Reservoir Characteristics of Dolomitized Intervals of the Arab Formation, a Case Study in one of the Hydrocarbon Fields of Persian Gulf

Dolomite successions are the main productive reservoir intervals in the upper Jurassic Arab Formation in many oil fields of the Persian Gulf. In this study, based on the results from core description, petrographic analysis, high mercury injection capillary pressure analysis and cores poroperm data, dolomite facies, and their reservoir characteristics have been investigated in an oil field of the Persian Gulf. On the basis of the dolomite characteristics, four dolomite groups including fabric preserving, fabric destructive, dolomite cements, and partial dolomitization are differentiated. In dolomites of the Arab Formation, intercrystalline and interparticle porosities are the main pore types. Evaporate mineralization is an important digenetic process associated with dolomitization which has had a significant impact on reduction of reservoir properties, i.e. porosity and permeability. Based on the integrated geological and petrophysical study, 12 dolomite facies were identified. The results of this study also demonstrated that in fabric preserving dolomite facies the main factors controlling reservoir quality are the nature of primary texture, and dissolution, and evaporate mineralization as the main diagenetic processes. In this dolomite group, dolomitization has not been significantly altered reservoir properties. In contrast, the dolomitization of dolomite facies related to fabric destructive group, in relation with dolomite crystal size and shape, had a profound destructive or constructive role on type and geometry of pore systems. In dolomite facies, partial dolomitization which have been characterized by the sparse distribution of dolomite crystal within mud dominated facies and pore filling of interparticle porosity have insignificant or negative effects on reservoir quality. Distribution of dolomitic facies in Arab A, B, and C members is mainly fabric preserving dolomite. Moreover, fabric destructive and partial dolomitization are prevailed in D member of Arab formation.