofacies paleogeography maps of each third-order sequence were reconstructed. The major paleogeographical units were the fluvial facies,delta facies,and la- custrine facies,with a northeastern provenance. The coal accumulation were favored in sequence III than sequence I and sequence II. The most favorable coal-forming environment was the inter-distributary bay of the delta plain,and the coal-accumulating center was located in the Shiwanzhen-Yujiawanzhen-Ansaixian area.