Gareth A. Williams

Quantitative analysis of time-lapse seismic monitoring data at the Sleipner CO2 storage operation

The CO2 storage operation at Sleipner in the Norwegian North Sea provides an excellent demonstration of the application of time-lapse surface seismic methods to CO2 plume monitoring under favorable conditions. Injection commenced at Sleipner in 1996 with CO2 separated from natural gas being injected into the Utsira Sand, a major saline aquifer of late Cenozoic age. CO2 injection is via a near-horizontal well at a depth of about 1012 m below sea level (bsl) some 200 m below the reservoir top, at a rate approaching 1 million tonnes (Mt) per year, with more than 11 Mt currently stored.

Cenozoic exhumation of the southern British Isles

Rocks that crop out across southern Britain were exhumed from depths of as much as 2.5 km during Cenozoic time. This has been widely attributed to Paleocene regional uplift resulting from igneous underplating related to the Iceland mantle plume. Our compilation of paleothermal and compaction data reveals spatial and temporal patterns of exhumation showing little correspondence with the postulated influence of underplating, instead being dominated by kilometer-scale variations across Cenozoic compressional structures, which in several basins are demonstrably of Neogene age. We propose that crustal compression, due to plate boundary forces transmitted into the plate interior, was the major cause of Cenozoic uplift in southern Britain, witnessing a high strength crust in western Europe.

Inversion and exhumation of the St. George's Channel basin, offshore Wales, UK

The western UK basins of the Irish Sea have provided one of the best natural laboratories for investigating the causes and consequences of intracratonic uplift and erosion (exhumation). To date, the emphasis has been on igneous underplating as the chief process driving their exhumation. In this paper, we demonstrate that tectonic inversion (the shortening of formerly extensional basins and reactivation of their constituent faults) dominated the exhumation of the St. Georges Channel basin, offshore Wales. Based on mapping of an extensive 2D seismic grid, evidence is presented for at least two major inversion episodes in the Late Cretaceous and the Neogene, plus minor shortening during the Eocene. Inversion was distinctly noncoaxial, especially during the Neogene when coeval transpression and transtension was focused at discrete bends and stepovers on the basin-bounding St. Georges, Bala and Northwest Flank faults. That the principal mechanism driving these uplift episodes was inversion (as opposed to igneous underplating) is corroborated by analysis of thermal history data (apatite fission track and vitrinite reflectance). They reveal late Cretaceous and Neogene geothermal gradients that were comparable with that at the present day, i.e. no significant increase in basal heat flow. Sonic velocity profiles logged in hydrocarbon boreholes constrain the minimum thickness of the eroded section, which varies between c. 1000 m in the centre and c. 2240 m at the margins of the basin. Given the strength of evidence for tectonic inversion in the St. Georges Channel basin, our favoured model invokes superimposition of the effects of inversion and igneous underplating to account for the complex exhumation history of the St. Georges Channel basin in particular, and the western UK basins in general.

UK data and analysis for shale gas prospectivity

Abstract Organic-rich shale contains significant amounts of gas held within fractures and micropores and adsorbed onto organic matter. In the USA shale gas extracted from regionally extensive units such as the Barnett Shale currently accounts for 6% of gas production and is likely to reach 30% by 2015. Shale gas prospectivity is controlled by the amount and type of organic matter held in the shale, its thermal maturity, burial history, microporosity and fracture spacing and orientation. Potential targets range in age from Cambrian to the late Jurassic, within the main UK organic-rich black shales: younger shales have been excluded because they have not reached the gas window, but they may possess a biogenic gas play. A geographic information system, showing the distribution of potential reservoir units, has been created combining information on hydrocarbon shows, thermal maturity, fracture orientation, gas composition, and isotope data to identify potentially prospective areas for shale gas. Some of these data are shown as graphs and maps, but crucial data is lacking because earlier exploration concentrated on conventional reservoirs. The prospects include Lower Palaeozoic shale basins on the Midland Microcraton (a high risk because no conventional gas has been proved in this play), Mississippian shales in the Pennine Basin (the best prospect associated with conventional fields and high maturity), Pennsylvanian shales in the Stainmore and Northumberland Basin system (high risk because no conventional gas discoveries exist) and Jurassic shales in Wessex and Weald basins (small conventional fields signify potential here).

