Systematic fracture (face cleat) strikes in Upper Cretaceous Fruitland Formation coal (San Juan Basin, Colorado and New Mexico) delineate two domains of regional extent that are separated by a boundary of variable cleat orientation near the Colorado-New Mexico border. South of the boundary, face cleats strike predominantly northward or northeastward; north of the boundary they strike northwestward. Such domains may determine fracture permeability pathways in coal seams. Prevalence of two directions of strongly developed fractures in the domain boundary region, close cleat spacing related to northward increase in coal rank, and resulting increased coal friability may enhance the success of well completion by open-hole cavity methods in the north-central part of the basin.
With the increasing importance of coalbed methane as a natural gas source, information on coal fracture (cleat) patterns is becoming critical for planning coalbed methane well placement. Fracture patterns in coal and adjacent rocks affect completion and stimulation techniques such as horizontal drilling, open-hole cavitation, and hydraulic fracture treatment. Regional mapping of coal fractures using outcrop and core data delineates important variations in fracture patterns that in turn affect the physical/mechanical properties of the coal.
The San Juan Basin of the east-central Colorado Plateau, New Mexico and Colorado, is a Late Cretaceous to early Tertiary structural basin that contains >4000 m of Paleozoic to Cenozoic marine and continental rocks. As defined by the Upper Cretaceous (Campanian) Fruitland Formation outcrop, the basin is roughly circular and~17,000km2 in area (Fig. 1). Over 1000 coalbed methane wells have been drilled in the Fruitland Formation in the past decade, making it one of the most important coalbed-methane-producing units in the country. The Fruitland consists of coastal plain deposits composed of sandstone, mud- stone, coal, and carbonaceous shale. It is > 100 m thick in the northwestern part of the basin, but it thins and disappears in the eastern part of the basin as a result of depositional thinning and erosion.
Fruitland Formation coal fractures include systematic and nonsystematic fractures and faults. Cleats are the systematic fractures in coal that are equivalent to joints in other sedimentary rocks. Fruitland cleats are extension fractures that are perpendicular to bedding, generally planar but locally strongly curved, commonly uniform in strike within an outcrop or core, and arranged in subparallel sets. Two cleat sets in an orthogonal pattern are designated face and butt cleat. Face cleats are planar, smooth- sided fractures that usually comprise the most prominent fracture set. Based on abutting relations, they are the earliest formed fractures. In the Fruitland Formation, they may be as much as several meters long in plan view. Butt cleats formed later and in most cases intersect and terminate against face cleats at 80 to 90° angles. A single face cleat set and an associated butt cleat set are present in most exposures. Their surfaces are irregular and rough, and they are less continuous and less well developed than associated face cleats.
