Ken Hitchen

Aspects of the Cenozoic deformational history of the Northeast Faroe–Shetland Basin, Wyville–Thomson Ridge and Hatton Bank areas

The nature and age of the Cenozoic compressional/transpressional deformation within the NE Faroe–Shetland Basin, the Wyville–Thomson Ridge and Hatton Bank areas have been investigated, primarily using seismic reflection data. In all three areas, the folds reach approximately 2 to 4k min amplitude and 40k min wavelength. Early and mid-Eocene compressional/transpressional deformation affected the Hatton Bank and Wyville–Thomson Ridge areas, and folding was locally active even earlier, during Paleocene/Cretaceous times. However, the main Cenozoic compressional/transpressional tectonism that affected the Hatton Bank area was coeval with development of the regional Late Eocene Unconformity (C30), and with changes in spreading geometries and a phase of accelerated subsidence in the Rockall Basin. Within the NE Atlantic margin, WNW-to NW-trending lineaments/transfer zones and associated oceanic fracture zones facilitate significant structural segmentation. Offsets in the continent–ocean boundary along Hatton Bank probably reflect inherited basin architecture, and many Cenozoic folds in the Hatton Bank, Wyville–Thomson Ridge and NE Faroe–Shetland Basin areas are considered to mainly reflect compressional buttressing against pre-existing structures. However, relatively small lateral displacements probably occurred along some reactivated transfer zones following continental break-up. Paleocene–Eocene compressional/transpressional deformation may have affected parts of the Faroe–Shetland Basin, but seismic resolution of this is largely masked by pervasive polygonal faulting. Significant, early to mid-Miocene compressional/transpressional deformation is recorded in the NE Faroe–Shetland Basin, and may also have exerted a major influence on the Wyville–Thomson Ridge and surrounding area. In particular, mid-Miocene growth of the Faroe Bank Channel syncline may have resulted in major changes in northern hemisphere deep-ocean circulation with associated impact on global climate. Compressional/transpressional deformation appears to have continued into Pliocene– ?Recent times and resulted in the development of features such as the Pilot Whale Anticline and associated mud volcanoes/diapirs.

Late Cretaceous basalts from Rosemary Bank, Northern Rockall Trough

Abstract A borehole drilled on the top of Rosemary Bank (Rockall Trough, NE Atlantic), recovered 16.72m of basalts and volcaniclastic sediments beneath a 1.53m thick limestone with basalt clasts, biostratigraphically dated as late Maastrichtian. Magnetostratigraphic data, constrained by the biostratigraphy, indicate extrusion of Rosemary Bank basalts took place during magnetochron C31R (71–69 Ma), or possibly earlier. This provides definite proof of pre-Tertiary volcanism in the Rockall Trough, and indicates that the onset of activity in the North Atlantic Igneous Province took place at least 7 Ma earlier than the postulated arrival of the Icelandic plume at 62 Ma. Geochemical and isotopic data suggest there was an enriched component to the magmatic source. Present data cannot resolve whether the Rosemary Bank basalts are an early manifestation of the Icelandic plume, whether they were associated with a different plume, or whether they were derived from enriched upper mantle without a significant thermal anomaly.

Evaluation of the CO2 Storage Capacity of the Captain Sandstone Formation

The volume of CO2 that can be stored in the Captain Sandstone formation in the North Sea was investigated by building a geological model and performing numerical simulations. These simulations were also used to calculate the best position for the injection wells, and the migration and ultimate fate of the CO2. The overall migration of CO2 and the pressure response over the entire formation was studied by the calculated injection of 15 million tonnes CO2 per year. The injection rate was restricted to a maximum of 2.5 million tonnes CO2 per year for each of a possible 15 wells considered. An important objective was to predict how to avoid flow of the injected CO2 toward potential leakage points, such as the sandstone boundaries and faults. The migration of injected CO2 towards existing oil and gas fields was also a determining factor. The summary conclusions are: - The Captain Sandstone formation has significant potential CO2 storage capacity. Even with all boundaries closed to flow, the probable storage capacity is calculated to be about 358 million tonnes, giving a storage efficiency of 0.6% of pore volume, with an expected operating life-span of 15-25 years. - The possible storage capacity of the formation may be at least four times greater if the aquifer boundaries are open. This increase would be a result of displacement of salt water, and not CO2. - The storage capacity if the sandstone is closed to flow may be increase from 358 to 1668 million tonnes of CO2 by significant additional investment in 15 to 20 water production wells. - Injection of up to 2.5 million tonnes CO2 per year in one well has an impact on the pressure throughout the entire formation, and thus interference between different injection locations must be considered.

