Tectonically Controlled Diagenesis and Fluid Evolution in Al Khlata Reservoir Sandstones, Sultanate of Oman

Abstract Late Carboniferous to early Permian glacigenic Al Khlata Formation reservoirs account for one of the largest hydrocarbon productions in the Sultanate of Oman. Tectonic activities caused a varied subsidence, uplift, and erosion history resulting in present-day formation depths between surface exposure and approximately 4000 m (13,125 ft). Furthermore, tectonism caused fluid incursions leading to mineral reactions and porosity-modifying processes. Since deposition, the pore water evolved from low-salinity and negative δ18OVSMOW value to a high-salinity brine with a positive δ18OVSMOW value. This change is related to Ara salt halokinesis beginning at approximately 200 Ma. At the eastern flank of the Ghaba Salt Basin, low saline intervals occurred isochronously with the obduction of the Hawasina Nappes-Samail Ophiolite. Dilution event(s) and changes to negative δ18OVSMOW values occurred between 60 and 50 Ma because of intraplate deformation creating new fluid pathways. Reservoir quality deterioration results from compactional porosity loss and heavy quartz cementation. Authigenic quartz precipitated as zoned syntaxial overgrowths cogenetically with calcite, Fe-dolomite, anhydrite, and kaolinite. Hydrochemical modeling of kaolinite precipitation indicates a link to CO2 migration rather than meteoric water infiltration. The onset of quartz cementation was contemporaneous with the obduction of the Hawasina Nappes–Samail Ophiolite irrespective of burial temperature, thus indicating a tectonic rather than a purely kinetic control.