The Importance of Deformation Models for Calculating
Reserves and Locating Wells in Overthrust
Belts: Examples from the Mississippian
Gas Fields of the NE British Columbia Foothills, Canada
Andrew C. Newson
Understanding the deformation model appropriate for an oil or gas field in an
overthrust
belt has major consequences for the exploration and exploitation of
these structures. Predicted reserves and well locations will be influenced by
the model chosen. With Mississippian gas wells costing $1.2 to $1.5 mm each, a
re-drill or even a whipstock to hit the target will negatively impact the bottom
line. This is especially true if the reserves are 3Bcf as opposed to the 20Bcf
originally predicted.
For the purpose of this talk we will define two end-member deformation models: fault bend folds and detachment folds. The fundamental difference between the two models is that the fault bend fold has nearly equal amounts of displacement on both sides of the structure, whereas the detachment fold has the displacement absorbed by the fold.
In the examples from British Columbia the reservoir formation is not directly imaged by seismic data. The top of it has to be "ghosted in" from the Permo/Penn event above it. If this is done using a constant thickness for the Permo/Penn to Mississippian interval this would be a more suitable technique for a fault bend fold model. As this area has a significant amount of detachment folding this technique is not appropriate. These differences in the deformation models will significantly impact both well location and reserve calculations.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California