Figure
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.
a.
An overall decrease (1° variations) in shale-free limestone porosity
that corresponds to increasing density with depth (that is, normal compaction).
b. A formation-scale density behavior (2° variations) that corresponds
to rock type; for example, higher density Austin Chalk overlies lower
density Eagle Ford Shale, which itself overlies higher density Buda
Limestone, etc.
c. Packages 10 to 100 ft thick within the Austin Chalk (3° variations)
that are individually characterized by internally consistent density
variations. For example, 3° packages No. 1 and No. 4 have density-based
porosities that are consistently less than 12 percent, while 3° package
No. 2 is characterized by porosities consistently greater than 15 percent.
d. Intervals 2 to 10 ft thick where density-based porosities vary by
1 to 10 percent (4° variations). In the No. 9 Wilson core interval,
these variations correspond to changes from relatively pure chalk (lower
density-based porosity) to chalk with an increased clay content. These
clay-enriched intervals may be depositional features that were deposited
as a drape on a pre-existing chalk bed, or they may be pressure-solution
features where insoluble clay particles are concentrated as stylolites
by solution of calcium carbonate components under burial loading. For
example, 3° interval No. 5 is generally characterized by two dense
beds (density porosities less than or equal to about 12 percent) that
are interbedded with two clay-enriched intervals where porosities are
greater than or equal to about 15 percent. However, the dense beds themselves
contain several 4° layers that exhibit more detailed density variations.
Many
of the well log responses that correspond to rock properties observable
in the No. 9 Wilson core can be correlated to log responses at the other
wells at the Devine Test Site (No. 2 Wilson and No. 4 Wilson). For example,
the gamma-ray and density porosity responses that correspond to two
marl-rich intervals at 2,410 and 2,424 ft in No. 9 Wilson (Figures 16
and 17) can be correlated to logs recorded in the other DTS boreholes.
Figure 17 presents some examples of correlative horizons, and correlation
of more subtle responses is possible.
Figure
17. Preliminary stratigraphy and facies model for the interwell P
and S propagation medium.
