The main reservoir facies of the Upper Dalan Member consists of grain-supported ooilithic facies. This facies has been studied in terms of reservoir characterization and a core interval of the Upper Dalan Member (about 272m thick) was studied in the subsurface section of the Persian Gulf. According to sedimentological study, the facies can be subdivided into three microfacies; fine-grained peloid / ooid grainstone, medium-grained ooid grainstone and coarse-grained intraclast / bioclast ooid grainstone. These facies belong to lower intertidal (beach ridge) and oolithic-bioclastic shoal sedimentary environment. Effective factors on the reservoir subdivided into two sets; textural and diagenetic factors. Some cross plots and petrographical studies showing the significant effects of each parameter on the reservoir. Textural factors consist of grain size, grain shape, sorting and fabric and diagenetic factors as negative and positive effects includes of compaction, cementation, dissolution, dolomitization, anhydritization and fracturing.