The Permian‐Triassic alluvial rift succession in the Horda Platform area is analysed to construct a refined depositional model. The study demonstrates how subsurface continental rift successions may be stratigraphically subdivided and correlated by integrating seismic and well‐log data in concert with conceptual models. Regional unconformities mark the top and base of the Permian‐Triassic succession, which is sub‐divided into six seismic stratigraphic sequences, delineated by erosional‐ and non‐depositional surfaces. The definition of seismic stratigraphic sequences is based on seismic facies trends and Gamma Ray log signatures. Time‐thickness maps combined with geometries in cross‐section display the depocentre development. During the Permian‐Triassic, the Horda Platform experienced faulting, shaping the Caledonian pre‐rift landscape into a series of N–S trending half‐graben basins, contemporaneous to a gradual climate change from arid in the Permian to humid in the latest Triassic. The basin underwent three phases of rifting during the Late Permian‐Middle Triassic: (1) disconnected heterogeneous depocentres with strain concentrated in the west; (2) depocentres expanded northward; and (3) mature half graben development with widely distributed strain. Vertical lithological changes in mudstone/sandstone dominance reflect varying rates of accommodation and sediment supply (A/S). Systematic A/S variation reflects strong climatic fluctuations, which controlled facies patterns during both tectonic quiescence in the Middle‐Late Triassic, and during syn‐rift sedimentation. The Permian‐Triassic tectono‐sedimentary development in the Horda Platform area provides valuable lessons on the influence of faulting on depocentre development, how the interplay between tectonic and climatic forcing is expressed in subsurface continental deposits, and aid the characterisation of reservoirs.

Fault throw gradients create transverse folding, and this can influence accommodation creation and sedimentary routing and infill patterns in extensional halfgraben basin. The Fanja half-graben basin (Oman) offers an excellent outcrop of an alluvial fan succession displaying cyclical stacking and basin-scale growthfold patterns. These unique conditions allow for an investigation of fault-timing and accommodation development related to fault-transverse folding. Our study combines geological mapping, structural analysis, sedimentary logging and correlation, and bulk mineralogical compositions. Mapping reveals that the basin is bounded by a regional-scale fault, with local depocentres changing position in response to transverse syncline and anticline development ascribed to faultdisplacement gradients. The alluvial Qahlah Formation (Late Cretaceous) is unconformably overlying the Semail Ophiolite, and is in turn overlain by the marine Jafnayn Formation (Late Palaeocene). Facies and stratigraphic analysis allows for subdivision of the Qahlah Formation into four informal units, from base to top: (i) laterite in topographic depressions of the ophiolite, (ii) greenish pebbly sandstones, deriving from axially draining braided streams deposited in the lowrelief half-graben basin. This green Qahlah grades vertically into the red Qahlah, formed by alluvial fanglomerates and floodplain mudstones, with drainage patterns changing from fault-transverse to fault-parallel with increasing distance to the main fault. The red Qahlah can be divided into (iii) the Wadi al Theepa member, found in a western basin depocentre, with higher immaturity and sand: mud ratio, suggesting a more proximal source, and (iv) the Al Batah member, located in the eastern part of the basin. The latter shows better sorting, a lower sand: mud ratio, and more prominent graded sub-units. It also shows eastward expansion from an orthogonal monocline, ascribed to accommodation developed in a relay EAGE WÜRTZEN et al.

Discrepancies in models of continental rift-basin dynamics and stratigraphic response calls on further investigation on the subject. Geometric- and lithological trends between stages of faulting is studied in the Permian- Triassic continental rift succession in the Horda Platform. The Horda Platform occupies the northeastern margin of the North Sea aulacogen where Late Permian-Early Triassic faulting shaped the Caledonian pre-rift landscape into a series of N-S trending half-graben basins, filled by Permian-Triassic strata. A tectonostratigraphic model developed from seismic- and well-data details the Permian-Triassic basin fill and structural basin development. Regional unconformities mark the top and base of the succession, while internally, six depositional sequences are delineated by erosional- and transgressive surfaces. Thickness maps reveal three syn-rift stages, where strain migrated and concentrated in different parts of the developing rift basin, from disconnected faults and scattered depocentres with varying accommodation space hosting deposits of different thicknesses, to fully linked faults bounding half-graben basins with expanded and connected depocentres. The lithology through the syn-rift stages reflect how sedimentation gradually outpace accommodation space creation. The overlying thick post-rift succession shows no evidence of rifting besides a minor fault-displacement along the Øygarden Fault Zone in the Late Triassic, reflected by slight wedging. Meanwhile distinct interchanging sand- and mud-dominated intervals reflect strong climatic fluctuations during the post-rift. The spatio-temporal heterogeneity in fluvial facies makes stratigraphic correlation challenging and is further complicated by discontinuous surfaces caused by basin floor tilting. With this study, classical sequence stratigraphic models are evaluated and revised against the observations from the Horda Platform. The stratigraphic model presented takes into account the dynamics of strain along faults through time and space and the resulting diachronic boundaries between the rift stages.

