Highly radioactive, black and organic-rich shales are present in the United States, in several geologic provinces, and elsewhere. Such organic-rich shales are not only potential source rocks but commonly owe their localized but significant production potential to natural fracture systems in an otherwise impermeable rock. These natural fracture systems are concentrated in the interbedded brittle, calcareous, cherty or silty zones.

Natural gamma-ray spectral information, such as that from the Spectralog, easily locates calcareous or silty zones, as both are characterized by low values of potassium and thorium, but excessively high values of uranium. These interpretive concepts have already assisted in many successful gas and oil well completion and recompletion attempts in the more permeable and/or fractured intervals of such shale formations.

The Spectralog allows a continuous monitoring of the source rock potential (SRP) of shales in open and cased boreholes. Hence, SRP variations can be studied both versus depth and on a lateral basis using appropriate mapping techniques. Gamma-ray spectral data also assist in detailed stratigraphic correlations, for, in addition to total gamma-ray counts, the Spectralog measures the individual gamma rays emitted by potassium (K⁴⁰), the uranium series nuclide bismuth (Bi²¹⁴), and the thorium series nuclide thallium (T1²⁰⁸).

Interpretive experiences with SRP, including SEM pore-system studies, in the Miocene Monterey Formation in California, the Cretaceous Niobrara and Pierre Shales of Colorado, the Lower Mississippian and Upper Devonian Woodford Shale of Oklahoma and West Texas, and the Eagle Ford Shale in the Cretaceous carbonate trend in south Texas indicate application potentials similar to those in the Devonian gas shales of the Appalachian basin.

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