Linking Depositional Facies to Diagenetic Events and Reservoir Quality of the Upper Sarvak Formation in a giant oil field, SW of Iran

The Cretaceous Sarvak Formation (Albian-Turonian) in the Azadegan oil Field in SW Iran is composed of a thick succession of carbonate deposits. Integrated macro and microscopic studies and seismic attributes show that reservoir interval of the Upper Sarvak Formation in the studied field consists of four depositional facies. These are: (1) Oligosteginid wackestone to packstone, (2) Bioclastic-echinoid packstone to wackestone, (3) Bioclastic-rudist boundstone and (4) Channel-filled carbonate deposits. These facies were deposited in distal open marine, proximal open marine, carbonate buildups/barriers and channel depositional setting, respectively. The comparison of these facies with the same worldwide carbonate successions show that the Upper Sarvak carbonates deposited on a carbonate ramp. Emergence of some parts of Sarvak carbonate platform at the end of Cenomanian stage resulted in development of channel deposition in the studied area. Reservoir quality of the Upper Sarvak Formation has good relationship with depositional facies, so that diagenetic controls on porosity and permeability are also facies-controlled. Mud-supported facies belong to distal open marine setting (facies 1) have low porosity and permeability. This is due to fine-grained texture and also lack of diagenetic effects. On the other hand, grain-supported facies belong to proximal open marine and carbonate build up/barrier setting (Facies 2 and 3) have relatively good reservoir quality. This is due to presence of primary interparticle pore spaces and also development of dissolution.This study highlights the facies-controlled diagenesis and its importance in prediction of the distribution of porosity and permeability ahead of exploration in hydrocarbon reservoirs as well as enhanced recovery. Therefore such study should apply to other oil and gas fields and sedimentary basins.