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The AAPG/Datapages Combined Publications Database
Four Corners Geological Society
Fracture Studies on the Upper Cretaceous Pictured Cliffs Sandstone and Fruitland Formation, Northern San Juan Basin, La Plata County, Colorado
A study of natural fractures at outcrops of the Upper Cretaceous Pictured Cliffs Sandstone and Fruitland Formation was conducted at several sites in La Plata County, Colorado, to determine the degree of fracturing and the orientation of fractures. The study was done to gain an understanding of the fracture network in the northern part of the San Juan Basin and to provide data for evaluating possible migration paths for coalbed methane and hydrogen sulfide gas from the subsurface to surface exposures. In general, there is one main joint set (J1) in each geologic unit in each area, and a second subordinate set (J2) that is oriented about 90° to the first set. The J1 and J2 sets are the most common and pervasive in all the areas studied; however, a third and fourth set are present in some areas. Comparable fractures in coal beds are referred to as cleats, which also occur in two sets, a face cleat and a butt cleat.
There appears to be a clockwise rotation from northwest to northeast of the J1 joint set in the Pictured Cliffs and in sandstone beds of the Fruitland. Face cleats display a slight counterclockwise rotation from north-south to northwest in the same area. The time of cleat development in Fruitland coals might have been shortly after burial, during coalification of the peat beds. The time of joint propagation in sandstones of the Pictured Cliffs and Fruitland was most likely during the Laramide orogeny in response to rotation of the Colorado Plateau. Joint and cleat trends observed at the outcrop probably extend southward into the subsurface of the San Juan Basin.
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