MRCSP Phase I Geologic Characterization Report - Midwest ...

100 CHARACTERIZATION OF GEOLOGIC SEQUESTRATION OPPORTUNITIES IN THE MRCSP REGION
studies on the reservoir aspects of the reefs (for examples, see Gill
and others, 1974; Gill, 1977; Sears and Lucia, 1979, 1980).
NATURE OF LOWER AND UPPER CONTACTS
The lower contact of the Niagara Group is gradational with the
underlying Manistique Group (Figure 5). The Salina Group, a
mixed interval of intercalated carbonates and evaporites overlies the
Niagara Group and provides a regional seal that is highly competent
(Figure A8-2). In the lower portion of the Salina Group, the A-1
evaporite formation overlies the inter-reef Niagara but not the pinnacle
reefs. The A-1 carbonate, A-2 evaporite and the A-2 carbonate
and B salt all overlie both the inter-reef and the pinnacle reefs (Sears
and Lucia, 1979).
LITHOLOGY
The reservoir facies consist primarily of porous and permeable
dolostone, although locally primary limestone has reservoir grade porosity
and permeability. Porosity is best developed in the pinnacle reef
core as well as the immediate off-reef facies (fore-reef, flanking beds)
and is characterized primarily by intercrystalline and vuggy pores.
DISCUSSION OF DEPTH AND THICKNESS RANGES
The pinnacle reefs range from 2,000 feet to more than 6,000
feet deep in the Michigan basin (Figure A9-1), with the majority
of reefs at depths that average approximately 3,500 to 4,500 feet.
Reservoir thickness may be highly variable and ranges from a few
feet to several hundred feet. An isopach map was not created for the
MRCSP Phase I project due to the small, high-relief reef features
that would not have been adequately illustrated by contours generated
by conventional gridding algorithims at the regional scale of
MRCSP mapping.
DEPOSITIONAL ENVIRONMENTS/
PALEOGEOGRAPHY/TECTONISM
The pinnacle reefs are located along a carbonate ramp generally
basinward of a shelf edge barrier reef complex. The reefs are characterized
by a complex interaction of biogenic growth and physicochemical
precipitation of carbonate cements. Common reef-builders
include various forms of stromatoporoids and corals indicative of
normal marine conditions during time of deposition. The reefs and
associated facies are generally subdivided into six readily recognizable
sub-facies (Gill, 1977): 1) biohermal mudmound consisting of
carbonate muds and skeletal components including crinoids and
bryozoans; 2) reef core consisting of a massive framework formed
by stromatoporoids, corals, algae, and a variety of subordinate biotic
elements combined with early submarine cements; 3) reef detritus
made up of detrital fragments of the reef core and deposited along
the flanks of the reef; 4) an inter-reef facies comprised of platform
carbonates; 5) restricted (lagoonal) facies consisting of laminated
and bioturbated, peloidal mudstones, and wackestones; and 6)
supratidal/island facies consisting of algal laminated sediments and
other features of high intertidal to supratidal deposition.
The pinnacle reefs were deposited in a tropical to subtropical latitudinal
belt. Subsequent diagenetic dolomitization has been attributed
to a number of mechanisms, including mixing zone processes,
Kohout convection, hypersaline reflux of brines, evaporative drawdown,
and hydrothermal circulation, although most workers agree
that reflux and hydrothermal processes were probably the main
mechanisms (Sears and Lucia, 1980). The relationship between
basin subsidence and eustatic changes at the time of pinnacle reef
deposition is presently unclear; there are a number of studies investigating
the relative timing of reef growth in response to relative sea
level changes.
SUITABILITY AS A CO 2
INJECTION TARGET OR SEAL UNIT
Niagaran reefs have been prolific oil-and-gas-producers After
their productive life, many are converted to gas storage units due
to their high porosity and permeability characteristics and effective
overlying seals. Despite the fact that reservoir-grade rock is
not regionally continuous, but is found in more localized reefs and
reef complexes, the Niagaran reefs should be considered high-quality
targets where CO 2 can be economically transported to the reef
trends. Porosity values can exceed 35 percent locally but typically
average 8 to 12 percent with the best porosity associated with dolomitized
reef cores and flank facies. The best reservoir rocks are
characterized by well-developed intercrystalline and vuggy porosity
with average permeability values of 3 to 10 md. Permeability
can be significantly higher where fractures intersect matrix porosity.
A high-quality sealing unit is provided by the overlying Salina
Group, characterized by abundant salt and anhydrite intercalated
with relatively thin carbonates. Cumulative oil production through
2004 was 336 million barrels of oil (MMbo) and 2.5 trillion cubic
feet (Tcf) of gas, indicating the high-quality porosity and permeability
available in many reefs. While individual reefs and reef
complexes are localized (averaging 50 to 400 acres), they can reach
up to 2,000 acres in size and have from 150 to 700 feet of vertical
relief . Also, the individual reefs are clustered close together within
trends. Thus, once a pipeline is brought to the trend, CO 2 injection
(and enhanced oil recovery) can proceed from reef-to-reef fairly
inexpensively.
A number of the Niagaran reefs are used for natural gas storage
operations in Michigan (Table A9-1). Such operations illustrate the
integrity of the reservoirs for storage operations. The relatively
small surface footprint of the reef-sand thick reservoir with large
capability for storage allow relatively large volumes of gas to be
cycled with few injection and withdrawal wells.
There is currently a project underway to utilize CO 2 from a gasprocessing
plant for enhanced oil recovery from three pinnacle
reefs along the northern Michigan trend. This work is being performed
with the sole purpose of oil recovery in mind, not optimal
sequestration of CO 2. The existence of the pipeline infrastructure
makes this area a highly attractive prospect for pilot sequestration
studies. Such a study would be favorable from logistical,
geotechnical, and economic standpoints, as much is known or can
be established using available data on reservoir heterogeneity and
compartmentalization.