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Gravity Fault Controlled Structural Traps, North-Central Montana

Mark S. Caldwell
Klabzuba Oil and Gas, Denver, CO

Within the Bearpaw Mountains and hidden beneath the surrounding plains lies a world class example of gravity-induced faulting within Upper Cretaceous marine sedimentary rocks. Seismic exploration, mostly for shallow Upper Cretaceous biogenic gas reservoirs, has yielded high quality 2D and 3D datasets spanning a large area of North-Central Montana. Mapping utilizing this seismic data reveals a complex pattern of faulting hidden beneath a glacial veneer north of the Bearpaw Mountains and into Canada. This fault pattern is very similar in geometry and origin to that mapped at the surface south of the Bearpaws.

Two principal decollements carry massive thrust sheets downdip off the flanks of the Laramide Bearpaw Uplift. The older, upper decollement is rooted in an organic-rich shale within the uppermost Colorado Group(1WS). Regional gentle NE structural dip on this thrust sheet is punctuated by regularly spaced fault-bounded horsts or “pop-up” structures that trap gas in the Eagle Sandstone. These “pop-up” structures were then carried piggyback by faulting associated with a lower decollement just above the Greenhorn (2WS). Several large thoroughgoing strike-slip faults offset earlier formed pop-ups with roughly 5000 feet right-lateral displacement.

Faults associated with both decollements trap biogenic gas in a variety of structural traps well imaged with 2D and 3D seismic. Most Eagle gas is trapped in comparatively small, fault-bounded horst blocks of 20-120 acres. Many one well fields have produced between 1 and 2 Bcfg from very high quality marine shoreface reservoirs. St. Joe Road field, discovered in 2001, covers 40 sq. mi. and has produced over 11 Bcfg from 74 wells in the Niobrara Sandstone. Analysis of gas-induced seismic anomalies serves to greatly reduce exploration risk.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado