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. The most common visible porosity types are mold ie, vuggy and intergranular in upper part, moldic, enlarged moldic, intrafossil and occasional fractures in middle part, and moldic and vuggy in lower part. The most suitable zone for hydrocarbon production is located in upper part coinciding with grain supported textures having intergranular porosity. Despite of the high total porosity occurring in middle part, the production potential is very low due to a pore network with narrow pore throats and microporosity which greatly reduces the permeability. Water saturation abruptly increases in the boundary of middle -lower parts probably due to variation in microfacies, higher clay conent and low porosity which altogether form a porosity system with tight pore throats and conduits, weakening the oil accumulation assumption in the ilam reservoir through vertical migration.