MRCSP Phase I Geologic Characterization Report - Midwest ...

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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.
-3000 APPENDIX A: MIDDLE SILURIAN NIAGARA GROUP REEFS 101 EXPLANATION Faults 500 ft contours High : 0 Low : -8600 -8500 -8000 -7500 -7000 -6500 -6000 -5500 -5000 -4500 -4000 -3500 -2500 -2000 -500 -1000 -1500 F A U L T S Y S T E M B O W L I N G G R E E N 20 10 0 20 40 Miles 20 10 0 20 40 60 Kilometers ³ Figure A9-1.—Generalized structure contour map drawn on the top of the Niagaran Group reefs.
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Characterization of Geologic Seques
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ABOUT THE MRCSP The Midwest Regiona
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CONTENTS About the MRCSP ..........
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CONTENTS Figure A14-2.—Structure
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1 CHARACTERIZATION OF GEOLOGIC SEQU
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BACKGROUND INFORMATION 3 (a minimum
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INTRODUCTION TO THE MRCSP REGION’
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GEOLOGIC MAPPING PROCEDURES, DATA S
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OIL, GAS, AND GAS STORAGE FIELDS 27
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CO 2-SEQUESTRATION STORAGE CAPACITY
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CONCLUSIONS AND REGIONAL ASSESSMENT
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REFERENCES CITED 47 National Confer
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APPENDIX A: REFERENCES CITED 151 Th
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