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Beneath the Precambrian Belt in the Deepest Well Ever Drilled in Montana

PETERSON, CHRIS H., and ROBERT C. NIMS, Unocal Corporation, Oklahoma City, OK

The Union 1-B30 Federal Canyon Creek well was drilled in the Montana thrust belt approximately 27 mi northwest of Helena to test Mississippian carbonates on a seismic structure in the footwall of the Eldorado thrust. The well penetrated over 12,000 ft of thrusted Precambrian belt metasedimentary rocks before encountering a faulted and fractured footwall section of Cretaceous through Mississippian shales, sandstones, and carbonates. The 17,818-ft test, the deepest well ever drilled in Montana, was plugged and abandoned after recovering fresh water during production tests in the Madison Group.

The original structural interpretation of the Canyon Creek prospect, based solely on 24- and 48-fold seismic data, was that of a relatively simple thrust-faulted, doubly plunging anticline in the footwall of the Eldorado thrust. The 1-B30 well indicated that a more complex duplex structure was present under the Eldorado thrust, and that the Madison was deeper than anticipated with a true vertical depth of 16,688 ft.

 

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)