The Miocene Gachsaran Formation evaporite cap rock, Zeloi oilfield, SW Iran

Abstract

Petroleum at the Zeloi oilfield (SW Iran) is produced from the Oligocene/Miocene Asmari Formation. Evaporites in the overlying lowermost member of the Gachsaran Formation form an efficient cap rock seal. In the present study, this cap rock was studied and interpreted on the basis of gamma-ray and sonic logs data from eleven boreholes. Geochemical data obtained from X-ray fluorescence analyses of the anhydrite samples and scanning electron microscope analyses of the halite samples have been interpreted in terms of depositional environment. Chemostratigraphy based on the use of Na2O/Al2O3, Na2O/Fe2O3, Na2O/TiO2, and Na2O/K2O ratios allowed division of the cap rock into three marker units: (1) a unit of weathering-resistant elements; (2) an index key unit; and (3) a unit of highly soluble elements. The stratigraphy of the cap rock suggested the occurrence of cyclic regression and transgression. Geochemical data in combination with textural analyses in scanning electron microscopy indicate arid depositional conditions for this part of the Gachsaran Formation. Cycles of marine transgression–regression taking place in a barred coastal sabkha-lagoon basin, must have been responsible for the formation of the evaporite sequences of the Gachsaran Formation.