Reservoir Characterization of Northern Michigan Niagaran Pinnacle Reefs
Stephen O. Sears 1 and F. J. Lucia2
1Louisiana State University, Craft and Hawkins Department of Petroleum Engineering, Baton Rouge, LA 70803
2Bureau of Economic Geology, University of Texas, Austin, TX 78713
Porosity and permeability distribution in Northern Michigan Niagaran pinnacle reefs is controlled by the geometry of depositional facies, which were later affected by exposure to meteoric water and hypersaline brines, followed by an episode of salt plugging. An understanding of depositional and diagenetic processes affecting porosity results in a model capable of predicting reservoir quality rock within individual reefs.
Three depositional facies have been recognized within the productive section of the reefs. These include mud mounds deposited below wave base, coral-stromatoporoid reefs and a restricted marine section reflecting increasing salinity in the basin. The mud mound facies always has very low porosity except when affected by isolated fractures. The coral-stromatoporoid and restricted marine facies contain porosity sufficient to be economically viable hydrocarbon reservoirs unless filled with halite salt crystals.
The stratigraphy of the reefs results in a tight mud mound core in the more basinward reefs, which does not exist in the shorter, more shelfward reefs. The tight mud mound facies, where present, is overlain by the porous coral stromatoporoid and restricted marine facies. Salt plugging is more prevalent in the basinward reefs, coincident with the presence of the mud mound cores. Where it occurs, it is more pervasive in the upper sections of the reefs. The basinward reefs commonly exhibit a "do-nut" of porosity encircling the mud mound below the base of salt plugging. The more shelfward reefs have no mud mound facies and salt plugging is minimal, resulting in reefs that consist of porous reservoir quality rock throughout.
AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York