Porosity Evolution In Carbonate Reservoir Rock Of Sarvak Formation, Zagros Basin, Iran

The Sarvak Formation with middle Cretaceous age (AlbianTuronian) is the second major oil carbonate reservoir rock in Zagros area. Thickness of the Sarvak Formation is 285 meters in well No. 8 of Kuh-e Mond. For investigation of microfacies and effect of diagenetic process on the porosity evolution of this formation are studied 329 thin sections colored with red Alizarin, core data. Based on petrographical studies, 15 microfacies were recognized. These microfacies were deposited in lagoon, back reef (leeward), reef, fore reef (seaward), shallow open marine and deep open marine settings. Six major transgressive regressive sequences are interpreted in the Sarvak Formation. In this study, porosity types were divided into primary and secondary porosity groups that affected by sedimentary environment, diagenetic process and tectonic. In high energy microfacies, primary porosity (interparticle and intraparticle types) was formed that interparticle porosity was decreased, but moldic and vugy porosities were increased in primary diagenetic stage (eogenetic). In mezogenetic stage, cementation lead to decrease porosity, but dissolution process, dolomitization and tectonic process lead to form vugy, intercrystalin and fracture porosities. The dissolution of unstable minerals (mainly aragonite) is the major process that improves porosity and then permeability by enlarging pores and pore throats. The best reservoir quality occurs in the upper parts of the regressive systems tracts, which contain primary grainsupported intervals or dissolution porosity. The studies show that porosity evolution controlled in sequence stratigraphy framework (A combination of depositional environment and diagenesis) in carbonate sequence of Sarvak Formation.