Structural Style and Timing in the Narraway Area, Foothills Belt, Alberta: Implications for Fracture Distribution and Hydrocarbon Exploration

Richards, David R.¹, Rob Scammell², Martin Armitage², Michael Doe¹ (1) Midland Valley, Golden, CO (2) Burlington Resources, Calgary, AB

The Narraway area is in the frontal foothills portion of the Alberta Rockies fold and thrust belt. The deformation is a mixture of detachment folds and small displacement thrusts in the Permian through Cretaceous section. It is an area of substantial gas production from several tight Cretaceous sandstone units. Fractures are critical to economic production in this area.

Details of the relative timing of the structural events are displayed on regional 2D seismic. A new depth-migrated 3D volume provided additional detail on the structures and timing. The depth migration included a considerable advance on the velocity model as well as insight into the anisotropy due to dipping shale above.

There is clear evidence of early thrusting on ramps through parts of the Cretaceous section. These faults are imaged basinward of the main Narraway folded zone. In the Narraway area some of the thrusts are folded by the later detachment folding, and they were inactivated by the change in geometry. There is disharmony vertically through the section, indicating the location of detachments in shales, and compartmentalizing the shortening into sections of harmonic detachment folding or ongoing thrusting. Steep limbs are primarily kink folds, rather than faults.

Existing wells show that production rates are related to fractures. 3D seismic, production and field examples suggest that, at least in part, fractures are related to faulting and folding. As production is not restricted to structural highs, detailed mapping of the faults and fold shapes in 3D can aid in exploration and step-out drilling.