East Shetland Basin Research Articles

Modelling the uncertainties in predicting produced water concentrations in the North Sea

A random walk model has been used to compute concentration distributions of dispersed oil in the North Sea resulting from produced water discharges. This formed part of a joint study commissioned by Statoil, OLF and BP International with modelling being undertaken by SINTEF and the Brixham Environmental Laboratory. The model has been set up using predicted tidal currents from the Norwegian Meteorological Office 20-km grid three-dimensional Continental Shelf model for the year 1990. Climatology data from the North Sea have been used to define the variation of the thermocline depth at monthly intervals over the year, and wind data from the East Shetland Basin have been used to compute the vertical mixing rates. Peak concentrations of dispersed oil were predicted to be approximately 3 μg l −1 in the East Shetland Basin, assuming no biodegradation; this value is consistent with the measurements of Stagg et al. (In: Reed, M., Johnsen, S. (Eds), Produced Water 2, Environmental Issues and Mitigation Technologies, Plenum Press, 1996), where the data were collected in the immediate vicinity of the discharges and the effects of degradation would be expected to be negligible. A matrix of model runs was undertaken with different parameter values for the degradation of dispersed oil, horizontal and vertical mixing, and the mixing across the thermocline. These results were used to estimate the sensitivity of the model and the uncertainty in the predicted concentration fields. The results indicate that for the areas of high concentration, a band approximately 150-km wide stretching up the centre of the North Sea, parameterisation of the vertical mixing is most important for predicting the concentration levels. For areas more remote from the discharges and closer to the land masses, the degradation rate of material has most effect on predicted concentrations.

Land–offshore tectonic links in western Norway and the northern North Sea

Aeromagnetic and marine gravity data sets for the northern North Sea and western Norway have been interpreted in terms of land to offshore tectonic links. The data allow identification of linking-structures both in crystalline basement and overlying sedimentary rocks and consequently facilitate a more comprehensive interpretation of regional structural features and trends. With the help of bathymetric data in the inshore region, major fault structures can be traced between western Norway and the offshore region west of the Øygarden Fault Zone. In this way spatial and chronological links have been established between tectonic events recognized on land and the development of the North Sea Basin. An interpretation of basement geology is proposed for the region between western Norway and the Uer Terrace. This is complemented with a depth to top crystalline basement map covering the North Sea Basin between the Uer terrace and the East Shetland Basin. Two regional NW–SE lineaments were recognized offshore western Norway and given the names Marflo Lineament and Troll Lineament. The Marflo Lineament is interpreted to be a Caledonian or older zone of weakness that accommodated development of a 20–30 km offset in the Jurassic Viking Graben axis. The lineament can be traced into the North Atlantic where it parallels oceanic fracture zones and transform faults. The Troll Lineament relates to a deep geological discontinuity crossing the Uer and Lomre Terraces. A grouping of earthquake epicentres along part of the lineament suggests that the discontinuity is tectonically active.

Permo-Triassic and Jurassic extension in the northern North Sea: results from tectonostratigraphic forward modelling

Abstract We have undertaken 2D forward modelling across the northern North Sea, based on reprocessed, interpreted and depth-converted deep reflection seismic lines NSDP84-1 and −2 (15 s twt) and refraction data. Two separate stretching phases, Permo-Triassic and Jurassic, are recognized. The cumulative stretching is consistent with the observed crustal structure and the overall basin configuration, as reproduced by forward modelling. Good agreement between observed and modelled top basement level, and crustal thickness below the platform areas are particularly emphasized. Crustal-scale modelling indicates that crustal thickness varied across the northern North Sea at the onset of the Permo-Triassic rifting, from c . 35 km in the platform areas to less than 30 km in the interior of the basin. This may be ascribed to Devonian(-Carboniferous?) crustal stretching. Thinning of the crust has progressively been narrowed, from post-Caledonian extensional collapse, to less regional Permo-Triassic basins, and finally development of the Viking Graben area in the Jurassic-early Cretaceous time. Most of the Permo-Triassic stretching occurred between the Øygarden Fault Zone to the east and the Shetland Platform (southern transect) and the Hutton Fault alignment to the west. The width of the Permo-Triassic basin was c . 120–125 km, with calculated β mean between 1.38 and 1.40. Permo-Triassic β mean estimates across the present Horda Platform vary between 1.33 and 1.39. The Jurassic β mean estimates for the same area vary between 1.08 and 1.13. Across the Viking Graben, Permo-Triassic β mean varies between 1.28 (southern transect) and 1.41 (northern transect). This is lower than estimates for the Jurassic β mean , which amounts to 1.53 and 1.42. Permo-Triassic and Jurassic β mean estimates across the East Shetland Basin are 1.29 and 1.11, respectively. Lithospheric thermal evolution reflects the general differences between Permo-Triassic and Jurassic stretching, with a much wider thermal perturbation during the former and a focusing and lateral migration towards the east of the peak thermal elevation during the latter. There are still uncertainties related to the degree of (de)coupling between the upper crust and upper mantle during the Permo-Triassic and the Jurassic rift phases. These uncertainties are related to the interplay between age, strain rate, crustal rheology, crustal thickness and long-lived zones of weaknesses.

