Geochemical study of Lower Miocene anhydrites of Central Iran and their effect on the reservoir properties

Abstract In the central basin of Iran, a clastic-carbonate reservoir, which is named Qom Formation, was deposited. Reservoir quality of this formation and effect of diagenesis on the reservoir properties evaluated in Yort-e Shah anticline; anhydritization is one of the diagenetic processes, which with occlude of pore system caused reduction of reservoir quality. Petrography study of the Qom Formation(c-4, d, e, and f members) indicates the presence of six anhydrite types including anhydrite layer, poikilotopic, pervasive, fracture filling cements, nodules and scattered and single anhydrite crystals. Diversity of anhydrite types show the effect of diagenesis processes in phreatic environment, near surface meteoric environment and burial environment, which they are affected to different degrees of diagenesis of reservoir facies. Based on geochemical study the environment towards top of Qom Formation become shallower and a lagoon with limited connection to see has been prevailed. Anhydritic layers with maximum thickness in “d” Member, act as a continues cap-rock for the lower reservoirs. Other types of anhydrite are sporadic in the Qom members and have a low percentage; therefore, they could slightly reduce the reservoir quality of this formation. Porosity distribution in the Qom members is affected by their facies that caused different rate of cementation. Due to changing facies in the members of Qom Formation, fracture porosity just extended in e and f members and it caused increasing permeability and consequently the reservoir qualities of them so they are suitable parts for gas storage project. Determination rate of anhydrite as one of the sources listed in H2S is helpful for reduction of risk assessment of gas storage project in Qom members.