Original mineralogy of the Ilam Formation, based on geochemical data in type locality, Payun Anticline and subsurface section DA # a

The Ilam Formation (Santonian to Campanian), as an oil reservoir rock in a Zagros field, composed of carbonate facies. In this study, two surface sections (Payun and Kabir Kuh anticline) and one subsurface section (DA#a), 6 microfacies have been recognized. These facies deposited in lagoon to deep marine environment. In this formation, 446 thin sections were studied petrographically and 93 powered samples geochemically analysed.Based on major and minor elements and oxygen and carbon isotope values in the Ilam Formation samples and their comparison with the results obtained from mixed aragonite and calcite fields published by different researchers, original carbonate mineralogy of the Ilam Formation was mixed aragonite and calcite. Oxygen and carbone isotopes indicate that these carbonates are mainly influenced by meteoric diagenesis. Paleotemperature calculation based on heaviest oxygen isotope value) -3.60 ‰ PDB), indicate that ambient water temperature during precipitations of the Ilam carbonates was 28°C. Major, minor and isotopic data in carbonates are depend on their mineralogy and mineralogy itself changes during diagenetic processes. By petrographic studies alone, cannot understand the original mineralogy and kind of diagenetic processes on carbonates, therefore must be use elemental and isotopic data as complementary studies. Different carbonate minerals are formed in different temperature, salinity and Ca/Mg ratio. In addition, major elements (Ca, Mg), minor elements (Sr, Na, Fe, Mn) and oxygen and carbon isotopes are also change.