AAPG Middle East Region Geoscience Technology Workshop:
3rd Edition Carbonate Reservoirs of the Middle East

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Diagenesis of the Wuchiapingian Unit in a Field in Saudi Arabia

Abstract

The Late Permian Wuchiapingian unit consists of shallow marine carbonates and evaporites, and it is around 90 meters thick in the studied field in Saudi Arabia. The Wuchiapingian unit is known for reservoir heterogeneity. This study integrates petrography, geochemistry, and core characterization to contribute to the overall understanding of this unit. A total of 24 diagenetic processes have been identified and are subdivided between the eogenetic and mesogenetic diagenetic environments. Each diagenetic process, to varying degrees, affects the reservoir quality positively or negatively. Based on their effect, they were classified into major and minor processes. The major processes include dolomitization, dissolution, compaction, calcite cementation, stylolitization, fracturing and anhydrite cementation. There are minor diagenetic processes that have no significant effects on porosity and permeability, which include micritization, chertification, pyritization, reservoir bitumen entrapment, celestite cementation, fluorite cementation and authigenic quartz formation. Dolomitization is the most porosity-altering process and occurs as eogenetic replacive dolomite and mesogenetic dolomite cement. The replacive dolomite is interpreted to have occurred as a direct result of the reflux of hypersaline waters derived from the up-dip evaporative lithofacies as indicated by heavier C-O-Sr isotopic ratios. This early dolomitization enhances porosity and permeability in the forms of fabric-destructive sucrosic dolomite in the shallow subtidal lithofacies and fabric-preserving dolomite forming intercrystalline porosity that connects interparticle pores and isolated molds of the grain-dominated lithofacies. The burial dolomite cement reduces porosity by filling moldic and interparticle pores. The C, O, Sr, and S isotopes indicate a contemporaneous marine origin of the Wuchiapingian carbonates and anhydrites. Enriched (C-O-Sr) isotopes are associated with the overdolomitized tidal flat and lagoon lithofacies. Depleted oxygen isotopes are associated with deeper subtidal limy carbonates and prominent (regional) subaerial exposures. The porosity types are primary (interparticle) and secondary (moldic, intercrystalline, vuggy, microporosity, and fracture porosity). The secondary porosity resulted from meteoric dissolution, replacive dolomitization, and fracturing. The porosity-modifying diagenetic processes and the identified porosity types suggest that lithofacies and diagenesis both strongly influence reservoir quality in the Wuchiapingian unit.