Peter N. Johannessen

Major coastal impact induced by a 1000-year storm event

Extreme storms and storm surges may induce major changes along sandy barrier coastlines, potentially causing substantial environmental and economic damage. We show that the most destructive storm (the 1634 AD storm) documented for the northern Wadden Sea within the last thousand years both caused permanent barrier breaching and initiated accumulation of up to several metres of marine sand. An aggradational storm shoal and a prograding shoreface sand unit having thicknesses of up to 8 m and 5 m respectively were deposited as a result of the storm and during the subsequent 30 to 40 years long healing phase, on the eroded shoreface. Our results demonstrate that millennial-scale storms can induce large-scale and long-term changes on barrier coastlines and shorefaces, and that coastal changes assumed to take place over centuries or even millennia may occur in association with and be triggered by a single extreme storm event.

Divergent development of two neighbouring basins following the jurassic north sea doming event: the danish central graben and the Norwegian-Danish Basin

The two neighbouring basins, the Danish Central Graben and the Norwegian-Danish Basin were both affected by the regional uplift of the North Sea and adjacent areas in the early Middle Jurassic that caused the formation of a regionally extensive unconformity. The uplifted area was not a simple dome structure but of a more irregular shape with an east-west oriented branch that included the Ringkobing-Fyn High and much of the Norwegian-Danish Basin and the Fennoscandian Border Zone. Late Aalenian-Bajocian deposition was confined to fault-controlled depocentres in both the Danish Central Graben and the Norwegian-Danish Basin. An initial southward slope in the Danish Central Graben changed to a north- to eastward slope before the end of the Middle Jurassic, and the change possibly coincides with the formation of a conspicuous sequence boundary in the Bathonian. The depositional area began to expand in the late Middle Jurassic as a result of a regional sea-level rise. In the Danish Central Graben, accelerating half-graben subsidence during the Callovian-Early Kimmeridgian enhanced the sea-level rise. Several periods of rapid subsidence during the Callovian-Volgian (mainly in the Oxfordian-Early Kimmeridgian and latest Kimmeridgian-Middle Volgian) gave accommodation space to more than four kilometres of marine mud. A break in subsidence in the late Kimmeridgian, probably related to a change of fault directions, resulted in deposition of shallow marine sandstones on platforms and hanging-wall slopes. The Norwegian-Danish Basin was characterised by a small rate of subsidence and continuous expansion of the depositional area throughout the Late Jurassic. The slow subsidence and a large supply of sediment from the Fennoscandian Border Zone caused repeated progradational events from the northeast. Hydrocarbon discoveries are known only from the Danish Central Graben where Middle Jurassic and Upper Jurassic reservoirs have been charged from Upper and to a smaller degree Middle Jurassic source rocks. Within the Norwegian-Danish Basin, reservoir rocks are abundant in the Upper Triassic-lowermost Jurassic, the Middle Jurassic and Upper Jurassic successions. The presence of mature source rocks, however, is the main risk factor as they most likely only occur within Lower Jurassic mudstones deeply buried in rim-synclines and in local grabens.

Geological Characterization of CO2 Storage Sites: Lessons from Sleipner, Northern North Sea

Publisher Summary This chapter draws some generic conclusions on reservoir characterization based on the Sleipner operation where CO 2 is being injected into the Utsira Sand. Regional mapping and petrophysical characterization of the reservoir, based on 2D seismic and well data, enable gross storage potential to be evaluated. Site specific injection studies, however, require precision depth mapping based on 3D seismic data and detailed knowledge of reservoir stratigraphy. Stratigraphical and structural permeability barriers, which are difficult to detect prior to CO 2 injection, can radically affect CO 2 migration within the aquifer. The Sleipner sequestration operation is the focus of the SACS (Saline Aquifer CO 2 Storage) project, whose aims include monitoring and modeling the fate of the injected CO 2 and regional characterization of the Utsira reservoir and its caprock. This chapter describes some of the results of the investigations and draws out some generic aspects of geological reservoir characterization, which are particularly applicable to CO 2 injection into flat-lying aquifers of regional extent.

