MRCSP Phase I Geologic Characterization Report - Midwest ...

APPENDIX A: LOWER DEVONIAN ORISKANY SANDSTONE
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low permeability carbonate rocks of the Helderberg Group, Keyser
Formation, Bass Islands Dolomite, or even the upper carbonates of
the Salina Group (Figure 5 and A11-1).
The upper trapping mechanism varies. In eastern Ohio, northern
Pennsylvania, and New York, the overlying rocks consist of lowpermeability
carbonates of the Onondaga or Bois Blanc Formations.
In the central plateau area of Pennsylvania and West Virginia, it
consists of the Huntersville Chert. The Needmore Shale forms the
cap in central Pennsylvania, Maryland, eastern West Virginia, and
western Virgina (Figure A11-8). The Bois Blanc Formation commonly
contains a basal sandstone, the Springvale Sandstone, that
substitutes for Oriskany where the actual Oriskany Sandstone is
absent, and often is called “Oriskany” by drillers. In those areas, the
Springvale can be as porous and permeable as the Oriskany itself.
The upper seal in these cases consists of the low-permeability carbonates
above the Springvale. The Huntersville Chert grades eastward
from a cherty limestone to a hard, massive, microcrystalline
chert (Flaherty, 1996). The Needmore Shale consists of dark gray
to black calcareous siltstone and shale, noncalcareous shale, and
argillaceous limestone.
Lateral trapping mechanisms consist of sealed faults and fractures,
juxtaposed with impermeable rocks (described above) across
fault planes, and permeability barriers within the sandstone as a
result of non-dissolution of cement or secondary precipitation of
authigenic quartz and other minerals.
The largest single storage problem for sequestration of CO 2 in the
Oriskany is the possibility of seal failure that would allow fluids to
escape from the sequestration reservoir. In fact, Johnson and others
(2004) consider cap rock integrity problems as the single most
important constraint on long-term sequestration in all target storage
sites. The integrity of Oriskany reservoir cap rocks and fracture
seals needs to be evaluated thoroughly for mechanical and, possibly,
chemical alteration potential before any pilot injection begins.
Mechanical seal problems would probably be more likely to
occur in areas where the structural complexity places a porous or
highly fractured rock in juxtaposition (vertical or lateral) with open
fractures or high porosity zones in the sandstone. For example, the
overlying Huntersville Chert is highly brittle and contains many
open fractures along anticlines where it has been bent and flexed.
It is these fractures that make the Huntersville a seductive target
for gas well drilling in the basin (Flaherty, 1996). But these fractures
often extend into the Oriskany, making the Huntersville and
Oriskany a single reservoir in many areas of Pennsylvania and West
Virginia. In such areas, utilization of the Oriskany for CO 2-storage
almost guarantees the fluids will also be injected into the Huntersville.
In such cases, the Huntersville must also be evaluated for seal
integrity. In addition, one or more seals could be ruptured under
high injection pressures (Friedmann and Nummedal, 2003; Johnson
and others, 2004).
Despite the potential setbacks one can envision within this
structurally complex setting, Oriskany gas storage fields have the
capability to store/deliver more natural gas than storage fields in
Figure A11-8.—Map showing the variations in lithology of the rocks overlying the Oriskany Sandstone (modified
from Oliver and others, 1971).