Abstract Hydraulic testing has revealed dramatic underpressures in Paleozoic shales and carbonates at the Bruce nuclear site in Ontario. Although evidence from both laboratory and field studies suggests that a small amount of gas-phase methane could be present in the shale, previous studies examining causal linkages between the gas phase and the underpressure have been inconclusive. To better elucidate processes in such a system, we used a highly simplified 1D representation of the site to test, by using iTOUGH2-EOS7C, the effects of various factors on the evolution of gas-phase methane and pressures within the system. Heterogeneity was represented by three stratigraphic regions with slightly different capillary pressure characteristics and, in one case, three thin distinct zones with very different characteristics. Underpressure occurred only when gas pressures set as an initial condition required it, and even in this case it was geologically short-lived. We conclude that the presence of multiple fluid phases is unlikely to explain the underpressure at the site; we suggest that the influence of gas-phase methane on porewater flow is minimal. This is consistent with prior conceptualizations of the underpressured section as a thick aquiclude, in which solute transport occurs extremely slowly, bounded by aquifers of significantly higher permeability.

ABSTRACT The A-1 Carbonate is the primary hydrocarbon source rock and an important reservoir component of the Silurian (Niagaran) pinnacle reef complexes in the Michigan Basin. The geology of the A-1 Carbonate, however, is not widely known because the majority of published research about this hydrocarbon system focuses on the pinnacle reefs. To gain a better understanding of the sedimentology and stratigraphy of the A-1 Carbonate, we integrated data from slabbed core, thin section petrography, gamma-ray logs, and energy-dispersive X-ray fluorescence spectrometry (ED-XRF). Thirteen distinct lithofacies within the A-1 Carbonate are recognized, with inferred depositional environments ranging from intertidal-sabkha to deep basin. The recognition of deep-water lithofacies contrasts significantly with previous interpretations of the A-1 Carbonate as a shallow, peritidal deposit. Lithofacies stacking patterns and ED-XRF elemental trends within the A-1 Carbonate are consistent with basinwide sea-level fluctuations that resulted in deposition of three major stratigraphic units, called the Lower A-1 Carbonate, Rabbit Ear Anhydrite, and Upper A-1 Carbonate. The basal part of the Lower A-1 Carbonate was deposited during a basinwide transgression, as evidenced by deep-water pelagic carbonate accumulation in the basin center, lithofacies that become progressively muddier from bottom to top, and higher concentrations of Si, Al, and K upward, which are interpreted to reflect the influx of continental sediments. The subsequent highstand deposits of the upper part of the Lower A-1 Carbonate are characterized by a decrease in Si, Al, and K, coupled with a shallowing-upward facies succession consistent with increased carbonate production rates. The Rabbit Ear Anhydrite, which bifurcates the Upper and Lower A-1 Carbonate units, exhibits a variety of anhydrite fabrics across a wide range of paleotopographic settings within the basin. The Rabbit Ear Anhydrite is interpreted to reflect a time-correlative sea-level drawdown, which caused basin restriction, gypsum deposition, and elevated concentrations of redox-sensitive elements, such as Mo and Ni. The Upper A-1 Carbonate represents sedimentation during another major basinwide transgression that culminated in the deposition of shallow-water microbialites on the crests of previously exposed Niagara reef complexes. Similar to the Lower A-1 Carbonate, the base of the Upper A-1 Carbonate exhibits elemental signatures indicative of continental influence, whereas the overlying highstand deposits are characterized by more normal marine conditions and lower concentrations of Si, Al, and K.

