Eighty years ago the first fractured shale production was found in the Rockies. Renewed interest in such objectives, especially in NW. Colorado and NW. New Mexico, has led to several important discoveries during the past five years in particularly the upper Cretaceous Niobrara calcareous shales, which are primarily being considered in this paper.

Fractured "shale" is the general oil-field term used to include not only shale but the generally brittle impermeable rocks ranging from calcareous shale, argillaceous limestone, siltstone to very fine-grained sandstone. In all cases the rocks are more brittle than the usual interbedded plastic shales. At points of abrupt flexure, in planes varying from vertical to horizontal, tensional cracks develop in the brittle rocks on the convex side of the flexure. Typical sites of flexure are sharply folded anticlinal axes and noses, abrupt changes in strike on any structure, monoclinal flexures and sharply folded synclinal axes. Faults also cause fracturing; most open fissures being adjacent rather than in the fault plane. In any case the interbedded plastic shales cause a trapping seal of th fractured brittle shale reservoir resulting in a modified stratigraphic trap which may occur independent of any structural closure. Downdip water is ordinarily absent.

Some factors which have hindered modern-day prospecting for these reservoirs include: the difficulty of evaluating past production histories; the problem of managements in evaluating future potential reserves of this non-homogeneous reservoir (thereby leaving the economics of the prospect and development in doubt); and present-day use of improper and inadequate methods of drilling, testing, completing, and producing these special reservoirs. A review of performance of "Niobrara" fields indicates that recoverable reserves of 5,000 barrels of oil per acre may be expected.

A recommended program for prospecting should include the following.

1. Establishment of the presence of proper brittle objectives in the subsurface at a depth commensurate with cost-productivity--expected return factors. Evidence of shows of oil or indirect indications of fracturing old wells drilled in the vicinity are vital.

2. Localization on an abrupt flexural trend, commonly coupled with cross-faulting.

3. A multiple-well program of evaluation and development to average out variability of fractured reservoirs, the wells being drilled on at least 40-acre spacing.

4. Careful attention to proposed techniques of drilling, testing, completion, and production suited to this special reservoir.

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