Timing, controls and consequences of compression in the Rockall-Faroe area of the NE Atlantic Margin

Abstract The simplest models of passive margins would suggest that they are characterized by tectonic quiescence as they experienced gentle thermal subsidence following the extensional events that originally formed them. Analysis of newly acquired and pre-existing 2D seismic data from the Rockall Plateau to the Faroe Shelf, however, has confirmed that the NE Atlantic Margin was the site of significant active deformation. Seismic data have revealed the presence of numerous compression-related Cenozoic folds, such as the Hatton Bank, Alpin, Ymir Ridge and Wyville–Thomson Ridge Anticlines. The distribution, timing of formation and nature of these structures have provided new insights into the controls and effects of contractional deformation in the region. Growth of these compressional features occurred in five main phases: Thanetian, late Ypresian, late Lutetian, Late Eocene (C30) and Early Oligocene. Compression has been linked to hotspot-influenced ridge push, far-field Alpine and Pyrenean compression, asthenospheric upwelling and associated depth-dependent stretching. Regional studies make it clear that compression can have a profound effect on seabed bathymetry and consequent bottom-water current activity. Bottom-water currents have directly formed the early Late Oligocene, late Early Miocene (C20), Late Miocene–Early Pliocene, and late Early Pliocene (C10) unconformities. The present-day Norwegian Sea Overflow (NSO) from the Faroe–Shetland Channel into the Rockall Trough is restricted by the Wyville–Ymir Ridge Complex, and takes place via the syncline (Auðhumla Basin) between the two ridges. The Auðhumla Basin Syncline is now thought to have controlled the path of the NSO into the Rockall Trough and the resulting unconformity formation and sedimentation therein, no later than the Mid Miocene.

Discussion on the location and history of the Walls Boundary fault and Moine thrust north and south of Shetland

J. D. Ritchie & K. Hitchen write: Flinn (1992) provides a comprehensive review of the published information on the location and history of the Walls Boundary Fault and its relationship to the Caledonian Front, which he equates with the Moine Thrust. However there are serious flaws and omissions in his arguments which invalidate his conclusions about the location of the Walls Boundary fault and Moine thrust north of Shetland. Walls Boundary fault. Flinn (1992) proposes that the fault is associated with Bouguer gravity anomaly ‘lows’ and magnetic negative anomaly ‘valleys’ and hence, north of Shetland, must trend northeastwards. This argument is unsound for the following reasons. (1) Gravity ‘lows’ do not necessarily mark the position of faults. If rocks of equal density are juxtaposed across a fault then no anomaly will be caused. (2) If, according to Flinn’s logic, the fault is aligned through gravity ‘lows’, why does this not apply south of Shetland where his fig. 2A shows the fault to miss completely a large –20 mgal low west of the southern tip of the Shetland mainland? This low is actually caused by the West Fair Isle Basin, whose eastern margin is the Walls Boundary fault (BGS Shetland solid geology map). (3) Flinn (1992, his fig. 2A) uses the published British Geological Survey (BGS) Bouguer gravity anomaly map, with selectively chosen contours, to trace the Walls Boundary fault north of Shetland. However, the ‘lows’ which Flinn claims to mark the location of the fault are actually caused by the

Using converted shear‐wave for imaging beneath basalt in deep water plays

We assess the feasibility of using Sub-Basaltic arrivals due to local Conversions (SBCs) to image structure beneath high-velocity basalt with both synthetic and real data. Detailed numerical modelling shows that in areas with water depths up to 1400m, for a 200m-thick basalt buried about 1km below the sea bed, a two-boat acquisition geometry with o set up to 12km may be used for optimum imaging based on the SBCs. Real data processing of a test line from the North Atlantic Margin reveals that the PP event from the top of the basalt, the PP and the SBC events from the bottom of the basalt form a triplet on the contour velocity spectrum. This feature can help in velocity analysis for SBC arrivals. Both the bottom of the basalt and the basement can be identi ed more con dently in the nal stacking results based on the SBCs, and the thickness of the basalt is determined to be approx 200m. The processing of the SBCs also con rms the presence of sediments beneath the basalt.

Prestack depth imaging via model-independent stacking

Most seismic reflection imaging methods are confronted with the difficulty of accurately knowing input velocity information. To eliminate this, we develop a special prestack depth migration technique which avoids the necessity of constructing a macro-velocity model. It is based upon the weighted Kirchhoff-type migration formula expressed in terms of model-independent stacking velocity and arrival angle. This formula is applied to synthetic sub-basaltic data. Numerical results show that the method can be used to successfully image beneath basalts.