Evaporite (Anhydrite and Celestite) Mineralization and its Effect on Reservoir Quality of the Dolostone Intervals of the Asmari Formation, Northern Part of Dezful Embayment, Southwest of Iran

Asmari formation, assigned to Oligocene-Lower Miocene, forms the most important hydrocarbon reservoir in the south and southwest of Iran. In the studied field, Asmari formation constitutes a sequence of carbonate (common at upper part) and clastic (common at lower part) sediments. The carbonate section belonging to early Miocene and being commonly assigned to shallow marine, high energy barrier and lagoon is severely subjected to diagenetic processes. Macro-microscopic studies (core and thin section) along with geochemical and conventional core analysis showed that, in dolostone intervals, the development of intercrystalline pore spaces and the presence of primary interparticle pore spaces on the one hand and the performance of compaction and evaporite mineralization on the other hand caused considerable changes in reservoir quality. This study aimed to investigate the effect of evaporite (anhydrite and celestite) mineralization on the Asmari reservoir properties. This research showed that the quantity and distribution of evaporitic minerals, namely anhydrite and celestite, played a very significant role in controlling the reservoir quality of dolostone intervals. The most common and efficient forms of anhydrite and celestite distribution occur as pore space cement-filling, poikilotopic, replacement, and occasional fracture-filling. Evaporitic cement-filling that occupies about 40% of intergranular (in early burial) and intercrystalline pore spaces, noticed in some samples, have reduced the porosity and permeability of dolostone intervals to nearly zero value. Primary and secondary nodular forms having considerable extent in all facies have not decreased the reservoir quality of Asmari formation.