Heterogeneity in the Austin Chalk
heterogeneity is caused by detachment along the same normal faults that
contribute to vertical heterogeneity. Fault dips are influenced by rock
type, as discussed previously. Although a broad range of fault orientations
may be expected, fault strikes subparallel, subperpendicular, and/or highly
oblique to the regional east-northeast to west-northwest trend probably
dominate (Figure 13). Marl smears (Figure 14) and rollover monoclines,
features closely associated with normal faults, also cause lateral heterogeneity
13. Marl smear associated with a normal fault (having 3-ft throw).
14. Joint located in upper Lower Austin Chalk, SSC tunnel. Joints
terminate at chalk-marl bedding planes; however, joints may reappear
in overlying chalk beds at the same location.
Where present in the
subsurface, joints (extension fractures in which no vertical movement
has occurred on either side of the fracture as shown in Figure 15) are
probably subparallel to the regional structural trend and may additionally
contribute to lateral heterogeneity. Joints are typically well developed
in chalk but are absent in intervening marl beds (Friedman and Wiltschko,
1992: Nance and others, 1994). Joints may be open or mineral filled. Some
joints are enlarged and contain water.
15. Small channel in lower Lower Austin Chalk, SSC tunnel. Maximum
thickness of chalk channel fill is approximately 2 ft. Channel fill
tapes laterally to about 1 ft. thick. Underlying chalk bed was eroded
during channeling but is approximately 6 inches thick away from channel.
Thin dark beds are marls.
lateral heterogeneity is caused by variations in bed thickness, depositional
facies that are often associated with syndepositional erosional truncation
of previously existing beds, and local mineralogical anomalies. A chalk
layer may thicken locally into shallow erosional submarine channels (Figure
16). Larger channels ranging from 20 to 125 ft wide and as much as 8 ft
deep may be filled with numerous thin cyclic chalk and marl layers.
16. Initial definition of the suite of rock facies that comprise the
interwell medium. This preliminary model uses the density log to depict
the seismic propagation medium as a series of distinct rock facies
that start as a coarse, first-order (1°) compaction trend and
end as a detailed assemblage of thin, fourth-order (4°) beds.
in the channel fills also produce vertical heterogeneity. At the SSC site,
channels are restricted to the lower Lower and Middle members of the Austin
Chalk, are sinuous, and are generally oriented along dip (paleobasinward).
Channels have been described from the Austin Chalk as far southwest as
Langtry, Texas (Lock, 1984). Distances between channels in the SSC tunnels
are greater than the channel widths, with 29 channels being identified
in 6 mi of tunnel length. The boreholes at the DTS are aligned along a
northwest to southeast dip (Figure 17); therefore, the probability of
intercepting a channel having the same general orientation is reduced,
although the typical sinuosity of a channel suggests some probability.
Also, pyrite (Fe-sulfide) nodules up to 1 inch in diameter are widely
disseminated in chalk and provide local variations in chalk composition.
17. Preliminary stratigraphy and facies model for the interwell P
and S propagation medium.