Mapping Formation-Top Offsets in the Southwestern Alberta Plains, Canada: Methodology and Results

This report presents the results of subsurface bedrock structure mapping in southwestern Alberta, Canada, using a methodology that assesses formation-top offsets. Three stratigraphic horizons were selected for mapping subsurface bedrock offsets: the base of the Fish Scales Formation, the top of the Milk River Formation, and the lower Bearpaw Formation flooding surface. The selected surfaces represent, or are close to, marine flooding surfaces in this area and can be considered to approximate a flat-lying surface (datum) and, therefore, are ideal for mapping post-depositional structures. A bedrock offset is defined in this report as any lineament or possible vertical displacement of a bedrock horizon that has been detected using geostatistical analysis of well-log data. This methodology identified numerous potential offsets which have been highlighted using residual maps of these three surfaces. The offset features identified from these surfaces were compared to each other, and to an isopach map of the interval from the base of the Fish Scales Formation to the top of Milk River Formation to evaluate whether these offsets represent potential growth faults. Some of the offsets mapped in this report were found to coincide with previously reported faults, which were identified at isolated riverbank outcrops and interpreted from seismic reflection surveys, and allow for determination of fault orientation and extent. The other offsets highlighted in this report may represent potential faults or differential compaction and highlight areas of geological interest for future structural analysis.