Michigan’s Pinnacle Reefs: From Prolific Producers to Carbon Storage

The Silurian (Niagaran) reefs of the Michigan Basin have been prolific producers of oil and gas, and now are perfectly suited for carbon storage. More than 500 MMBO and 3.1 TCF of natural gas have been produced from small pinnacle reefs extending in bands across northern and southern lower Michigan. The reefs average 0.2 square miles in area, with thicknesses from 300 – 700 ft. They were discovered by geoscientists who understood both the paleoecology of the Silurian, as well as how reflection seismology can serve as a tool for their detection.?

The pinnacle reef discoveries were not the first oil discoveries in Michigan, which had its first commercial production in the 1920s in southern Michigan. However, the reefs are uniquely prolific, and their discovery set off a boom in geoscience as well as drilling and completion technology. The first wave of pinnacle reef exploration in the prolific northern belt occurred in the 1960s and continued through the 1970s and early 80s. Not all reef-prospective areas were developed, as part of the Pigeon River State Forest was deemed a wilderness area and off-limits to drilling.?

EOR AND CCUS: Core Energy is currently storing CO2 produced in the processing of the Antrim Shale by injecting it into the Silurian (Niagaran) pinnacle reefs in the northern Niagaran pinnacle reef trend (NNPRT). In addition, miscible CO2 EOR operations are ongoing in depleted pinnacle reef reservoirs. Operations have been highly successful. In some instances, the EOR recovery has exceeded 50% of volumes produced during the primary production phase.

In an article that evaluates the CO2 storage potential across the NNPRT, Autumn Haagsma and co-authors conclude that previous studies underestimated the full storage potential and that the ultimate potential for storage and EOR could be much greater.

THE IMPORTANCE OF PALEOECOLOGY: The reefs formed in a particularly fascinating time in Earth history.?The Ordovician mass extinction had occurred and during the late Silurian, the Michigan Basin was located in the equatorial zone. Reef-building occurred within a shallow shelf less than 100 ft deep. In this environment emerged, rugose corals, crinoids, algal stromatolites, stromatoporoids, trilobites, gastropods, brachiopods, and other marine life. Over time, the environment changed, and the seas became more shallow, and evaporites formed.?The microbialites constituted a perfect source rock, while the reefs created locations for oil to accumulated during their travel through migration pathways. The porous zones often consisted of porous grainstones or diagenetically enhanced reef facies. The best productive facies consists of crystalline dolomite, peloidal packstone, stromatolitic rudstone, and thrombolytic bindstone. The evaporites (including anhydrites) which intercalated with algal laminites, constitute an excellent seal.?

KEYS TO SUCCESS:?Successful exploration for the pinnacle reefs involved deep understanding of how the growth in the reefs was driven by sea level changes, and how that impacted the development of porous carbonate sediments. Vertical heterogeneity characterizes the pinnacle reefs, with high-porosity grainstone (deposited during transgressive high-energy phases). Dolomitization enhanced porosity while diagenetic cementation could occur during regressive stages. Advances in 3D seismic assisted pinpointing the reef structures, while a study of the paleoecology enabled modeling the reef core, talus, bioherms, reef apron, tidal flats, etc. which help determine where to drill and perforate.?

Core analysis has been vital to the understanding the depositional environment, namely the paleoecology, and then to understand the pore volumes of each facies, at each stage of sea level. In addition, geomechanical studies of the same core samples can determine exactly how much volume of miscible CO2 can safely be injected and at what rate. Microseismic passive monitoring and the use of distributed acoustical sensing (DAS) fiber networks are useful to detect induced seismicity.?Further, work has been done to see if there are surface expressions of microseepage or underlying geomagnetic signatures of the reefs.?

