Vertical Heterogeneity in the Austin Chalk
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).
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).
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 fact.
Sedimentological 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, below).
Distribution 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).