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.
From a regional subsurface study, it becomes apparent that there are three lithilogic units of lower Pennsylvanian age, which are of paramount economic importance. These units are the Marble Falls, Comyn (Duffer), and Big Saline formations. These formation names were used because of their prominence in published literature and their descriptive nature.
The Comyn (Duffer) and overlying Marble Falls are very similar, both in lithologic character and depositional history, but can be separated in the subsurface on the basis of electric log correlation. It is the contention of the author (John G. Nikkel) that these units are equivalent to the undifferentiated Marble Falls out crops in San saba Llano an burnett counties and that they are Morrowan in age.
Drill cuttings descriptions of the Comyn (Duffer) limestone in the southwest Stephens County area are as follows: tan, brown, and shades of gray, with a salt & pepper appearance, Micro crystalline to medium crystalline crystal size, Chert & calcite can be secondary material associated with the rock cuttings. Fluoroscope shows are often dull to medium green, with a dull cut.
Porosity tends to develop within three facies of the Marble falls Formation of which the Comyn (Duffer) limestone are a unit of this formation: phylloid algal limestone, stromatoporoid limestone, and oolitic cacareniste. At the outcrop the algal, stromatoporoid, and oolitic facies are most prevalent near the edge of bank complexes. However, they are by no means restricted to the outer part of the banks. Fractures related to a system of mostly down to basin normal faults enhance permeability in many places. (According to Jerry Namy)
Hydrocarbon production from the Comyn (Duffer) along the western edge of Eastland and Stephens counties appears to be limited to porosity traps and has been minor and near its western edge. The Marble Falls Deviates from this pattern and produces gas from porosity development along its depositional axis in Comanche and Hamilton counties. According to John G. Nikkel.
References: John G. Nikkel Pan American Corp., Abilene Texas and Jerry Namy, Texland Inc., Fort Worth, Texas, Combined Abstracts from the above authors with sample description from Sterling Sanders, Stivers Consulting, Inc.