Lithofacies Description and Reservoir Characterization of a Niagaran Pinnacle Reef, Macomb County, Michigan

P. Mescher¹ and D. J. Schultz^2
1Geological Resources Co
²Veritas DGC Inc. Exploration Services

Reservoir characterization and lithofacies description of a continuous core from the Silurian “Brown Niagaran” (Guelph Formation) pinnacle reef and the overlying Salina Group evaporites and carbonates (A-1 and A-2 Formations) show that these formations contain a variety of depositional facies with variably developed reservoir quality, reservoir geometries, and seal capability. Acquisition of a whole core provided detailed depositional facies information and petrographic data for an analysis of both the reservoir facies and reservoir seals within the reef complex. This data was used to examine the interrelationships of lithology, pore types, rock properties from core analysis, and log data. This geological information was used to help delineate the pinnacle reef geometry and identify the edges of the very steep reef flanks in the subsurface.

The pinnacle reef portion of the cored interval (“Brown Niagaran” Guelph Formation) consists of several depositional cycles that include the reef core facies, overlain by reef detritus (storm apron) deposits, and followed by intra-reef deposits. The reef core lithofacies is comprised of variably developed encrusting stromatoporoids, digitate branching stromatoporoids, hemispherical stromatoporoids, and corals. These rocks contain intercrystalline, skeletal moldic, intraskeletal, and fracture pores. Geopetal fill, in combination with exposure surfaces observed in the core, indicate that karst processes may have created cavernous pores in some portions of the framework reef core – further substantiated by the lost circulation zone at the base of the reef core facies at the bottom of the cored interval. The reef detritus facies, consisting of burrowed dolopackstone and dolowackestone with floating clasts of eroded reef debris, has porosity that is characterized by intercrystalline, fine skeletal moldic, fracture and solution vug pores. Intra-reef deposits include burrowed skeletal peloidal dolopackstone and dolowackestone that exhibit geopetal fill in some skeletal moldic pores, as well as open intercrystalline and intraskeletal pores. Because of the mud-rich character of this facies, the intra-reef deposits have fewer fractures and consequently lower permeability.

An argillaceous, stylolitic weathered surface marks the contact of the “Brown Niagaran” with the overlying Salina Group carbonates and evaporites. The A-1 carbonate consists of shallow subtidal, algal doloboundstones and dolopackstones that represent the flooding of the reef. It includes a thin interval of imbricated flat pebble conglomerate that represents rip up clasts derived from desiccated polygons. Commonly considered a seal, porosity in the A-1 interval consists largely of poorly connected intercrystalline pores. However, a basal dolopackstone in the A-1 also contains a combination of intercrystalline pores and relic interparticle pores, and it may be considered a potential reservoir. Coastal sabkha deposits of the A-2 Evaporite and A-2 Carbonate units (main reservoir seals) include anhydritic doloboundstones, enterolithic anhydrite, algal mats, peloidal dolomudstone, and other features indicative of deposition in supratidal to evaporative lagoon environments. Porosity in the sabkha deposits is limited to poorly interconnected microcrystalline voids resulting in low permeability.

AAPG Search and Discovery Article #90900©2001 AAPG Eastern Section Meeting, Kalamazoo, Michigan