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PAUL A. THAYER AND MARY K. HARRIS
Calcareous Quartz Sand
This lithfacies occurs at the base of the Santee in P-21
and at the top of the unit in P-22. It consists of friable, fine to
medium quartz sand with subordinate echinoderm, molluscan,
and foraminiferal allochems. Terrigenous mud matrix
forms 5-15% of the total unit. Most of the echinoderm and
molluscan fragments are broken and rounded, suggesting
transport. Locally, aragonitic molluscan shells have been dissolved
forming moldic pores. Rare sparry calcite cement
forms syntaxial overgrowths on echinoderm fragments and
partially fills moldic pores. Porosity in this lithofacies is 20-
30% depending on the amount of mud matrix, and is mainly
intergranular. Permeability is excellent because of the large
pore throats connecting intergranular pores.
Figure 4. Symbols used in graphic logs.
maximum size averages lower coarse sand (x = 0.89φ, s =
0.22φ) in P-22 and upper medium sand (x = 1.26φ, s = 1.13φ)
in P-21. Sorting of the terrigenous and fraction is moderate,
and is slightly better in P-22 (x = 0.56φ, s = 0.19φ) than in P-
21 (x = 0.64φ, s = 0.25φ).
Principal lithofacies of the Santee Limestone in downdip
SRS wells include calcareous mud, calcareous quartz
sand, skeletal wackestone-packstone, and quartz-rich skeletal
grainstone. Non-carbonate grains common to all lithologies
include quartz, feldspar, heavy minerals, muscovite,
collophane, and glauconite (Fig, 2 and 3). Detrital heavy
minerals are ubiquitous and include garnet, brown tourmaline,
rutile, zircon, sillimanite, staurolite, kyanite, monazite,
ilmenite, and magnetite.
Calcareous Mud
This lithofacies occurs only in P-21 where it is dominant
above 187 ft (Fig. 2); it grades into muddy, skeletal wackestone
below. The unit consists of terrigenous mud with subordinate
skeletal allochems and minor, very fine, subangular
quartz sand. Molluscs and foraminifers are the main
allochems; most are sand size and abraded. Woody organic
debris, diatom valves, sponge spicules, collophane, and muscovite
are common, but form less than 1% of the total volume.
Authigenic minerals include glauconite, pyrite,
clinoptilolite, chalcedony, and opal-CT lepispheres. Porosity
of this lithofacies is typically less than 5%. Pore types are
chiefly moldic formed by dissolution of aragonitic molluscan
shells and intraparticle in foraminifers. Permeability is
very low because mesopores are isolated from each other by
microporous mud.
Skeletal Wackestone-Packstone
This is the dominant lithofacies in both wells and consists
of molluscan-echinoderm wackestone interbedded with
subordinate molluscan - echinoderm packstone. Micrite
matrix forms 20-65% of whole-rock volume, and in places
has aggraded to microspar. Terrigenous mud forms 5-49%
and is most abundant in P-21. Fine and very fine quartz sand
ranges from 1-35%, and is more common in P-22 than in P-
21.
Skeletal allochems constitute 15-50 of whole-rock volume
and consist of whole, broken, and abraded echinoderm
and pelecypod fragments with subordinate bryozoans, foraminifers,
gastropods, serpulid worm tubes, ostracodes, and
comminuted skeletal debris. Most allochems are sand size,
although gravel size pelecypods and bryozoans are dominant
locally. Sponge spicules, diatom valves, collophane, woody
organic debris, pyrite, clinoptilolite, and muscovite are common
in P-21, but rare in P-22. Authigenic chalcedony is
common in both wells and partly replaces calcitic molluscan
allochems. Rare opal-CT lepispheres have grown within
intraparticle and moldic pores. Glauconite, as discrete grains
and fossil replacements, is common in both wells and is most
abundant in the middle of the Santee, where there is a relatively
high percentage mud (Figs. 2 and 3).
Porosity in this unit ranges from 3-25% and is chiefly
mesomoldic and mesovug. Moldic pores formed by dissolution
of aragonitic molluscan grains in the freshwater meteoric
realm; hence, total porosity is controlled by the number
and size of aragonitic allochems available for solution. In
places, moldic pores have been reduced or closed by growth
of sparry calcite cement. Enlargement of moldic pores by
dissolution of surrounding allochems or micrite produced
vugs. Other pore types include: intraparticle in foraminifers
and bryozoans; channels formed by enlargement of molds
and vugs; and intercrystal in spar-reduced molds. Permeability
in this lithofacies is low because molds and vugs are isolated
from each other by microporus micrite or microspar,
which have small pore-throat diameters. Porosity and perme-
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