Document Type: Research Paper
Department of Petroleum Geology, Research Institute of Petroleum Industry (RIPI) 14857-33111 Tehran, Iran
School of Geology, College of Science, University of Tehran, 14176-14411 Tehran, Iran
School of Earth Sciences, Damghan University, 36716-41167 Damghan, Iran
The Upper Cretaceous Ilam carbonate Formation has been analyzed for its coated grains (fine ooids and rhodoids) in oilfields of SW Iran. The recognized coated grains are morphologically classified into several types. Petrographic and geochemical characteristics indicate that the ooids were originally composed of low-magnesium calcite (LMC; consistent with global observations), but rhodoids consisted originally of high-magnesium calcite (HMC). The overall primary mineralogy of the intervals containing coated grains has been a mixture of HMC and LMC. Co-occurrence of these mineralogies and allochemical (both ooids and rhodoids and other bioclasts) components indicates a rhodalgal-like grain association and a relatively temperate paleoclimatic conditions.