Dissertations / Theses on the topic 'Shetland Basin'

Seismic stratigraphic analyses of the late Palaeocene-Present transitional to post-rift succession in the Faroe-Shetland Basin and Faroe Graben (FSC) on the NE Atlantic volcanic passive continental margin have revealed the occurrence of a early Eocene dendritic palaeo-drainage system and Middle Eocene-Miocene contractional inversion structures. The palaeo-drainage system consists of a significant NNW-SSE trending distributary channel (40km long, 5km wide and up to 400m deep), fed by numerous tributaries (100m deep). The drainage system incised into a major delta system (Colsay Sandstone Member) and was subsequently unfilled and draped by estuarine deposits (Hildasay Sandstone Member and Balder Formation). The excellent preservation of the palaeo-valleys indicates that uplift, incision and subsequent infilling of the drainage system occurred relatively rapid (biostratigraphically constrained to 1.45 My). The uplifting responsible for the incision event at c. 54.75 Ma, earliest Ypresian) was widespread and extends as far as the North Sea (Bressay area) and SE England (London Basin). Furthermore, coeval volcanic activity is consistent with the drainage system having resulted from transient uplift driven by a mantle-plume. This transient uplift event (incision and infill) in the FSC provides important new evidence for the evolution of the ancestral Iceland mantle plume and its influence on stratigraphic development. The inversion structures, developed in Middle Eocene, Oligocene and Middle Miocene, are marked by folding with the syn/post inversion stratigraphy onlapping and thinning over the structures. The location and orientation of the inversion structures suggest that the underlying Mesozoic structural configuration, especially the NW-SE transfer zones, influenced their development. The timing and nature of the movement of the inversion structures in the FSC provide new temporal constraints which help to understand better the controlling mechanism of passive continental margins.

The investigation of the role of basin-scale forcing on the circulation of the Faroe-Shetland Channel (FSC) is important to further understanding of the inter-annual variability of the Atlantic water (AW) fluxes in this region. The FSC plays a key role in the transfer of warm and saline AW towards the Nordic Seas that is an integral part of the Atlantic Meridional Overturning Circulation which is projected to decline over the twenty-first century and might reduce the oceanic heat and salt transports towards the Arctic. So far little attention has been paid to the mechanisms driving the AW fluxes in the FSC, reliable estimates of AW temperature and salt transports time series are lacking. This study presents a new time series of the AW fluxes based on the combination of hydrography and altimetry data. The mechanisms involved in driving the variability of AW fluxes are considered based on observational data and the output from a high-resolution ocean model (VIKING20). The hydrographic observations from 1993 to 2015 show an increase in temperature and salinity of AW. However, there is no evidence of trends in AW volume, temperature or salt transports during the observed period. This analysis confirms that the amount of heat and salt transported through the FSC is dominated by the volume transport. Moreover, this study identifies a bias in the standard deviation of the geostrophic velocity at a depth associated with referencing the geostrophic calculations to the sea surface geostrophic velocity from satellite altimetry. This finding does not strongly influence the AW volume transports in the AW layer, however, it has important implications for estimates of the geostrophic volume transport at depth. This study shows that the Ekman driven up/downwelling and the differential Ekman pumping mechanisms driven by the local wind forcing may influence sea surface height (SSH) and the displacement of isopycnals in the channel, leading to AW volume transport variabilit However, due to the large associated error bars on the surface and subsurface parameters, there is no clear evidence that these mechanisms are significantly responsible for the AW volume transport variability in the FSC. Lagrangian trajectories show evidence of two pathways from the North Atlantic to the FSC that may explain AW variability in the FSC: one pathway involves the flow of warm and saline waters from the Rockall Trough that corresponds to high temperatures and low AW volume transport in the channel, and the other pathway involves the flow of relatively cooler and less saline waters from the Iceland Basin that is linked to low temperatures and stronger volume transport in the FSC. Moreover, we show that the first (second) pathway is associated with the negative (positive) phases of the North Atlantic Oscillation (NAO) and the ocean gyre contraction (expansion). The changes of the NAO index phases explain 26 % of the AW volume transport variance in the FSC. Another important mechanism that leads to stronger (weaker) AW volume transport is stronger (weaker) pressure gradient across the Greenland-Scotland Ridge, reflected by the SSH changes. This mechanism explains 29 % of AW volume transport variance in the FSC.

