Radioactivity and Organic Content of Some Paleozoic Shales

By measurement of the total beta activity, the total alpha activity and the radium content, a new method has been developed for the routine determination of radioactive substances in sedimentary rocks. The important radioactive elements in these rocks are uranium, thorium, and potassium. Earlier work by Beers and Goodman has shown that these radioactive elements may be found largely in three principal loci: (1) in association with the heavy minerals of sands and sandstones, (2) in K40, the active isotope of potassium which is found in evaporites, oil-field brines, clastic fragments of micas and other potassium-bearing minerals, and in clays and shales which may contain up to 6.5 per cent potassium, (3) in the uranium and thorium content of shales and clays, impure limestones and marls, shaly sandstones, and organic sediments. Pure limestones and pure quartz sands are found to exhibit practically no measurable radioactivity. Black shales containing up to 16 per cent organic matter have been found to contain high concentrations of the three principal radioactive elements. Excellent correlations are shown to exist between the uranium content, the thorium-uranium ratio and carbon content in individual shale formations. These studies throw new light on three important problems: (1) the sedimentation environment of Paleozoic black shales, (2) the evolution of petroleum source beds, (3) the relationship of radioactive and organic matter in these beds.