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The lower Pennsylvanian formations which produce oil and gas along the Bend flexure and in the Fort Worth basin crop out in the Llano uplift area. These 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 lithologic units of lower Pennsylvanian age which are of paramount economic importance. These units are the Comyn, Marble Falls, and Big Saline formations. These formation names were used because of their prominence in published literature and their descriptive nature.
The Comyn 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 correlations. It is the contention of the author that these units are equivalent to the undifferentiated "Marble Falls" outcrops in San Saba, Llano, and Burnett counties and that they are Morrowan in age.
Hydrocarbon production from the Comyn has been minor and appears to be limited to porosity traps near its western edge in Eastland and Stephens counties. The Marble Falls deviates from this pattern and produces gas from porosity development along its depositional axis in Comanche and Hamilton counties.
The Big Saline outcrops in McCulloch County are believed to be Atokan in age due to rather wide lithologic variation and depositional history between it and the underlying Morrowan units. This is substantiated by paleontological evidence. The Big Saline is lithologically heterogeneous, with graded sediments in Jack and Wise counties. Both limestone and coarse clastic reservoirs are basically gas-bearing, but oil production is commonly found in either lithologic type of rock.
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