Sequence stratigraphy, diagenesis and geological zonation of the lower Triassic carbonate reservoir of the Kangan formation from the central to Northern Persian Gulf

Abstract

The lower Triassic Kangan Formation is one of the most important carbonate reservoir rocks in southern Iran and the Persian Gulf. This formation is part of the Dehram Group and consists of a carbonate-evaporite sequence, including limestone, dolomite and anhydrite. This carbonate sequence has been deposited on a gently-sloping homoclinal carbonate ramp in a warm and dry climate conditions. In this study, by carefully examining geological reservoir zones (GRZs) in three wells in the central (wells A and B) and northern (well C) Persian Gulf, in terms of facies changes, sedimentary environment, diagenesis and sequence stratigraphy, it is possible to determine the reservoir quality by considering the reservoir heterogeneity. Petrographic observations show that the Kangan Formation consists of fifteen microfacies related to four facies belts, including sabkha, lagoon, tidal flat and shoal environments. Facies and environmental changes in the Kangan Formation indicate three third—order and seven fourth-order sequences in the central and northern Persian Gulf. Each sequence includes TST (Transgressive System Tract) and HST (Highstand System Tract) related to sabkha, intertidal, lagoon and shoal environments. The main diagenetic processes in the reservoir are dolomitization, dissolution and cementation. The connection between the depositional facies, sedimentary environment and diagenetic processes (dolomitization, anhydrite and calcite cementation and dissolution), allowed for the identification of seven geological reservoir zones (GRZs) related to the fourth-order sequences. These sequences and GRZs demonstrate vertical and lateral heterogeneity of the reservoir, observed as variation in GR log changes, lithology, facies frequency, diagenetic features and reservoir properties among the studied wells. GRZ-1 to GRZ-3 in the northern Persian Gulf and GRZ-4 to GRZ-7 in the central Persian Gulf show better reservoir quality. Facies analysis of the Kangan Formation indicates higher energy conditions for the central parts than the northern parts.