Regional variation in Cretaceous mudstone compaction trends across Haltenbanken, offshore mid-Norway

Abstract

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

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

“…They also ruled out variations in geothermal gradient as the main cause, because when porosities were plotted against temperature or against depth below the base of the Naust Formation, the range of porosity trends was only slightly reduced. Cicchino et al (2015) reported XRD results from four wells and and grain size analyses from two wells, and could not identify any lithological factor that might contribute significantly to the lateral variation in compaction trends.…”

“…2b). Cicchino et al (2015) investigated possible causes. They ruled out exhumation as the main cause of the lateral variation in compaction trends because the stratigraphic evidence from well and seismic data shows that the sediments are currently at their maximum burial depths across the area covered by Fig.…”

“…11 Cicchino et al (2015) compared the density-depth trends with the porosity-depth trend determined by Hansen (1996) for hydrostatically pressured Cretaceous and Tertiary mudrocks on the Norwegian Shelf (Fig. 5a) and inferred that the lower porosity mudstones at Haltenbanken are close to being normally compacted, even though they are now overpressured.…”

“…5a) and inferred that the lower porosity mudstones at Haltenbanken are close to being normally compacted, even though they are now overpressured. Cicchino et al (2015) interpreted the lateral variations in mudstone porosity as the result of lateral variations in the efficacy of water escape following rapid Naust burial, and suggested that the mudstones subsequently became better sealed by ongoing clay diagenesis and lithification. They also speculated that lateral variations in overpressure were greater in the early Pleistocene, following deposition of the majority of the Naust Formation.…”

“…Exploration wells were selected by Cicchino et al (2015) where the top Kvitnos-base Lange interval is thicker than 300 m and both density and natural gamma logs had been recorded throughout the interval. After quality control, these criteria yielded a set of 36 wells from the area shown in Fig.…”