Rock types of the Kangan Formation and the effects of pore-filling minerals on reservoir quality in a gas field, Persian Gulf, Iran

Abstract

One of the major problems with carbonate reservoirs is to understand the relationship between porosity, permeability, and irreducible water saturation (Swir). The quality of a carbonate reservoir can be directly related to porosity types, varieties of pore throat size, and various diagenetic minerals. This has made it difficult to distinguish the areas with the highest permeability and the lowest Swir. Accordingly, the Kangan Formation comprises three types of replacive dolomite textures (Rd1, Rd2, Rd3), two types of dolomite cement textures (Cd1, Cd2), two types of anhydrites structures (noduls and beds), six types of anhydrites textures (needle, crystalline, radial, disordered, fibrous, mixture), and four types of calcite cements (fiber/bladed, blocky, mold filling, fracture filling). Among these, the replacive dolomite types improved the reservoir quality (porosity and permeability) and the dolomite cements reduced the porosity without having had any major effects on permeability. Anhydrite nodular structure had no major effect on reservoir quality while anhydrite-bedded structure affected it through creating some barriers in flow path. Moreover, fiber/bladed and mold-filling cements had no major effect on reservoir quality but blocky cements reduced it. And finally, fracture-filling cements may or may not affect the reservoir quality. The best types of porosities are intercrystalline and interparticle, prevailing in lower part of the Kangan Formation. In this study, six reservoir rock types (RRTs) have been defined on the basis of special core analysis (SCAL) as well as thin section description. The RRT-1 is the best type of reservoir, and toward the RRT-6, the reservoir quality reduces.