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Devonian oil-bearing shale (Chattanooga Shale) occurs over a wide area of north-central Alabama and south-central Tennessee. Four counties, Limestone and Madison Counties, Alabama, and Giles and Lincoln Counties, Tennessee, appear to have the best potential for future development in the region. In this area, the shale ranges from 0 to 15 ft (4.5 m) thick and has less than 100 ft (30 m) of overburden. The shale is typically dark gray to black with pyrite laminae and nodules. Very small lenses and interbeds of sandstone and siltstone, calcite streaks, phosphate nodules, and cherty layers occur locally. The unit is correlative with the Gassaway Member of the Chattanooga Shale as recognized in Tennessee. Shale samples of high oil content are typically very dark gray to black, slightly pyritic, and some have a petroliferous odor. Sandy and silty shale samples show a sharp decrease in oil yield values. In this region, the shale appears to have accumulated in a shallow (less than 100 ft, 30 m) marine reducing environment with a highly irregular shoreline and some scattered islands. If this depositional hypothesis is correct, it may account for the sporadic occurrence, variable thicknesses of the formation, and vertical variations of chemical data for the shale unit.
Reconnaissance sampling throughout the four-county area, augmented by widely spaced core holes in Alabama, indicate that the shale has an average oil yield potential of 13 gal/ton by modified Fischer Assay. Maximum oil yield obtained was 23 gal/ton from a sample in Lincoln County, Tennessee. For samples having more than 7 gal/ton oil, whole rock and trace metals (Co, Cr, Mo, Ni, V, Ti, Zn, and U) analyses were made. Uranium values range from 0.0 to 70 parts per million (ppm) (av. 20 ppm). Values for other trace metals are as follows: Co 4 to 300 ppm (av. 91 ppm); Cr 5 to 200 ppm (av. 81 ppm); Mo 0 to 845 ppm (av. 230 ppm); Ni 10 to 600 ppm (av. 234 ppm); V 12 to 540 ppm (av. 257 ppm); Ti 3,000 to 11,500 ppm (av. 7,439 ppm); Zn 30 to 910 ppm (av. 228 ppm). Fixed carbon ranges from 0.60 to 12.97%, with an average of 9.58%. Total organic carbon averages 16.93 wt. % with a hydrocarbon index of 305 mg/g. The predominant clay mineral is illite, but mixed layered clays are common. The kerogen and organic content appears to coat and be interstitial to the quartz grains and clay particles. Coaly fragments were noted in some samples, but amorphous material suggestive of algal origin was noted in many samples. The components of the kerogen fraction are 29% aromatics, 65% resin and asphaltenes, and 6% saturates. Analyses of the extracted oil indicate 11% paraffins-naphthenes, 45.7% aromatics, 4.2% sulfur, 25.9% eluted NSOs, 1.5% non-eluted NSOs, and 11.7% precipated asphaltine. Solidstate 13C NMR spectra suggest a poor conversion of organic carbon to oil.
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