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.