Accretionary, forced regressive shoreface sands of the Holocene-Recent Skagen Odde spit complex, Denmark — a possible outcrop analogue to fault-attached shoreface sandstone reservoirs

Spit systems are often important components of Recent deltas and coastal plains, where waves redistribute fluvial-derived sediments and they show large variations in terms of size, volume and grain size. However, spit systems are likely to form along any wave-influenced coastline and may form where faulted terrains of poorly to moderately lithified rocks are exposed to wave erosion. In such areas large spit systems are likely to be formed down-current from fault escarpments, along fault crests and on fault ramps, but appear to have been overlooked in the geological record. The Holocene-Recent Skagen Odde spit complex has developed within the last 8000 years and is still growing. The most important factors that have governed its growth are well-known, including climate, sea-level changes, palaeo-relief, free fetch, tidal range, progradation and sediment transport rates. The spit system has developed down-current from a steep slope comparable to a fault escarpment. It shows a triangular form with sides of 30-40 km and it contains 5-10 km 3 of sand. The main part of the spit system consists of five sedimentary units that constitute a shallowing-upward shoreface succession: (lower) storm sand; bar-trough sand; beach sand; peat and (upper) modern aeolian sand. The succession was formed during a forced regression as uplift outpaced eustatic rise. As the average basin-floor gradient has been steeper than the shoreline trajectory, the accommodation increased during basinward progradation of the spit system, and the resulting succession represents an accretionary, forced regression. The spit-system model may serve as an analogue to ancient fault-related shoreface sandstones without associated fluvial or deltaic deposits.

Tsunami propagation over a wide, shallow continental shelf caused by the Storegga slide, southeastern North Sea, Denmark

The large Storegga slide, which occurred on the Norwegian Atlantic shelf ∼8150 yr ago, triggered a tsunami that has been identified in sediment deposits along the coasts of Greenland, Norway, the Faroe Islands, the Shetland Islands, Scotland, and the northernmost coasts of England, but hitherto not along the southeastern shores of the North Sea. It has generally been assumed that the shallow continental shelf of the North Sea attenuated and dissipated the energy of the tsunami before it reached those coastlines. We used radiocarbon and optically stimulated luminescence dating as well as stratigraphic, lithologic, chemical, and palynological analyses of sediment cores to identify tsunami deposits on the barrier island of Romo located on the southwestern North Sea coast of Denmark. We show that tsunami sediments were deposited in a freshwater paleolake that is located ∼16 m below present-day mean sea level. The tsunami sediment run-up was between 1.5 m and 5.5 m above the contemporaneous sea level. Our results demonstrate that the Storegga slide tsunami propagated across the wide (>500 km) and relatively shallow (depth <95 m) continental shelf of the North Sea and resulted in run-up along adjacent coastlines. In contradiction to earlier theoretical studies, the coastline of the southeastern North Sea cannot be regarded as being sheltered from impacts of North Atlantic tsunami incidents.

Architecture of an Upper Jurassic barrier island sandstone reservoir, Danish Central Graben: implications of a Holocene-Recent analogue from the Wadden Sea

Abstract An unusually thick ( c . 88 m), transgressive barrier island and shoreface sandstone succession characterizes the Upper Jurassic Heno Formation reservoir of the Freja oil field situated on the boundary of Denmark and Norway. The development and preservation of such thick transgressive barrier island sands is puzzling since a barrier island typically migrates landwards during transgression and only a thin succession of back-barrier and shoreface sands is preserved. Investigation of the development and geometry of the Freja reservoir sandstones is problematic since the reservoir is buried c. 5 km and seismic resolution is inadequate for architectural analysis. Description of the reservoir sandstone bodies is thus based on sedimentological interpretation and correlation of seven wells, of which five were cored. Palaeotopography played a major role in the position and preservation of the thick reservoir sandstones. Using the nearest maximum flooding surface above the reservoir as a datum for well-log correlations, the base of the barrier island succession in the wells is reconstructed as a surface with steep, seaward-dipping palaeotopography. The relief is c . 270 m over a distance of c . 8 km and dips WNW. As a complementary approach to investigation of the reservoir architecture, a Holocene–Recent barrier island system in the Danish part of the NW European Wadden Sea has been studied and used as an analogue. The barrier island of Romo developed during a relative sea-level rise of c. 15 m during the last c. 8000 years and is up to 20 m thick. To unravel the internal 3D facies architecture of the island, an extensive ground penetrating radar (GPR) survey of 35 km line length and seven cores, c. 25 m long, was obtained. Although the barrier island experienced a rapid relative sea-level rise, sedimentation kept pace such that the island aggraded and even prograded seawards and became wider and longer due to the large surplus of sand.

Regional Study of the Neogene Deposits in the Southern Viking Graben Area - a Site for Potential CO2 Storage

Since 1996 Statoil has injected CO2 separated from gas of the Sleipner Vest field into saline reservoir sands of the Utsira Forrnation in the central North Sea at a depth of approximately 900 m. This is the first case of industrial scale CO2 storage in the world (1 million tons per year). The Saline Aquifer CO2 Storage (SACS) project is conducted by a consortium of oil companies and research institutions. This paper presents regional geological interpretation in the southem Viking Graben area. Sands of the Mio-Pliocene Utsira Formation and the Nordland Shale have been identified and their stratigraphical architecture outlined from seismic sections and well-logs. Detailed geophysical and geological aspects at the injection site are treated in Arts et al. (this volume) and Brevik et al. (this volume).