No abstract

Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details.  The study used well data from Cretaceous formations at Haltenbanken. Overpressure is due to disequilibrium compaction and clay diagenesis. Mechanical compaction continues up to temperatures of at least 130 C. The hypothesis that mudstones only compact chemically above 100 C is falsified.

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.

Abstract.Vintage 2-D (two-dimensional) seismic reflection surveys from the sparsely explored Mentelle Basin (western Australian margin) have been reprocessed and integrated with a recent high-quality 2-D seismic survey and stratigraphic borehole data. Interpretation of these data sets allows the internal geometry of the Mentelle Basin fill and depositional history to be reanalysed and new insights into its formation revealed. Basin stratigraphy can be subdivided into several seismically defined megasequences separated by major unconformities related to both breakup between IndiaMadagascar and Australia-Antarctica in the ValanginianLate Hauterivian and tectonically-driven switches in deposition through the Albian.Resting on the Valanginian-Late Hauterivian breakup unconformity are several kilometre-scale mounded structures that formed during Late Jurassic to Early Cretaceous extension. These have previously been interpreted as volcanic edifices although direct evidence of volcanic feeder systems is lacking. An alternative interpretation is that these features may be carbonate build-ups. The latter interpretation carries significant climatic ramifications since carbonate build-ups would have formed at high palaeolatitude, ∼60 • S.Soon after breakup, initial subsidence resulted in a shallow marine environment and deposition of Barremian-Aptian silty-sandy mudstones. As subsidence continued, thick successions of Albian ferruginous black clays were deposited. Internally, seismic megasequences composed of successions of black clays show previously unresolved unconformities, onlapping and downlapping packages, which reflect a com-

Knowledge of compaction behaviour underpins basin modelling and pore-pressure estimation for drilling wells. Mudstones of the Cretaceous Lange and Kvitnos formations at Haltenbanken are diagenetically mature and overpressured with a pressure-depth profile that shows little lateral variation. From density logs, we made the unexpected discovery that porosities vary by a factor of two at depths of around 2700 m below seafloor, with greater porosities in the west, so we investigated possible causes for the variation. Exhumation cannot be the cause because the Cretaceous mudstones are presently at their maximum burial depths across most of the area. Nor are lateral variations in geothermal gradient high enough for diagenesis to be responsible for the lateral porosity differences. X-ray diffraction and grain-size analyses were conducted on cuttings but no significant lithological variations were found. We infer that the lateral differences in compaction trends developed because porewater escape was more inhibited in the west during recent rapid burial by glaciogenic sediments. Associated lateral variations in overpressure may subsequently have decayed. The novel finding in this study is that diagenetically mature mudstones at Haltenbanken display large lateral variations in porosity that cannot be attributed to lateral differences in overpressure, exhumation, temperature or lithology. Received