The Middle- Late Triassic Dashtak Formation acts as one of the best seal for giant gas reservoirs entrapped within the Dehram group inthe Southwest of Iran and the northern Persian Gulf because ofthick anhydrite deposits. The facies analysis of Dashtak Formation led to recognition of 14 microfacies, which are related to coastal plain, sabkha)supratidal(, tidal flat,lagoon, and shoal environments. Examination of the vertical and lateral facies changes and thire comparison with their modern and ancient counterparts, all indicate that the Dashtak Formation was deposited in a shallow carbonate ramp platform. The sequence stratigraphic studies showed that the Dashtak Formation consists of four third-order depositional sequences, each consisting of the transgressive and highstand systems tracts. The thickness of this formation increases from the Kuh-e Surmeh toward the boreholes d, c, b and a. A strontium isotope dating of the Dashtak Formation indicated a Triassic age from Ansian to Norian stages.