Controls on Late Jurassic, subtle sand distribution in the Tampen Spur area, Northern North Sea

The role of fault growth and linkage is an important, but poorly understood, factor in controlling the distribution of sediments within extensional settings. Tectonostratigraphic analysis of the Statfjord East fault system, located in the East Shetland Basin of the Northern North Sea, demonstrates the importance of Late Jurassic normal faulting in controlling the nature of subtle, hanging wall traps. Wells in Norwegian Block 34/7 have penetrated prospective sandstone reservoirs that correlate with several thin, but seismically resolvable, units within syn-rift sediments of the Statfjord East hanging wall. Detailed 3D seismic interpretation and mapping of amplitude anomalies in Block 34/7 and the adjacent part of Block 33/9 reveal a number of spatially discontinuous stratigraphic units within the Late Jurassic, syn-rift Draupne Formation. These depositional units are spatially related to sub-basins, which are controlled by interaction and linkage of several en echelon fault segments. The main control on the distribution of local depocentres is the along-strike displacement variation associated with fault propagation by segment linkage. A close association between sandstone occurrence and structural position, suggests a model in which shoreline migration is controlled by the dynamics of fault lengthening through time.

Fluid inclusion constraints on conditions and timing of hydrocarbon migration and quartz cementation in Brent Group reservoir sandstones, Columba Terrace, northern North Sea

Abstract Fluid inclusion data have been obtained from Brent Group sandstones from the Columba Field on the margin of the East Shetland Basin, northern North Sea. Cogenetic primary aqueous and hydrocarbon inclusios trapped during the initial stages of late diagenetic quartz overgrowth are common, indicating that the pore fluid present during the onset of a major phase of quartz-kaolinite(-illite) diagenesis consisted of immiscible saline aqueous and petroleum phases. Similar inclusions are also observed in abundant planar arrays cross-cutting detrital and authigenic quartz. Homogenization temperature and salinity data from the two types of aqueous inclusions are statistically indistinguishable, as are the properties of the two types of hydrocarbon inclusions, suggesting that the same two-phase fluid was present probably throughout the main phase of quartz cementation. Fluid inclusion thermobarometry shows that the majority of the fluids were trapped in the range 104 ± 6°C and 300 ± 33 bars. These data are consistent with either (1) trapping of a warm, over-pressured hydrocarbon + aqueous fluid, probably in thermal and chemical disequilibrium with the host reservoir rocks, derived from a Kimmeridgian source in the deep Viking Graben basin to the east of the reservoir within the period 85–55 Ma, or (2) trapping of a hydrostatically pressured fluid, in thermal equilibrium with the host reservoir rocks, with hydrocarbon ± aqueous fluids derived by lateral up-dip migration from a local Kimmeridgian hydrocarbon source in the East Shetland Basin within the period 65–40 Ma. A maximum duration of 10 Ma for the fluid flow event can be estimated based on a typical burial history model for the area.

