Application of Sonic Logs to Estimate the Magnitude of Exhumation in the Foredeep Section of the Western Canada Foreland Basin
In the Western Canada Foreland Basin (WCFB), techniques using coal rank, vitrinite reflectance and other proxies have been used to estimate the magnitude of exhumation accompanying post-orogenic uplift. The results generally agree in terms of trends, where the exhumation increases towards the deformation front. The estimation of magnitude, however, varies up to several thousand metres in the same area depending on technique. These differences introduce significant uncertainty into the calibration of hydrocarbon generation models and resulting petroleum system analyses.
Compaction of sedimentary rocks is largely irreversible, therefore uplifted strata retain the imprint of burial. The degree of compaction as a function of burial depth, furthermore, follows well-documented trends for clastic mudstones (Japsen, 2000). This makes it possible to calculate the magnitude of exhumation by comparing a compaction curve or baseline from rocks at maximum burial depth with a compaction curve from rocks that have been uplifted; the vertical distance between these two curves will yield the magnitude of exhumation.
Compressional velocity obtained from sonic logs is strongly dependent on porosity, which is reduced with compaction. In this study, compressional velocity is used as a proxy for compaction to estimate the magnitude of exhumation. The compaction curve or baseline representing maximum burial depth used in this study was proposed by Japsen (2000) and is appropriate for marine mudstones, like the Upper Cretaceous Colorado Group in the WCFB, that are dominated by smectite/illite clays. This curve relates compressional velocity to burial depth in the absence of erosion and overpressure.
Twenty six wells distributed in the foredeep depozone of the basin were used to calculate the magnitude of exhumation, following the principles explained above. In general, the exhumation magnitude increases from NE to SW, similar to trends observed by previous authors. This study, however, reveals exhumation patterns are collocated with basement terrain structures; the largest exhumation magnitude, for example, overlies a crustal reverse fault.
AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016