Organic-rich, dark, and highly radioactive shales are encountered worldwide in several geologic sequences. These shales are potential source rocks and frequently owe their significant but localized production potential to natural fracture systems that are concentrated in brittle, calcareous, cherty, or wilty zones interbedded in an otherwise impermeable rock.

Under these circumstances, standard interpretive well logging techniques do not provide satisfactory formation evaluation results. However, such calcareous, silty, and cherty zones, frequently characterized by low concentrations of potassium and thorium but excessively high uranium content, are located easily with natural gamma ray spectral logging techniques. Gamma ray spectral logging, in addition to total gamma ray counts, measures and records the individual contributions of potassium (in percent), and uranium and thorium (both in ppm) in open and/or cased well bores.

The typical log response in organic-rich shales shows high potassium and thorium content plus excessive uranium enrichment. Such logging information correlates with the organic carbon content, as illustrated by field data.

Basic concepts of natural gamma ray spectral logging devices are reviewed, and field experiences in the Cretaceous Niobrara and Pierre Shales in the Rocky Mountains, Woodford Shale of Oklahoma and west Texas, and Devonian shales of the Appalachian basin, etc will be discussed.

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