The giant oil and gas fields in the northern Persian Gulf correspond to reservoir intervals which comprise Surmeh and Dalan formations. The main production units in these formations are their upper carbonates with typically ooid grainstone to dolostone lithofacies. As a comparative study the petrophysical characteristics of these formations were considered based on core logging, well logging and petrography of thin sections. The upper carbonate unit of the Surmeh Formation was studied in the Tabnak Field and the Dalan Formation in the South Pars Field. The average porosities were 17.15% and 8.50% for the Surmeh and Dalan reservoirs respectively. Their permeability’s were 308 md for the Surmeh and 30 md for the Dalan reservoirs. This contrast in porosity-permeability data of the studied reservoirs correspond to their depositional and diagenetic history. The high reservoir potential of the Surmeh Formation correspond to its interparticle primary porosity partly remained after partial early cementation and its intercrystalline porosity due to dolomitization. This is different in the Dalan Formation which had a different diagenetic history and its porosity mainly reduced during compactional processes during the burial as well as burial cementation. However, although the Surmeh reservoir has a higher porosity and permeability, it has a lower production yield, because of its limited source rock and migration pathways.