Dolomite Types of the Asmari Formation in the Aghajari Oil Field and their Effect on the Trend of Mercury Injection Curves

The Asmari Formation, in the Aghajari Oilfield (located in the Dezful Embayment) with about 400-meter thickness, was deposited in a homoclinal carbonate ramp and affected by various diagenetic processes. Core studies, microscopic thin sections and Scanning Electron Microscope images (SEM) from the 5 wells indicate that dolomitization is one of the most effective diagenetic processes of the reservoir quality of the Asmari Formation, and dolomitization mainly affects the upper parts of this Formation. In addition, five types of dolomites including (1) Dolomicrite (62 micron), (4) floated dolomites in the fine grain matrix, and (5) dolomite cements are recognizable. Each type of these dolomites have different effects on the trend of mercury injection curves (Pc), and therefore on the reservoir quality of the Asmari. Moreover, dolomicrospars are the most common type and have high porosity and permeability (average 16% and 35 mD respectively) due to the presence of intercrystaline porosities. Therefore, they can be saturated by mercury under low entry pressures (about 5 psi), and then show high-steep capillary pressure curves. Dolomicrites due to fine-grain nature have initially high porosity and relatively low permeability (11.5% and 4 mD respectively). But in along with dissolution, they show reservoir characteristics similar to Dolomicrospars, and their Pc curves shift to the left. Floated dolomites in matrix have no considerable role in the reservoir quality. Moreover, dolospars due to over-dolomitization and locking dolomite crystals, and dolomite cements due to disconnecting pore spaces led to decreasing in porosity and permeability. It seems zones 1 to 3 of the Asmari Formation in the Aghajari Oilfield have more potential for drilling and well-completion in the future because of more concentration of dolomicrospars and vuggy dolomicrites.