Evidence for kilometre-scale Neogene exhumation driven by compressional deformation in the Irish Sea basin system

Abstract Large tracts of the NW European continental shelf and Atlantic margin have experienced kilometre-scale exhumation during the Cenozoic, the timing and causes of which are debated. There is particular uncertainty about the exhumation history of the Irish Sea basin system, Western UK, which has been suggested to be a focal point of Cenozoic exhumation across the NW European continental shelf. Many studies have attributed the exhumation of this region to processes associated with the early Palaeogene initiation of the Iceland Plume, whilst the magnitude and causes of Neogene exhumation have attracted little attention. However, the sedimentary basins of the southern Irish Sea contain a mid–late Cenozoic sedimentary succession up to 1.5 km in thickness, the analysis of which should permit the contributions of Palaeogene and Neogene events to the Cenozoic exhumation of this region to be separated. In this paper, an analysis of the palaeothermal, mechanical and structural properties of the Cenozoic succession is presented with the aim of quantifying the timing and magnitude of Neogene exhumation, and identifying its ultimate causes. Synthesis of an extensive apatite fission-track analysis (AFTA), vitrinite reflectance (VR) and compaction (sonic velocity and density log-derived porosities) database shows that the preserved Cenozoic sediments in the southern Irish Sea were more deeply buried by up to 1.5 km of additional section prior to exhumation which began between 20 and 15 Ma. Maximum burial depths of the preserved sedimentary succession in the St Georges Channel Basin were reached during mid–late Cenozoic times meaning that no evidence for early Palaeogene exhumation is preserved whereas AFTA data from the Mochras borehole (onshore NW Wales) show that early Palaeogene cooling (i.e. exhumation) at this location was not significant. Seismic reflection data indicate that compressional shortening was the principal driving mechanism for the Neogene exhumation of the southern Irish Sea. Coeval Neogene shortening and exhumation is observed in several sedimentary basins around the British Isles, including those along the UK Atlantic margin. This suggests that the forces responsible for the deformation and exhumation of the margin may also be responsible for the generation of kilometre-scale exhumation in an intraplate sedimentary basin system located >1000 km from the most proximal plate boundary. The results presented here show that compressional deformation has made an important contribution to the Neogene exhumation of the NW European continental shelf.

Pressure constraints on the CO2 storage capacity of the saline water-bearing parts of the Bunter Sandstone Formation in the UK Southern North Sea

The Bunter Sandstone Formation (BSF) in the UK sector of the Southern North Sea is thought to have a significant potential for the injection and storage of anthropogenic CO2 within periclines that lie above salt domes and pillows formed by halokinesis in underlying Zechstein strata. During the formation of the periclines, the BSF and its overlying top seals were subjected to extensional stresses and, in consequence, are commonly cut by seismically resolvable faults that present a risk to the containment of gas and buoyant fluids such as supercritical CO2. Although most of the closed structures in the BSF are saline water-bearing, eight gas fields (total gas initially-in-place >72 bcm (billion cubic metres)) have been discovered to date. The seismically resolved structure of these gas fields demonstrates that two different top seals, the Haisborough Group and the Speeton Clay, can seal gas columns of up to 128 and 104 m respectively, despite the presence of faults with small displacements above the field gas–water contacts. The observed gas columns are equivalent to CO2 columns of up to around 100 m in height. Simple geomechanical modelling suggests that existing optimally orientated faults may dilate or be reactivated if the pore-fluid pressure increase as a result of CO2 injection exceeds a gradient of about 13.4 MPa km–1, potentially resulting in loss of storage integrity.

Utilizing spectral decomposition to determine the distribution of injected CO2 at the Snøhvit Field

Time-lapse 3D seismic reflection data, covering the CO2 storage operation at the Snohvit gas field in the Barents Sea, show clear amplitude and time-delay differences following injection. The nature and extent of these changes suggest that increased pore fluid pressure contributes to the observed seismic response, in addition to a saturation effect. Spectral decomposition using the smoothed pseudo-Wigner–Ville distribution has been used to derive discrete-frequency reflection amplitudes from around the base of the CO2 storage reservoir. These are utilized to determine the lateral variation in peak tuning frequency across the seismic anomaly as this provides a direct proxy for the thickness of the causative feature. Under the assumption that the lateral and vertical extents of the respective saturation and pressure changes following CO2 injection will be significantly different, discrete spectral amplitudes are used to distinguish between the two effects. A clear spatial separation is observed in the distribution of low- and high-frequency tuning. This is used to discriminate between direct fluid substitution of CO2, as a thin layer, and pressure changes that are distributed across a greater thickness of the storage reservoir. The results reveal a striking correlation with findings derived from pressure and saturation discrimination algorithms based on amplitude versus offset analysis.

