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COLLINS, EDWARD W., Bureau of Economic Geology, University of Texas at Austin, Austin, TX

ABSTRACT: Fracture Zones Between Overlapping, En Echelon, Normal Fault Strands: Outcrop Analogs Within the Balcones Fault Zone, Central Texas

Areas between overlapping normal faults contain abundant fractures and are potential targets for hydrocarbons in fractured reservoirs, as well as being potential areas for preferential groundwater recharge and flow. Cretaceous limestone outcrops within the Balcones fault zone contain two types of fault overlap: a relay ramp between overlapping master faults dipping the same direction and a structural bridge between overlapping faults dipping in opposite directions. These outcrops reveal a variety of fracture characteristics including fracture type, geometry, spacing, and connectedness within these structural zones.

The fault overlap areas are up to about 1 km wide, and the en echelon master faults may overlap by as much as 2 km. Strata within these fault overlap zones are cut by abundant small normal faults, commonly having throws of less than 0.5 m, and joints. These deformed areas consist of a mosaic of intermingled fracture sets having 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. Fractures also do not have uniform spacing within any given unit. Spacing of single small faults and fracture swarms commonly is between 2 and 45 m along traverses perpendicular to and oblique to the master overlapping faults. Swarms of small faults are usually as much as 6 m wide, and may contain as many as 15 faults. Joint swarms that are as much as 12 m wide have fracture spacings of 0.6 to 1.5 m.

AAPG Search and Discovery Article #90989©1993 GCAGS and Gulf Coast SEPM 43rd Annual Meeting, Shreveport, Louisiana, October 20-22, 1993.