Heterogeneity in the Austin Chalk
is probably the main factor at the Devine Test Site. Structural vertical
heterogeneity in Austin Chalk is caused by detachment along steep (average
60° dip) normal faults and by associated features such as fault
breccia, marl smears, and rollover monoclines (Figures 5 and 6) and
5. Complex normal fault zone in upper Lower Austin Chalk at the
SSC site, showing large normal faults (F) and smaller antithetic
faults (A), mainly calcite filled. Almost 70 ft of throw across
this zone has been interpreted from borehole data. Rollover monocline
(R) is found on the far hanging wall. Water (W) was produced from
only one small (3-ft throw) fault.
6. Small normal fault (2-ft throw) in SSC tunnel, with changes in
dip caused by variations in chalk composition. Lower dips occur
in strata containing or composed entirely of marl. Note expansion
of marly chalk into the fault aperture.
Actual dip along continuous
fault surfaces varies such that steeper dips (locally, up to nearly vertical)
occur in purer chalk, whereas flatter dips (as low as 30°) occur in
marls (Figure 7).
7. Small normal fault (0.5-ft throw) in lower Lower Austin Chalk,
with steep and listric segments that reflect lithologic variation
between chalk and marl.
Some faults have steep
and listric segments, responding to lithologic variations (Figure 8).
Some larger faults at the SSC have been interpreted from seismic data
and well data to become listric in the Eagle Ford Shale, which underlies
the Austin Chalk (The Earth Technology Corporation, 1989).
8. Small (1-inch throw) antithetic faults located between bends in
larger associated faults. Additional space created along fault surface
is filled with calcite.
Faults occur individually
or in widely spaced (hundreds to thousands of feet) clusters of 2 to 10,
or more, fractures. Small antithetic faults often are concentrated near
bends in their associated larger faults (Figure 9), which is a common
development in other extensional terranes (for example, see Laubach and
others, 1992). Faults may be open, locally mineral filled with calcite,
or totally mineral filled. Fracture-related water production is local
at the SSC site and may be restricted to a single fault within a larger
fault cluster (Figures 6 and 10). The low probability of intercepting
widely spaced high-angle faults with vertical boreholes is an accepted
9. Cluster of small intersecting normal faults in SSC tunnel with
water production restricted to one fault.
10. Submarine channel in lower Lower Austin Chalk, SSC tunnel. Channel
is also shown in background of Figure 4. Approximate width of channel
is 125 ft. Note inch-scale laminations of chalk and marl in channel
vertical heterogeneity at the bed scale (0.1 to 6 ft) is caused by interbedding
of large-scale (3 to 4 ft), brittle, relatively pure chalk layers that
have small scale (0.1 to 1 ft), ductile, marl-dominated layers (Figure
5). Even so-called pure chalk beds may include a marl admix that is more
concentrated in the upper or lower parts of the bed (Figure 7). At the
intrabed scale (<1 inch), vertical heterogeneity is caused in some intervals
by vertical variations in sediment texture. When not churned by burrowing
organisms (bioturbation), this process forms fine laminae (Figure 11,
11. Association of partially mineralized (calcite) normal fault and
submarine channel (upper left of fault) is the cause of complex local
heterogeneity in this section of lower Lower Austin Chalk.
of primary porosity and attendant diagenetic effects such as calcite cementation
or dissolution may then be texturally influenced (Hovorka and Nance, 1994).
Flatlying pelecypod fragments also contribute to vertical heterogeneity
in Austin Chalk. Responding to this vertical heterogeneity, vertical permeability
in unfractured carbonate rocks is generally lower than horizontal permeability,
on the basis of whole-core analyses. Bed-scale (>0.5 ft) vertical heterogeneity
is generally detectable using high-resolution well logs. Structural and
sedimentological heterogeneities at several scales in the Austin Chalk
are locally compounded (Figure 12).
12. (a) Frequency histogram of fault strikes, SSC tunnel. Dominant
strike is approximately northwest, or subnormal to the dominant trend
of the Balcones Fault Zone. Compare with (b) northeast strikes of
major faults (throw >20 ft) mapped at the surface. (c) Frequency
histogram of joint strikes, SSC tunnel. Joint strikes, tightly clustered
toward the northeast, are parallel to the dominant structural trend
of the Balcones Fault Zone.