Hydrography of the East Shetland Basin in relation to decadal North Sea variability

Three hydrographic sections running east from the coast of Shetland into the central northern North Sea have been surveyed since 1989. These sections cross the East Shetland Atlantic Inflow, which is one of the three northerly oceanic inflows to the North Sea. The East Shetland Atlantic Inflow has previously been discounted as of great importance to the hydrography and ecology of the North Sea. However, results from the hydrographic sections, current meter deployments and acoustic Doppler current profiler surveys reveal that the current is a persistent feature of North Sea circulation during the summer months. This inflow may form the western boundary of a cyclonic gyre in the bottom waters east of Shetland. When considering the ecological significance of the inflow it is evident that there have been substantial changes in the salinity of the northern North Sea, and of the adjacent oceanic waters during the past decades. A 20-year record of hydrographic parameters along the standard JONSIS section (59°17′N) suggests that these changes are related to decadal variability of the two western North Sea inflows; the Fair Isle Current and the ESAI. These changes are in turn related to changes in the oceanic waters at the entrance to the North Sea, as demonstrated by long-term time series generated from two standard sections in the Faroe-Shetland Channel. It is directly via the ESAI, and indirectly via the Fair Isle Current, that oceanic variability is communicated to the North Sea.

A Magnus opus: Helium, neon, and argon isotopes in a North Sea oilfield

This study of the Magnus oilfield, located in the East Shetland Basin, northern North Sea, represents the most detailed investigation of noble gas isotope systematics in a liquid hydrocarbon reservoir yet undertaken. Samples from nine producing wells across this Middle Jurassic field were taken and the helium, neon, and argon isotopic ratios and abundances in the oil were determined. Both the helium and the neon isotope systematics require a contribution from a mantle source. If the mantle endmember is modeled using mid-ocean ridge (MOR) values, 2.3–4.5% of the 4He and 4.3–6.2% of the 21Ne is mantle-derived. The remainder of the 4He and 9.0–12.0% of the 21Ne is crustal-radiogenic and the remaining 21Ne is atmosphere-derived. The resolved mantle-derived He/Ne ratio is quite distinct from upper-mantle values estimated from MOR samples, but indistinguishable from values resolved in other regions of continental extension. This result provides strong evidence that the subcontinental lithospheric mantle not only has a different noble gas inventory to the convecting mantle under MORs, but also has a near uniform composition. The quantity of radiogenic noble gas associated with the Magnus oil/groundwater system can only be accounted for by production predominantly from outside the volume of the Magnus Sandstone aquifer/reservoir drainage area and the associated Kimmeridge Clay source rock formation and together with the mantle-derived noble gases, provides strong evidence for cross-formational communication with deeper regions of the crust. The 20Ne and 36Ar must have been input into the oil phase by interaction with an air-equilibrated groundwater. Noble gas partitioning between a seawater-derived groundwater and the oil phase at the average Magnus Sandstone aquifer temperature requires a subsurface seawater/oil volume ratio of 110 (±40) to account for both the 20Ne and 36Ar concentrations in the central and southern Magnus samples. The volume of groundwater which has equilibrated with the Magnus oil is indistinguishable from the static volume of water estimated to be in the down-dip Magnus aquifer/reservoir drainage volume. This suggests that the Magnus oil has obtained complete equilibrium with the groundwater in the reservoir drainage volume, probably during secondary migration, and further suggests that concurrent cementation of the Magnus sandstone aquifer has occurred with little or no large-scale movement of air-equilibrated groundwater through the aquifer system. Higher concentrations of 20Ne and 36Ar in the N. Magnus oil cannot be accounted for by equilibration with a seawater-derived groundwater. This is qualitatively consistent with the earlier and more mature oil in this section equilibrating with freshwater, which is known to have been trapped in the crest of the reservoir structure.

Influence of relative sea-level on facies and reservoir geometry of the Middle Jurassic lower Brent Group, UK North Viking Graben

