Abstract

The Marble Falls Group is a Lower Pennsylvanian carbonate complex that crops out in a discontinuous arcuate belt rimming the east, north and west sides of the Llano uplift. The limestone accumulated on the eastern part of the Texas craton, which sloped toward the adjacent Fort Worth basin. Three units within the Marble Falls Group have been mapped in the outcrop area. They are informally referred to as the “lower limestone”, “middle shale” and “upper limestone”. The same tripartite subdivision is evident in the subsurface to the north of the Llano uplift in Lampasas, Mills, Hamilton, Comanche and Brown counties.

Marble Falls gas production in the southern part of the Fort Worth basin is almost exclusively from the “upper limestone”. The “upper Marble Falls” forms several northeast to southwest trending carbonate bank complexes. The bank complexes terminate abruptly and pass laterally into shale and dark spiculitic limestone. Both structural and stratigraphic traps are evident within the bank complexes. Pottsville field in Hamilton County is a steep-sided structural trap from which ten wells have produced approximately 33 billion cubic feet of gas at depths of 2,600 to 2,900 feet (793 — 884 m). Santa Anna field in Coleman and Brown counties is a large stratigraphic trap, discovered in 1928, in which more than 100 gas wells were completed. Early production records are not available; however, the incomplete data that are published substantiate the fact that Santa Anna is a large economically attractive field, in which many wells have produced at least 1 to 2 billion cubic feet of gas at depths of 2,100 to 2,400 feet (640 – 732 m).

Porosity tends to develop within three facies of the “upper Marble Falls limestone”: phylloid algal limestone, Komia limestone and oolitic calcarenite. On the outcrop the algal, Komia and oolitic facies are most prevalent toward the edges 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.