Abstract: The lithofacies and paleogeography of the Early-Middle Ordovician in the western Ordos Basin were reconstructed
based on the stratigraphic comparison and core and thin section observation of many exploration wells, combined with
measurements of multiple outcrops. The methodology is the single factor analysis and multifactor comprehensive mapping. Results
of characterization of stratigraphic thickness show that the Early-Middle Ordovician in the western basin has a palaeogeographic
pattern of three oldlands, one depression and one trough, i. e., the Alxa oldland, Iran oldland, Qingyang oldland, Dingbei
depression and Helan sea trough. Field measurement of lithologic profiles and comparison with drilling data show that along with
the extension of the Helan Trough and the extrusion to the North China plate eastward, the western basin experienced multiple
tectono-sedimentary stages. These include the successively sedimentation of carbonate rocks in the Sandaokan and Zhuozishan
periods, weakly striped platform deposition in the Kelimoli period, the slope and basin sediments in the Wulalike period.
Accordingly, carbonate reservoirs were developed, including the platform shoal reservoirs represented by the Zhuozishan Group,
the reef shoal reservoirs in the weakly striped platform margin represented by the Kelimoli Group. They are generally dolomitized
In the Early-Middle Ordovician, large amounts of deep-sea turbidites were developed in the study area, forming on extremely
thick flyschoid sedimentary formation, which has few hydrocarbon generation abilities. Only the graptolite-bearing shales that
formed in the Upper Ordovician Wulalike and Pingliang formations have some hydrocarbon generation abilities..

Key words: Ordovician, Paleogeography, Ramp slope, Weakly rimed carbonate platform, Grain shoal, Turbidite flow