A detailed investigation of the Eocene stratigraphy in the Faroe-Shetland Basin was undertaken with a view to developing a basin-wide seismic-stratigraphic framework in order to describe the palaeogeographic evolution and depositional architecture of the basin. This study has tested the applicability of sequence stratigraphy on a regional and local scale and outlines the major depositional controls on the succession. Integration of seismic, well and biostratigraphic data has allowed for the identification of four regionally correlatable seismic-stratigraphic units which document the basin fill history. Depositional systems are controlled by the interaction of fluctuating relative sea-level and local tectonic controls. Early Eocene uplift on compressional fold structures, inherited Mesozoic palaeobathymetry, dynamic uplift from mantle plume activity and prograding lava deltas all control the distribution, thickness and facies of the Eocene succession. Case studies from a basin margin and intra-basinal setting have provided detailed evidence of the localised sedimentary response to changing basin conditions. The use of high resolution 3-D seismic data has enabled depositional sequences and the complex seismic geomorphology of palaeo-drainage systems to be recognised. A cyclicity of sedimentary response is observed in a deltaic environment which documents the evolution of relative sea-level on the southern margin of the basin. Classical sequence-stratigraphic techniques have, in places, provided a useful guide to stratigraphic interpretation and analysis. However, attempts to test the widely used sequence stratigraphic model of sequences as regionally correlatable stratigraphic surfaces have failed largely because of the impracticability of correlating seismic reflections on a regional scale. The conclusion from this is that sequence stratigraphy is a powerful analytical tool that can be applied locally, but is likely to encounter significant difficulties on a basinal length scale. This larger scale correlatability is a corollary of the link between sequence development and eustatic sea level changes. In this thesis, this link cannot be substantiated, and local factors predominate.

This study has explored the phenomenon of basin-wide sand intrusion. Two case studies from regions with very different tectonic and depositional histories, host rock successions and intrusion geometries have been used to examine sand intrusion in the subsurface. Striking first-order similarities between the two contrasting case studies are apparent 1) the basin-wide nature of the sand intrusion event 2) the strong temporal link between the intrusion of sand and major tectonic events within the basin. It therefore appears likely that sand remobilisation in the subsurface and subsequent sand intrusion was triggered by earthquake activity. The case studies are presented as four papers produced for publication, supported by a review of previous work and a discussion of the case study results. A broad-based approach using a range of geological techniques was used to examine problems in our understanding associated with basin-wide fluid escape and sand intrusion. The first of the two case study areas consists of a region (in excess of 10,000 km<super>2 </super>) of large-scale conical sandstone intrusions hosted within a deep-water Eocene-Oligocene succession consisting of claystone and bio-siliceous ooze in Tranche 6 of the Faroe-Shetland Basin. The emplacement of the conical sandstone intrusions occurred during the Late Miocene, coincident with a phase of basin inversion. Polygonal faults were exploited as conduits to feed the conical intrusions, however intrusion propagation occurred along new fractures rather than exploiting the pre-existing polygonal fault network. The second case study is a region of sandstone pipes hosted within the sand-dominated rocks of the Middle Jurassic Carmel Formation and Entrada Sandstone in SE Utah. Pipes formed before the deposition of the Middle Jurassic Cannonvillle Member of the Entrada Sandstone across the region. Pipe formation was coincident with increased tectonic shortening and the inception of eastward tilting of the basin due to uplift to the west. Fluid-flow velocity during pipe formation is constrained to around 1.6 cms"1 based on calculations of settling velocity of grains present within the pipe fill.

Geochemical analysis of the volcanic interval in Well 214/4-1, Faeroe-Shetland Basin, has enabled a correlation to the Lower Basalt Formation of the Faeroe Islands, ca. 240 km to the W. The volcanic interval consists of a ca. 450 m thick sequence of hyaloclastites, which are overlain by a ca. 50 m thick subaerial lava sequence. This volcanic interval is interpreted to have formed at a palaeoshoreline environment, where subaerial lavas flowed from the land surface into a substantial body of water at least 450 m deep (i.e. the Faeroe-Shetland Basin at that time), resulting in the quenching and fragmentation of magma to product the hyaloclastities. Well 214/4-1 is <50 km to the SE of the Faeroe-Shetland Escarpment, which has previously been interpreted as a hyaloclastite delta, thus implying that there a number of unrecognised hyaloclastite units within the Faeroe-Shetland Basin and that the coastline was steadily encroaching W/NW, towards the Faeroe Islands during the volcanic interval. The overlying ca. 10 m thick Coal-bearing Formation (CBF) represents a significant hiatus in the volcanic activity at the end of LBF times. Erosion and subsidence of the lava field led to the development of an expansive lacustrine environment, which resulted in the accumulation of plant material and associated detritus and chemical sediments, mainly ironstones, and the formation of mineable coal seams. Petrographic and geochemical analysis of siderite spherules within the ironstone beds from two localities on Suðuroy have helped to define margin- and centre of-lake environments, at least 10 km apart. Contemporaneous fluviatile lithologies in West Suðuroy are composed of reworked palagonitised tephra, basalt lava clasts and plant material.

