Facies analyses and depositional setting of the Lower Cretaceous Shurijeh–Shatlyk formations in the Kopeh Dagh–Amu Darya Basin (Iran and Turkmenistan)

Abstract The Lower Cretaceous siliciclastic fluvial to shallow marine Shurijeh–Shatlyk formations together form one of the main reservoir successions in the Kopeh Dagh–Amu Darya Basin (NE Iran, Turkmenistan, and north of Afghanistan). Sedimentary facies and thickness changes in the Shurijeh–Shatlyk formations show complex variations throughout the basin, which highlight the importance of understanding the controlling factors on facies distribution and depositional setting for future hydrocarbon exploration. Integration of 3D seismic data, 13 boreholes, and five surface stratigraphic sections led to identification of lithofacies and facies associations. The results allow subdivision of this siliciclastic succession into four stratigraphic units (A, B, C, and D). Our study indicates that base‐level changes interacted with palaeotopography inherited from Middle Jurassic basin rifting, Upper Jurassic carbonate reefs/patch reef distributions, and reactivation of the deep basin faults, to control the Shurijeh–Shatlyk depositional setting and sedimentary facies distribution. Our findings show that the Aghdarband and Badkhyz–Maimana palaeo‐highs affected the Kopeh Dagh–Amu Darya Basin throughout deposition of the Shurijeh–Shatlyk formations. Allogeneic processes and active tectonics, related to the Valanginian Late Cimmerian Orogeny, caused a base‐level fall, exposure of Upper Jurassic carbonate sediments, and deposition of a carbonate pebbly conglomerate unit in the western part of the study area. This base‐level fall led to the deposition of thick evaporites in the western area and conglomerate in the eastern parts of the Kopeh Dagh. Due to a Hauterivian sea‐/base‐level rise, several deltas formed in the basin, which form the main reservoir zone in the Murgab Depression and Khangiran Field in Turkmenistan and Iran.