Pinnacle Evaporite 2
Carol Gudes
In the Williston Basin, the Winnipegosis Formation is the major carbonate unit of the initial transgressive-regressive pulse of the Kaskaskia sequence. Twenty-two lithofacies were identified by well-log, core, and thin-section studies of Winnipegosis rocks; these belong to seven environments of deposition which include deep basin, deep shelf, shallow shelf, reef, lagoon, tidal flat, and evaporite basin. The deposition of the Winnipegosis and Prairie Formations were inter-related and occurred during six episodes. Following a brief hiatus separating the underlying Ashern Formation from the Winnipegosis, a clear, quiet, shallow-marine environment became established in the North Dakota portion of the Elk Point Basin. Beds deposited during this First Episode are found in the central, deepest part of the Basin and pinch out laterally.
Beds of the Second Episode overstep those below indicating the extent of maximum, marine transgression. At this time, the basin differentialted into two distinct regions due to rapid sea-level rise accompanied by varying amounts of carbonate production: 1) a deep basin where slow deposition gave rise to a deep-marine environment with scattered, atoll-like, pinnacle reefs, and 2) a peripheral, carbonate shelf where, proceeding landward, there developed shallow-marine, patch-reef, lagoon, and tidal-flat environments.
Evaporite beds that formed during the Third (dolostone and anhydrite) and Fourth (Prairie; halite) Episodes were restricted to the initially deep, inter-reef basin, filling it to the level of the top of the pinnacle reefs. In this evaporite basin, there was increase in salinity and decrease in water depth due to evaporative drawdown. Beds formed during the Fifth Episode were tidal-flat deposits that accumulated over sites of former, pinnacle reefs and shelves with continued deposition of halite elsewhere. Additional Prairie halite (Sixth Episode) filled the remainder of the Basin.
Diagenesis of Winnipegosis rocks took place during six stages of diagenesis: syndepositional 1, eogenetic 1, telogenetic, syndepositional 2, eogenetic 2, and mesogenetic stages of diagenesis. Dolomitization is the major diagenetic process in Winnipegosis rocks. It has altered rocks from the pinnacle reefs and shelf-margin, patch reefs producing intercrystal porosity that, along with intraparticle, moldic, growth-framework, and solution-vug types of porosity produced rocks with the greatest oil potential.
Outcrops of sulfate and mixed sulfate-carbonate rocks are common every where in Canada outside of the Shield province. Interstratal salt deposits are abundant in the interior lowlands. Types of karst that occur are determined chiefly by relations between (i) formation thickness and purity, (ii) regional topography and hydraulic gradient, (iii) effects of receding Wisconsinan and earlier glaciers, and (iv) extent of modern permafrost.
Exposures ofbare karst on thick, pure sulfate formations are comparatively rare. Two principal landform types found on them are: (1) high-density polygonal karst (micro-sinkhole densities of thousands per km2), where hydraulic gradients are high and tills are thin; (2) hills and ridges of blocks uplifted and fractured by hydration (anhydrite) tectonics at paleo-icefront positions where hydraulic gradients are low.Deeply till-mantled karst dominated by collapse and suffosion sinkholes in the mantling detritus is well developed in southwestern Newfoundland and in central and northern Nova Scotia.Covered karst is abundant on sulfates conformably overlain by carbonate or clastic strate; collapse sinkholes are the principal landform. Very large breccia pipes (up to 25 15 km) are associated with deep subrosion of salt during glacier recessions.Syngenetic breccia karst is a fourth, distinct category created in some formations of thin, interbedded dolostones and sulfates. Where these are exposed to high hydraulic gradients, deep calcite-cemented breccias were formed in a first generation, upon which sinkhole and pinnacle karsts and dissolution drape topographies were able to develop rapidly in late-glacial and post-glacial conditions.
Halite is salt and will melt on contact with high moisture, including high humidity. However, these specimens are not evaporite crystals and are quite stable if protected from high humidity. Of course, they are best in dry, arid climates. It's a good idea to mount in a plastic see-through box and keep in a sealed cabinet. Just don't expose to your tea kettle or steam bath, and they will last your lifetime. CLOSED LOCALITY. See article in Mineralogical Record February 2013.
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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.
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.
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