Internal Stratigraphy and Organic Facies of the Devonian-Mississippian Chattanooga Shale in Kansas and Oklahoma

Abstract

The Late Devonian-Early Mississippian Chattanooga Shale of the Midcontinent is popularly referred to as a black shale, but it can be gray or green as well. The Chattanooga is a dark gray to black shale in Oklahoma, becoming lighter in color to the north in Kansas. Internal divisions due to differences in organic content account for the variation in color and suggest the presence of several organic facies within the formation. Three information subdivisions of the Chattanooga Shale (upper, middle, and lower shale members) were first recognized by examining geophysical logs of the Chattanooga-equivalent Woodford Shale of western Oklahoma (Hester et al., 1988). All three members are commonly present throughout western Oklahoma, but generally only the upper and middle members are found in Kansas (Lambert, in press). The middle member consistently has a higher gamma-ray log response than either the upper or lower members. Density log calculations correlated to organic geochemical analyses led Hester et al. (1989, 1990) to conclude that this member is the most organic-rich unit within the Woodford. Total organic carbon (TOC) content within each member decreases northward, but the middle member generally has the highest TOC. Rock-Eval analyses reveal that the upper and lower members commonlymore » contain type III kerogen in Oklahoma, while kerogen in the middle member can be type I or II. In Kansas, the upper member is characterized by type III kerogen and the middle member by either type I, II, or III kerogen.« less