Genesis of anhydrites in the dolomite facies of the oligo-miocene Asmari formation, Zagros basin: implication for reservoir quality

The type and amount of distribution of evaporite minerals within reservoir rocks play important role in reservoir quality. This study focuses on thin section petrography, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, aqueous fluid inclusions microthermometry, and UV fluorescence microscopy of hydrocarbon fluid inclusions, to study the genesis of anhydrites within dolomite facies of the Oligocene-Miocene Asmari Formation, Zagros Basin. Petrographic studies of primary (dolomicrite) and secondary (dolosparite) dolomite-dominated facies of the Asmari Formation showed that the associated diagenetic anhydrites occurred in different morphological textures (forms) including poikilotopic, pervasive pore-filling, scattered pore-filling, fracture-filling, and vein cements, nodular and layered forms. Fluid inclusion studies of anhydrites revealed that the diagenetic anhydrites particularly within the dolomicrite facies were formed at an average temperature of 68.2 °C, and by mineralizing solutions with salinity of about 23.5 wt% NaCl. Indeed, in the dolomicrite facies, the anhydrites were associated with gypsum dewatering and/or direct anhydrite precipitation during eodiagenesis. In contrast, in the dolosparite facies, anhydrites were formed at an average temperature of 100.1 °C, and by mineralizing solutions with salinity of about 16.4 wt% NaCl. This latter condition was very likely related to the dolomitization processes including the creation of calcium-rich and magnesium-poor pore fluids with presence of sufficient sulfate ions during the mesodiagenesis. Occurrences of eugenic and mesogenic anhydrites within the dolomite facies were largely controlled the petrophysical parameters as well as the reservoir quality of the Asmari Formation.