Kevin Gillen¹

(1) Vox Terrae, Calgary, AB

ABSTRACT: Predicting Trends in Both Fracture Intensity and the Direction of Open Fractures Using Strain

Eight cores from the Schooler Creek Group of N.E. British Columbia and Alberta were examined to characterize the distribution, morphology and orientations of fractures. Of primary interest were the controls on both the intensity of fracturing and the direction of open fractures within select zones of producing regions.

Select intervals of core were first assembled into continuous segments and analyzed for fractures. None of these cores were scribed for orientation at the time that they were cut. Thus core orientations were derived using paleomagnetic analysis. Paleomagnetism involves the use of the magnetic signal within rock as a "fossil" compass needle. This allowed the fracture data to be restored to in situ orientations.

Many cores showed strain in the form of both slickensides and high-angle to subvertical stylolites. Slickensides on fracture surfaces were used to delineate variations in the directions of shear, while subvertical stylolites were used to infer the direction of paleo-compression. These two variables were combined to derive a picture of strain variations within the rock. Both fracture intensity and the development of porosity along fracture surfaces appear to be related to strain. There appears to be a general increase in the intensity of high-angle to subvertical permeable fractures as one moves from the south to northern-most well, which coincides with changes in the nature of strain indicators. The strike of these open fractures matches the model for paleo-compression variation within select regions of structure.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado