Diagenesis and its effect on the reservoir quality of the Asmari Formation, Aghajari Oilfield, SW Iran

The Asmari Formation is the main reservoir rock for the Aghajari oilfieild. It is composed of about 400-meter limestone, dolostone and interlayers of sandstones. Study of 1200 meters drilling cores, 2800 microscopic thin sections, 12 SEM analyses along with 125 routine core tests in 5 cored wells indicate that the Asmari Formation was effected by various diagenetic processes such as micritization, compaction, cementation, fracturing, dissolution and dolomitization. Some of these processes (e.g. dissolution, dolomitization and fracturing) have constructive effects on the reservoir quality and created wide variety of porosity types including vuggy, intercrystaline and channel in upper parts of the Asmari Formation. Destructive diagenetic processes (micritization, compaction and cementation) have destroyed pore spaces and make the lower parts of the Asmari (specially zone 5) to a non-reservoir unit. Porosity-permeability plots on the Lucia's diagram show sandstones and carbonates rocks with interparticle porosities have good reservoir qualities and always plot on upper parts of classes 1-3. Samples with fracture porosity mainly plot on upper part of class 1. This shows fractures has no considerable role in promoting the porosity, but they strongly increase permeability. Dolostones and the rocks with vuggy porosity have plotted on classes 2 and 3 (high porosity, relatively high permeability). Paragenetic secession of the Asmari Formation shows the diagenetic processes occurred syn-sedimentary on sea floor, after sedimentation during the low-deep burry and uplift. The results of this study can be useful in detection of reservoir zones, increasing of hydrocarbon production and enhanced recovery of this oilfield.