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Stratigraphic and Depositional Analysis of the Faridah Reservoirs in the Central and Northeastern Parts of Saudi Arabia

Abstract

The log and core analysis provides a depositional and stratigraphic synthesis of the Faridah unit in the study area. It is based on a review of 2000 feet of core data from 17 wells. These wells are located in the onshore and adjacent coastal waters. The work was intended for establishing a correlative stratigraphic framework and understanding the depositional settings of the Faridah interval from the outcrop belt to the subsurface. The Middle Jurassic Dhruma Formation is bounded above and below by the Tuwaiq Mountain and Marrat formations, respectively, and consists predominantly of carbonates and a lesser amount of shale. The formation has been subdivided into six members; D1-D6, with D-4 containing the Faridah unit. The type section, W-5 well, comprises four stacked and distinct layers, termed the Faridah-D, -C, -B and -A, in ascending stratigraphic order. The sedimentologic analysis and lithologic breakdown of the Faridah sediments led to the identification of 12 primary lithofacies. These are designated LF1 to LF12 in broadly shallowing upward, basin to platform, order. The lateral distribution of the Faridah lithofacies suggests that the environment of deposition range from platform interior to basinal setting. The stratigraphic architecture and facies stacking patterns suggest that the Faridah interval spans parts of at least two long-term depositional sequences, together forming a large-scale northward-prograding and thinning shelf-margin succession. This thinning trend is accompanied by a gross change in depositional facies, from (a) thick platform-top peloid-oncoid packstone and grainstone, to (b) shelf-edge micro-gravels, coral boundstones and slumped foreslope facies, to (c) thin bituminous basin-floor laminites in the northern parts. The D4 sediments are stacked to form four correlatable high-frequency cycles (Faridah D-A), representing a mid- to late-highstand systems tract. These four cycles progressively thin basin-ward, and their correlatability becomes insecured and less distinguishable. The best primary interparticle macroporosities are associated with the Faridah shelf-margin grainstone facies at the Faridah, Ash Shihiyah and southern Juraybi'at areas, with fair to good permeabilities. The northward transition from clean porous shelf-edge facies to tight basin-floor laminites occurs over a distance of just a few tens of kilometers, and provides opportunities for stratigraphic traps in the area.