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ABSTRACT: Practical Approach to a Fractured Chalk Play: Untapped Reservoirs in the Cretaceous Niobrara Formation

K. M. Wilson

Although the Niobrara oil play has been active since the late 1960s, success rates have generally been low and completions expensive. Drilling location and completion decisions have commonly ignored fractured reservoir theory and proven practice. An example of how not to conduct a fractured play may be found in the largely disappointing Silo field near Cheyenne, Wyoming, which was discovered in 1983 and developed pell-mell after discovery. However, although many wells were not economic, several produced huge quantities of oil. Why were these latter wells successful? What can be learned about the nature of the trap that could be applied elsewhere? How can success rates be improved?

The answers to these questions may be provided by a detailed geophysical, subsurface, and surface geochemical study conducted over an area of some 80 townships in Colorado and Wyoming. Aeromagnetic and seismic data support a correlation between fracturing and basement structure. Landsat image interpretation suggests that fracture trends that are presently oil filled can be mapped with reasonable accuracy.

Subsurface mapping of empirically defined net/gross pay strongly supports the geophysical data. Isopach maps suggest that the time of fracturing invoked by previous workers may be in error. Surface geochemical data suggest a tentative correlation between soil helium content and fracture trend. Extrapolation of the data from known producing areas to prospect areas indicates that perhaps two major Niobrara fields remain untapped. Application of the model presented here should allow these prospects to be drilled successfully.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990