Geochemical Perspective on the Diagenesis of the Buah Carbonates from the Jabal Al Akhdar, Oman Mountains

The Buah Formation has been recognized as an active exploration target due to the potential for exposed carbonate rocks, which can be used as a proxy for hidden subsurface reservoir rocks. Petrographical observations and geochemical investigations of carbonate rocks provide useful insight into the diagenetic alterations and their effects on reservoir quality. The distribution of carbonate sediments is controlled by a variety of environmental parameters, including temperature, substrate, salinity, and the presence of siliciclastic materials. Metastable minerals, such as aragonite and high-Mg calcite (HMC) tend to transition to low-Mg calcite (LMC) and dolomite. To study diagenetic alterations, geochemical data of carbonate rocks from outcrops of Wadi Bani Awf (WBA) and Wadi Bani Kharus (WBK) in Jabal Al Akhdar, Oman, have been obtained using the ICP-OES technique. The Mg/Ca and Sr/Ca ratios of studied samples are plotted to show the relationship between hypothetical carbonate endmembers and Buah carbonates along with literature data. The samples from WBA and WBK outcrops exhibit three distinct groups that are anticorrelated, i.e., a decrease in Sr/Ca with an increase in Mg/Ca. The trend is consistent with the fact that the rhombohedral calcite can accommodate smaller cations (e.g., ionic radius of Mg²⁺ = 86 pm), whereas orthorhombic aragonite prefers larger cations (e.g., ionic radius of Sr²⁺ = 132 pm). Furthermore, the data are compared with samples collected in the Gulf of Mexico (e.g., 4193 and DIS2 from AC601 and GC53 sites, respectively). Some of the WBA samples match the data of sample 4193 precipitated from seepage of hydrocarbon-rich brine fluids with abundant micropores and shell material. While a few of the WBK samples have data that matches with micrite D1S2 sample precipitated from hydrocarbon-free brine fluids. The effect of diagenesis by incorporating Mg into the calcite mineral structure is represented by the third group, having samples from both WBA and WBK with the lowest Sr/Ca and highest Mg/Ca ratios. Our findings suggest that the anticorrelation of Mg/Ca and Sr/Ca ratios between three distinct groups of Buah carbonates may have been caused by varied diagenetic conditions ranging from micritization to mechanical and chemical compaction leading to dolomitization.