ABSTRACT Silurian-age (Niagaran) reefs in the Michigan Basin have long been interpreted as relatively homogeneous units, despite production histories that strongly suggest the reefs are heterogeneous in both lateral and vertical dimensions. In an attempt to better illustrate reservoir heterogeneity in these reefs, a three-dimensional (3-D) sequence stratigraphic model was produced for the Ray Reef field. The resulting 3-D Petrel model incorporates 28 wells in the field using a combination of gamma-ray and neutron logs, porosity and permeability data from whole-core analysis, and facies descriptions from eight cores evenly distributed within the reef complex. Comparison of porosity and permeability values within the diverse depositional facies clearly shows trends related to the individual facies and positioning within the sequence hierarchy. Incorporation of the sequence stratigraphic framework into the 3-D model illustrates the episodic nature of reef growth as exhibited by the stacked nature of reef and capping grainstones, often separated by well-developed exposure horizons. The model also suggests a distinct difference between windward and leeward margins in both the geometry of the reef complex and the distribution of reservoir-prone facies. Windward margins are steeper due to higher rates of aggradational growth, and they typically contain higher percentages of reservoir-quality rock in both the reef core and forereef facies. Utilization of the sequence stratigraphic approach illustrates that the vertical reservoir heterogeneity often predicted from production in these reefs may be controlled in large part by the combination of vertical stacking patterns of facies within third- and fourth-order sequences.

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.