To understand interrelationship between geological and petrophysical parameters, the petrophysical and petrographical data from Ilam reservoir in one of the oil fields in North Dezful (SW of Iran) were integrated. 10 different microfacies were recognized via detailed petrographical studies. In upper Ilam the Ooidal-peloidal grainy fabrics are abundant indicating a high energy shoal environment whereas, in middle and lower parts the muddy textures are common, wackestone, rarely packstone in middle part and lime mudstone to wackestone in lower part, suggest low energy and fairly deep depositional environment. The most important diagenetic processes are early stage physical compaction and calcite cementation in upper part and dolomitization and pyritization in middle/lower parts. The grain- supported fabrics in upper part are intensely cemented, occluding the intergranular pore spaces in most cases. Therefore, the diagenesis has a serious destructive effect on the reservoir quality comparing to the depositional environment.