ABSTRACT

This study describes two types of fault overlap within the Balcones Fault Zone: a relay ramp between overlapping master faults dipping in the same direction and a structural bridge between overlapping faults dipping in opposite directions. Cretaceous limestone outcrops within these structural zones have been described for this study, and the outcrops reveal a variety of fracture characteristics, including fracture type, geometry, spacing, and connectivity, that are important in understanding the framework of fractured strata. Areas between overlapping normal faults contain abundant fractures and therefore are potential targets for hydrocarbons in fractured reservoirs, as well as potential areas for preferential ground-water recharge and flow in fractured aquifers.

The fault overlap areas are up to 0.6 mi (1 km) wide, and the en echelon master faults may overlap by as much as 1.2 mi (2 km). Strata within these fault overlap zones are cut by joints and by abundant small-displacement normal faults, commonly having throws of less than 1.6 ft (0.5 m). These deformed areas consist of a mosaic of intermingled fracture sets that have multiple strikes; thus, fracture connectivity is locally high.

Fracture spacing is variable within fault overlap areas. Some individual beds and multiple-bed packages are more fractured than other beds of similar thickness and composition. Also, fractures do not have uniform spacing within any given unit. Spacing of single small faults and fracture swarms is commonly between 6.5 and 150 ft (2 and 46 m) along traverses perpendicular and oblique to the master overlapping faults. Swarms of small faults, commonly as much as 20 ft (6 m) wide, may contain as many as 15 faults. Joint swarms that are as much as 40 ft (12 m) wide have fracture spacings of 2 to 5 ft (0.6 to 1.5 m).