Technical Luncheons
Exploration for Gravity Fault Controlled Structural Traps, North-Central Montana
Speakers:
Mark S. Caldwell
Klabzuba Oil & Gas Inc., Denver, CO
Date/Time: Thursday, June 19, 2008 - 11:30 am
Location: Telus
Convention Centre - Calgary, Alberta
The cut-off date for ticket sales is 1:00 pm, Monday, June 16th, 2008*
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ABSTRACT
The Bearpaw Uplift of north-central Montana hosts a world-class example of gravity-induced faulting within Upper Cretaceous marine sedimentary rocks and their overlying Tertiary volcanic and sedimentary cover. Seismic exploration, mostly for shallow (less than 640 metres, 2100 foot depth) Upper Cretaceous biogenic gas reservoirs, has yielded high-quality 2D and 3D datasets. Mapping 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 Bearpaw Mountains.
Laramide basement-involved uplift of the Bearpaw Arch was accompanied by extensive outpouring of surface lava flows and volcaniclastics and the emplacement of a wide variety of intrusive bodies in the middle Eocene (53-42 mya). Thrust sheets loaded with thick volcanic and sedimentary cover moved off the flanks of the Bearpaw Arch along over-pressured organic-rich shale decollements. Rocks beneath the decollements are mostly undeformed with dips of 1 to 3 degrees away from the arch. The head ward or updip portion of the thrust sheets is dominated by extensional structures in a chaotic fault array of normal faults and grabens, while the basinward downdip portion is characterized by compressional thrust-faulted structures.
The detailed study area, located in northern Hill and Blaine Counties, is dominated by north directed compressional structures that trap biogenic gas in a variety of structural traps well imaged with 2D and 3D seismic. Two principal decollements have been identified and mapped in the study area. The older, upper 1WS decollement is rooted in an organic-rich shale within the uppermost Colorado Shale (First White Specks). Regional gentle NE structural dip on this thrust sheet is punctuated by regularly spaced fault-bounded horsts or "pop-up" structures that are up to 8 km (5 miles) long. These structures were then faulted and folded by deformation associated with the lower, younger 2WS decollement, located in organic rich shale just above the Greenhorn (Second White Specks). Several long (up to 64 km, 40 mi.) strike-slip faults offset earlier formed pop-ups with roughly 1500 m (5000 feet) right-lateral displacement as measured by piercing points at mapped fault intersections.
Historically, the primary exploration objective around the Bearpaw Uplift has been the Upper Cretaceous (Campanian) Eagle Sandstone. Much of the Eagle gas in the study area is found in structures associated with the (older) 1WS decollement. Structural traps include north vergent fault-bend fold traps, backthrusts (south vergent thrusts), pop-up blocks, blind thrusts, and footwall cutoffs. Several one well Eagle fields have produced between 1 and 2 Bcfg from very high quality marine shoreface reservoirs (30% porosity, 200-1000 md perm). The largest Eagle gas accumulation in the study area has produced over 6 Bcfg from a dome formed above a blind thrust.
Recent exploration has led to the discovery of shallow gas reserves in the underlying Niobrara (Santonian Medicine Hat) Sandstones. Klabzuba's St. Joe Road field, discovered in 2001, covers 40 sq. mi. and has produced over 11 Bcfg from 74 wells in the Niobrara Sandstone. Current daily production for the field is over 10 million cfgpd. Niobrara gas is trapped downdip of a major right-lateral strike slip fault that formed above the 2WS decollement.
BIOGRAPHY
Mark Caldwell received his B.Sc. degree in Geology from the University of Michigan and a M.Sc. degree in Geology from Western Michigan University. Mark began his career with Northern Michigan Oil & Gas in Traverse City, Michigan exploring the Michigan Basin. Mark has been employed as a geologist for Klabzuba Oil & Gas, Inc. since 1988 and has been actively working both the subsurface geology and geophysics of the Bearpaw Uplift since 1994.
