--> ABSTRACT: Influence of Faults on Ground-Water Flow in a High-Porosity Quartz Sandstone Aquifer in Central Texas, by Leslie C. Randolph and Brann Johnson; #91022 (1989)

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Influence of Faults on Ground-Water Flow in a High-Porosity Quartz Sandstone Aquifer in Central Texas

Leslie C. Randolph, Brann Johnson

Faults in porous sandstone aquifers can measurably influence fluid flow because the permeability of the associated fault gouge is two to six times smaller than that of the undeformed rock. Faults with sufficient displacement (reservoir rocks still juxtaposed across fault) tend to hydraulically compartmentalize the aquifer, with the bounding faults representing leaky boundaries. Unfortunately, only limited field-scale data exist to allow establishment of quantitative relationships between the geologic characteristics and hydraulic characteristics of a fault or fault zone. In this presentation, we document an example of hydraulic compartmentalization of an aquifer by faults.

The study area is in northern Mason County in central Texas and focuses on the Hickory Sandstone aquifer, which is a 130-m thick, high-porosity quartz sandstone unit. The aquifer contains numerous dip-slip and oblique-slip faults of varying displacement. Detailed geologic mapping, gravity surveys, and well control together with structural analysis of the fault system allow us to characterize the structural configuration of the aquifer. This detailed characterization is critical to a proper analysis of the well data. Analysis of temporal and spatial variations of water levels in uncased wells demonstrates that discrete faults or small fault zones with net displacement of 30 m, or less, measurably influence ground-water flow. On a time scale of at least several years, if not greater, sustained water-level differences of from 3 to 6 m occur across faults with 30 to 50 m of displacement. Although data are more limited, the hydraulic head difference appears to be greater across faults with g eater displacement. In addition to head differences, water-level gradients also differ across the faults. Regions of the aquifer bounded on all sides by moderate-displacement faults exhibit much smaller hydraulic head gradients compared to regions only partially bounded by faults.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.