Abstract

Using wireline logs to estimate pore pressure in mudstones at the chemical compaction stage is not straightforward because clay diagenesis proceeds independently of effective stress, and neither density nor velocity is uniquely related to the maximum effective stress experienced by the mudstones. We propose the Budge–Fudge method, in which we assume there is a unique trend on the sonic–density cross-plot for mudstones at the chemical compaction stage that have not been unloaded. In addition to the sonic–density chemical compaction trend, an initial guestimate of maximum effective stress previously experienced by the mudstones is required. Additional overpressure from unloading processes is then estimated from the sonic log, referenced to the density response. The initial guestimate of maximum effective stress may be adjusted to fit any available measured pressures or pressures estimated from geological knowledge. We have applied the Budge–Fudge method to Cretaceous mudstones at Haltenbanken, and find that estimated pressures match measured pressures and expected pressure–depth profiles. Furthermore, the analysis suggests that the lateral variations in mudstone porosity, previously reported, result from lateral variations in overpressure build-up immediately following rapid burial by glaciogenic sediments; subsequently, overpressures have increased through clay diagenesis and equilibrated laterally across the area.