Rikke Weibel

The influence of rifting on the composition of Kimmeridgian to Barremian mudstones from northern Wollaston Forland, North-East Greenland is investigated by petrographic and mineralogical analyses of the Brorson Halvø-1 and Rødryg­gen-1 cores, and provenance of analysis by zircon U-Pb age dating of nearby sandstones. Mudstone composition varies systematically as a function of the timing of rifting progression and position in the half-graben depositional system. Pyrite primarily precipitated in the early rift to rift climax phases. Euhedral pyrite overgrowths on framboids formed only during the rift climax phase (Lindemans Bugt Formation). Dolomite is the dominant carbonate cement, except for the sediments deposited in the early waning rift phase (Palnatokes Bjerg Formation) where calcite is dominant, and in the late waning rift phase (Stratumbjerg Formation) where siderite dominates. The highest-temperature reactions with precipitation of illite, quartz, ankerite and barite signify sediment burial depths of >2 km prior to exhumation. Uplift-induced fracturing occurred mainly in the early rift to rift acceleration succession (Bernbjerg Formation). Mudstones in the proximal part of the half-graben (Rødryggen-1) include more detrital kaolinite than the distal mudstones (Brorson Halvø-1), which contain more mixed-layer illite-smectite and illite. Vermiculite was deposited only in the proximal part of the basin in the rift climax and waning rift successions. Chlorite was deposited proximally and distally during the waning rift phase, though supply began earlier in the distal part. Fine-grained sediment in the distal part of the half-graben was therefore probably supplied by axial transport from Palaeoproterozoic crystalline rocks and Meso- to Neoproterozoic metamorphic rocks located to the north and north-west. This agrees with the zircon provenance signature from outcropping sand-rich facies, where zircon grains with U-Pb ages of 2.0–1.6 Ga are dominant, in addition to common 1.6–0.9 Ga ages, and fewer 2.8–2.6 Ga and 0.47–0.36 Ga ages.

The significance of mineralogical maturity as a provenance indicator has long been debated and we use this study to demonstrate that it can indeed be a powerful tool to track the distribution of sandstone reservoirs. We investigate the cause of the pronounced geographic and stratigraphic differences in mineralogical composition that are found in the Upper Triassic–Lower Jurassic Gassum Formation across the Norwegian–Danish Basin and surrounding areas. Zircon U-Pb dating of 46 sandstone samples including analysis of 4816 detrital grains are combined with quantifications of the detrital mineralogical composition and placed in a sequence stratigraphic framework. The results show that the Gassum Formation can be divided into a southeastern region with high mineralogical maturity and a less mature region to the northwest with more feldspars, rock fragments, micas, and heavy minerals. Both the mineralogical assemblage and the provenance signature have been thoroughly homogenized in the SE region where sediment supplies from the Fennoscandian Shield and the Variscan Orogen are evident. In the NW region, sediment was initially supplied from Fennoscandia only, but the provenance abruptly changed from the Telemarkia Terrane to comprising also the more distant Caledonian Orogen resulting in a different mineralogical assemblage. The change occurred during a basinwide regression and may be caused by tectonic movements in the hinterland that permanently changed the composition of the sediment supplied to the basin.

Evaluation of the regional geotectonic impact of the High Arctic Large Igneous Province (HALIP) in the present‐day northern Atlantic region has been hindered by poor correlation between the Svalbard–Barents Shelf region and eastern North Greenland. New sedimentological and biostratigraphic data from Peary Land and Kronprins Christian Land (Kilen), North Greenland reveal that the Lower Cretaceous palaeogeographic and sequence stratigraphic development of this area is closely comparable to that of Svalbard. The succession records Hauterivian – early Barremian regional uplift and emergence followed by fluvial sedimentation and subsequent transgression in the late Barremian – early Aptian. Recognition of this tectonically forced regression in North Greenland provides a link to a coeval well‐known tectonostratigraphic event in the Svalbard region, and hence to regional tectono‐magmatic uplift heralding the HALIP and the initiation of the Amerasia Basin.

