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.