MRCSP Phase I Geologic Characterization Report - Midwest ...

52 CHARACTERIZATION OF GEOLOGIC SEQUESTRATION OPPORTUNITIES IN THE MRCSP REGION
One of the most striking features in the Appalachian basin east
and south of Ohio is the Rome trough (Figures and A1-2). The
Rome trough is a graben that extends from the Lexington fault system
in central Kentucky to at least as far northward as southwestern
Pennsylvania (Woodward, 1961; McGuire and Howell, 1963; Harris
1975; Wagner, 1976; Cardwell, 1977; Beardsley and Cable, 1983;
Shumaker, 1986, 1996; Harper, 1989; Drahovzal and Noger, 1995;
Gao and others, 2000; Wilson, 2000; Harris and others, 2004). The
trough is the result of a Cambrian rifting event that down-dropped
basement rocks thousands of feet. The apparent highly faulted nature
of the Rome trough in eastern Kentucky is interpreted from the
mapped surface geology and a relatively large number of basement
tests that have been integrated with available seismic data (Drahovzal
and Noger, 1995). Farther northeastward, in West Virginia
and Pennsylvania, where the basin is deeper, there are fewer basement
wells and less publicly available seismic data, making these
areas appear to be less faulted. In eastern Kentucky, the structural
relief on the top of the Precambrian is greater than 13,000 feet from
the northern boundary of the Rome trough along the Kentucky
River fault system to the trough’s deepest part near the center of
the graben on the West Virginia state line. The trough in Kentucky
steps down from a high northern boundary fault across a series of
down-to-south normal faults into the center of the graben and then
up again across a lower southern shoulder bounded by a smaller
displacement fault, the Rockcastle River fault. The Rockcastle
River fault changes from a low-displacement normal fault to a highdisplacement
thrust fault in southern Kentucky where it was reactivated
during the Alleghany orogeny (Drahovzal and Noger, 1995;
Drahovzal and White, 2002). Many of the faults of the Rome trough
were reactivated during subsequent Paleozoic orogenic movements
and are expressed in Quaternary-age surface materials.
The relatively symmetrical graben structure of the Rome trough,
as mapped in Kentucky, changes near the West Virginia line to an
asymmetric half graben, dipping southeast and bounded on the
southeast by the major East-Margin fault (Gao and others, 2000;
Wilson, 2000). Offsets along the East-Margin fault are up to 7,000
feet. The half-graben structural character continues north into southwestern
Pennsylvania to the cross-strike structural discontinuity referred
to as the Pittsburgh-Washington lineament (Lavin and others,
1982; Parrish and Lavin, 1982; Alexander and others, in press).
North of the lineament, the structural character of the basement
changes to a southeast dipping monocline cut by down-to-southeast
dipping normal faults. Offsets across the lineament range from 0 to
2,500 feet. Offsets along the down-to-southeast normal faults are
generally less than 500 feet. Several other northwest-oriented lineaments
cross the basement monocline farther to the northeast but
exhibit relatively small offsets. The presence of the Rome trough
north of this lineament is equivocal, but it has been postulated by a
number of workers (Harris, 1975; Wagner, 1976; Shumaker, 1986,
1996; Harper, 1989; Jacobi and others, 2004) to extend into New
York in the north-central part of the state.
The basement rocks of the MRCSP study area are extremely important
because of their influence on the overlying Paleozoic rocks.
Basement structure, in particular, has influenced the subsequent
structural history of the Paleozoic rocks. In many cases, major basement
faults were reactivated by later orogenic movement to fault
the younger overlying rocks. Other basement faults show little or no
reactivation and exhibit no shallow expression in Paleozoic rocks.
Still other faults in Paleozoic rocks apparently have no basement
roots, being solely the result of shallow tectonics. Careful study is
required when assessing the faulting in a region to distinguish these
three types of faults. The distinction between the types of faults and
related structures is critical in the siting of sequestration targets in
order to assure the integrity of seals associated with potential CO 2
reservoirs. However, it is known that not all faults are permeable,
or represent points of leakage, because mineralization may seal the
faults, making them impermeable barriers rather than fluid pathways.
Thus, site-specific investigations and testing are required to
ascertain the integrity of any proposed CO 2-storage site.
During Phase II of the Regional Carbon Sequestration Partnership
studies, additional data on the basement structures will be
required for those areas that are being considered for CO 2 injection.
Reflection seismic profiles will be required to provide information
on structure, especially faults that could compromise the reservoir
seals. The data may also provide an indication of subsurface lithology,
facies/permeability changes, and may even reveal some
permeable basement zones, which may themselves be candidates
for sequestration. Such initial seismic data will be critical in determining
whether further examination of the area is warranted based
on basement faulting and structure, as well as the structure of shallower
Paleozoic sequestration target zones. Seismic reflection data
will also act as a guide to the location of any subsequent drill holes,
indicating best placement for injection, or where to drill to provide
the most useful analytical information. Subsequent drill-hole and
core data will not only provide critical information on reservoir and
seal properties in the target sequestration zones, but will provide
important basement lithologic and structural data that may have
bearing and influence on overlying reservoirs.
2. CAMBRIAN BASAL SANDSTONES
The basal sandstone interval includes some of the most promising
targets for CO 2 sequestration within the MRCSP study area. The
Mt. Simon Sandstone, which is part of this mapped interval, has the
largest sequestration potential of any individual geologic unit within
the MRCSP study area (see volume calculations section). However,
the Mt. Simon has also been one of the most misunderstood geologic
units within the region. Many previous workers have assumed
the Mt. Simon was present across much of the region (Janssens,
1973; Havorka and others, 2000; Gupta, 1993) when, in fact, recent
research has shown that the true Mt. Simon pinches out in central
Ohio (Baranoski, in preparation). East of this pinch-out, thinner, less
continuous sandstones are found in this “basal sand” position. Thus,
for the Phase I MRCSP assessment, these sandstone intervals have
been mapped as one group across the entire region.
Cambrian stratigraphic nomenclature for the MRCSP study area
is problematic. A cursory examination of the geologic correlation
chart (Figure 5) demonstrates a lack of regional consistency in
currently accepted geologic terms as well as a number of states
simply using the term “basal sandstone.” In general, this problem
grew from the practice of taking geologic contacts and formalized
terms from outcrops studied in the upper Mississippi Valley and Appalachian
fold and thrust belt and carrying them many miles into the
deep subsurface of the region by analyzing drilling cuttings from
sparse well control and interpreting correlative surfaces. The advent
of modern geophysical logs allowed more detailed regional correlations;
however, geologic terms remain entrenched provincially,
and numerous correlation difficulties are not resolved. While the
same can be said for many younger geologic units, the problem is
especially acute within the deep Cambrian strata because of the low
number of wells, and great distances between wells, that have been