Effects of Microfacies and Diagenesis on Petrophysical Properties of Sarvak Formation, Fars Area, Southern Iran

Abstract

The Lower to Upper Cretaceous (Albian- Turonian) Sarvak Formation, the second major oil and gas reservoir in Zagros Basin of southern Iran is principally composed of carbonates with minor shale. 15 microfacies were recognized from 287 meters of core and 329 thin sections (colored with red Alizarin) were collected for petrographic analysis, together with analysis of core and well logs. Petrophysical properties of carbonates are controlled in part by the original depositional texture, but also largely by subsequent diagenetic processes. The sedimentary and diagenesis processes together control the arrangement, distribution and orientation of the major constituents, the open space and pathways, the fractures and the stylolites in the rock. When working with reservoir quality of carbonate reservoir rocks, these main fabric elements have to be considered. In this study, the microfacies were deposited in lagoon, back reef (leeward), reef, fore reef (seaward), shallow open marine and deep open marine settings. The petrographic analyses indicate that the Sarvak Formation carbonates have undergone a complex diagenetic history which includes compaction, cementation, dissolution, dolomitization, neomorphism and fracturing. Cementation and compaction reduce porosity and leads to low permeability and poor reservoir quality. Dissolution, dolomitization and fracturing diagenesis processes improved reservoir quality. Dissolution process with generating of secondary porosity consists of vuggy and muldic has important effect on increasing of porosity, but most important factor in development of reservoir has been fracturing. In lagoonal deposits, single unit was distinguished with moldic and vuggy porosities. In shoal/reef deposits, two units were distinguished in terms of dissolution and grain frequency. In shallow openmarine deposits, two units were identified with different degrees of fracturing and dolomitization; while deep open-marine deposits were characterized by a three unit in terms of stylolitization and dolomitization. Consequently, the shoal/reef deposits with Rudist grainstone and rudstone textures and interparticle and moldic porosities had the best reservoir quality. The key challenge in this reservoir analysis was to predict the vertical distribution of petrophysical properties to improve r eservoir characterization. This research improved our understanding of geologic controls on the reservoir performance.