The Yanchang Formation in Zhenjing area

southwestern Ordos Basin

contains typical low-porosity and low-permeability tight sandstone reservoirs.Core observation and logging data analyses of more than 20 wells drilled in the reservoirs were combined with experimental means like microscope and scanning electronic microscope to study the litho-logy and sedimentology of the reservoirs.Results show that the rocks are mainly lithic～arkoses with small amount of feldspathic litharenites.Sedimentary facies are dominated by the subfacie of braided fluvial delta

of which the microfacies-underwater distributary channel and mouth bar-have the best and second best petrophysical properties for hydrocarbon pooling.Analyses of relationship between reservoir physical properties and grain size of sandstone indicate that sandstone is the best in terms of reservoir physical properties.Diagenesis study of the area shows that three factors had played critical roles

namely compaction

cementation and dissolution.Compaction caused porosity(and permeability)of the rocks to decrease to less than 10% at a depth deeper than 2 400 m.Cementation was both destructive and constructive in that it reduced porosity of the rocks but meanwhile it also restrained further compaction and kept the particles apart.Dissolution was favorable for reservoir physical properties as the porosity was greatly improved during two stages of dissolution in deep reservoirs.