Abstract A coral-stromatoporoid reef has been recognized from a core in the Lockport Dolomite of West Virginia, and interreef dolomite was recovered in a second core from a nearby well. Submerged topographic highs on the sea floor, a relic of sand bars in the underlying Keefer Sandstone, provided an optimum site for Lockport organic growth. Local relief protected the reef community from being smothered by terrigenous material. Simultaneously, argillaceous interreef sediments accumulated in deeper water between topographic highs. The reef, though small, consists of three vertical biofacies. The buildup began as a thicket of current-baffling crinoids. They colonized the muddy, shallow-subtidal sea floor after an initial transgression. Skeletal debris of this bafflestone facies gradually changed the consistency of the sea floor; the substrate became firmer and the crinoids were supplanted by a community of stick corals. The substrate coarsened with the admixture of coral skeletons, and the reef was eventually capped by a frame-building stromatoporoid community. In contrast to other Niagaran reefs, the vertical succession of biofacies was not due to upward reef growth above wave base. The patch reef in the Lockport Dolomite developed in a quiet, protected shelf setting, and community succession was controlled intrinsically resulting from continual alteration of the sea floor by the reef organisms themselves.

Abstract Most of a 450 foot (150 m), gas-bearing Niagaran reef was cored by the Shell, St. Union 1-8. The well is located approximately in the center of the pinnacle reef belt (Fig 1). Three distinct facies described by Sears and Lucia (1979) are present in the core. The facies are, in ascending order, mud mound, coral-stroinatoporoid, and restricted marine. Crinoid bryozoa mudstone-wackestone (Fig. 2) which contains stromatactis-like vugs is typical of the mud mound facies. A more diverse fauna including corals and stromatoporoids (Figs. 3 and 4) colonized the mound as it accreted upward from deeper water. Immediately prior to post-pinnacle evaporite deposition in the Michigan Basin a restricted fauna, mostly comprised of pentamerid brachiopods, inhabited the reef.

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.