Abstract Detailed sequence stratigraphy of several neighbouring fields in the East Shetland basin indicates that a higher order of cyclicity is superimposed onto the lower Brent Group. Both relative falls and rises in sea-level affected the strongly progradational nearshore/barrier-island system and induced important variations in the geometry, distribution and stacking patterns of the reservoir facies. The majority of the shallow marine strata belong to the transgressive and highstand systems tract and consist of the lower-shoreface Rannoch, the upper shoreface and strand-plain Etive and the back-barrier and lagoonal Lower Ness. High frequency increments of shoreline progradation are preserved in the Rannoch as gently seaward-dipping bedsets. The small-scale units mark asymmetric cycles in grain size and mica content and together, with stratiform cement, impart a strong permeability anisotropy on the Rannoch. The nearshore Etive Formation represents a sediment-laden, barred nearshore complex and is subdivided into (1) a uniformly thick upper shoreface and (2) an overlying emergent strand plain. The two subfacies are commonly separated by a thin interval of foreshore which is locally enriched in heavy minerals. The shoreface deposits show systematic variations in stacking geometries ranging from strongly progradational to aggradational. Two relative falls in sea-level led to the subaerial erosion of nearshore Etive deposits and the juxtaposition of coastal-plain mudstones, coals and fluvial deposits with shale-clast lags on lower shoreface Rannoch strata. Although lithostratigraphically part of the Etive, these deposits bear no resemblance to the well ordered nearshore-Etive deposits. The channel complexes are vertically and laterally heterogeneous and are confined to incised valleys separated by non-depositional interfluves. In Cormorant and Tern fields, well-performance anomalies such as early water breakthrough and unpredictable water injection and oil production rates are associated with these lowstand incised valley deposits. An enigmatic coarse-grained unit occurs in the lower Rannoch in Eider field and is provisionally interpreted as the distal expression of the lowstand shoreface deposits associated with the older incised valley. Following the lowstand deposition nearshore-marine sedimentation reestablished during the transgressive/early highstand systems tract. Under the influence of rising relative sea-level the barrier complex aggraded in place with minimal landward translation of the shoreline position. The volume of sediment supplied to the Etive shoreface was sufficiently high to keep pace with phases of increased accommodation. The shoreline system subsequently stacked with a strongly progradational geometry as the high sediment supply filled the available accommodation space during the latter part of the highstand systems tract.

Simulation of Mineral Diagenesis in Reservoirs. Application to Illite Formation in Feldspathic Sandstones: ABSTRACT

Petroleum geologists and production engineers are faced with reservoirs where porosities and permeabilities (poroperm) have been reduced by mineral phases precipitated during the geological evolution. Diagenesis of sandstones is influenced by many factors : initial composition of the sediment, burial history, composition of infiltrated waters. An appraisal of poroperm decline due to mineral diagenesis only can result from an integration of these factors. A quantitative evaluation of diagenetic phenomena is possible using numerical modelling. A first approach of the mineral transformations can be made using a new geochemical modelling software (NEWKIN) applied to closed cells, where aqueous solution and minerals are not in equilibrium initially. Cements of illite and quartz frequently occur in sandstones bearing feldspar, such as Middle Jurassic reservoirs of the Brent Group (East Shetland Basin, North Sea) which today lie between 3500 and 4500 in depth. Results of closed cells simulations are presented, which explore the conditions of illite and silica authigenesis in this Province, particularly in terms of temperature, water composition, and kinetics (oversaturation of the waters with respect to quartz, low pH). Another key of non-equilibrium, in pervious rocks, is the flow of interstitial water. Its role must be appraised by a -[open quotes]reaction-transport[close quotes]more » code. A new software is presented (DIAPHORE), able to solve, at the reservoir scale, in a coupled way : (1) advection of water and chemical elements in the porous volume; (2) mass balance of the considered chemical elements in the rock volume; (3) dissolution-precipitation phenomena occurring locally (using the geochemical code precedently described); (4) a feedback of the mineral transformations on permeability and reactive surface areas through a [open quotes]textural[close quotes] model at the grain scale.« less

NUMERICAL MODELLING OF DIAGENETIC QUARTZ HYDROGEOLOGY AT A GRABEN EDGE: BRENT OILFIELDS, NORTH SEA

Oilfields in the Middle Jurassic Brent Group of the East Shetland Basin contain abundant quartz cement, which reduces porosity by 10–15%. This quartz cement is believed to have started to grow at depths of around 2‐3km. Oxygen isotope signatures in diagenetic quartz imply that it grew from meteoric pore waters; however, fluid inclusion temperatures are 30–50d̀C higher than temperatures which should have prevailed at the inferred palaeo‐depth. Large‐scale circulation of hot water has been proposed to explain this “temperature anomaly”, but this is geologically difficult in terms of heat and fluid budgets. Finite‐element fluid‐flow models of a generic fault‐block rift‐basin stratigraphy are used to evaluate two alternative hydrogeological models for deep quartz diagenesis in Brent Group sandstones: Meteoric waters may have penetrated several kilometeres into the crust before rising rapidly as a hot, isotopically‐evolved fluid; Cool meteoric waters from the palaeo‐land, fed directly into the Brent Group aquifer, moving tens of kilometres laterally into the basin. Numerical models show that in this case gravitational penetration of moving fluids to considerable depth in the crust was indeed feasible with a driving head of only 200 m. Fluid velocities were so low (≅ 0.01 m/yr) that the heating effects of the fluids were minimal. Long‐range lateral transport of heat was also inefficient, for fluids tend to emerge along the first major fault plane encountered. Large‐scale mixing within aquifers resulted in a uniform fluid during the start of quartz diagenesis in the Brent Group. This mixing was not sensitive to permeability anisotropy. The slow infiltration of meteoric fluids into Brent Group aquifers is inferred to have maintained a low‐salinity, meteoric isotopic dominance to the pore fluids. These slow, large‐scale lateral flows may have exited from the Brent aquifer at “leak points” on shallow‐buried structures, such as Gullfaks. Degraded oil on the downdip side of Gullfaks also independently suggests an active deep aquifer in the Early Tertiary. Consequently, diagenetic quartz grew at equilibrium temperatures and adopted a meteoric‐dominated isotopic signature. Quartz supply was decoupled from such fluids, and was probably locally‐sourced.

