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A detailed lithologic reservoir study was conducted to aid Texaco personnel in designing and monitoring an experimental surfactant-polymer flood in the Mississippian Benoist Sandstone, one of several producing formations in the Salem field of south-central Illinois. Twelve elongated five-spot patterns are distributed over the 60-acre (24 ha.) project area. The Benoist Sandstone averages about 49 ft (15 m) of net pay at a depth of about 1,800 ft (550 m). Cores from eight wells were studied in detail. Particular attention was paid to variations in sedimentary structures, lithology, and mineralogy that could influence reservoir performance. Techniques employed in this study included examination of slabbed cores, thin-section petrography, X-ray diffraction (XRD) mineralogy, a d scanning electron microscopy/energy dispersive spectrometry (SEM/EDS).
The Benoist Sandstone is one of several Late Mississippian deltaic sandstone units deposited in the subsiding Illinois basin. These sandstones are bounded above and below by fossiliferous marine limestone and shale. Delta-front sandstones, hereafter referred to as bar-finger sandstones, comprise the bulk of the formation. Channel-fill deposits are found near the base of the unit and nonreservoir, tidal-flat deposits near the top. The bar-finger deposits are moderately to well-sorted, fine to medium-grained sandstones with horizontal to inclined planar bedding and some ripple and planar cross-bedding. The planar bedding is accentuated by clay and mica-rich layers, one millimeter to several centimeters thick. These shale layers increase and thicken upward, and separate the bar-finger sa ds into several reservoir units.
The Benoist sandstones are quartzose, containing 70 to 98% monocrystalline and polycrystalline quartz and small amounts of detrital feldspar and shale clasts. Cement is predominantly quartz in the form of syntaxial overgrowths, with minor calcite. Small amounts of clay occur as detrital laminae, authigenic pore fillings, and sand-grain coatings. The percentage of detrital and authigenic clay increases near the top of the bar-finger sandstone and significantly reduces permeability. Illite, the dominant clay,
occurs as both grain coats and pore bridgings, kaolinite forms scattered pore fills, and Fe-chlorite coats a few grains.
The channel-fill deposits are well to poorly sorted, very fine to medium-grained sandstones, commonly conglomeratic, and contain calcite-cemented zones. These deposits are mineralogically similar to the bar-finger deposits but contain abundant shale and fossil fragments at the base. Noncalcareous channel sandstones are characterized by scattered Fe-chlorite grain coatings and pore-filling illite.
Intergranular porosity is well developed and has not been severely reduced by the pervasive quartz overgrowth cementation. The eastern part of the project area contains a higher quality reservoir section because of the sparsity of clay zones in the bar-finger sandstone and the thicker channel-fill deposits in this area. Secondary porosity, produced by the dissolution of feldspar grains, has slightly enhanced the overall quality of the Benoist Sandstone.
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