Mineralogy and geochemistry of alteration induced by hydrocarbon seepage in an evaporite formation; a case study from the Zagros Fold Belt, SW Iran

Leaking hydrocarbon and associated fluids produce a reduced environment which initiates diagenetic reactions in the rocks and soils overlying hydrocarbon reservoirs. This article introduces mineralogical and geochemical alterations induced by hydrocarbon seepages in the Gachsaran evaporite Formation, Masjed Soleiman, Zagros Folded Belt, SW Iran, using various geochemical methods. X-ray diffraction (XRD) analysis revealed two dominant groups of mineralogical changes in the anomalous zone including: (1) spotty occurrences of jarosite, natroalunite, and sulfur; (2) simultaneous decrease of sulfate and increase of carbonate. The δ13C values of carbonate phases vary between −8.9‰ and −32.3‰ which support a partial contribution of hydrocarbon in these phases. pH measurements of rock samples indicated that hydrocarbon seepage causes decrease of pH, though not very significantly. However, pH in areas that show ongoing sour hydrocarbon seepage among rarely pure gypsum beds decrease to −0.09 which produces acid sulfate soil. Trace element concentrations indicated that Cr, Co, Ni, V, Cd, U, Cu, and Zn increase in various patterns around the hydrocarbon seepages excepting the areas with very low pH.