Detailed petrographic, geochemical and strontium isotope studies, combined with field observation and rnicrofacies investigations indicate that syngenetic celestite at the base of Asmari Formation at the Bangestan anticline, Behbahan region, was deposited directly from Tertiary seawater in an intrasupratidal to supratidal environment. During diagenesis, the precipitated celestite underwent several textural and mineralogical modifications, the most important of which was the formation of diagenetic crystallization rhythmites. Fluid inclusion geotherrnometry and rmR analyses suggest that the fluid medium, in which recrystallization of the celestite took place, was among other constituents rich in immature hydrocarbons. It is demonstrated that the celestite concretions were subjected to several low temperature thermal events, the maximum of which did not exceed 255?C