Correlation of the Upper Dalan/Kangan Formation between offshore Iran and the Zagros: Impact of lateral changes in facies and diagenesis on reservoir quality

Abstract

Abstract The Dalan/Kangan (Khuff) Formation is a major reservoir in Iran and the rest of the Middle East Gulf region, and contains some of the world's biggest gas reserves. The reservoir facies of this formation developed on a very large regional carbonate platform which had a very low topographic relief. This large geographic extent of the platform system is responsible for the development of very extensive facies tracts - in order to encounter major facies changes, large areas need to be investigated. This is often not possible in detailed reservoir-scale studies. In such cases, integrated models that extend from proximal positions to more oceanward locations are difficult to apply. In order to address these issues a large multidisciplinary and multiscale subsurface study has been launched on a large database including fields from onshore and offshore Iran. The aims to better understand the facies distributions, sequence stratigraphic architecture and the regional reservoir development. Twenty-two principal facies have been recognized including massive to laminated anhydrite, mudstone with anhydritic nodules, dolobreccias, green shale, massive to bioturbated mudstone, laminated dolomudstone, coarse lithoclastic and bioclastic grainstone, bioclastic grainstones, oolitic grainstones, peloidal grainstones, dark mudstones, and thrombolitic limestone. These facies have been interpreted in terms of depositional environment including: evaporitic flats or saltern (shelf supratidal/intertidal to subtidal setting), tidal flats (intertidal to supratidal setting, with numerous subenvironments such beach ridges, intertidal flats, tidal channels), subtidal lagoon (subtidal setting), leeward shoals (subtidal to intertidal setting), oolitic to oobioclastic shoal belts (subtidal to intertidal setting), composite sandwave constructions (subtidal setting), and middle shelf deposits (subtidal setting). From the Dalan K4 through to the Kangan K1 significant changes in platform type/geometry, facies organization and climate occurred. Consequently, different depositional models need to be created for each of the major stratigraphic interval. Conceptual geological models have been constructed for the large-scale stratigraphic architecture, sedimentological organization and the palaeoecological systems. The correlations show significant changes in sedimentological and reservoir facies across the study area, which are not evident at smaller scales. These large-scale geographic and stratigraphic facies trends provide a regional framework which can then be used to help constrain reservoir-scale studies.