Recent drilling in the Rattler Butte area of central Montana has renewed interest in the Pennsylvanian Tyler Formation as a drilling

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objective. New production in this area, coupled with the surrounding well density, provides an ideal situation for further development of Tyler stratigraphic-seismic exploration concepts and methods.

Both geologic and geophysical Tyler thickness maps have proven to be useful tools in delineating eroded Heath and subsequent lower Tyler deposition. Seismic modeling has revealed a series of possible Tyler-Heath erosional edge characteristics, providing another tool for Tyler-Heath boundary definition. In modeling specific seismic sand signatures, it was found that seismic character and amplitude are dependent upon both formation thickness and lithology.

Detailed mapping of the study area also revealed a new environmental interpretation of the Tyler. Unlike the fluvial system to the north, the Tyler regime in the Rattler Butte area appears to have fluctuated among fluvial, deltaic, and marine systems.

Two hydrocarbon occurrence patterns have been noted within the Tyler: (1) although reservoir quality sands are present throughout the Tyler, those within the lower Tyler are more likely to contain hydrocarbons, and (2) close proximity to the Tyler-Heath erosional edge increases the chances of discovering oil-filled Tyler sands.

Combined use of these exploration tools should greatly enhance the chances for successful lower Tyler exploration.

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