ABSTRACT Mississippian (Tournaisian–Viséan) carbonate mounds in the Compton and Pierson limestones, Ozark region, North America, have been called Waulsortian. However, European Waulsortian mounds contain features such as geopetals with multigenerations of carbonate mud (polymuds) and stromatactis cavity systems that are rare to absent in Ozark mounds. To determine similarities and differences, examine their origins, and clarify nomenclature, mounds in the Compton and Pierson limestones are compared with Waulsortian mounds in the Feltrim Limestone, Ireland. Features considered included mound size, geometry, style of aggradation, composition, depositional setting, and diagenetic history. Mounds in the Compton and Pierson limestones are <10 m (33 ft) thick and form singular knoll-form or aggregates with a strong lateral growth component. In contrast, individual Waulsortian mounds in the Feltrim Limestone range from 5 to > 30 m ( 16 – 100 ft ) thick, but coalesce and vertically aggrade to form complexes that exceed 500 m ( 1600 ft ) . Pierson mounds are crinoidal and grain-rich, whereas Compton and Feltrim mounds are bryozoan-rich and mud-dominated. All mounds have similar cement stratigraphy and diagenetic histories. Mud-rich Compton mounds and Feltrim mounds are interpreted as deeper water than skeletal-rich Pierson mounds. Limited accommodation constrained Compton and Pierson mound size and forced lateral aggradation. Subsidence-driven accommodation in the Dublin Basin allowed Feltrim mounds to grow larger, coalesce, and aggrade vertically. Three types of mounds are recognized: true Waulsortian in the Feltrim Limestone, mud-cored Waulsortian-type Compton and Pierson mounds, and Pierson transported bioaccumulation mounds. Small dimensions of Waulsortian-type Pierson and Compton mounds limit their potential as oil and gas reservoirs, whereas Pierson crinoidal sediment piles are known to form reservoir-size accumulations.

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.