Abstract: Fracture Analysis of the Upper Devonian Antrim Shale, Michigan Basin

James A. Richards, Joyce M. Budai, Lynn M. Walter, Linda M. Abriola

The Antrim Shale is a fractured, unconventional gas reservoir in the northern Michigan basin. Controls on gas production are poorly constrained but must depend on the fracture framework. Analyses of fracture geometry (orientation, spacing, and aperture width) were undertaken to better evaluate reservoir permeability and, hence, pathways for fluid migration.

Measurements from nearly 600 fractures were made from outcrop, core, and Formation MicroScanner logs covering three members of the Antrim Shale (Norwood, Paxton, Lachine) and the Ellsworth Shale. Fracture analyses indicate pronounced reservoir anisotropy among the members. Together related with lithologic variations, this leads to unique reservoir characteristics within each member.

There are two dominant fracture sets, northeast-southwest and northwest-southeast. Fracture density varies among stratigraphic intervals but always is lowest in the northwest-southeast fracture set and is greatest in the northeast-southwest fracture set. While aperture width decreases markedly with depth, subsurface variation in mean aperture width is significant. Based on fracture density and mean aperture width, the Norwood member has the largest intrinsic permeability (2.0E-11 m² northeast-southwest and 1.4E-11 m² northwest-southeast) and the Ellsworth Shale the lowest intrinsic permeability (4.0E-15 m² northwest-southeast and 1.3E-15 m² northwest-southeast). The highest intrinsic fracture permeability in all intervals is associated with he northeast-southwest fracture set. The Norwood and Lachine members thus exhibit the best reservoir character. This information is useful in developing exploration strategies and completion practices in the Antrim Shale gas play.

AAPG Search and Discovery Article #90984©1994 AAPG Annual Convention, East Lansing, Michigan, September 18-20, 1994