Microfacies Analysis, Geochemical and Petrographic Evidences for Recognition of Original Carbonate Mineralogy of Dalan Formation in the South Pars gas field

The Late Permian Dalan Formation in folded Zagros region, as well as the Persian Gulf, contains giant gas resources, so that this formation along with the Triassic Kangan Formation in the Persian Gulf contain about 50% of total gas reservoirs. Based on petrographic studies, the thickness of the Dalan Formation in the X well in the South Pars gas field is 370 meters with dominant dolomite, dolomitic limestone, limestone lithology and anhydrite interbeds. The identification of original composition of carbonates is difficult due to the effects of diagenetic processes, as over time aragonite and high magnesium calcite change into a low magnesium calcite and caused changes in further texture and original composition of the rock. Therefore elemental studies should be used as an integral to the petrographic studies. Concentration of the major and trace elements in carbonates are depend on their mineralogy composition and the mineralogy composition itself, changes by the influence of diagenetic processes. Study of major elements (Ca, Mg), trace elements (Mn, Fe, Na, Sr) and compare them with the published results by different researchers for aragonitic mineralogy, and considering petrographic evidence such as oomold porosity, high frequency of bivalves, selective dolomitization, spalled ooids, isopachous fibrous marine cement, implied that original mineralogical composition of Dalan Formation was dominantly aragonitic. Eighteen microfacies are determined for the Dalan Formation. Based on the identified microfacies, this carbonate succession was developed on a homoclinal carbonate ramp.