MRCSP Phase I Geologic Characterization Report - Midwest ...

APPENDIX A: LOWER SILURIAN MEDINA GROUP/“CLINTON” SANDSTONE
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LITHOLOGY
The Medina Group is composed of interbedded sandstones, siltstones,
and shales, with some carbonates (Laughrey, 1984; Laughrey
and Harper, 1986; McCormac and others, 1996). The sandstones of
the Grimsby Formation are very fine- to medium-grained, monocrystalline,
quartzose rocks, with subangular to subrounded grains,
variable sorting, and thin, discontinuous, silty shale interbeds.
These sandstones vary in color, from white to gray to red; hence,
the reference to these units by drillers as “Red Clinton” and “White
Clinton”, particularly in eastern Ohio (Figure A7-1). Cementing materials
include secondary silica, evaporites, hematite, and carbonates
(Piotrowski, 1981; McCormac and others, 1996). The Cabot Head
Shale is a dark green to black, marine shale with thin, quartzose, siltstone
and sandstone laminations that increase in number toward the
top of the unit (Piotrowski, 1981; Laughrey, 1984). The Whirlpool
Sandstone forms the basal unit of this sequence, and, in the greater
part of the Appalachian basin, is composed of a white to light gray to
red, fine- to very fine-grained quartzose sandstone that is moderately
well sorted and has subangular to subrounded grains (Piotrowski,
1981; Brett and others 1995; McCormac and others, 1996). This
basal unit becomes dolomitic in localized areas within the northwestern
part of the basin (i.e., the Manitoulin Dolomite) (Laughrey,
1984; McCormac and others, 1996; Castle, 1998).
DEPTH AND THICKNESS RANGES
The Medina Group crops out at its type locality in New York; in
central Pennsylvania, outcrops of the equivalent Tuscarora Sandstone
are present. In the remainder of the Appalachian basin, however,
the Medina and equivalent units remain in the subsurface. The
depth to this reservoir ranges from less than 1,000 feet to 6,700 feet,
with wells located offshore in central Lake Erie reporting depths of
over 2,200 feet (McCormac and others, 1996).
Figure A7-3 illustrates the structure of the Medina Group throughout
the basin. Structure contours are given in subsea elevations using
an interval of 500 feet. The structure on top of the Medina Group
strikes northeast-southwest and dips toward the southeast at a rate of
approximately 40 to 70 feet per mile, with more shallowly dipping
strata toward the north and west. A low point (subsea elevation of
-9,000 feet) exists in southwestern Pennsylvania and northern West
Virginia. East of this point, toward the Appalachian structural front,
the Medina Group dips steeply to the northwest at rates of 70 to
about 180 feet per mile.
Figure A7-4 illustrates the thickness of this sequence across the
basin using a contour interval of 50 feet. Gross thicknesses range
from 0 feet in the northwestern portion of the basin to more than
700 feet in eastern West Virginia. These thicknesses are generally
consistent with those previously published by McCormac and others
(1996) and Laughrey and Harper (1986). The actual pay zones of the
Medina Group (i.e., where reservoir porosity and permeability are
favorable) comprise only a portion of these thicknesses, however.
Pay zones range from 3 to 50 feet and average 23 feet in thickness
(McCormac and others, 1996).
The structure and thickness of the Medina Group and equivalent
units, as presented in Figures A7-3 and A7-4, are consistent with
the interpretation that during early Silurian time, an influx of siliciclastic
material came from eroding Taconic highlands and an island
arc located at the eastern edge of the Appalachian basin (i.e., central
Pennsylvania and eastern West Virginia). These sediments were
shed across the basin to create a clastic wedge that is thickest in the
southeast and thins toward the northwest. The low area on the structure
contour map (Figure A7-3) is indicative of the foreland basin
that existed adjacent to the eroding highlands.
DEPOSITIONAL ENVIRONMENTS/
PALEOGEOGRAPHY/TECTONISM
The depositional history of the Medina Group dates back to the
latter part of the Taconic orogeny in early Silurian time. During
this period, clastic material eroded from both foreland fold-belt
highlands adjacent to the eastern edge of the Appalachian basin
and the plutonic igneous rocks of the island arc orogen (Laughrey,
1984; Laughrey and Harper, 1986; McCormac and others, 1996).
The directions of sediment transport from these highlands were
both parallel (i.e., northeast-southwest) and perpendicular (i.e.,
to the northwest) to the shoreline (Laughrey and Harper, 1986),
which ran from northern Beaver County to central Warren County
in Pennsylvania (Piotrowski, 1981). The Medina depositional
system is that of a shelf/longshore-bar/tidal-flat/delta complex.
The Whirlpool Sandstone is the basal transgressive unit of this
system and is overlain by shelf muds and transitional silty sands
of the Cabot Head Shale. These sediments were overlain by shoreface
and nearshore sands of the lower Grimsby Sandstone, which
grade into argillaceous sands at the top of this unit (Laughrey,
1984; Laughrey and Harper, 1986; McCormac and others, 1996).
Laughrey (1984) divided the Medina Group’s depositional system
into five facies: 1) tidal flat, tidal creek, and lagoonal sediments;
2) braided fluvial-channel sediments; 3) littoral deposits; 4) offshore
bars; and 5) sublittoral sheet sands. Facies 1, 2, and 3 sediments
comprise the Grimsby Sandstone, which was deposited in a complex
deltaic to shallow marine environment. The deeper, offshore
mud and sand bar deposits of Facies 4 were reworked by both storm
and tidal currents to become transitional sandstones of the Cabot
Head Shale. The Whirlpool Sandstone is included in Facies 5, which
was formed in nearshore marine and fluvial, braided river environments
in existence during the beginning of a marine transgression
(Piotrowski, 1981; Laughrey, 1984; McCormac and others, 1996).
STRUCTURE/TRAP TYPES
Throughout the Appalachian basin, stratigraphic traps have been
shown to control the occurrence of gas in the Medina Group, although
in localized areas (e.g., Portage County, Ohio, and Mercer
County, Pennsylvania), gas production may be enhanced by geologic
structure (Piotrowski, 1981; Laughrey and Harper, 1986; McCormac
and others, 1996). The overall heterogeneity of this reservoir is
evidenced by the variety of mechanisms forming the stratigraphic
traps, which include sandstone pinchouts, porosity changes, gaswater
contacts, and diagenesis (Laughrey and Harper, 1986).
SUITABILITY AS A CO 2
INJECTION TARGET OR SEAL UNIT
The MRCSP considers the Medina Group/ “Clinton” sandstone
as a sequestration target (Figure 5), particularly for its prevalence
throughout the Appalachian basin as a reliable oil-and-gas-producing
reservoir (Figure A7-2), its sandstone lithologies, and the
presence of less permeable confining rocks above and below the
sequence. However, several factors, including the variability in
lithology, the “tight” nature of this reservoir (with respect to both
porosity and permeability), and discontinuity of sandstone lenses in
the northwestern portion of the basin, may limit the overall success
of the Medina Group as a CO 2 sequestration target.