A study of the clay mineralogy of the Devonian black shales of the Appalachian basin was undertaken to help predict areas for potential gas resources. More than 2,100 samples from 84 drill holes representing 11 shale units were analyzed for clay mineralogy or whole—rock mineralogy or both. Approximately 1,300 samples were from drill cores and about 800 samples were from drill cuttings. Illite (2M) recrystallized during diagenesis is more or less uniformly present throughout all shale units. Chlorite formed during low grade metamorphism is least abundant in the younger units and more abundant in the older units. Illite—smectite mixed—layer clay, which supplied the material for the formation of chlorite, is most abundant in the younger units and least abundant in the older units. Illite—chlorite mixed—layer clay occurs as a trace in all shale units. Kaolinite, the only unaltered detrital clay mineral, occurs in about 25 to 30 percent of the samples. The source of the kaolinite appears to have been to the east and northeast of the basin of deposition. The color of the shale units is primarily due to the organic content; however, those shales that contain calcite are darker than those that do not for equal amounts of organic carbon.
