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 Numerous field and laboratory studies over the past two decades claim that microbes catalyze nucleation and growth of dolomite at temperatures common in low-temperature geologic environments (25–60° C). However, a critical reexamination of the X-ray diffraction (XRD) data presented by these studies indicates that the laboratory products are not dolomite but rather a mixture of minerals, including very high-magnesium calcite (VHMC). Because VHMC can be “compositionally” indistinguishable from dolomite (i.e., 50 mol% MgCO 3 ), the positions of the principal (104) XRD reflection for VHMC and dolomite can be identical. Nevertheless, published XRD patterns of products derived from microbial experiments lack convincing evidence of cation ordering, which is a unique characteristic of carbonate minerals exhibiting R 3 (dolomite) symmetry. The lack of cation ordering in laboratory precipitates instead indicates that the products are VHMC, which possesses R 3 c (calcite) symmetry. Hence, previous laboratory studies have misidentified VHMC for dolomite. Despite the failure to synthesize dolomite in microbial experiments, the low-temperature laboratory results remain interesting. High-temperature (60–300°C) dolomitization experiments have long shown that ordered dolomite is invariably preceded by disordered VHMC precursors that recrystallize to dolomite over time. Although recrystallization from VHMC to ordered dolomite has not been documented in the low-temperature microbial experiments, it may be common in natural settings where higher surface temperatures and longer time periods overcome kinetic barriers to dolomite formation. Mineralogical arguments aside, petrological observations show that VHMC products from microbial laboratory experiments are dissimilar to both natural dolomites and high-temperature synthetic dolomites. First, the published microbial experiments produced VHMC or other carbonates as cements via direct precipitation from solution rather than by replacement of a CaCO 3 precursor, whereas the latter is demonstrated in high-temperature synthetic dolomites and inferred for most natural dolomites. Second, these precipitates tend to be spheroidal and/or dumbbell shaped, and as such they are fundamentally different from both the dominant rhombohedral form and the mimetic replacement textures observed in natural and high-temperature synthetic dolomites. Thus, the microbial products are not only mineralogically unlike natural dolomites, they also differ with respect to their mode of formation and their morphological characteristics.

Abstract The Niagara-Lower Salina reef complex reservoirs of the Michigan Basin host significant hydrocarbon volumes and have recently been identified as promising targets for enhanced oil recovery and carbon sequestration. Although these carbonate buildups have been studied extensively since the late 1960s, there is still wide uncertainty and disagreement concerning their morphology and internal stratigraphic and facies architecture. The prevailing paradigm depicts the reef complexes as tall, symmetric “pinnacles” with heterogeneous internal facies distributions that are patchy and unpredictable. The current study challenges this model of the reefs by examining four Silurian reef reservoirs with abundant core and petrophysical wire-line logs. New and existing subsurface data show that Silurian reefs in the Michigan Basin are highly asymmetric with internal facies distribution patterns that are strongly influenced by east-northeast paleowind direction. Six major depositional environments are identified during the main stage of reef complex growth based on sedimentological characteristics observed in core, as well as the vertical progression (stacking) of facies observed both in core and wire-line log signatures. A central reef core environment is identified based on interspersed coral-stromatoporoid boundstone and skeletal wackestone facies consisting of frame-building organisms such as tabulate corals and stromatoporoids, as well as intrareef faunal assemblages of bryozoans, brachiopods, crinoids, and rugose corals. Environments to the east (windward) of the central reef core are steeply inclined to the east (~40°) with narrow facies belts characterized by coarse reef talus. In contrast, environments to the west (leeward) of the central reef core have shallower slopes that dip to the west (< 15°) and are characterized by wide facies belts composed of carbonate mud and skeletal debris that become finer and thinner in the leeward direction. Application of this new Silurian reef model to reef complexes throughout the basin demonstrates remarkable consistency with respect to the overall asymmetric shape of the reef complexes, as well as the windward-leeward internal facies architecture. The asymmetric architecture and windward-leeward facies distribution patterns described in the new model offer a significant improvement upon preexisting models for Silurian reefs in the Michigan Basin and more accurately reflect our modern understanding of how environmental controls affect reef development and architecture. Furthermore, this new reef model can be used to more accurately predict the shape and internal facies distributions for other Silurian reef complex reservoirs within the Michigan Basin, particularly those that lack abundant well control.

Abstract Recent advances in remote sensing technology and digital image analysis have been leveraged to significantly increase the level of complexity and precision that can be captured in maps of modern carbonate depositional settings. Satellite mapping efforts in a variety of modern carbonate environments have generated a spectrum of facies distribution characterizations that more accurately reflect the natural complexity of carbonate systems in terms of facies body size and shape. Predictive quantitative relationships are being harvested from this robust data set. Foremost among these is the globally observed power law scaling relationship between facies body size and frequency of occurrence. The scaling dimension, an attribute derived from the slope of the power law trend, describes the rate of change for the probability of encountering a facies body of a particular size. It also provides a robust measure for characterizing and comparing genetically distinct groups of facies. We propose here that the scaling dimension is a quantitative manifestation of the natural complexity inherent in carbonate depositional systems and can be used to test the fidelity of facies maps generated from outcrop, seismic, or other subsurface tools to natural facies patterning. For this study we compare modern carbonate facies trends observed in a series of global carbonate settings to evaluate the natural range of the scaling dimension and the sensitivity of the parameter to changing depositional controls. To demonstrate the persistence of the scaling relationship across widely varying settings, case studies from the Hawaiian Islands, Arabian Gulf, Caribbean Sea, Great Barrier Reef, the Flores Sea, and southern Indian Ocean are presented. Initial results suggest that the scaling dimension ranges between 0.6 and 1.6. Relatively higher scaling dimension values are associated with robust reef margin systems and lower values are associated with platform interior patch reef systems.

Abstract Using Landsat 7 spectral data in conjunction with cluster analysis algorithms and field-collected sediment sample data, a surficial sediment texture map is created for the Caicos Platform. Cluster analysis utilizes statistical algorithms to group Landsat picture elements with similar spectral signatures into relatively homogeneous thematic classes. Carbonate sediment textures are subsequently assigned to each thematic class as part of a manual geologic interpretation that is calibrated using sediment data. Results indicate that the Caicos Platform is heavily grain-dominated. Geospatial calculations show that 7% of surficial sediments on Caicos are grainstone, 59% are mud-lean packstone, 18% are packstone, 5% are wackestone, and 1% are reef, with the remaining 10% characterized by exposed Pleistocene islands. Sediment distributions are highly asymmetric with most platform-scale facies variations following major depositional trends. At the 28.5 meter scale, the Landsat-derived facies map for Caicos demonstrates greater than 84% correlation with sediment data. This agreement indicates that the Landsat-derived facies map accurately characterizes the spatial dimensions and distribution of platform-scale depositional features, like grainstone shoals and tidal flats. Local-scale textural heterogeneity within individual depositional regimes is also identified. Surficial sediment distributions on the Caicos Platform are interpreted to be controlled by platform physiography and island orientation relative to the dominant hydrodynamic forces, such as the Antilles Current, easterly Trade Winds, and Atlantic swells. We also demonstrate that the coupling of statistical algorithms, Landsat data, and sediment data offers a powerful quantitative approach for investigating the spatial distribution of surficial sediments on modern carbonate platforms.