Analytic and petrologic studies of both detrital and chemical components of the Pennsylvanian Excello Shale Member of the Senora Formation in northeastern Oklahoma divide the shale member into a northern carboniferous facies and a southern bituminous facies. The change occurs just south of a limestone reef in the underlying Breezy Hill Limestone Member. Continued study and shale-facies mapping will aid in determining the subsurface position of the reef and the distribution of environments of shale deposition. In shales, clay mineralogy is only a general indicator of environment. Other components such as iron, phosphate, carbonate, organic matter, zeolites, and trace elements are more sensitive to environmental change.