Depositional and diagenetic controls on reservoir quality of the uppermost Jurassic–Lower Cretaceous sequences in the Persian Gulf; a focus on J–K boundary

Abstract

This study presents reservoir characterization of the uppermost Jurassic–Lower Cretaceous successions of the Manifa and Yamama formations in four wells in the Persian Gulf. Detailed sedimentological, petrophysical and geochemical analyses are integrated to evaluate the controlling factors on reservoir quality of these sequences. Microfacies analysis has resulted in the recognition of 10 facies types deposited in lagoon, algal mounds, shoal, and open marine settings. Micritization, dissolution, various types of cementation, dolomitization, and compaction are notable diagenetic alterations indicating marine/hypersaline, meteoric and shallow to deep burial realms. Stable carbon and oxygen isotopic compositions indicate a meteoric diagenetic alteration at the topmost part of Manifa unit, beneath the Jurassic–Cretaceous (J–K) disconformity. Sequence stratigraphic analysis led to the identification of four transgressive–regressive (T–R) sequences in the Yamama Fm. and a regressive hemi-sequence in the Manifa unit. Reservoir quality study resulted in the recognition of eight hydraulic flow units and four types of reservoir facies with specific and predictable sequence stratigraphic framework. Dissolved grain-dominated shoal facies at the upper part of regressive systems tracts and algal reef/mound facies in transgressive systems tracts provide the best reservoir units of the studied sequences. Microporous mud-dominated facies of open marine settings are dominant reservoir facies in the upper Yamama unit, with moderate to low reservoir quality.