ABSTRACT Pennsylvanian red beds are the youngest known rocks in the Michigan Basin. Two new formation-level units, the Pewamo Formation and the Haybridge strata, have recently been described. The Pewamo Formation, composed of Pennsylvanian red sandstones and minor laminated mudstones, is known from outcrops, abandoned quarries, and one core in Ionia County. The Haybridge unit is located in the shallow subsurface and in coal mine tailing piles in Shiawassee County. It consists of red sandstone, red mudstone, coal, and gray mudstone, all hosting Pennsylvanian macroscopic plant fossils. Neither the Pewamo nor the Haybridge rocks have any demonstrated relationship to red core cuttings reported as Jurassic from the central Lower Peninsula of Michigan. No firm evidence exists for Jurassic, or any other post-Pennsylvanian rocks in the Michigan Basin. The red core cuttings may be glacial sediments with reworked palynomorphs from rocks transported from elsewhere. A shallow coring project, followed by detailed sedimentologic, petrographic, mineralogic, and paleontologic studies, is necessary to: (1) refine the vertical and lateral stratigraphy of the Pennsylvanian rocks in Michigan; (2) solve the “Jurassic red bed problem”; and (3) understand the late Pennsylvanian–Pleistocene history of the Michigan Basin.

ABSTRACT The objective of this study was to investigate the geological controls on stratigraphic and lithologic variability in the Ordovician Utica Shale and related Collingwood Member in the Michigan Basin in order to assess CO 2 sequestration cap rock (seal) potential, including petrophysical properties and mechanical fracture responses. Twelve conventional cores and hundreds of modern well logs from the Michigan Basin were analyzed to correlate and calibrate wireline log signatures with whole-rock mineral composition (from X-ray diffraction analysis) and mechanical properties (from core analysis) to identify brittle, fracture-prone zones, and to validate the Utica Shale as a regional geologic seal. Analysis using scanning electron microscopy with Quantitative Evaluation of Minerals by Scanning Electron Microscope (QEMSCAN ® ) software was employed to image pores and for quantitative analysis of mineralogy, texture, and porosity. Mercury injection capillary pressure and triaxial strength testing was conducted to assess petrophysical properties and mechanical responses. The results suggest the Utica Shale could reliably contain upwards of 1500 m of buoyant, supercritical CO 2 stored in underlying Cambrian and Ordovician reservoirs.

ABSTRACT Previous work shows that the Burnt Bluff Group was deposited as a series of shallow- to moderate-water-depth facies in a tropical marine setting in the Michigan Basin during the Llandoverian. New interest in the unit for both hydrocarbon resources (subsurface) and aggregate resources (outcrop) is driving research in this poorly understood unit. New cores, as well as investigation of the outcrop belt in the Upper Peninsula of Michigan, allow further elaboration of the depositional model. The traditional interpretation of the Burnt Bluff Group consists, in stratigraphic order, of the open-marine deposits of the Lime Island Formation, the restricted lagoonal–tidal flat Byron Formation, and the open-marine deposits of the Hendricks Formation. The contacts with the underlying Cataract Group and overlying Manistique Group are gradational. Identification and description of facies in both cores and outcrop sections provide new constraints on the stratigraphic nomenclature of the Burnt Bluff Group. New outcrop and core data require the revision of the depositional model for portions of the group because typical Byron-like facies are found interbedded with Hendricks-like facies in both the Byron and Hendricks Formations. Limited age data from published research combined with the new facies model for the Burnt Bluff Group suggest that the unit was deposited as a time-transgressive package on a carbonate ramp during the Llandoverian.

ABSTRACT Based upon the construction of a high-resolution sequence stratigraphic framework, this paper interprets the evolution of Niagaran (Silurian) reefs in the Michigan Basin as being characterized by episodic reef growth in response to three distinct, third-order-scale, eustatic sea-level fluctuations. The fluctuations are observed in both the northern and southern reef trends and are interpreted to coincide with Silurian eustatic sea-level fluctuations defined at a global scale. The resulting episodic reef growth model, based upon subsurface core and wireline log analysis, is characterized by at least two orders of stratigraphic cyclicity (probably third and fourth order) that likely formed in response to eustatic sea-level change as well as relative sea-level variations. A detailed sequence stratigraphic analysis of the reefs utilizing facies stacking patterns and identification of key surfaces highlights the punctuated growth of these reefs and provides insight into the lateral and vertical facies variability observed in the subsurface. The sequence hierarchy is manifested by thicker (third- and fourth-order) sequences (tens of meters thick) controlled by globally recognized sea-level changes, and thinner (fifth-order) cycles (few meters thick) driven by relative sea-level variations. Local changes in relative sea level were likely controlled by the combination of higher-frequency eustatic variations along with subsidence and autocyclic mechanisms related to reef growth. The higher-frequency (fourth-order) cyclicity, likely due to eustatic sea-level change, played a major role in controlling the lateral and vertical heterogeneity of reservoir facies in these reefs. Understanding of the growth of these reefs utilizing a modern sequence stratigraphic approach provides new insight into the development of the Niagaran reefs while providing evidence for a complex and episodic depositional model that explains the variability observed in the stratigraphic architecture of these reefs.

