MRCSP Phase I Geologic Characterization Report - Midwest ...

76 CHARACTERIZATION OF GEOLOGIC SEQUESTRATION OPPORTUNITIES IN THE MRCSP REGION
TRAPS/STRUCTURE
Figures A5-4 and A5-5 show only the grossest structures in the
MRCSP study area—the Michigan and Appalachian basins, and the
central arches area of eastern Indiana, western Ohio, and central
Kentucky. Other, somewhat more subtle geologic structures occur
throughout the region that have affected the siting of wells targeting
deeper Ordovician and Cambrian production. Many good hydrocarbon
producing areas within the region are associated with flexures
and structural noses mapped on higher formations that are not apparent
on regional maps (Potter, 1978).
Figure 6 illustrates some of the major structural features affecting
the MRCSP study area that are not readily apparent in Figures
A5-4 and A5-5. Those of prime importance include the Grenville
front, East Continental rift basin, Rome trough, Bowling Green
fault zone, Findlay arch, Cincinnati arch, Lexington fault system,
and the Pine Mountain thrust fault. Some production is associated
with fractures stemming from these structures, and some occur as
a result of subtle diagenetic affects related to equally subtle highs.
Ordovician carbonate reservoirs, for example, are coincident with
subsurface features associated with these structures. Some structural
traps occur in narrow, linear, dolomitized zones along normal and
strike-slip faults associated with basement faults. For example, the
Albion-Scipio trend in Michigan is a model for fault-related fractured
and dolomitized reservoir rocks in the Ordovician carbonates.
Wickstrom and others (1992) speculated that much of the fracturing
associated with Ordovician carbonate reservoirs probably resulted
from reactivation of deeper structures. Oil and gas fields associated
with fractured Ordovician carbonates in eastern Kentucky and central
West Virginia lie along mapped faults associated with the Rome
trough, a large Cambrian extensional feature extending from central
Kentucky through West Virginia and Pennsylvania (McGuire and
Howell, 1963; Harris, 1978), possibly into New York (Harper, 1989;
Jacobi and others, 2004).
Similarly, Lacazette (1991) indicated that the Bald Eagle Formation
was highly fractured in the limited area of north-central
Pennsylvania where the formation produces natural gas. Henderson
and Timm (1985) identified deep-seated (basement-involved),
down-to-the-north, normal faulting on seismic data extending
upward through Upper Ordovician carbonates and shales (Trenton
and Utica). Fracture porosity in the Bald Eagle sandstones occurs in
zones containing vertical to subvertical fracture sets associated to
the deep-seated faulting (Laughrey and Harper, 1996).
SUITABILITY AS A CO 2
INJECTION TARGET OR SEAL UNIT
Sealing Units
Figure A5-3.—Representation of Upper Ordovician clastic lithofacies
(based on Thompson, 1970). The bracketed interval indicates the range of
color-boundary fluctuation within the Bald Eagle interval.
The Knox to Lower Silurian Unconformity Interval has many
properties favorable for a confining unit. The carbonate rocks above
the Knox unconformity typically serve as a fluid barrier (Baranoski
and others, 1996), and the shales above provide an extra seal in areas
where fracture porosity and permeability exist in the carbonates.
Faults, fractures, and the Knox paleokarst system provide the
major migration avenues for fluids within the Ordovician carbonate
sequence (Nuttall, 1996), so these rocks would have the most effective
sealing properties in areas where fracturing has not occurred,
or where the fractures have been sealed by mineralization. Figure
6 shows the major basement structures in the MRCSP study area,
structures that are known to have affected fracturing in the carbonates.
The best potential areas for seals should occur where these