Abstarct: Understanding types and amount of porosity in hydrocarbon reservoirs is one of the most important and fundamental parameters in reservoir evaluation. In this research, petrographic analysis of thin sections, velocity deviation diagrams, and FMI image logs were employed to identify the types of porosity in the Ilam carbonate formation, in one of oil field in southwestern Iran. In the first phase of the research, petrographic studies of the Ilam Formation revealed that the wackstone to packstone facies in this formation exhibit various types of porosities, including intercrystalline, intergranular, vuggy, intragranular, moldic, micro porosity, and fracture porosity. In the secondstage of research, baxsed on the produced synthetic velocity deviation log, the Ilam Formation was divided into four different depth zones. The main reservoir zones of the Ilam Formation have near-zero velocities, indicating the presence of intercrystalline and intergranular porosities that exhibit relatively high permeability. Zones with negative velocities are associated withcaved or washed outpartsof the borehole.Zones with positive velocity deviations are lixnked to intragranular and moldic porosities, which are mostlypresentin the secondary reservoir sections. In the third phaseof research, using the FMI image log of the IlamFormation, secondary porosities caused by open fractures and vuggy porosity were identified. Accordingly,the average secondary porosity in this reservoir is 5.7%,the average fracture porosity is 0.5%,andthe average vuggy porosity is 5.2%. This studyrevealedthat intergranular and intercrystalline porosities have the most significant impact onreservoirquality of the IlamFormation, while intragranular, moldic, vuggy, and fracture porosities are less abundant and have a lesser effect on qualityof the reservoir. Therefore, the best reservoir qualityof the Ilam Formationexists where primaryor depositionalporosities have been preserved, or inzoneswhere intercrystalline porosities haveextendeddue to diagenesis and dolomitization processes.