Use of scintillometer and gamma-ray logs for correlation and stratigraphy in homogeneous black shales

INTRODUCTION: Widespread, thick, uniform black-shale units may defy all but the coarsest stratigraphic subdivision. The devonian and Mississippian black-shale formation such as the Ohio, New Albany, and Chattanooga Shales of the eastern and central United States are good examples. Exposures of these black organic-rich shale formations appear lithologically uniform except for a few interbeds of greenish-gray shale. These black shales make up as much as one-fourth of the Devonian sedimentary sequence in the Appalachian basin and are important sources of natural gas Avila, 1976). Yet, because most means of stratigraphic control are absent and paleontologic correlation is laboriously difficult in these shales, internal subdivision and correlation among Devonian-Mississippian black shales of some areas are wholly inadequate. In parts of Ohio, Indiana, and Tennessee, widespread bentonites or thick green-shale beds are present in the Devonian-Mississippian black-shale sequence, providing a basis for its subdivision (Campbell, 1946; Hass, 1956; Hoover, 1960; Conant and Swanson, 1961; Lineback, 1968, 1970). In eastern Kentucky (Fig. 1), however, similar marker beds are either very thin or absent. Thus, the Devonian-Mississippian black-shale sequence of eastern Kentucky has not been divided previously, because apparent homogenity impeded attempts to compare and correlate the sequence with subdivision in the better known black-shale sequences of adjacent states.