Porosity evolution in dolomitized Permian–Triassic strata of the Persian Gulf, insights into the porosity origin of dolomite reservoirs

Abstract

Permian–Triassic Kangan and Dalan formations and their time equivalents in the Persian Gulf host the largest gas reservoirs of the world. Most parts of these reservoirs are dolomitized. This study focuses on the origin and evolution of porosity in these reservoirs. Thin sections from two wells were studied for analyzing petrographic characteristics including lithology, depositional texture, pore types and diagenetic processes. Porosity measured on all plug samples and scanning electron microscopy analysis helped to better understand small scale properties. Results showed that both limestones and dolomites can have high porosity. In limestone samples, grain-dominated textures are commonly porous. Dolomite samples inherit the porosity of the precursor limestones, but the ratios between mud and grain-dominated textures are different for these samples. So, at least parts of the porosities in these samples have been created during dolomitization process. Completely dolomitized intervals include both low and high porosity samples. Comparing anhydride and dolomite content, it can be concluded that pore spaces have been occluded by anhydrites cements along with dolomitization. This type of cement is also responsible for low porosity of moderately dolomitized facies. Overdolomitization is not present in the samples. With respect to the reflux dolomitization model of these formations, limited circulation of the dolomitizing fluid could not precipitate released carbonate as crystal overgrowth and so the high porosity of the samples retained to the later stages of diagenesis. This was not affected by the size or boundary shape of the dolomite crystals.