Sedimentology, sequence stratigraphy, and diagenesis of the lower Wuchiapingian Khuff unit in a field in Saudi Arabia

Abstract

The lower Wuchiapingian Khuff unit consists of shallow marine carbonates and evaporites, and has two parts: the upper Wuchiapingian and the lower Wuchiapingian; the lower unit has a thickness of around 90 meters in the studied field in Saudi Arabia. This study integrates sedimentology and sequence stratigraphy to contribute to the exploration and exploitation of lower Wuchiapingian Khuff unit. The lithofacies include subaerial exposure, sabkha/saline anhydrite, tidal flat, lagoon, shallow subtidal, shoal flank, shoal crest, and open marine. The lithofacies succession represents an arid, homoclinal ramp within an extensive tectonically-stable low-relief epeiric sea. The study area, during the time of deposition, exhibited no significant depth variations or clear landward seaward directions, and was dominated by shallow subtidal, lagoon and high-energy shoal deposits distributed between interpreted three structure-induced paleo-highs. The lower Wuchiapingian Khuff unit covers a time span of approximately 3.5 My. (from the boundary of the Capitanian-Wuchiapingian; based on strontium isotopic analysis). The lower Wuchiapingian Khuff represents a third-order composite sequence. The four constituent depositional sequences (WK-1 to WK-4) represent third-order eustatic cycles, and each is bounded by regionally mappable sequence boundaries with varying degrees of exposure features. The systems tracts within these depositional sequences represent parasequence sets. The unit consists of 17 shallowing-upward parasequences, grouped into symmetric and asymmetric fifth-order cycles. The WK-2 to WK-3 sequences have the highest reservoir quality and core coverage, and were used to construct lithofacies distribution maps for both transgressive systems tracts and highstand systems tracts. Eustatic sea level changes controlled the formation of accommodation space during a period of diminished tectonic activities, showing only subtle variations in antecedent-paleotopography. Evident south to north trends include lateral lithofacies changes, increasing dolomitization, increasing anhydrite cementation, thinning in the bounding anhydrite units, and strontium enrichment. The lower Wuchiapingian Khuff unit is known for reservoir heterogeneity. The detailed petrographic, geochemical, core and imaging analyses revealed 24 diagenetic processes that have influenced pore types, pore throats (permeability) and pore size distributions. The processes 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 early dominant replacive dolomite and late dolomite cement. The replacive dolomite is interpreted to have occurred as a direct result of the reflux of hypersaline waters derived from the updip evaporative lithofacies that show heavier C-O-Sr isotopic ratios. This early (near surface) 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 Khuff carbonates and anhydrites. Enriched (C-O-Sr) isotopes are associated with the overdolomitized tidal flat and lagoon lithofacies. Depleted oxygen isotopes are associated with 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 pore size distribution, derived from 2D image analysis of QEMSCAN BSE images, is dominated by upper micropores for the good reservoir rocks; the lower micropores dominate the poor reservoir rocks. The porosity-modifying diagenetic processes, identified porosity types, and pore size distributions suggest that lithofacies and diagenesis have strong influence on reservoir quality in lower Wuchiapingian Khuff unit.