The Danish onshore subsurface contains very large geothermal resources that have the potential to make a significant contribution to transforming Danish energy consumption toward a more sustainable energy mix. Presently, only a minor fraction of this green energy is exploited in three small plants. The main factors that have hampered and delayed larger-scale deployment are related to uncertainties in the geological models, which inevitably lead to high economic risks that are difficult for smaller district heating companies to mitigate without support from a compensation scheme. To facilitate and stimulate much wider use of the Danish geothermal resources, the Geological Survey of Denmark and Greenland (GEUS) and other research institutes have conducted several regional research projects focusing on the geological and geochemical obstacles with the principal objective of reducing the exploration risks by selecting the best geological reservoirs.One of the most important geological factors causing uncertainty is the quality of the reservoirs and their ability to produce the expected volume of warm geothermal brine. Thus, great emphasis has been placed on investigating and understanding the relationships between reservoir sandstone, porosity, permeability, petrography, diagenetic processes and alterations related to variable sediment sources, basin entry points, depositional systems and climate, burial and thermal history. Mesozoic sandstones comprise the most important geothermal reservoirs in Denmark. Details concerning the reservoir quality are compiled and compared for the Lower Triassic Bunter Sandstone, Triassic Skagerrak, Upper Triassic – Lower Jurassic Gassum and Middle Jurassic Haldager Sand formations. The Bunter Sandstone Formation contains extensive aeolian and more confined fluvial sandstones with high porosity and permeability. However, highly saline formation water could be unfavourable. The Skagerrak Formation comprises well-sorted braided stream sandstones in the centre of the basin, and is otherwise characterised by muddy sandstones and alluvial fan conglomerates. An immature mineralogical composition has caused intensive diagenetic changes in the deepest buried parts of the basin. The Gassum Formation consists of shoreface, fluvial and estuarine sandstones interbedded with marine and lacustrine mudstones. In the upper part of the formation, the sandstone beds pinch out into mudstones towards the basin centre. Pervasive siderite- and calcite cement occurs locally in shallowly buried sandstones, and with burial depth the maximum abundances of quartz and ankerite cement increase. Sandstones of shallow burial represent excellent reservoirs. The relatively coarse grain size of the Haldager Sand Formation results in high porosity and permeability even at deep burial, so the formation comprises a high-quality geothermal reservoir.Substantial progress has been made, and a well-established regional geological model combined with reservoir quality is now available for areas with cored wells. This has enabled an improved estimation of reservoir quality between wells for exploration of geothermal reservoirs.

The end-Triassic mass extinction is considered to have been caused by voluminous and repeated emissions of CO2 and/or methane and other gases from magmatic activity in the Central Atlantic Magmatic Province. Despite improved geochronological dating and correlation between the magmatic activity and the extinctions, exactly how the biotic crisis commenced remains poorly understood. Here, we compile palynological and palaeobotanical data, bulk organic δ13C, biomarkers, mercury and other geochemical proxies, charcoal, and sedimentology, from a Rhaetian terrestrial succession in southern Sweden. Our results provide an insight into the climatic, environmental and ecosystem changes that took place at the onset of the mass extinction event.

In order to meet the increasing demand to decarbonize the atmosphere, storage of CO₂ in subsurface geological reservoirs is an effective measure. To maximize storage capacity, various types of saline aquifers should be considered including dynamic storage options with open or semi-open boundaries. In sloping aquifers, assessment of the immobilization potential for CO₂ through dissolution and mineralization along the flow path is a crucial part of risk evaluations. The Gassum Formation in the Skagerrak is considered a nearshore CO₂ storage option with sloping layers, facilitating buoyant migration of CO₂ northwards along depositional and structural dip. In this study, petrographic data and provenance analysis provide the basis for estimating reactivity of the sandstones. Immobilization of CO₂ in the reservoir through fluid dissolution and mineral reactions reduces risk of leakage. Petrographic analyses are integrated with seismic and well-log interpretation to identify sedimentary facies and to estimate mineral distribution with corresponding reactivity in the proposed injection area. Here the Gassum Formation comprises south-prograding, shoreface-fluvial para-sequences, sourced from northern hinterlands. Pronounced differences in the mineralogical maturity in the studied area are identified and explained by the sediment transport distances and the type of sediment source. This is possible because the U-Pb ages of zircon grains in the sediments can be used to pinpoint the areas where they originate from in the Fennoscandian Shield, such as the Telemarkia or Idefjorden terranes. Albite and Fe-rich chlorite are identified as the most reactive mineral phases in the Gassum sand, of which feldspar comprises the largest weight fraction and the grain-coating chlorite has largest surface area. Their distribution is partly controlled by provenance, so their abundance decreases basinwards with increasing sediment maturity. The abundance of fluvial sandstones presumably increases northwards in basal parts of para-sequences, while shoreface sandstones comprise the top part of sandy units. CO₂ injected in the proposed area will migrate upwards within the reservoir, toward higher proportions of Telemarkian-derived sediment and up-dip along the seal, toward more immature sediments. Thus, the reactivity of sediments increases in younger deposits and up depositional dip, while kinetic reaction rates will decrease in shallower, lower temperature regions. Identifying these parameters is important to estimate the CO₂ mineral sequestration potential as a function of sedimentary facies and ensure safe storage of CO₂. This approach can advantageously be applied to all reservoirs considered for CO₂ injection to improve the estimation of the possible CO₂ storage volume by taking the provenance dependence of the mineralization potential into account.