Abstract The Triassic of the Central North Sea is a continental succession that contains prolific hydrocarbon-bearing fluvial sandstone reservoirs stratigraphically partitioned by mudstones. Within the Skagerrak Formation of the UK sector, hydrocarbon accumulations in the Judy, Joanne and Josephine Sandstone members are top sealed by the Julius, Jonathan and Joshua Mudstone members, respectively. However, UK and Norwegian stratigraphic correlations have been problematical for decades, largely due to biostratigraphic challenges but also due to the non-uniqueness of the lithotypes and because the cross-border stratigraphic nomenclature differs and has yet to be rationalized. This study focuses on mudstones rather than sandstones to unify cross-border correlation efforts at a regional scale. The mudstone members have been characterized by integrating sedimentological, petrophysical and geophysical data. The facies are indicative of playa lakes that frequently desiccated and preserved minor anhydrite. These conditions alternated with periods of marshy, palustrine conditions favourable for the formation of dolostones. Regional correlations have detected lateral facies changes in the mudstones which are important for their seismically mappable extents, resulting palaeogeographies and, ultimately, their competency as intraformational top seals. Significant diachroneity is associated with the lithological transitions at sandstone–mudstone member boundaries and although lithostratigraphic surfaces can be used as timelines over short distances (e.g. within a field), they should not be assumed to represent timelines over longer correlation lengths. Palaeoclimatic trends are interpreted and compared to those of adjacent regions to test the extent and impact of climate change as a predictive allogenic forcing factor on sedimentation. Mudstone member deposition occurred as a result of the retreat of large-scale terminal fluvial systems during a return to more arid ‘background’ climatic conditions. The cause of the member-scale climatic cyclicity observed within the Skagerrak Formation may be related to volcanic activity in large igneous provinces which triggered the episodic progradation of fluvial systems.

Abstract A review of recent Triassic research across the Southern Permian Basin area demonstrates the role that high-resolution stratigraphic correlation has in identifying the main controls on sedimentary facies and, subsequently, the distribution of hydrocarbon reservoirs. The depositional and structural evolution of these sedimentary successions was the product of polyphase rifting controlled by antecedent structuration and halokinesis, fluctuating climate, and repeated marine flooding, leading to a wide range of reservoir types in a variety of structural configurations. Triassic hydrocarbon accumulations form an important energy resource across the basin, not only in the established Buntsandstein fairway but also in Rogenstein oolites and Muschelkalk carbonates. In addition, sand-prone sections in the Late Triassic, such as the Schilfsandstein, have the potential to be hydrocarbon reservoirs. Several Triassic intervals are now the focus for developing geothermal projects. A detailed understanding of Triassic reservoir quality and distribution is one of the main keys to efficiently unlocking the geothermal and remaining hydrocarbon potential across the basin.

Abstract The Paleocene–Eocene-aged Sele Formation is developed across the basinal region of the Central North Sea. The section comprises a number of deep-marine fan systems that expanded and contracted across the basin floor in response to relative sea-level changes on the basin margin and fluctuating sediment yield off the Scottish landmass modulated by climate and hinterland uplift. Persistent sediment entry points to the basin resulted in the development of discrete axial and transverse fan fairways with a geometry dictated by an irregular bathymetry sculpted by differential compaction across Mesozoic faults, halokinesis and antecedent fan systems. A high-resolution biostratigraphic framework has allowed the evolution of fan-dispersal systems in response to these effects to be tracked across the basin within four genetic sequences. The proximal parts of the fans comprised channel complexes of low sinuosity, high lateral offset, and low aggradation. The development of these systems in a bathymetrically confined corridor of the Central Graben ( c. 65 km wide), combined with high sediment supply, resulted in the eventual burial of any underlying relief. The behaviour of sand-rich reservoirs in this region is dominated by the permeability contrast between high-quality channel fairways and more heterolithic overbank regions, with the potential for early water breakthrough and aquifer coning in the channel fairways, and unswept volumes in overbank locations. Compartmentalization of compensationally stacked channel bodies occurs locally, with stratigraphic trapping caused by lateral channel pinch-outs, channel-base debrites, mud-rich drapes and abandonment fines. Towards the southern part of Quadrant 22, approximately 150 km down-palaeoflow, the systems became less confined and in this region are dominated by channel–lobe complexes, which continued to interact with an irregular bathymetry controlled by antecedent fans, mass-transport complexes and halokinesis in the form of rising salt diapirs. Reservoirs in this region are inherently stratigraphically compartmentalized by their heterolithic lithology and compensational stacking of lobes, and further complicated by structuration and instability induced by the diapiric or basement structures needed to generate a trapping structure in these settings.

