We studied the microfacies types and sedimentary settings of carbonate rocks of the Upper Ordovician Qrebake and Lianglitage Formations from 12 wells of the Yubei area

by means of microfacies analysis.Eight kinds of carbonate microfacies have been identified based on the microscopic observation of grains

biological assemblies

matrix and structural characteristics.Based on how the microfacies types are associated to each other

combined with drilling and logging data

as well as seismic profile

palaeontological and geochemical analyses

we believed that the Upper Ordovician carbonate rocks are mainly developed in the sedimentary environments of open platform

platform margin and marginal-platform slope.The sedimentary facies in the Late Ordovician differ from the east to the west.The boundary is delineated along Well YB5-Well SH2-the south of Well PSB2

with marginal-platform slope-continental shelf facies in the east

and carbonate platform facies in the center and the west.In conclusion

the Upper Ordovician in the Yubei area has evolved from open platform

to platform margin

to marginal-platform slope

and then to continental shelf facies in vertical sequence

and the variation of sedimentary environments in the area is mainly controlled by regional tectonic movements.