Budge–Fudge method of pore-pressure estimation from wireline logs with application to Cretaceous mudstones at Haltenbanken

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.

Compaction of diagenetically altered mudstones – Part 2: Implications for pore pressure estimation

“…Compared to the analysis shown by Sargent et al (2015), the trends on the crossplots for all wells are more consistent as a result of applying the lithological corrections to the sonic log. From a point shortly below the top of the Kvitnos Formation in this well, the data track a linear trend.…”

“…The Budge-Fudge approach of Sargent et al (2015) offers a partial solution to the difficulties in applying the two-step method to pore pressure estimation in diagenetically altered mudstones, and could be a starting point for a completely deterministic two-step solution using density and sonic logs. The method is applicable to mudstones at temperatures above 100 C, and makes use of the crossplot between sonic transit time and density (Fig.…”

“…This plot helps us to visualize why the vertical effective stress is not uniquely defined by density and sonic transit time in diagenetically altered mudstones, even if their depositional lithology is known and they have been uniaxially consolidated. We then compare pore pressure estimates from wireline logs in the Cretaceous mudstones at Haltenbanken obtained by the widely used method of Eaton (1975) and the Fudge approach proposed by Sargent et al (2015). We argue that the Budge-Fudge approach is preferable because it requires explicit accounting for both disequilibrium compaction and unloading mechanisms of overpressure generation, although it does require geological input to help estimate the overpressure contribution from disequilibrium compaction.…”

Compaction of diagenetically altered mudstones – Part 1: Mechanical and chemical contributions

“…After quality control, these criteria yielded a set of 36 wells from the area shown in Fig. 2b, of which ten wells containing at least one direct measurement of pressure were selected by Sargent et al (2015) in developing the Budge-Fudge method of pore pressure estimation, revisited in Part 2. The logs were edited and data points from sandy intervals were removed using the natural gamma log as a discriminator.…”

“…We found large differences between compaction profiles and concluded that the vertical effective stress history was the principal factor responsible for the differences, even though the vertical effective stress shows little spatial variation at the present day . Sargent et al (2015) analysed density and sonic logs together to show how the amount of overpressure due to unloading processes can be estimated from these logs.…”