Abstract Fluvial reservoirs are inherently heterogeneous. They typically have a complex connectivity between sandbodies of limited predictability and have highly variable reservoir properties at a range of scales. Attention is typically focused on the connectivity of fluvial channels because this primarily determines the feasibility of hydrocarbon recovery. However, in sand-rich fluvial reservoirs, where connectivity is less of an issue, the internal heterogeneity related to deposition and preservation of the fluvial deposit still results in uneven fluid movement and presents a challenge for prediction of reservoir behavior. The Triassic Skagerrak Formation provides an example of a sand-rich, dryland fluvial reservoir that would, prior to production, be regarded as having no critical issues relating to depositional architecture. The Skagerrak was deposited as widespread, coarsening-upward sheets by terminal fluvial systems extending several hundred kilometers from the basin margins. These sheets are typically playa and splay dominated in the lower part and become increasingly channel dominated upwards. Multistory channel-belt packages at the top of coarsening-upward sheets form the main producing intervals, and were likely to have been the product of mobile, avulsive, multiple channel systems which generated kilometer-scale channel belts. Well-test and pressure data indicate that these channel belts constitute a dual-permeability system consisting of a network of higher-permeability bodies that are variably distributed within a lower-permeability matrix. This behavior is a product of the channel belts comprising discontinuous, coarse-grained thalweg and lower bar bodies embedded in a finer-grained matrix of upper bar and splay sands. Intervals where these coarse-grained facies are well connected form high-permeability pathways which dominate early production, and these tend to be located at the bases of multistory units. However, long-term production rates are ultimately determined by inflow to these depleting pathways from the finer-grained, lower-permeability upper bar and splay facies. Well tests typically encounter boundaries, which reflect the common presence of internal barriers and baffles within the channel belts. These boundaries are composed of abandonment plugs, bar-draping fines, mud-chip conglomerates, and cemented calcrete-clast lags. However, despite the apparent abundance of flow baffles, long-term production indicates that the channel belts are fully connected laterally and that such features are likely to be discontinuous. Whilst the high sand-shale ratio of the Skagerrak would suggest that there should be few problems related to the connectivity of the fluvial sandbodies, the vertical connectivity is considerably reduced as a result of compartmentalizing shales. The origin of these shales is variable. Predictable shale packages of semi-regional extent mark intervals of terminal fluvial contraction, resulting in regional interfingering of fluvial sand sheets and floodbasin fines. In addition, more localized bar-top and floodplain shale remnants which scale with the areal extent of a field introduce more random flow barriers which are less predictable. Despite developments in the conceptual understanding of the facies architecture of such fluvial reservoirs, prediction of reservoir behavior is hindered by a paucity of quantitative and qualitative data on the geometry of preserved fluvial lithosomes with which to construct 3D models of complex bar architectures at a sub-channel-belt scale. This is particularly important when such belts are larger than the field extent, and the heterogeneities which need to be modelled are therefore at a finer hierarchical scale. The prediction of effective permeability of the reservoir requires information on the grain-size architecture within these lithosomes, together with the geometry and distribution of flow-baffling fines drapes and mud-chip lags. However, there are currently insufficient data from good-quality outcrops to be able to fully capture the potential impact ofthe natural variability of this architecture and construct predictive, quantitative models with a range of realistic geometries and properties.

ABSTRACT The Rotliegend Group in the Netherlands provides a depositional record of fluvial, aeolian, and playa interaction within a major Permian dryland basin. Ephemeral fluvial systems drained off the London–Brabant and Rhenish Massifs and flowed northwards towards the Silverpit Formation desert lake, whilst marginal dune fields expanded and contracted in response to changing aridity and fluvial runoff. There are few modern parallels to the scale of the Southern Permian Basin depositional system as a whole, but recent dryland analogues provide a valuable means to understand the depositional processes which locally operated across the basin during the Late Permian. A variety of modern analogues is required to adequately sample the range of climatic conditions that the Rotliegend depositional systems encountered, with examples selected from modern ergs, fluvial and alluvial fans, playa, lacustrine, and saline-lake settings. However, although the long-term allocyclic controls on deposition and preservation of the Rotliegend have long been recognised, the contrast between the diversity of surficial facies seen in modern dryland settings and that preserved in the ancient record suggests that the Rotliegend also failed to preserve much of the expected facies diversity through aeolian deflation and sustained, polycyclic reworking of interacting fluvial, lacustrine, and aeolian systems. Widespread fluvial activity and lacustrine shoreline facies, which form a visible record of relatively recent pluvial episodes in modern basins, have limited preservation potential, and maps of gross facies belts in the Rotliegend are not true palaeogeographic facies arrangements but time-averaged associations of those facies which ultimately entered the stratigraphic record.