MRCSP Phase I Geologic Characterization Report - Midwest ...

INTRODUCTION TO THE MRCSP REGION’S GEOGRAPHY AND GEOLOGY
9
GEOLOGIC UNCERTAINTIES
Our knowledge about the sequestration potential in deep geologic
units is limited by the availability of data on the various subsurface
attributes of the region. For example, in making broad regional assessments,
such as this Phase I task, our assessment is constrained,
and thus limited, by the availability of oil-and-gas-well data, accessibility
to seismic data, and our previous experience and working
knowledge in and of the region.
In general, the amount of data available for mapping and analysis
of any particular unit is directly proportional to its depth below the
surface. Thus, the deeper the unit, the less certain is our understanding
of the various parameters related to, and needed for, assessing
geologic CO 2 sequestration targets in the subsurface of the MRCSP
region. Unfortunately, since our primary data set is based mainly
on oil and gas wells, the control points used to map the various
units discussed herein is limited by where and how deep companies
in this industry drill. The deeper the well, the more costly it is to
drill; hence, overall, there are fewer deep wells. This is especially
true once the well depth exceeds about 6,000 to 7,000 feet. Consequently,
our knowledge on the deepest portions of the region is
limited—to date, no wells are known to have been drilled to the
deepest extreme of the Appalachian basin, a depth thought to exceed
45,000 feet. These depths are not practical, in any event, for current
sequestration consideration.
Another deep feature of the region that may represent a significant
potential sequestration target is the region containing the Rome
trough, a inadequately known structural feature in the subsurface of
the Appalachian basin (Figure 6). The Rome trough is a large, deep
feature that occurs in Kentucky, West Virginia, and Pennsylvania
and is approximately parallel to the Ohio River (it is thought the
current location of sections of the Ohio River are controlled by
structural irregularities related to this feature). Seismic data and a
limited number of deep wells drilled in this area indicate the Lower
Paleozoic geologic section rapidly expands within this feature. For
example, it is known that several thousand feet of sedimentary rock
occur in the Rome trough proper, that are not known to exist outside
the boundaries of the feature. These same data indicate sandstones,
some of which may have good storage reservoir potential, occupy
many portions of this expanded section. However, what is not
known is how extensive these potential reservoirs are. Nonetheless,
some of these potential sandstone sequestration targets are within
the economic limit of feasibility making them a possible target for
consideration as a large injection target (perhaps in the 9,000 to
12,000-foot range).
It is beyond the scope or economic abilities of this project to test
these deep regions. However, their presences should be mentioned
because, should the Rome trough contains the sandstone intervals
that some believe to be there, this deep feature could easily double
the sequestration potential within the MRCSP region.
STRATIGRAPHIC CORRELATION
Assessing the regional potential for CO 2 sequestration requires
an understanding of the many stratigraphic units (groups and formations)
in the MRCSP region and their geologic and stratigraphic
relationships between various areas of the partnership (Figure 5).
Therefore, a regional correlation chart was one of the first, and most
significant, undertakings accomplished by the geologic team.
Each state has, over the past 150 years or so, developed its own
stratigraphic nomenclature in order to explain the geologic history
and stratigraphic succession of rocks within each state—some of
these terms are unique to rocks that occur only in the subsurface.
The changing geologic character of many of these rock units, or at
least their position within a geologic basin, has created some differences
in the nomenclatures used in each state (see Figure 5). Other
variations between states are related to different methods used for
establishing the placement of unit boundaries or how a unit is classified
(ranked, i.e., group, formation, member) within a specific rock
interval or in a different area of the region. Prior to the development
of this correlation chart, no detailed chart showing the correlations
between the individual MRCSP states existed. We continue to refine
this chart as work progresses. More detailed correlation charts,
where needed, are presented in the discussion of the individual
units/intervals in Appendix A.
SELECTION OF MAPPED UNITS AND LIMITATIONS
Using the regional correlation chart and our knowledge of these
units as a basis, an initial list of potential CO 2 sequestration reservoirs
and seal (cap rock) intervals was chosen for further consideration.
Known stratigraphic intervals of saline formations, petroleum-producing
units, gas-generating (source rock) carbonaceous
shales, and coal-bearing units were identified in each state. Many
of these intervals can be readily correlated between states or basins;
however, others are restricted to a single basin or regions with a
basin, and determining their relationship to other more established
units is problematic.
Phase I of all DOE regional partnership projects called for a regional
assessment of the CO 2 sequestration potential in each partnership
area within a defined time frame. To expedite our evaluations,
a list was developed that consists of wide-ranging stratigraphic
intervals (often composed of multiple groups and formations) and
was the basis for an initial assessment of potential reservoirs and
cap rock units that could be regionally mapped using existing data
sources. Maryland and Michigan were later added to the partnership
and the process and list had to be repeated and slightly modified.
Where possible, we adapted previously chosen mapping units to
for use within the new states. However, the addition of the coastal
plain and inclusion of the entire Michigan basin required adding additional
units to the selection list.
After the list of geologic intervals and/or individual units to be
mapped was finalized and a database schema devised, individual
states of the team started collecting the data available to them. This
included, amoung other things, oil-and-gas-well data files (both
electronic and paper), previously completed geologic mapping databases,
published and unpublished studies within individual states,
and miscellaneous data (i.e., core and sample records, geochemical
analyses, miscellaneous geologic data files). As time permitted and
as data sources were discovered, some individual units were added
to the mapping list—an example of this addition is the inclusion of
the Niagaran reefs and Sylvania Sandstone. Table 1 is a final list of
all mapped units in the MRCSP project.
Nine potential reservoir horizons and five potential cap-rock intervals
(including organic shales that can also be considered potential
reservoirs) were chosen for regional mapping and further analyses
after our initial screening. Our selection of which reservoir and seal
intervals to map is by no means all-inclusive for the region. On the
contrary, throughout our Phase I analysis, several other prospective
reservoirs were noted. Additionally, the selected intervals do not
necessarily represent laterally continuous zones of homogenous reservoirs
or seals. Many assumptions are necessary when mapping at
such a regional scale. Considering the magnitude of this project, the
calculated volumes of potential CO 2 that can sequestered may vary