Mixed evaporite/carbonate characteristics of the Triassic Kangan Formation, offshore area, Persian Gulf

Abstract

The Early Triassic Kangan Formation is a mixed carbonate-evaporite succession that is considered to be part of the largest carbonate gas reservoir in the Persian Gulf region. This stratigraphic succession is characterized by alternating limestone, dolostone, evaporite and shale that have been investigated in terms of evaporite facies characteristics in the Iranian offshore area. The main body of Kangan carbonates was deposited in a shallow-marine, restricted carbonate ramp platform, and underwent intense near-surface diagenesis and minor burial modification. Evaporitic facies consist of anhydrite, secondary anhydrite after gypsum, and mixed carbonate-evaporite, which are dominant in the different parts of Kangan Formation, as result of arid climate and abrupt eustatic sea level changes. This article focuses on the evaporite successions, in which diverse evaporitic lithofacies have been recognized that can be categorized in two main classes: (1) carbonate-dominated facies with evaporite contents (C facies types), and (2) evaporite-dominated with minor carbonate contents (A facies types). Fine-crystalline anhydrite laminae and bands, bedded evaporite (bedded pseudomorphs); interstitial anhydrite pseudomorphs after gypsum and nodular anhydrite are common features. The evaporite successions in microscopic and macroscopic scales indicate that dolomitization and anhydrite precipitation took place contemporaneously, as ascribed to sabkha/seepage-reflux models. The evaporite sedimentation has mainly occurred in a subsiding coastal basin of a salina or hypersaline lagoon. In this setting, the subaqueous precipitation of the carbonate and evaporite lithofacies was followed by the interstitial growth of diagenetic, secondary anhydrite. As a whole, the evaporite succession reflects an infilling and diagenetic process. Gypsum was converted to anhydrite pseudomorphs following shallow to deep burial diagenesis events.