About This Item
Share This Item
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 carbonate beds 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 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 10 wells have produced approximately 33 Bcf of gas at depths of 2,600 to 2,900 ft (792 to 884 m). Santa Anna field in Coleman and Brown Counties is a large stratigraphic trap discovered in 1928. More than 100 gas wells have been completed in Santa Anna field. Early production records are not available. However, the inco plete 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 Bcf of gas at depths of 2,300 to 2,400 ft (700 to 732 m).
Porosity tends to develop within three facies of the upper Marble Falls limestone: phylloid algal limestone, stromatoporoid limestone, and oolitic carcarenite. 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.
Structure is complicated and difficult to perceive without seismic data. Even where seismic data are available, standard isopach mapping techniques cannot be employed because there are no continuous shallow seismic reflectors. Moreover, topographic highs capped by Edwards Limestone outcrops tend to yield poor-quality records.
In spite of the problems inherent in exploring a structurally and stratigraphically complex area, there are undoubtedly undiscovered, commercially attractive gas fields in the southern part of the Fort Worth basin. The high exploration risk is to some extent offset by shallow depth, low acreage costs, and the attractive nature of potential targets.
End_of_Article - Last_Page 1427------------