REPOSITORIES: The Milwaukee Public Museum has digitized their extensive collection of Niagaran pinnacle reef fossils which were collected from numerous quarries in the area. In Kalamazoo, the Michigan Geological Survey houses a massive core repository at Western Michigan University. The Michigan Geological Repository for Research and Education was founded in 1982 by William Harrison, and has been critical in both oil and gas development and both planning safe and effective EOR and carbon storage. Other geologists who have studied the Niagaran pinnacle reefs in Michigan include G. Michael Grammer, Autumn Haagsma, Amy Noack,?Karen Cercone, and others.

PERSONAL NOTE:?I had the opportunity to visit the core repository in Kalamazoo in 2008. It was incredibly impressive! Saving, storing, and studying the core will continue to pay dividends as the reservoirs are both revitalized and repurposed. William Harrison’s vision has been a game-changer.?

USEFUL REFERENCES

Cercone, Karen Rose; Kyger C. Lohmann; Late Burial Diagenesis of Niagaran (Middle Silurian) Pinnacle Reefs in Michigan Basin. AAPG Bulletin 1987;; 71 (2): 156–166. doi: https://doi.org/10.1306/94886D62-1704-11D7-8645000102C1865D

Core Energy (2018). Northern Niagara Pinnacle Reef Trend (NNPRT) CO2 Monitoring, Reporting, and Verification (MRV) Plan. Traverse City, Michigan.?https://www.epa.gov/sites/default/files/2018-10/documents/coreenergyniagaran_mrvplan.pdf?

Cotter, Z. M. K. (2020). Depositional controls of a Guelph Formation pinnacle reef debris apron and their effect on reservoir quality: A case study from northern Michigan. Master’s Thesis. Bowling Green University.?

Field Museum (2023). The Virtual Silurian Reef. https://silurian-reef.fieldmuseum.org/narrative/398?

Friedman, G. M., and Kopaska-Merkel, D. C., 1991. Late Silurian Pinnacle Reefs of Northern Michigan. In: Catacosinos, P. A., Daniels, P. A. Jr. (Eds.), Early Sedimentary Evolution of the Michigan Basin. Geological Society of America Special Paper 256, 89- 100.?

Grammer, G. Michael, Harrison, W. B., Barnes, D., Voice, P. (2018). Introduction to the Paleozoic stratigraphy and resources of the Michigan Basin. The Geological Society of America Special Paper 531, pp 1-8.

Grammer, G. M., Noack, A. K., Qualman, H., Ritter-Varga, A., Wold, J., Sandomeirski, A., E, and Harrison, W. B. III. Reservoir Characterization of Silurian (Niagaran) “Pinnacle” Reefs in the Michigan Basin. AAPG Search and Discovery Article #50286 (2010).?

Haagsma, Autumn, et al. (2020) A comparison of carbon dioxide storage resource estimate methodologies for a regional assessment of the Northern Niagaran Pinnacle Reef Trend in the Michigan Basin. Environmental Geosciences. V. 27, No. 1, p. 9-23.?

Harrison, W. (2020). Assessment of Enhanced Oil Recovery Using Carbon Dioxide in Michigan Basin Silurian Pinnacle Reefs. Batelle. DOE MRCSP Project #DE-FC26-05NT42589?https://www.osti.gov/servlets/purl/1773069?

Lang, Kelly. (2019). Rock Characterization and Carbon Isotope Analysis of a Silurian Reef Trend in the Michigan Basin after CO2 Injection. Ohio State University. Undergraduate Thesis.?

Lowenstam, H.A. 1957. Niagaran reefs of the Great Lakes area. Geological Society of America Memoir 67, p. 215-248.

The History of Michigan’s Oil and Natural Gas Industry. https://www.cmich.edu/research/clarke-historical-library/explore-collection/explore-online/michigan-material/oil-gas-industry-michigan/history-of-michigan's-oil-and-natural-gas-industry#a1?

Tresnak, J. P. (2017). Rock magnetic investigation of the MIchigan basin soils and sediments overlying the oil-bearing Silurian pinnacle reefs. Master’s Thesis. Michigan Technological University.?

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