Shales are believed to have facies just as distinctive as those of limestone and sandstone. Shale, however, requires careful detailed examination of both detrital and chemical components to delineate facies. To test this hypothesis a study was made of the Pennsylvanian Excello Shale Member of the Senora Formation, a seemingly uniform black shale unit in the Mid-Continent area. The Excello can be divided into a bituminous facies on the south and a carbonaceous facies on the north. The facies change occurs just south of a limestone reef in the underlying Breezy Hill Limestone Member of the Senora Formation. Hence, the subsurface location of the reef probably can be determined by a study of shale cuttings and by mapping the distribution of the two shale facies.

The Excello Shale Member is a black, phosphate nodule-bearing shale unit, 4-8 ft thick, which crops out from northeastern Oklahoma northeastward across Kansas, Missouri, and Illinois. At its exposures near Tulsa, Oklahoma, the average composition of the Excello is 17.9 percent organic matter, 2.5 percent iron oxides, 0.5 percent phosphate (as calci-fluor-apatite), 9.4 percent acid-soluble matter which was not identifiable, 24.2 percent quartz, and 42.5 percent illite. Kaolinite is present near formation boundaries and in nodules formed of carbonate-fluor-apatite, containing pyrite. Pyrite occurs only in the nodules and is generally absent from the shale.

The environment of deposition of the Excello Shale is believed to have been a marine basin having an Eh between 0 and -0.1 and a nearly neutral pH. Where dead organisms settled out or where fecal pellets collected, the Eh and pH were lower and there phosphate was deposited as nodules.

The shale is believed to be a sediment type deposited in a transgressive sequence offshore from a coal swamp, but partly separated from the swamp by a Breezy Hill reef at about the present Oklahoma-Kansas border. Seaward from the reef (south), the shale is bituminous; landward (north) the Excello facies is more micaceous and carbonaceous.

In the subsurface west of Tulsa, at about R. 16 W., the Excello Shale and the Little Osage Shale Member of the overlying Fort Scott Formation merge. Both shales were deposited in the same sea as it transgressed and regressed across the Pennsylvanian surface.