This paper investigates the role of the effective thickness of the Asmari reservoir formation zones on oil production in one of the Iranian carbonate oil fields. Effective thickness is a term that includes the total gross thickness of rocks by lithofacies for a selected wellbore. The lithology of the Asmari Formation in the studied area consists of dolomite, sandstone, lime, dolomitic-lime, sandstone-shale, and shale limestone dolomites. Based on the existing well-logs, the average shale volume, the effective arithmetic means of porosity in the gross intervals, and average water saturation or hydrocarbon-bearing increments of the studied field is calculated from well-logs. The depth interval of 2214 to 2296, in wellbore #A shows 9.6% average shale volume, 27.2% average water saturation, and 20.9% average porosity. The depth interval of 2213 to 2280, in wellbore #B, shows 6% average shale volume, 21.25% average water saturation, and 28.5% average porosity. Based on our petrophysical assessments we divide the Asmari reservoir in the studied field into eight zones. Zone 1 is made of carbonate (calcareous and dolomitic), zones 2 to 5 are mainly sandstone, zones 7 and 8 are calcareous and shale and zone 6 is a mixture of all the above-mentioned rocks. Among these eight zones, there are two main hydrocarbon productive zones. The numerical calculation of in situ oil volume showed that zone two contains 65% of oil volume in this reservoir. This zone with more than 80% of sand has the highest net hydrocarbon column.