ABSTRACT Despite extensive research on Silurian (Niagaran–Wenlockian) reefs, most studies concerning faunal abundance and distribution have been qualitative studies with an emphasis on taxonomy, paleoecology, and evolution. This study is the first quantitative study of relative abundance and distribution of fauna throughout a single Wenlockian reef located in the southern trend of the Michigan Basin. Building on an established sequence stratigraphic framework with wind directions surmised from known paleogeographic location, the purpose of this study was threefold: (1) to quantitatively determine the relative abundances of fauna from subsurface cores of Ray Reef and show how they are tied to the established sequence stratigraphic framework; (2) to determine if the probable wind and current directions, along with water depth, influenced the morphology and distribution of fauna on the reef; and (3) to analyze the influence of wind and current on syndepositional marine cementation. Relative faunal abundance differed among the leeward, windward, and reef crest locations. Overall faunal density was highest in the crest and lowest along the leeward side of the reef complex. Diversity was highest in the crestal portion of the reef complex and in the reef core facies, in general. Changes in faunal morphology and community replacement were seen repeatedly through all cores in association with shallowing-upward conditions, which coincided with third-order stratigraphic and higher-frequency sequence stratigraphic cyclicity. The percentage of syndepositional marine cement was highest on the windward side and lowest on the leeward side. As has been reported in other reef complexes of varying geological ages, results of this study indicate that the core of the Silurian reef was composed mostly of rubble or debris, relative to the smaller proportion of in situ fauna.

ABSTRACT The Michigan Basin is one of the world’s important sedimentary basins that contains significant quantities of evaporites. Here, evaporites are found in deposits of Ordovician through Mississippian age rocks; however, most of the thick evaporite accumulations occur in Silurian and Devonian intervals. Halite is most significant in the Silurian Salina Group, with a maximum aggregate thickness of halite exceeding 650 m (2150 ft). During the earliest evaporite deposition in the Salina Group (A-1 Evaporite), sylvite was widely deposited in the north-central portion of the basin within the upper 91.4 m (300 ft) of the formation. Devonian salt is also present in the north-central portion of the basin in the Horner Member of the Lucas Formation, where maximum aggregate net thickness of halite reaches 125 m (410 ft). Recrystallization of much of the halite obscures the primary depositional crystal geometry; however, some well-preserved beds do show crystal growth that is interpreted as bottom-growth chevrons, which likely suggest shallow-water deposition. Throughout the rest of the Michigan Basin, in both space and time, the evaporite phase deposited is CaSO 4 . In the shallowest portions of the Mississippian Michigan Formation, the sulfate mineral phase is gypsum; everywhere else in the basin, all the evaporitic sulfate deposits are anhydrite. Although the dehydration of the gypsum to anhydrite has slightly altered the original depositional morphology, some primary growth geometry is still evident. Subtidal and sabkha morphologies can be documented in all the anhydrite/gypsum deposits of the Michigan Basin. Based on historic production, evaporite minerals have added an estimated $15.5 billion (in 2013 dollars) to the industrial mineral economy of Michigan since the first commercial development in the 1860s.