Oil and gas production of tight chalk reservoirs frequently rely on the presence of natural fractures, which increases the effective permeability of the reservoirs. Fracture characterization is therefore imperative in optimizing production schemes and obtaining economically viable recovery factors. Subsurface fracture characterization is often deemed challenging as the available data is typically of varying age and quality, and represents different scales.We have developed an integrated workflow for fracture characterization in chalk to address these challenges. The workflow is based on data from borehole images, cores and seismic. These data are typically available for most chalk (and hydrocarbon) fields. The interpreted borehole image dataset contains over 17 000 manual dip picks, ensuring a statistically viable base. A total of 150m of core is available from 3 wells. The applied 3D seismic cube covers a 8 x 5km hydrocarbon chalk field in the Danish North Sea.

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For the first time, this study presents interpretations of alluvial fans in spectral decomposition RGB Blends, analysed in seismic time slices from the Middle Triassic to Lower Jurassic stratigraphical interval of the Horda Platform (northern North Sea). The time slices record a shifting alluvial fan front, fluvial variability, uplift and erosion and reveals depositional elements that may set the common conception of the geological development of this area during the Early–Middle Mesozoic up for discussion. Results show that the Upper Triassic Lunde Formation in the eastern margin of the Horda Platform was characterized by the deposition of coalesced alluvial fans. A variable extent of the fan front through the Upper Triassic is linked to interplaying allogenic factors: uplifted source areas in the aftermath of Early–Middle Triassic rifting determined sediment availability; climate transitioning from arid to semi‐humid, with increasingly fluctuating precipitation, controlled sedimentation, and run‐off; provenance dictated bulk sedimentology and affected prevailing alluvial processes. An overall retreat of the fan system through the Late Triassic coincided with a significant change in landscape characteristics at the transition into the overlying Statfjord Group. Uplift and initial tilting of the Horda Platform caused landscape degradation and the formation of plateaus and incised valleys, contemporaneous with increased humidity and marine transgression, forming estuaries. The shift in depositional style has implications for reservoir properties, creating complexity and heterogeneity in terms of facies distribution and connectivity, which may benefit potential CO2 storage. Upper Triassic alluvial fan development in the Horda Platform (northern North Sea) depicted through spectral decomposition of seismic time slices Enhanced level of seismic stratigraphic interpretation with RGB blending Impact of allogenic factors on alluvial depositional development Implications of alluvial variability on reservoir properties

<p>The Triassic continental succession located in the eastern side of the Horda Platform (northern North Sea) represents a potential supplementary CO<sub>2</sub> storage formation to the principal Jurassic sandstones above. Several 1-2.5 kilometer-thick packages of Triassic sediment lie within a series of large eastward-dipping half-grabens east of the Viking Graben. The deeply buried Triassic deposits (1.8-3 km) within the prospective area are confined between the Øygarden Fault Zone to the east and the Vette Fault Zone to the west, the latter separating the prospective area from the Troll hydrocarbon fields. Despite extensive petroleum exploration within the Horda Platform, the entire Triassic interval remains largely untested and its storage potential poorly understood. The lack of wellbore penetrations and 3D seismic coverage means that reservoir quality can only be assessed using conceptual predictions and analogue studies.</p><p>A seismic stratigraphic model is built using available 2D and 3D seismic and integrated well log data to discern the Triassic basin fill history and structural development of the area. The stratigraphic succession is subdivided into seismic facies, where reflection patterns infer depositional characteristics. A shift in log facies trend between mud- and sand-rich intervals indicates a variance in subsidence rate and sedimentation supply related to tectonic displacement rate and climate. Visual analyses of the seismic data along key horizons also reveal depositional features such as channels, hanging wall fans, and footwall fans. The location and distribution of the channels are mapped in order to assess the connectivity of possible storage bodies.</p><p>Analogous Triassic sandstone reservoirs of the Snorre field (Tampen Spur) roughly 125 km away are similar in terms of stratigraphic facies and mineralogy. The Snorre field reservoirs are dominantly subarkosic and arkosic sandstones with illite-smectite and chlorite-smectite and lesser amounts of kaolinite and chlorite clay minerals within the matrix and pore spaces. Furthermore, the Triassic succession is composed of sandstones and mudstones deposited in fluvial systems with subordinate alluvial plain environments. The sandstones possess highly variable poro-perm values closely related to the depositional facies. As the Horda Platform sediments were deposited more proximal to the source than those of the Tampen Spur, it is expected that coarser grain sizes persist within the prospective interval, but perhaps at a lower degree of maturity and higher grain size variability. Overall, this preliminary assessment suggests that thick Triassic channel sandstones present in the eastern Horda Platform have promising CCS potential.</p><p> </p><p><strong>Keywords</strong>. Triassic, Horda Platform, CCS, basin fill history, seismic facies, tectonic displacement, climate, reservoir quality</p>