MRCSP Phase I Geologic Characterization Report - Midwest ...

84 CHARACTERIZATION OF GEOLOGIC SEQUESTRATION OPPORTUNITIES IN THE MRCSP REGION
grained textures are present throughout the section. Cross bedding,
bioturbation, and rare shell fossils have been observed in outcrop
and core material. Porosity is usually good in outcrop and the shallow
subsurface; however, burial compaction and cementation significantly
reduce porosity in the deeper subsurface. In some facies
in the Michigan basin, early carbonate cement and feldspar grains
have been dissolved, producing extensive secondary porosity. Porosity
as high as 14 percent has been measured from core at depths
greater than 11,000 feet in central Michigan. Throughout Ohio, the
St. Peter, where present, is a relatively clean, well-rounded, fine- to
medium- to coarse-grained, friable, quartz arenite. Drillers in Ohio
encounter flows of brine from this thin, yet highly porous and permeable
unit that typically washes out during drilling.
DISCUSSION OF DEPTH AND THICKNESS RANGES
The St. Peter Sandstone is thinnest (10 to 100 feet) and shallowest
(depths of less than 2,500 feet) across the arches of Indiana
and Ohio (Figures A6-1 and A6-2). The unit is thickest and deepest
in the Illinois and Michigan basins. The thickness of the St. Peter
Sandstone exceeds 1,100 feet in the center of the Michigan basin,
which is ten times greater than the maximum thickness across the
arches in Indiana and Ohio. No significant thickening is noted in
the southwestern portion of the region at the margin of the Illinois
basin. In Michigan, the St. Peter ranges from approximately 3,000
feet deep at the southern state line to more than 11,000 feet deep in
the basin center.
DEPOSITIONAL ENVIRONMENTS/
PALEOGEOGRAPHY/TECTONISM
Large areas of the North American craton were exposed during
Knox unconformity time, thus providing ample sediments to be
reworked and deposited as part of the St. Peter Sandstone. Penecontemporaneous
subsidence in the Michigan and Illinois basins
resulted in thick accumulations being preserved in those areas.
Subsequent erosion may have removed much of the sand deposited
across Ohio and adjacent areas. Subsidence along the Rome trough
in northern Kentucky also resulted in locally thick St. Peter Sandstone
(Price, 1981; Humphreys and Watson, 1996). Studies from
the outcrop of the St. Peter Sandstone in Wisconsin (west of the
MRCSP study area) suggest that it was deposited in a terrestrial to
shallow marine shelf facies belt that transgressed across the upper
Midwest (Dott and Byers, 1981). Aeolian facies are known from
central Wisconsin and marine facies have been documented from
other areas of the outcrop belt. In the Michigan basin, the facies
range from shoreface to inner and outer marine shelf (Barnes and
others, 1992).
SUITABILITY AS A CO 2
INJECTION TARGET OR SEAL UNIT
Because of its widespread occurrence, depth, and porosity in the
Michigan basin, the St. Peter Sandstone should make an important
sequestration target there. Northern and eastern Kentucky has good
potential, where the St. Peter is locally thick, although more data are
needed. Ohio and Indiana have marginal to spotty potential, due mainly
to the shallower depths and thin nature of the unit in these areas.
Injectivity rates are not well known for the St. Peter Sandstone in
Michigan; however, it is an important oil- and gas-producing unit.
High flow rates of natural gas (5 to 30 million cubic feet per day
[MMcfpd]) and gas condensate have been recorded from Michigan
wells. Porosity above 10 percent is present at various intervals
throughout the formation. Permeability ranges from less than 10
to over 300 md. Porosity and permeability have been shown to be
related to primary depositional facies and formation of secondary
porosity (Barnes and others, 1992).
In the Michigan basin, porosity decreases proportionally with
burial depth due to compaction and quartz cementation, so that,
below 4,000 to 5,000 feet, there is usually very little porosity in this
unit. In some facies that have a high feldspar grain content that had
been cemented early with carbonate minerals, however, significant
secondary porosity has formed at depth by dissolution of carbonate
cement and grains. At depths below 7,000 feet, the only apparent
porosity is from this secondary dissolution processes (Barnes and
others, 1992). Although permeability generally increases with porosity,
the precipitation of authigenic clay minerals in the secondary
porosity may severely reduce permeability and create microporosity
that may not be very effective. The porosity and permeability at
depth will require further analysis.
7. LOWER SILURIAN MEDINA GROUP/“CLINTON” SANDSTONE
The Medina Group (Figure 5) consists of interbedded sandstones,
siltstones, and shales with some carbonates of Early Silurian age
(McCormac and others, 1996). The stratigraphic nomenclature of
this unit is somewhat complex, due to the influence of both facies
changes across the Appalachian basin and drillers’ terminology.
Specifically, this sequence is known as the Medina Group in northwestern
Pennsylvania and western New York; the Cataract Group in
southern Ontario and eastern Ohio; and erroneously as the “Clinton”
sandstone by basin drillers, particularly in eastern Ohio and northern
Kentucky (see discussion below). The Medina Group of Pennsylvania
and New York is comprised of three major stratigraphic units,
in descending order: 1) the Grimsby Formation; 2) the Cabot Head
Shale (sometimes called the Power Glen Shale); and 3) the Whirlpool
Sandstone (Figure A7-1). In eastern Ohio, drillers’ terminology
predominates, so the Grimsby is called the “Clinton” sandstone, and
the Whirlpool is known as the “Medina” sandstone. The “Clinton”
undergoes a facies change in central Ohio (Figure A7-2), where the
porous (petroleum-producing) sandstones associated with this unit
pinch out and are replaced by the Cabot Head Shale and limestone
of the Brassfield Formation. The MRCSP discussion of this interval
revolves around its potential as a CO 2 sink, thus this interval is not
mapped or discussed separately west of this facies change. A fourth
unit, known as the Manitoulin Dolomite, is equivalent to the basal
Whirlpool Sandstone and is present in southern Ontario, eastern
Ohio, and near the shores of Lake Erie in Pennsylvania (Laughrey,
1984; McCormac and others, 1996; Castle, 1998). In the southern
and eastern portions of the basin, the Medina Group nomenclature
is lost. In northern Kentucky, the “Clinton” and Tuscarora Sandstones
are equivalent to the Grimsby and Whirlpool sandstones of
northwestern Pennsylvania, respectively (McCormac and others,
1996). In West Virginia and southcentral and central Pennsylvania,
the stratigraphic equivalent of the Medina Group is the Tuscarora
Sandstone, and in eastern Pennsylvania, the Shawangunk Formation
is equivalent to this sequence (Piotrowski, 1981; Avary, 1996).