oa Fracture-controlled Dolomite Reservoirs in Late Cretaceous Carbonates of the Sarvak Formation, Anaran Anticline, Islamic Republic of Iran

Late burial dolomite of hydrothermal origin replaces Cretaceous carbonate shelf sediments of Albian to Turonian age (Sarvak Formation) in the NW closure of the Anaran Anticline, Zagros Mountains, Simply Folded Belt, Iran. The outcrops, spectacularly exposed along deep river canyons, offer the possibility for a 3D reconstruction of the geobodies, combining both field data (sedimentary logs, cross sections and samples for diagenetic and petrophysical studies) and LIDAR derived photorealistic model of the most representative dolomite bodies. The aim is to quantify the impact of hydrothermal dolomitisation on the reservoir quality. The hydrothermal origin of the dolomitising fluids will be discussed integrating different geochemical data (fluid inclusions, stable and radiogenic isotopes, minor and trace elements). Dolomite replaces carbonate rocks characterised by different facies, showing different geometries: 1) massive plus stratabound (Lower Sarvak); 2) pipes (Upper Sarvak). Dolomite distribution decreases vertically with at least three discrete breaks corresponding to two main aquitard intervals: 1) Ahmadi Shales, separating massive dolomite form from plume-like bodies; 2) Turonian Mudstones; 3) Surgah Fm., above, which no dolomite has been observed. Fractures and faults play a major role in controlling the distribution of the dolomite bodies, in particular the ENE-WSW and WNW-ESE conjugate fault systems, which are often associated with dolomitic halos. A geological conceptual model is built taking into account different fracture and matrix porosity models for limestone and dolomite.