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The Raton basin contains up to 3,000 ft (900 m) of marine shale and subordinate carbonate rocks of Cretaceous age, including (in ascending order) the Graneros Shale, Greenhorn Limestone, Carlile Shale, Niobrara Formation, and Pierre Shale. Clastic reservoir rocks are sparse in this part of the section and drilling for them in the Raton basin has led to disappointing results. However, brittle siltstone and carbonate-rich interbeds within the Cretaceous shale intervals are capable of providing fracture reservoirs under the right conditions.
Fracture reservoirs in other Rocky Mountain basins occur where there is maximum curvature of brittle interbeds within shale sequences at fairly shallow depths. Relatively low confining pressures found at shallow depth facilitate development of open fractures in the brittle interbeds. Anticlines, synclines, and monoclines can have favorable fracture systems. It should be kept in mind that if the axial surface of a fold is inclined, the hinge will migrate laterally with depth, and the hinge is generally the part of the fold having the maximum curvature. There are numerous folds in the Raton basin that could have excellent fracture systems. It is necessary to determine the areas of maximum curvature of the shale interval having brittle interbeds capable of fracturing.
Carbonate-rich beds of the Greenhorn Limestone and Niobrara Formation appear to be the most widespread and thickest intervals that might develop fracture reservoirs. Siltstone or orthoquartzitic interbeds in the Graneros, Carlile, and Pierre Shales may provide other zones with fracture systems. Hydrocarbon shows have been reported from the Graneros, Greenhorn, Niobrara, and Pierre Formations in the New Mexico parts of the Raton basin. Also, minor gas was produced from the Garcia field near Trinidad, Colorado. Fracturing appears to have enhanced the reservoir characteristics of the Wagon Mound Dakota gas field in the southern part of the basin.
Structure contour maps and lithofacies maps showing brittle interbeds in dominantly shaly sequences are the basic tools used in exploration for fracture reservoirs. These maps for the Raton basin indicate numerous exploration targets.
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