Martyn Quinn

The geological evolution of the Falkland Islands continental shelf

Abstract The Falkland Islands are surrounded by four major sedimentary basins: the Falkland Plateau Basin to the east, the South Falkland Basin to the south, the Malvinas Basin to the west, and the North Falkland Basin to the north. The four main basins appear to have formed initially as Triassic through earliest Cretaceous extensional rifts associated with the break-up of Gondwana. A ?Valanginian end to rifting was followed by thermal sag. There is evidence of Cenozoic uplift in at least the North Falkland Basin, possibly coincident with Andean compression and the development of overthrusting along the plate boundary to the south of the islands resulting from opening of the Scotia Sea. There is no evidence from offshore seismic and gravity-magnetic data to support interpretations that the Falkland Islands have rotated clockwise through up to 180° during Gondwana separation. With the exception of the South Falkland Basin all the major basins probably underwent initially, more or less east-west extension, and had a similar orientation to adjacent South American and western southern African basins. The Falkland basins probably shared a similar geological history with the offshore southern African and South American basins.

The effects of Cenozoic compression within the Faroe-Shetland Basin and adjacent areas

Abstract The effects of Cenozoic compression within the Faroe–Shetland Basin and surrounding areas are mainly manifested in the form of growth folds. The scale and orientation of the folds varies significantly, with axial trace lengths ranging between less than 10 to over 250 km and trends including east, NE-, NNE-, ENW-, NNW- and WNW. The NE-trending features are the most numerous, though they are mainly restricted to the NE Faroe–Shetland Basin where an inherited Caledonian structural grain is most prevalent. Limited evidence exists for late Paleocene and early Eocene activity along the Wyville Thomson Ridge, whereas mid–late Eocene and Oligocene fold growth is more common in the SW Faroe–Shetland Basin. Although the effects of well-defined early–mid Miocene deformation appear to be mainly constrained to the NE Faroe–Shetland Basin, this phase of activity is also inferred to have been responsible for major growth of the Wyville Thomson Ridge. Early Pliocene fold growth is observed within the Faroe–Shetland Basin and adjacent areas, with raised seabed profiles over some of the anticlinal features suggesting that the effects of compressional stress continue at the present day. Despite the variation in trend and size of growth folds, there is, we believe, similarity in their local mechanism of emplacement, with buttressing of sedimentary successions against pre-existing basement architecture and igneous intrusions being of particular significance. However, the lack of obvious spatial or temporal pattern to fold growth development within the NE Atlantic margin as a whole mitigates against a single regional driving mechanism being able to explain the current distribution, orientation and timing of the folds.

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.

Lough Neagh: the site of a Cenozoic pull-apart basin

Synopsis The Lough Neagh Basin in Northern Ireland is the site of a Cenozoic depocentre defined by gravity measurements and a thick succession of Paleocene basaltic lavas and Upper Oligocene clays. Much of the Cenozoic outcrop is concealed by Lough Neagh, but the rhombic outline of the lough provides some indication of the underlying structural control of the depocentre. Several authors have inferred that a Cenozoic pull-apart basin lies within the Lough Neagh area and suggest it is one of a number of transtensional basins, including the Bovey and Petrockstow basins in SW England, associated with NW to NNW-trending strike-slip fault zones. Solid geology maps and gravity data show that the structure of the Lough Neagh Basin is dominated by a segmented, orthogonal pattern of offset NNW and NE-trending faults. It is proposed that pull-apart basin formation took place in the Mid-Paleocene by dextral movement on these offset faults. The potential link between strike-slip tectonics and Cenozoic volcanism in the north of Ireland is briefly considered.

Carbon dioxide storage in the Captain Sandstone aquifer: determination of in situ stresses and fault-stability analysis

The Lower Cretaceous Captain Sandstone Member of the Inner Moray Firth has significant potential for the injection and storage of anthropogenic CO2 in saline aquifer parts of the formation. Pre-existing faults constitute a potential risk to storage security owing to the elevated pore pressures likely to result from large-scale fluid injection. Determination of the regional in situ stresses permits mapping of the stress tensor affecting these faults. Either normal or strike-slip faulting conditions are suggested to be prevalent, with the maximum horizontal stress orientated 33°–213°. Slip-tendency analysis indicates that some fault segments are close to being critically stressed under strike-slip stress conditions, with small pore-pressure perturbations of approximately 1.5 MPa potentially causing reactivation of those faults. Greater pore-pressure increases of approximately 5 MPa would be required to reactivate optimally orientated faults under normal faulting or transitional normal/strike-slip faulting conditions at average reservoir depths. The results provide a useful indication of the fault geometries most susceptible to reactivation under current stress conditions. To account for uncertainty in principal stress magnitudes, high differential stresses have been assumed, providing conservative fault-stability estimates. Detailed geological models and data pertaining to pore pressure, rock mechanics and stress will be required to more accurately investigate fault stability.

‘Laramide’ events: Mid North Sea High (UK Quadrants 38 & 39)

The Mid North Sea High is a broad, approximately east-trending structural high that extends from the coast of the Scottish Borders region to the UK North Sea median line. Recently acquired seismic reflection profiles were used to map isopach patterns of the Chalk Group (Upper Cretaceous and lowermost Paleocene), of the combined Montrose and Moray groups (Paleocene and lowermost Eocene), and of the lower Stronsay Group (lower Eocene) in UK Quadrants 38 and 39, on the eastern Mid North Sea High. In an anomalous and undrilled part of this area, the Chalk Group thins over an area broadly centred on a long lived, buried, NNW-trending structural high with a core largely comprised of Devono-Carboniferous sediments. In contrast, the combined Montrose and Moray groups and the lower Stronsay Group sediments thicken over the buried high. We informally name the buried high the α Ridge. Signal processing techniques were applied to the original stack sections to help clarify the geometry of seismic reflections, especially from within the Chalk Group and Palaeogene successions of the anomalous zone, and to interpret deep erosion at the top of the Chalk Group that probably occurred in Danian times. The area of deeply eroded Chalk was subsequently buried beneath an anomalously thick Palaeogene succession. The area of eroded Chalk is more extensive than the α Ridge and extends northwards beyond the survey area. From a regional perspective, the processes that may have controlled the observed depositional pattern are discussed. It is considered whether rapidly changing tectonic stress may have caused local ‘Laramide’ (mid Paleocene–Danian) uplift and erosion of the α Ridge and its immediately surrounding area, swiftly followed by enhanced subsidence (inversion) and deposition in the mid Paleocene (Danian), or whether the stratal pattern is better explained by the infilling of a large erosional hollow; mechanisms involving the local halokinesis and/or dissolution of Zechstein salt are discounted. We conclude that additional seismic and palaeogeographical information is required to fully test the models proposed in this paper.