A New Approach to Shale Compaction and Stratigraphic Restoration, Beaufort-Mackenzie Basin and Mackenzie Corridor, Northern Canada (1)

Abstract

Despite a long history of study, porosity reduction in shales and mudstones is still poorly understood. Many published shale compaction curves from different sedimentary basins exist, but it is difficult to make comparisons because of variations in sediment composition, geologic age, tectonic history, and experimental technique. The Beaufort-Mackenzie basin provides a unique opportunity to study shale compaction because it contains shales with rather uniform physical and chemical properties, deposited under highly variable sedimentation rates. The sonic log provides a sensitive measure of porosity change with depth in the Beaufort-Mackenzie basin. A new porosity-sonic transit-time equation, calibrated using shale and mudstone core porosity measurements from this basin, yields an acoustic formation-factor exponent of 2.19 and a matrix transit time of 220 microseconds/m, which are believed to be valid for low TOC (<2%), noncalcareous shales. Log-determined shale compaction curves define distinctive compaction zones that correlate with sedimentation rate, pore pressure, and erosion. Normal shale compaction is linear below 500 m at 1%/100 m decrease in porosity for hydrostatic pressure conditions. In eroded areas, the amount of missing strata can be estimated by reference to the normal compaction curve. In compositionally heterogeneous shale sequences, a knowledge of shale composition and its effect on log response is essential to avoid introducing significant error into calculated compaction curves. Preliminary results from the Brackett basin imply that maximum burial is more important than maximum paleotemperature in reducing shale porosity. This suggests that shale porosity could be a useful temperature-insensitive paleodepth indicator for eroded sedimentary sequences.