Abstract: The principles of sedimentology were used to study the sedimentary characteristics,forming process,dictating factors and petroleum exploration significances of reworked sands in the Ordovician Pingliang Formation at southwestern margin of Ordos Basin based on outcrops,restoration of paleocurrent and geochemistry data.The result shows that reworked sands are relatively well developed in the study area and they are mainly affected by factors including provenance,relative sea level,topography and tectonic activities.The Pingliang Formation in the study area is unitary in lithology and various in sedimentary structures and can be divided into three typical facies.Facies 1 of mudstone with horizontal bedding represents deep water autochthonous sediments.Facies 2 of sandstone with cross bedding indicates turbidity sediments.Facies 3 of sandstone with bimodal cross bedding points to reworked sands.The typical characteristics of reworked sands are as follows:1) relatively well-sorted and -rounded grains of bimodal grain-size distribution;2) two predominant directions of paleocurrent,with one going down along slopes,indicating the direction of turbidity current movement,and the other running parallel to slopes,showing the direction of contour current flowing;3) obvious bioturbation;4) one to tripartite divisions of cumulative frequency curves,showing both gravity and tractive current features;5) upward-fining normal grading with erosive top;and 6) relatively abundant in trace elements (B,Cr,V,and etc).While flowing westward roughly parallel with the slope,the contour current in the study area could have helped to improve reservoir properties of the reworked sands through winnowing,transporting,reworking and re-deposition of the southeastward running turbidity sediments down the slope.Both turbidity sediments and coarse-grained contour current sediments could serve as good oil and gas reservoirs.At the same time,fine-grained deep water autochthonous sediments and contour current sediments could be quality source rocks.It is very likely for the reworked sands,contour current sediments and autochthonous sediments to combine with one another to form ideal hydrocarbon generation-accumulation-sealing assemblages in the study area.

Key words: turbidity, contour current, reworked sand, Pingliang Formation, Ordovician, Erdos Basin