An integrated tectonic and sequence stratigraphic analysis of the Cretaceous and Danian of the Danish Central Graben has led to significant new insights critical for our understanding of the chalk facies as a unique cool-water carbonate system, as well as for the evaluation of its potential remaining economic significance.A major regional unconformity in the middle of the Upper Cretaceous chalk has been dated as being of early Campanian age. It separates two distinctly different basin types: a thermal contraction early post-rift basin (Valanginian–Santonian), which was succeeded by an inversion tectonics-affected basin (Campanian–Danian). The infill patterns for these two basin types are dramatically different as a result of the changing influence of the tectonic, palaeoceanographic and eustatic controlling factors.Several new insights are reported for the Lower Cretaceous: a new depositional model for chalk deposition along the basin margins on shallow shelves, which impacts reservoir quality trends; recognition of a late Aptian long-lasting sea-level lowstand (which hosts lowstand sandstone reservoirs in other parts of the North Sea Basin); and, finally, the observation that Barremian–Aptian sequences can be correlated from the Boreal to the Tethyan domain. In contrast, the Late Cretaceous sedimentation patterns have a strong synsedimentary local tectonic overprint (inversion) that influenced palaeoceanography through the intensification of bottom currents and, as a result, the depositional facies. In this context, four different chalk depositional systems are distinguished in the Chalk Group, with specific palaeogeography, depositional features and sediment composition.The first formalization of the lithostratigraphic subdivision of the Chalk Group in the Danish Central Graben is proposed, as well as an addition to the Cromer Knoll Group.

Kilometre-scale geobodies of diagenetic origin have been documented for the first time in a high-resolution 3D seismic survey of the Upper Cretaceous chalks of the Danish Central Graben, North Sea Basin. Based on detailed geochemical, petrographic and petrophysical analyses, it is demonstrated that the geobodies are of an open-system diagenetic origin caused by ascending basin fluids guided by faults and stratigraphic heterogeneities. Increased amounts of porosity-occluding cementation, contact cement and/or high-density/high-velocity minerals caused an impedance contrast that can be mapped in seismic data, and represent a hitherto unrecognized, third type of heterogeneity in the chalk deposits in addition to the well-known sedimentological and structural features. The distribution of the diagenetic geobodies is controlled by porosity/permeability contrasts of stratigraphic origin, such as hardgrounds associated with formation tops, and the feeder fault systems. One of these, the Top Campanian Unconformity at the top of the Gorm Formation, is particularly effective and created a basin-wide barrier separating low-porosity chalk below from high-porosity chalk above (a Regional Porosity Marker, RPM). It is in particular in this upper high-porosity unit (Tor and EkofiskFormations) that the diagenetic geobodies occur, delineated by "Stratigraphy Cross-cutting Reflectors" (SCRs) of which eight different types have been distinguished. The geobodies have been interpreted as the result of: (i) escaping pore fluids due to top seal failure, followed by local mechanical compaction of highporous chalks, paired with (ii) ascension of basinal diagenetic fluids along fault systems that locally triggered cementation of calcite and dolomite within the chalk, causing increased contact cements and/or reducing porosity. The migration pathway of the fluids is marked by the SCRs, which are the outlines of highdensity bodies of chalk nested in highly porous chalks. This study, thus, provides new insights into the 3D relationship between fault systems, fluid migration and diagenesis in chalks and has important applications for basin modelling and reservoir characterization.---