ABSTRACT This research determined that frequency attenuation can be used as a potential porosity/permeability detector within a carbonate reservoir in a Silurian pinnacle reef in the northern portion of the Michigan Basin. This reef, the Charlton 30/31 Field in Otsego County, Michigan, produced 2.6 million barrels of oil during its primary production phase and was selected for enhanced oil recovery operations in the early 2000s. As part of a U.S. Department of Energy study of this enhanced oil recovery project, a four-dimensional (4-D) P-wave seismic survey was acquired in an attempt to monitor the flow of CO 2 through the reservoir. A three-dimensional (3-D) P-wave seismic survey was acquired prior to the injection of the CO 2 , which served as the baseline for the 4-D survey. Various interpretation methods were performed on this initial 3-D data set in order to develop a reservoir characterization and a dynamic model for multiple reservoir simulations. During the initial phase of the project, a relationship was identified between the low instantaneous frequency derived from the full-azimuth seismic volume and zones of high porosity and permeability in the reef identified from well logs. This relationship was used for the reservoir characterization. Forward models developed to approximate the reservoir’s characteristics support the relationship. Multiple analyses and data sets acquired following the creation of this reservoir characterization confirmed the interpreted matrix porosity distribution developed using this relationship. Following the injection of 29,000 tons of CO 2 into the Charlton 30/31 reef, a second 3-D P-wave seismic survey was acquired that served as the monitor survey for the 4-D survey. Amplitude analysis of the monitor survey provided a good correlation to zones predicted to be filled with CO 2 by the final, predictive reservoir simulation. However, one particularly strong amplitude anomaly immediately to the northeast of the CO 2 injection point was observed that suggested this reef may contain fracture porosity as well as zones of high matrix porosity. Although few individual, open fractures have been reported in these reefs, the available data sets are not optimum for the recognition of vertical fracture systems. In an attempt to identify and characterize any open fracture trends that may exist within this reef, four azimuthal seismic volumes were developed and analyzed. Isofrequency volumes were developed from these azimuthal volumes through spectral decomposition, and then they were compared in an attempt to identify frequency attenuation within the reservoir. In theory, zones of high matrix porosity and permeability affect all azimuthal volumes in an isotropic manner, producing zones of frequency attenuation in all azimuthal volumes to approximately the same degree. However, due to their often linear nature, fracture trends produce anisotropy that is potentially recognizable in an azimuthal volume. Increased amplitude in some lower frequencies, specifically 15 Hz, in the 160° azimuthal volume was observed to the northeast and southwest of the CO 2 injection point on the same time slice corresponding with the CO 2 flow interpreted on the 4-D monitor survey. This suggests that some open, northeast-trending fractures exist near the injection point, resulting in increased directional system permeability at that location that channels more CO 2 to the northeast than is explained by the matrix porosity and permeability alone.

ABSTRACT The Middle Devonian Sylvania Sandstone in the Michigan Basin, United States, is noteworthy for its mixed, cherty dolomitic carbonate, limestone, and sandstone composition and excellent reservoir quality and fluid-flow properties in many areas of the basin. Substantial commercial brine production was initiated in the middle twentieth century, and liquid waste disposal continues today in the Sylvania Sandstone, although little or no hydrocarbon production is known from this unit. The Sylvania Sandstone pinches out to the south and west in the basin and overlies either the Bass Islands Group at the regional base Kaskaskia unconformity to the south or the Bois Blanc Formation, with which it is proposed to be in facies relationship to the northeast. The Sylvania Sandstone is overlain by and apparently interfingers with the Meldrum Member of the Amherstburg Formation throughout the basin. The Sylvania unit predominantly consists of siliciclastic rocks in the southeastern Michigan Basin and in outcrop in Ohio, but it is transitional to predominantly cherty carbonate in the northwest along a depositional hinge striking from southeast to northwest through the central basin. This hinge zone is dominated by mixed carbonate and siliciclastic strata deposited in normal-salinity, tidally influenced paralic and shallow-marine environments. Excellent reservoir quality is present in quartz sandstones and, especially, mixed sandy dolomite lithofacies in the central basin. Cherty facies may also possess significant porosity, but typically with low permeability. Limestone lithofacies are commonly non-reservoir-quality facies. Multiple high-frequency, low-magnitude relative sea-level cycles within the Sylvania Sandstone are suggested from regional stratigraphic analysis. The Sylvania Sandstone in the Michigan Basin is interpreted as a mixed carbonate and siliciclastic, basin-margin facies assemblage deposited in an overall transgressive systems tract above the base Kaskaskia unconformity and in conformable relationship with more basinal facies of the Bois Blanc Formation.