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 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.