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PETROLOGY AND POROSITY PERMEABILITY CHARACTERISTICS OF TERTIARY AQUIFER SANDS
shows that the unit consists of 98.4% framework quartz,
0.3% feldspar, 0.5% heavy minerals, 0.4% opaque minerals,
0.3% muscovite, and 0.1% glauconite. Staurolite, zircon,
garnet, tourmaline, and rutile are the major detrital heavy
minerals.
Monocrystalline quartz from 99% of the total quartz
population; of this 65.5% is non undulatory and 33.5% is
undulatory. Robertson and Thayer (1992) also indicated that
there is little or no variation in mineral composition between
updip and downdip wells in the SRS area. Plots of quartz
types on the Basu and others (1975) quartz provenance diagram,
along with the presence of garnet, kyanite, staurolite,
and sillimanite indicate a medium to high rank metamorphic
provenance for most Gordon sands.
Strom and Kaback (1992) did semi quantitative X-ray
diffraction analyses on 18 Gordon sands from the SRS p-
wells. Their results show that the sands consist mostly of
quartz with 5 – 18% clay. Smectite and kaolinite are the
dominant clay minerals with minor to trace quantities of
illite. Authigenic minerals include clinoptilolite, 2 – 5 um
diameter cristobalite lepispheres, and fibrous chalcedony
(Strom and Kaback, 1992; Thayer, 1989, 1992). X-ray diffraction
analyses of four sandy mud samples from intra aquifer
confining layers show that they consist of quartz and 51 –
64% clay minerals. Kaolinite is the dominant clay mineral in
the muds with minor illite and smectite.
UPPER THREE RUNS AQUIFER
The Upper Three Runs aquifer consists of all strata from
the water table to the top of the Gordon confining unit (Fig.
1), and includes the sandy and sometimes calcareous sediments
of the Santee Limestone, and all of the Barnwell
Group (Aadland and others, 1992). The Upper Three Runs
aquifer consists mainly of unconsolidated quartz sand intercalated
with thin, discontinuous layers of sandy mud and
carbonate. Figure 2 shows the percent gravel, sand, and mud
for 102 samples from the Upper Three Runs aquifer, and
Figure 3 gives the percent sand, silt, and clay for 9 representative
sand samples.
The Upper Three Runs is a subangular, lower coarse and
medium grained, slightly gravelly, immature quartz sand.
Gravel ranges from zero to 21.7% and averages 1.7 + 0.4%.
The gravel is pebble and granule size grains of rounded and
subrounded quartz and quartzite. Sand averages 91.2 + 0.9%,
and ranges from 58.0 to 99.5%. Mud (silt + clay) averages
6.6 + 0.7%, and ranges from 0.4 to more than 25% (Fig. 2).
About 50% of the sampled sands contain less than 5% mud.
Hydrometer analyses of the mud fractions of 9 Upper Three
Runs sands indicate that clay-size particles are more abundant
than silt-size grains (Fig. 3).
The average grain size of 90 Upper Three Runs sands is
1.69 + 0.06φ (lower medium sand). Sands range from 0.57φ
(upper coarse grained) to 3.58φ (very fine grained); mean
Figure 5. Cumulative frequency curves showing the distribution
of mean permeability values for the Gordon, Upper
Three Runs, and Steed Pond aquifer units.
size of slightly more than 50% of the sampled population is
lower coarse and upper medium sands. The average sorting
of Upper Three Runs aquifer sands is 1.14 + 0.07φ, which is
poorly sorted on the Folk and Ward (1957) verbal scale.
Sorting ranges from 0.40 φ (well sorted) to 3.19φ (very
poorly sorted). Almost 70% of the sampled population is
moderately and poorly sorted, and about 25% is well and
moderately well sorted. Sorting is controlled primarily by the
amount of mud matrix in these sands. Typically, the greater
the percentage of mud matrix the poorer the sorting. Upper
Three Runs aquifer sands are fine-skewed (x = 0.13 + 0.03);
skewness ranges from –0.49 (strongly coarse-skewed) to
0.73 (strongly fine-skewed), and is controlled by the amount
of mud and gravel in the sands. Sands with high mud contents
are usually fine and very fine skewed, whereas those
with high percentages of gravel are coarse and strongly
coarse-skewed. Mean Kurtosis of 90 Upper Three Runs
aquifer sands is 1.57 + 0.08, which is very leptokurtic on the
Folk and Ward (1957) scale. Kurtosis values range from 0.59
(very platykurtic) to 4.26 (extremely leptokurtic); most of
the sampled sands are leptokurtic and very leptokurtic.
Strom and Kaback (1992) determined the composition
of 28 sand and muddy sand samples from SRS P-wells penetrating
the Upper Three Runs aquifer. The sands consists of
quarts with 2 to 40% detrital clay. Smectite and kaolinite are
the dominant clay minerals with minor to trace quantities of
illite. Trace amounts of chlorite, calcite, plagioclase, and K-
feldspar were also reported. X-ray diffraction analysis of
four mud samples from intra – aquifer confining layers
shows that they consist of silt-size quartz and 52 – 64% clay,
mainly kaolinite with minor to trace amounts of illite and
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