The Middle Jurassic Brent Group was deposited within northward prograding wave dominated delta system which developed in the North Viking Graben-East Shetland Basin. Five formations are readily distinguished in core and on wireline logs, each representing a distinctly separate depositional environment. Poorly sorted, subarkosic sandstones comprise the Broom Formation at the base of the Brent Group and are interpreted as having been deposited within an alluvial to shallow marine fan delta system, from proximal to distal gravity flows. In contrast to other formations of the Brent Group, the Broom Formation thins towards the centre of the Viking Graben and its rarely of reservoir quality. The Rannoch Formation comprises of a series of stacked upwards coarsening very fine to fine grained micaceous sandstones which in the north of the Brent Province are underlain by a thick sequence of prodelta shales. Planar laminated and hummocky cross stratified sandstones of the Upper Rannoch Formation were deposited within a storm dominated shoreface environment during a period of rapid delta progradation. Shoreface sandstones of the Rannoch Formation form laterally continuous sheet sandbodies, and have good reservoir potential. Tidal channel and beach/foreshore sandstones of the Etive Formation form some of the main reservoir intervals within the Brent Group having a clean well sorted nature and good lateral and vertical connectivity. Both the Rannoch and Etive Formations thicken northeastwards towards the northern limits of the Brent Delta. Interbedded sandstones, siltstones and shales of the Ness Formation respresent the establishment of an extensive delta plain across the East Shetland Basin during Upper Brent Group times. Interdistributary bay fill deposits comprise the majority of the Lower Ness Formation and are typically of poor reservoir quality. In contrast, the sand rich Upper Ness Formation is dominated by distributary channel, mouth bar and lagoonal beach shoreline sandstones which form important reservoir units in the Ness Formation. The Tarbert Formation represents the final stages of Brent Group deposition. Facies and thickness variations reflect the gradual retreat and subsequent drowning of the Brent Delta. Reservoir quality is highly variable as a consequence of interdigitation between barrier shoreline and washover sandstones with back barrier lagoonal shales.

Petroleum migration pathways through a basin are determined by the three-dimensional distribution of discontinuous sealing surfaces, which are usually parallel to bedding. The petroleum migrates below the sealing surface taking the structurally most advantageous route. The three-dimensional distribution of migration pathways within the petroleum system can be modelled on a personal computer using a program based on the parameters developed during the research summarised in this thesis. Application of the model to the Paris, Williston, West of Shetlands and Wessex Basins demonstrates that a good correlation can be made between predicted pathways and discovered accumulations using simple models. Migration pathways form a dense network overlying hydrocarbon generating areas in the central parts of basins. Towards the basin margins they commonly become increasingly focused into discrete pathways by the sealing-surface morphologies. The Paris and Williston Basin research showed how relatively minor structuring of geological strata can result in a significant focusing of pathways. Eventually these pathways may reach the surface as shown by seepages. Research in the Wessex Basin revealed that reverse modelling of pathways from seeps assists in the prediction of the location of leaking accumulations. Deflection of the pathways from the structurally most advantageous route below the sealing surface may be caused by lateral sealing barriers due to facies variation in the carrier rock below the seal, fault juxtaposition, or cross-formational seals such as salt intrusions. Deflection of pathways also occurs where there are hydrodynamic conditions in response to topography-driven groundwater flow.

The Heidrun field is located in the Halten Terrace of the Mid-Norwegian Continental Shelf and is one of the first giant oil fields found on the Norwegian Sea. Modern 3D seismic reflection data acquired over the field, as well as well data were used to define the key structural and stratigraphic elements within the study area. The basic geologic history of the Heidrun field is typical of most North Sea plays, and includes Triassic rift sequences that are masked by the reactivation of bounding faults that were active during the Jurassic rift phase. This rifting phase was followed by deposition of marine black shales and subsequent carbonaceous shales during the Latest Jurassic to Earliest Cretaceous. The next sequence was characterized by the deposition of Paleocene-Eocene boundary tuffs, which were formed due to volcanism associated with a rifting event that separated Norway and Greenland. Finally, an Eocene to present passive margin marine sequence is dominant over the study area that is mainly composed by glacial deposits. Traditional reservoir intervals within the Heidrun field are located within the Jurassic age inter-rift sequence. However, most recently Cretaceous-age turbidites have been explored in the Norwegian and North Sea as possible targets with some success. These Cretaceous turbidites are traditionally found as basin floor fan deposits within rifted deeps along the Norwegian continental shelf and are believed to be sourced from localized erosion of Jurassic- age rifted highs. Data within our study area revealed the existence of a deep-water Cretaceous age wedge located within the downthrown hanging wall of several smaller half-grabens formed on the Halten Terrace. Seismic attribute extractions taken within this Cretaceous wedge show the presence of several elongate to lobate bodies that seem to cascade over fault-bounded terraces associated with the rifted structures. These high amplitude elongated bodies are interpreted as proximal sedimentary conduits that are time equivalent to the Cretaceous basin floor fans located in more distal portions of the basin to the west. Several wells penetrate the updip, tilted half-graben hanging walls which are believed to be sourcing these turbidite systems. These half graben fills have the potential to contain high quality Cretaceous sandstones that might represent a potential new reservoir interval within the Heidrun field.<br>text