Depositional and sequence stratigraphic controls on diagenesis in the Upper Cambrian-Lower Ordovician Barik Formation, central Oman: Implications for prediction of reservoir porosity in a hybrid-energy delta system

This study aims to employ an integrated depositional and sequence stratigraphic approach to assess the control of diagenesis on reservoir porosity of a hybrid-energy delta system. The study focuses on examining the outcropped Upper Cambrian-Lower Ordovician Barik Formation representing a highstand system tract of a hybrid-energy delta in the Haushi-Huqf region of Central Oman, which is equivalent to the subsurface gas reservoirs within the Interior Oman Salt Basins. The petrographic, mineralogical, and geochemical assessments revealed that the Barik Formation primarily consists of moderately to very well-sorted coarse-grained siltstones to fine-grained sandstones, with a composition mainly comprisingfeldspathic- and subfeldspathic arenite and the reservoir porosity is controlled by various degrees of diagenetic processes. Eodiagenetic alterations comprise mechanically infiltrated clays and kaolinitization of silicate grains, including detrital feldspars, micas, and mud matrix. The mechanically infiltrated clays (e.g., smectite) were introduced into the sandstones by the percolation of muddy waters under the control of tidal pumps. Although these clays may affect reservoir porosity during eodiagenesis, they positively preserve reservoir porosity during deep burial (mesodiagenesis) by inhibiting cementation by feldspar and quartz overgrowths. Kaolitization of silicate detrital grains was facilitated by the influx of meteoric waters into the sandstones as the delta progrades during the highstand system tract. During progressive burial and lack of extensive eodiagenetic cement, compaction continued and reduced reservoir porosity. Illitization and chloritization of the mechanically infiltrated smectitic clays took place; however, they limited the cementation by quartz and feldspar overgrowths. The quartz and feldspar overgrowths on detrital grains with discontinued smectitic clay coats supported the detrital grains from further compaction and, thus, helped in reservoir porosity preservation. Moreover, extensive feldspar dissolution and kaolinitization has occured during mesodiagenesis and enhanced the reservoir porosity. Subordinate amounts of calcite, dolomite, and gypsum cement have a minor reduction in the reservoir porosity of the Barik Formation. Hence, it can be concluded that, on the one hand, lack of eodiagenetic cementation favored intense reservoir porosity reduction due to compaction. Nevertheless, on the other hand, the mechanically infiltrated clays during the eodiagenetic stage helped preserve the reservoir porosity by hindering the formation of quartz and feldspar overgrowths during the mesodiagenetic stage. This study may serve as an analog model for a similar hybrid-energy delta sandstone reservoirs to better understand the type, and spatial and temporal of diagenetic alterations and their role in controlling the reservoir quality in deeply buried sandstone reservoirs. © 2023