SURFACE TO SUBSURFACE CORRELATION OF NATURAL FRACTURES IN ORDOVICIAN TO PENNSYLVANIAN LIMESTONE UNITS IN SELECTED AREAS OF CENTRAL AND NORTH-CENTRAL TEXAS

The Mississippian Barnett Shale is the major gas producing rock unit in the Fort Worth basin of north-central Texas. Gas production from the Barnett is enhanced by the complexity of its internal natural fracture networks. Stratigraphically, the Barnett lies between two prominent limestone units: the underlying Ordovician age Ellenberger formation and the overlying Pennsylvanian age Marble Falls.

Few exposures of the Barnett Shale exist in outcrop, and those that do are poorly preserved, making it difficult to conduct a detailed fracture analysis. The Marble Falls and Ellenberger formations contain the same major fracture sets and are better preserved in outcrop.

We conducted detailed fracture analysis in four localities, within these limestone units to study the geometry and origin of surface fractures. These locations are (1) Possum Kingdom Lake; (2) Lake Brownwood Spillway; (3) Bend River; (4) Archer Ranch. In all four locations, two prominent sets were found: (1) an older east-west trending set; and (2) a younger northeast-southwest set. We have found that the proximity to major geologic structures such as the Llano Uplift, as well as minor structures such as local faults or folds can have an effect on the orientations of fracture sets. Additionally, variations in clay content of a particular rock unit can effect fracture orientation and spacing.

Fracture orientations obtained from outcrop analysis were compared to subsurface data from two horizontal Formation Micro Imaging (FMI) logs in the Forth Worth Basin. The subsurface fracture sets observed in the FMI data are similar to the surface fracture sets in regards to their orientation, and are interpreted as being formed by the same tectonic stresses.

The first set formed by Mode 1 opening of fractures, related to east-west compressive stresses during the Pennsylvanian Ouachita Orogeny. The second set formed from tensile stresses that affected much of central Texas during the opening of the Gulf of Mexico.