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Unconventional Jurassic Carbonate Source Rocks, Saudi Arabia

Abstract

Jurassic carbonate source rocks within the Jurassic Tuwaiq Mountain, Hanifa, and basal Jubaila formations supplied vast amounts of oil to Jurassic carbonate reservoirs. These source rocks contain 1% to 14% TOC, abundant organopores, and very low to no clay content. Deposition was in an outer ramp to basin depositional environment, beneath fair-weather wave base and within storm wave base. Storms swept sediment down-dip into the outer ramp/basin and appear to have waxed and waned in a cyclic manner. Sedimentary structures include: gently undulating parallel lamination (GUP lamination) or sinuous lamination; micro-hummocky cross lamination; ripple lamination; micro cut and fill lamination; and micro-topographic infill lamination. TOC appears to be concentrated in fecal pellets that were transported down-dip by storms. Three lithofacies have been recognized: 1) anoxic, black, laminated, wackestone to mud-dominated packstone; 2) dysoxic, black, horizontally micro-bioturbated, laminated to very thin bedded, wackestone to mud-dominated packstone; and 3) oxygenated, gray, bioturbated, thin bedded, wackestone to mud-dominated packstone. Fecal pellets are the most common grains. Skeletal constituents include: Bositra buchi bivalves that are whole, shell halves, and fragmented to highly fragmented; and abundant non-descript highly fragmented skeletal detritus. A pycnocline divided the water column into: 1) anoxic water beneath; 2) dysoxic water at the contact; and 3) oxygenated water above. The pycnocline moved up and down in the water column, creating apparent cyclicity within the strata, and may have been controlled by: relative sea level change; variable restriction of circulation; or a combination of both. Diagenetic products include: dolomite crystals (0% to 5%); anhydrite crystals (0% to 5%); pyrite cubes and finely disseminated crystals (1% to 3%). All of this data has been utilized to characterize and explore for unconventional Jurassic carbonate source rocks in Saudi Arabia.