Mesozoic–Cenozoic exhumation events in the eastern North Sea Basin: a multi‐disciplinary study based on palaeothermal, palaeoburial, stratigraphic and seismic data

Abstract

Four Mesozoic–Cenozoic palaeothermal episodes related to deeper burial and subsequent exhumation and one reflecting climate change during the Eocene have been identified in a study of new apatite fission‐track analysis (AFTA®) and vitrinite reflectance data in eight Danish wells. The study combined thermal‐history reconstruction with exhumation studies based on palaeoburial data (sonic velocities) and stratigraphic and seismic data. Mid‐Jurassic exhumation (ca. 175 Ma) was caused by regional doming of the North Sea area, broadly contemporaneous with deep exhumation in Scandinavia. A palaeogeothermal gradient of 45 °C km−1 at that time may be related to a mantle plume rising before rifting in the North Sea. Mid‐Cretaceous exhumation affecting the Sorgenfrei–Tornquist Zone is probably related to late Albian tectonic movements (ca. 100 Ma). The Sole Pit axis in the southern North Sea experienced similar inversion and this suggests a plate‐scale response along crustal weakness zones across NW Europe. Mid‐Cenozoic exhumation affected the eastern North Sea Basin and the onset of this event correlates with a latest Oligocene unconformity (ca. 24 Ma), which indicates a major Scandinavian uplift phase. The deeper burial that caused the late Oligocene thermal event recognized in the AFTA data reflect progradation of lower Oligocene wedges derived from the uplifting Scandinavian landmass. The onset of Scandinavian uplift is represented by an earliest Oligocene unconformity (ca. 33 Ma). Late Neogene exhumation affected the eastern (and western) North Sea Basin including Scandinavia. The sedimentation pattern in the central North Sea Basin shows that this phase began in the early Pliocene (ca. 4 Ma), in good agreement with the AFTA data. These three phases of Cenozoic uplift of Scandinavia also affected the NE Atlantic margin, whereas an intra‐Miocene unconformity (ca. 15 Ma) on the NE Atlantic margin reflects tectonic movements of only minor amplitude in that area. The study demonstrates that only by considering episodic exhumation as an inherent aspect of the sedimentary record can the tectonic evolution be accurately reconstructed.

Pre-drilling assessments of average porosity and permeability in the geothermal reservoirs of the Danish area

“…The Mesozoic deposition is influenced by differential uplift across the basin and occasionally, halokinesis has resulted in local deviations from the regional uplift trend (Japsen et al 2007). The regional trend points to gradually increasing uplift toward the northeast.…”

“…The average core permeability (gas permeability) is about 4300 mD, whereas the well test permeability (liquid permeability) is about 6300 mD. Data originate from an excellent sandstone reservoir in the lower part of the Gassum formation (zone 6) and Bidstrup 1999; Japsen et al 2007), and hence the standard depth scale is replaced by 'estimated maximum burial depth' .…”

“…Our study indicates, however, that the sandstone porosity more likely is related to geological factors such as burial depth, lithofacies, clay content and diagenesis. In addition, the porosity-depth trend is affected by differential uplift and erosion; thus present-day depths were corrected for uplift using exhumation data presented in Japsen and Bidstrup (1999) and Japsen et al (2007). For simple modelling purposes in undrilled areas, the porosity may be considered to be controlled by depth alone, provided that 'present-day depths' are replaced by 'estimated maximum burial depths' .…”

Filling the North Sea Basin: Cenozoic sediment sources and river styles

“…Uplift of Fennoscandia, coupled with a marked deterioration in climate, led to a major increase in sediment supply to the south-eastern embayment of the North Sea via the Baltic River (Overeem et al, 2001), although sediment did not reach the southern North Sea until the Late Miocene (Knox et al, 2010;Rasmussen & Dybkjaer, 2014;Thöle et al, 2014). The South Swedish Dome also appears to have undergone relative uplift (Japsen & Bidstrup, 1999;Japsen & Chalmers, 2000;Lidmar-Bergström & Näslund, 2002;Japsen et al, 2007), while subsidence accelerated in the North Sea region (Fig. 6).…”

“…Elsewhere, the effects of relative sealevel rise were countered by continued uplift, most notably of the Scottish source areas, as shown by a major phase of southward land progradation. Shortly after, the shelf underwent uplift and erosion associated with a major phase of exhumation of Fennoscandia including the continued rise of the South Swedish Dome and southern Norway (Japsen & Chalmers, 2000;LidmarBergström & Näslund, 2002;Japsen et al, 2002Japsen et al, , 2007Cloetingh et al, 2005). Discussions regarding the potential interplay of uplift and climatic deterioration of the Scandinavian mountains is still unresolved (e.g.…”