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CAROLINA GEOLOGICAL SOCIETY
Guidebook for 1992 Annual Meeting
Pages 49-55
PETROLOGY AND RESERVOIR CHARACTERISTICS OF MIDDLE AND LATE EOCENE CARBONATE
STRATA IN DOWNDIP WELLS AT THE SAVANNAH RIVER SITE, SOUTH CAROLINA
Paul A. Thayer
Department of Earth Sciences
UNCW, 601 South College Road
Wilmington, N.C. 28403-3297
Mary K. Harris
Environmental Restoration Department
Westinghouse Savannah River Company
Box 616
Aiken, S.C. 29802
INTRODUCTION
Carbonate strata of middle and late Eocene age were
continuously cored in two downdip wells at the Savannah
River Site (SRS) (Fig, 1). Petrographic study of the core was
done to: 1) delineate lithofacies, 2) assess the effects of
diagenesis on pore system evolution, and 3) determine depositional
environments.
Figure 1. Location of wells used in this study.
Figures 2 and 3 are graphic logs showing the stratigraphy,
lithology, fossils, and grain-size parameters of insoluble
residues from carbonate intervals in SRS wells P-21TA and
P-22TA. Figure 4 explains the symbols used in the graphic
logs. Our stratigraphic terminology is modified slightly from
Fallaw and others (1992). Stratigraphic relationships of
Eocene carbonate strata in this region have been reviewed by
Powell (1984), Huddlestun and Hetrick (1985), Prowell and
others (1985), Gohn (1988), Nystrom and others (1991), Fallaw
and others (1992), and Harris and Zullo (1992). Papers
concerned with petrology of Eocene carbonate strata in this
area include Powell and Baum (1981), Powell (1984),
Thayer and Miller (1988), and Thayer and others (1988).
SANTEE LIMESTONE
The Santee Limestone unconformably overlies the Congaree
Formation (Figs. 2 and 3). The basal contact occurs in
an interval of missing core in well P-22. In P-21, the lower
contact is marked by a phosphatic crust; the underlying Congaree
sand is calcareous and contains thin silcrete layers.
The top of the Santee is an unconformity, which Harris and
Zullo (1992) identified as the 40 Ma type 1 boundary. Harris
and Zullo (1992) assigned the Santee to the upper middle
Eocene TA3.5/3.6 cycle and suggested that the lower part of
the unit represented a transgressive systems tract and the
upper part a highstand systems tract.
The Santee is usually moderately to well indurated, but
is friable in sandy and muddy parts of both wells. Colors are
yellowish-gray and light and very light gray, except in the
upper third where grayish-green and greenish-gray predominate.
Weight percent carbonate ranges from 14.7 to 97.7% (x
= 64.2%, s = 24.1%) and is higher in P-22 (x = 70.2%, s =
19.5%) than in P-21 (x = 57.3%, s = 27.1%). Carbonate content
decreases upward in P-21 with a concomitant increase in
percent terrigenous mud (Fig. 2), whereas, P-22 shows no
systematic trend (Fig. 3). Weight percent terrigenous mud
(silt plus clay) averages 22.4% and is much higher in P-21 (x
= 35.5%, s = 26.8%) than in P-22 (x = 10.2%, s = 6.2%). In
fact, the Santee becomes a calcareous mud above 187 ft in P-
21 (Fig. 2). Subangular quartz sand averages 13.6 weight
percent and is more abundant in P-22 (x = 19.7%, s = 17%)
than in P-21 (x = 7.1%, s = 10.1%).
Mean size of terrigenous quartz is lower fine sand in P-
21 (x = 2.91φ, s = 0.73φ) and upper fine sand in P-22 (x =
2.18φ, s = 0.22φ). Maximum quartz size greatest near the
base of the Santee and attains lower pebble size (-2.39φ)
directly above the unconformity in P-21. Overall, quartz
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