This study re-examines large and deep U-shape reflections (2-4 km wide, 100-200 m deep) within the Upper Cretaceous-Danian Chalk Group in the inverted Roar Basin of in the Danish North Sea, previously interpreted as a moat associated with a contour-parallel current system and/or erosive channels by gravity-driven turbidites. Improved 3D seismic data quality and seismic interpretation techniques helped to identify overlooked reflection terminations that suggest that rather than a linear depression, the U-shape reflections outline several bowl-shaped depressions. In addition, vertical high-amplitude columns and vertical discontinuity zones within and below the depressions were recognized and interpreted to indicate the presence of small fluid pipes, suggesting that the formation of the depressions is more complex. Carbon isotope analysis of high acoustic impedance beds within the underlying Lower Cretaceous chalk shows negative δ13C values down to −20‰, and are interpreted to indicate sediments influenced by methane-derived authigenic carbonates. Permo-Triassic half-grabens seem to have been a major source of gas-bearing fluids, as evidenced by hydrocarbon leakage phenomena within Triassic to Lower Cretaceous strata. In areas where Zechstein salt is present, the leakage root lies at salt welds, causing the formation of focussed seismic reflection wipe-out and dim zones. In areas where salt was absent, the leakage root comprises a much more diffuse zone across the fault boundaries of the Permo-Triassic half-grabens, and gas chimneys are characterized seismically as broad vertical dim zones up to 10 km wide. Campanian inversion tectonics caused fault reactivation and several 100s of meters uplift of the Roar Basin, which created an instability of the trapped gas-bearing fluids. Gentle fluid venting through observed pipes caused sediment entrainment, which could be carried away by bottom current activity, causing localized zones of non-deposition and formation of individual depressions. This model thus does not disregard the role of bottom current activity in the formation of the depressions, yet it includes a fluid venting element that fits better with the architecture and overall evidence for fluid venting features in pre-chalk strata as well as in the Chalk Group. Importantly, it shows that prior to the thermogenic maturation of the main source rock (i.e., the Bo Member of the Farsund Formation in the Late Miocene), fluid venting already occurred on the Late Cretaceous seafloor from deeper source rocks that are at present overmature.

This study documents a variety of deposits created by submarine landslides within the Upper Cretaceous to lowermost Palaeocene Chalk Group in the Danish Central Graben and investigates the impact of remobilization on porosity. Improved visualization of the landslides in 3D seismic data compared to previous studies was facilitated by better seismic data quality for the Chalk Group, the availability of a large stack of stratigraphy-consistent horizons, and the use of spectral decomposition data. The illustrated examples are chosen to reflect the spectrum of deformation styles seen in the chalk and are all having a well penetrating the affected succession. They include a large collapse (375 km2) of an inversion ridge within the Kraka and Gorm formations, a field of large slide blocks (100–1000 m, 10–26 m) of likely lowermost Danian age embedded in the uppermost Ekofisk Formation, a debris flow system within the uppermost Tor Formation likely originating from the Ringkøbing-Fyn High, and fine-grained bottom current sediment waves within the lowermost Danian Ekofisk Formation. In general, porosities are higher (10-25 porosity units) in the remobilized chalk compared to time-equivalent pelagic chalk in nearby reference wells. In earlier studies this has been linked to lack of bioturbation (resulting in limited grain repacking) in the remobilized chalks due to high sedimentation rates, resulting in a relative open fabric during initial burial. In contrast, surrounding and covering pelagic deposits could be much more effectively bioturbated leading to tighter grain packing during burial. The insights of this study help in seismic characterization of mud-grade carbonate oozes and find important applications reservoir modelling of mud-grade carbonate reservoirs (also in the light of carbon capture and storage), and in palaeo-reconstructions of pelagic seafloors since submarine landslides provide kinematic indicators.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5830839

This study proposes a unique workflow to unravel complex burial diagenetic histories of overpressured basins based on the integration of seismic and well log data, biostratigraphy, petrography, clumped isotope analyses and basin model-EAGE SMIT et al.

The mapping of faults provides essential information on many aspects of seismic exploration, characterisation of reservoirs for compartmentalisation and cap-rock integrity. However, manual interpretation of faults from seismic data is time-consuming and challenging due to limited resolution and seismic noise. In this study, we apply a convolutional neural network trained on synthetic seismic data with planar fault shapes to improve fault mapping in the Lower and Upper Cretaceous sections of the Valdemar Field in the Danish North Sea. Our objective is to evaluate the performance of the neural network model on post-stack seismic data from the Valdemar Field. Comparison with variance and ant-tracking attributes and a manual fault interpretation shows that the neural network predicts faults with more details that may improve the overall geological and tectonic understanding of the study area and add information on potential compartmentalisation that was previously overlooked. However, the neural network is sensitive to seismic noise, which can distort the fault predictions. Therefore, the proposed model should be treated as an additional fault interpretation tool. Nonetheless, the method represents a state-of-the-art fault mapping tool that can be useful for hydrocarbon exploration and CO2 storage site evaluations.

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