Investigation of Anhydrite Distribution and its Impact on Reservoir Quality of the Qom Formation in Yortshah Anticline

In this study, the Qom Formation in Yortshah anticline has been evaluated as one of the reservoirs near Tehran city for gas storage. Formation of anhydrite is one of the diagenetic processes that can have a negative impact on reservoir properties. Petrographic investigation of c-4 to f members of the Qom Formation in Yortshahh anticline indicates the presence of 6 anhydrite textures including anhydrite layer, poikilotopic, pervasive and fracture filling cements and nodules and spare anhydrite crystals. Based on the results of petrophysical logs, anhydrite includes 3–8% of this reservoir. With the exception of anhydrite layer with the thickness up to 6 m in d member, which can be a potential cap rock for the lower reservoirs, anhydrite cements along with other cements have reduced the porosity of the reservoir. The porosity distribution in the members of the Qom Formation is affected by their different lithologies. As the e and f members due to different lithology than the c-4 member, in addition to the micropores, they also have fracture porosity, which increases the permeability and reservoir properties of these members.