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Hydrocarbon production at Tubal field is from secondarily enhanced, preserved primary porosity in an interval of carbonate grainstones at the top of the Upper Jurassic Smackover Formation. These grainstones were deposited as an ooid shoal over a topographic high on the south Arkansas shelf. The degree of porosity and permeability preservation in these sediments ranges greatly and is controlled by both depositional and diagenetic processes.
Porosity and permeability are reduced mostly by pressure solution and the precipitation of late sparry cements. The chemical stability of a grain type determines its resistance to pressure solution, whereas pore size is critical to retaining porosity and permeability after cementation. Therefore, the ability of particular facies types to preserve permeability is controlled chiefly by the types, sizes, and sorting of the constituent grains. Using these parameters, pay facies (facies in which porosity and permeability are commonly preserved) can be defined and identified. Pay facies at Tubal field include the poorly sorted coarse ooid facies, the ooid-composite grain facies, and the pellet-grainstone facies. By studying the distribution of analogous modern facies, the ability to predict the distributions of upper Smackover pay facies, and thus production, can be improved.
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