The Lower Pennsylvanian formations, which produce oil and gas along the Bend flexure and in the Fort Worth basin, crop out in Llano uplift area. The outcrops have been studied extensively from both paleontological and lithological viewpoints from which several conflicting classifications have evolved. With the addition of subsurface terminology, stratigraphic classifications vary with individuals and companies.
Author: David E. Noller
The Mississippian sample cuttings observed in north central Texas are from the
Barnett Shale, the Chester detrital limestone, and the Chappel reef. The Barnett is a
black, very carbonaceous and methane-rich shale. The Chester is a brown, oolitic,
chert-rich limestone that represents the inner-reef facies. The Chappel reef is
composed of white, fine to microcrystalline, chalky limestone, with porosity
occurring primarily in fractures. Chappel sample hydrocarbon shows are subtle,
with a trace to forty percent (40%) of the sample cuttings having bright
fluorescence, no stain, no to fair odor, and a subtle solvent-crush cut. Hot wire gas
detectors usually record small gas measurements while Chappel reefs are being
drilled, due to Barnett Shale gas contamination, and because of the fractured nature
of the reservoir. Hydrocarbons in the Chappel reef are indicated by increases in
Propane (C3), Isobutane (IC4 ), and Normal Butane (NC4) on the chromatagraph.