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Distribution and Origin of Regional Coal Fracture (Cleat) Domains in Upper Cretaceous Fruitland Formation Coal: Possible Effects on Coalbed Stimulation and Methane Production

LAUBACH, S. E., Bureau of Economic Geology, University of Texas at Austin, Austin, TX, and C. M. TREMAIN, Colorado Geological Survey, Denver, CO

Fracture permeability is of primary importance to producibility of coalbed methane. To evaluate controls on fracture (cleat) patterns in coal beds in the Upper Cretaceous Fruitland Formation in the San Juan basin, we studied fractures in the coal and adjacent sandstones in 11 cores and 90 outcrop stations along the basin margin and mapped fractures in selected areas. Cleats are perpendicular to bedding, planar, usually uniform in strike within an outcrop or core, and arranged in closely spaced subparallel sets. Face cleats are the first formed (and generally most prominent) fractures; butt cleats formed later, and in most cases strike perpendicular to the face cleat. Within a given bed, a hierarchy of fracture sizes exists, ranging from primary cleats that extend across beds, to verti ally discontinuous secondary and tertiary cleats. Face cleats delineate two principal fracture domains of regional extent separated by a boundary that extends east-northeastward from the Colorado-New Mexico border at approximately T32N R12W to T32N R7W and southward into New Mexico. South of the domain boundary, face cleats predominantly strike northward or northeastward; north of the boundary face cleats predominantly strike northwestward.

Timing of cleat development is constrained by the age of cleats relative to dated folds and the burial history of the Fruitland Formation. Where beds are reoriented by folds, cleats remain orthogonal to bedding, indicating that they formed prior to fold development during the early Tertiary. Cleat crosscuts bedding-parallel coal compaction fabrics, indicating that cleat formed later. Cleat development was contemporaneous with orogenesis in the Cordilleran belt, in progress during late Campanian time. Patterns reflect lateral stresses associated with northeast- and southeast-directed Cordilleran orogenic movements. Domains may represent separate deformation events or contemporaneous paleostress provinces.

Contrasts in cleat development in individual domains can affect hydraulic fracture treatment or cavity completion. The prevalence of strongly developed face and butt cleat in the domain-boundary region and resulting increased coal friability may increase the success of cavity completions in this area.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)