Stratigraphic framework and diagenetic features related to reservoir quality of the triassic carbonates in central to northern Persian Gulf

Abstract

The Early Triassic Kangan Formation in southern Iran and Persian Gulf, equivalent to the Upper Khuff Formation in the neighboring Arab countries hosts vast natural gas resources in the Persian Gulf. The current study discusses sedimentary facies, diagenesis, depositional setting and reservoir characteristics of the Triassic Kangan Formation within a sequence stratigraphic framework across a section passing from the central to northern Persian Gulf. For this purpose, a comprehensive analysis was conducted using well logs, cores and thin sections from four wells. In this respect, fifteen microfacies representing four facies’ belts ranging from peritidal to lagoon and barrier (shoal) settings of a shallow marine carbonate ramp environment were identified. The Kangan Formation has undergone marine, meteoric and burial diagenesis severely impacting its reservoir quality through dolomitization, dissolution and cementation. Analysis of sedimentological and petrophysical characteristics enabled the recognition of three third-order and seven fourth-order depositional sequences. This unit’s persistent and complex diagenetic history, spanning since the Lower Triassic, has significantly altered reservoir quality by reshaping pore types, size and geometry. Dolomitization and dissolution have notably improved the reservoir quality of the Kangan Formation in the central Persian Gulf. However, towards the northern Persian Gulf, the presence of anhydrite in the form of cement, nodules and interbeds has adversely affected reservoir quality. Consequently, the Kangan Formation behaves as a tight gas carbonate reservoir in the Coastal Fars structural zone of Zagros. Therefore, the Coastal Fars and the northern Persian Gulf can be conceptualized as the inner part of a large-scale paleo-ramp environment (inner ramp), whereas the central Persian Gulf lies within its mid-ramp and basin environment.