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The Raton basin in New Mexico contains up to 4,700 ft (1,430 m) of marine shale and subordinate sandstone, limestone, coal, and terrestrial shale of Cretaceous age. Clastic reservoir rocks are sparse in the Cretaceous section below the Trinidad Sandstone, and drilling them has, to date, led to disappointing results. However, brittle carbonate- and silica-rich interbeds in the shale intervals are capable, under the proper conditions, of providing highly productive fracture reservoirs containing commercial deposits of oil and/or gas.
Other Rocky Mountain basins have fracture reservoirs that have proven to be productive of hydrocarbons where there is maximum curvature of brittle interbeds in shale sequences at fairly shallow depths. Fracture reservoirs can occur along anticlines, synclines, monoclines, and in areas where there is an abrupt change in dip and/or strike. Numerous flexures in the Raton basin could have excellent fracture systems.
Carbonate-rich beds of the Greenhorn Limestone, the Codell Sandstone Member, the Fort Hays Limestone Member, and the overall Niobrara Formation appear to be the most widespread and thickest intervals that might develop fracture reservoirs. Silica-rich siltstone and orthoquartzite interbeds in the Graneros, Carlile, and Pierre Shales may provide additional zones capable of fracturing. Minor gas was produced from fracture reservoirs in the Niobrara, Carlile, and Greenhorn formations at the Garcia field in the Raton basin in southern Colorado. In addition, there have been at least 35 shows of oil and/or gas from Cretaceous rocks of the New Mexico part of the Raton basin, with 23 of the shows from the Graneros, Greenhorn, Carlile, Niobrara, and Pierre formations--units mostly lacking indi enous porosity. Cored Pierre Shale in 1 well has vertical fractures with oil stain, and it seems likely that, in most of the other wells, the shales and the limestones, which lack indigenous porosity, have fractures that may contain hydrocarbons. Development of natural gas from fracture reservoirs in the Niobrara Formation is currently taking place in the Denver-Julesburg basin immediately northeast of the Apishapa arch, which forms the northeastern flank of the Raton basin. It should be noted that the Codell Sandstone Member, primarily on the western steeply dipping limb of the Denver-Julesburg basin, has become a new target as a result of several recent commercial oil discoveries in a fractured reservoir environment.
Favorable areas for development of fracture reservoirs within the Cretaceous rocks of the Raton basin are along the synclinal bend at the foot of the western limb of the basin, along the hinges of smaller flexures within the basin, and where there are abrupt changes in dip and/or strike. Detailed structure-contour and lithofacies maps are needed to refine the broad targets outlined here.
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