Addressing anisotropy in 3-D prestack depth migration: A case study from the Southern North Sea

This is a case study of a 3-D anisotropic prestack depth migration (APSDM) of data from Block L10 of the Dutch sector of the North Sea. Producing gas reservoirs in L10 are typical of the area in that they are contained in horst and tilted fault blocks of the Rotliegend Formation. Imaging these fault blocks is made difficult by a complex overburden greatly influenced by salt tectonics. In particular, the overburden includes chalk characterized by fast velocities and a strong vertical velocity gradient.

Refraction microtremor (ReMi) to determine the shear-wave velocity structure of the near surface and its application to aid detection of a backfilled mineshaft

Abstract Passive refraction microtremor (ReMi) surveys utilize standard field seismic-refraction recording equipment and linear geophone arrays to record ambient background noise owing to microtremors caused by natural and anthropogenic activities. The technique relies upon the detection of coherent phases of Rayleigh waves that have propagated along the axis of the geophone array, which is the same mode of propagation that causes ground roll on standard refraction surveys. Rayleigh-wave propagation is confined within one wavelength of the surface, causing dispersion because waves with longer wavelengths (lower frequencies) are controlled by ground stiffness and density properties at greater depths. Field records that include coherent modes of dispersive Rayleigh-wave propagation along the field array are processed using slowness (reciprocal of the phase velocity)–frequency transformations to extract the phase velocity–frequency dispersion curves. A series of dispersion curves are extracted by processing the field records of sub-groups including 6–8 geophones, from which 1D shear-wave velocity–depth profiles are constructed and attributed to the centre of each array sub-group. In this survey, nine overlapping sub-groups of eight geophones were selected along the whole field array of 24 geophones equi-spaced over 69 m. A 2D shear-wave velocity section was created by infilling a grid between each of the velocity–depth profiles using an anisotropic inverse distance weighting algorithm. Interpretation of the 2D section included the identification of: (1) reworked ground comprising colliery spoil and clay to around 5 m below ground level associated with shear-wave velocities from 100 to 700 m s−1; (2) deeper strata within the host formation associated with higher velocities that increased with depth to above 1000 m s−1 at depths below 10 m; (3) a backfilled mineshaft and a backfilled sandstone quarry at depths below 7 m associated with low-velocity perturbations within the background host velocity structure. Key recommendations from this case study include the use of low-frequency geophones to increase the depth of investigation and recording of high frequencies at reduced geophone spacings to increase near-surface resolution.

Comparison of surface wave techniques to estimate shear wave velocity in a sand and gravel sequence: Holme Pierrepont, Nottingham, UK

This study evaluated the application of surface wave methods to aggregate variability and thickness determinations. We compared the results of field assessments of sand and gravel sequences using three surface wave survey approaches. The first was a seismic refraction approach, the second, a continuous surface wave (CSW) survey approach, and the third adopted a multi-channel analysis of surface waves (MASW) technique to the original refraction field set-up and records. The sand and gravel sequences were highly heterogeneous and the shear wave profiles were not normally dispersive (i.e. did not exhibit a monotonic increase in velocity with depth), which had a significant effect upon the performance of the three field approaches. Both CSW and MASW approaches provided information over a broad spectrum from which velocity–depth profiles were produced, but the upper frequency of operation was limited in both methods because of poorer signal quality at higher frequencies. Shear wave velocity profiles obtained using vertically vibrating sources during CSW surveys were different from profiles obtained using a horizontally polarized source in the refraction survey. This was attributed to different propagation paths and modes of propagation, which were illustrated via additional tomographic inversion of the refraction travel times but could also be attributed to data inversion methods. Probing using an ultra-lightweight cone penetrometer, continuous reflection profiling using ground-penetrating radar, and also an active extraction programme at the field site provided the opportunity to directly observe the subsurface geology and verify field results. Within the sand and gravel sequence, high-velocity layers were associated with matrix-supported coarse gravel lenses, some of which were weakly cemented. Localized high- and low-velocity zones within the underlying bedrock were interpreted as being related to lithostratigraphic heterogeneity and the development of an upper, weathered zone.