Quantitative analysis of Triassic extension in the northern Viking Graben

It has been recognized for over a decade that large-displacement, pre-Jurassic faults are present in the Northern Viking Graben, part of the North Sea rift. These faults define a series of major fault-blocks below the more obvious Jurassic rift basin. We attempt here to quantify the amount of extension associated with this older rift event, which is probably of early Triassic age. Quantitative modelling of the Triassic rift and the succeeding period of thermal subsidence has been undertaken, using a combination of flexural backstripping and flexural-cantilever forward modelling. These techniques suggest that Triassic extension across the Horda Platform (Norwegian sector) reached c. 40% ( ß = 1.4). The consequences of this extension were deposition of a thick (>3 km) Triassic-Upper Jurassic syn-rift and post-rift sequence, prior to renewed, but minor, extension in the Late Jurassic-earliest Cretaceous. The thickness of the Viking Group reservoirs in the Troll area appears to have been almost entirely controlled by sediment loading during post-Triassic thermal subsidence. Jurassic extension on the Horda Platform was <5%, an order of magnitude less than the Triassic event. The Horda Platform is therefore principally an area of Triassic extension marginal to the main Jurassic rift further west. In the UK sector of the Viking Graben, Triassic structures are less obvious than those below the Horda Platform, because of Jurassic overprinting. They are, however, still present. Average Triassic extension across the East Shetland Basin was c. 15%, comparable with the magnitude of Jurassic extension in the same area. We believe that the Tern/Eider and Cormorant fault-blocks, with proven shallow basement, comprised a large eroded horst during the early Triassic, uplifted in the footwalls of major faults flanking the Magnus and Statfjord half-graben. During the Triassic, the Magnus half-graben was contiguous with the Unst Basin, now situated in the western footwall of the younger Jurassic basin. The presence of the Unst Basin suggests that Triassic extension occurred across the area that is now the northern Shetland Platform, continuing into the West Shetland area. Although the more obvious structures in the Viking Graben are Jurassic in age, the earlier Triassic event was equally as important in controlling the structural and stratigraphic history of the basin.

Industry – inputs to the environment

Sullom Voe Terminal has been in operation since November 1978, processing crude oil from the East Shetland Basin. Gas processing was initiated in April 1981, with full LPG production commencing in May 1982. The terminal is operated by BP Exploration, while port operations are controlled by Shetland Islands Council. This paper will summarise both terminal and port operations, to highlight those areas which involve a discharge or emission to the environment. Variation of the discharges with time will also be indicated to provide a background against which the results of the environmental monitoring programme can be set.

Structure and tectonics of the northern North Sea: new insights from deep penetration regional seismic data

Abstract Recently acquired, deep penetration regional seismic data provide important new insights into the structure and tectonic evolution of the northern North Sea. Improved data quality permits better understanding of fault geometries and salt tectonics in the graben areas and reveals the pre-Tertiary geology of the surrounding platforms for the first time. This paper presents seismic data from five structurally distinct areas. (1) South Viking Graben . This relatively narrow graben formed the principal sedimentary basin during the Mesozoic period and contains a thick Triassic-Cretaceous succession. Low-angle, eastward-dipping faults developed at the western basin margin control a relatively simple half-graben geometry. The Beryl Embayment is notable for the local rotation of Triassic-Jurassic strata independent of the underlying basal Permian reflectors, indicating local Mesozoic salt movement in this area. Faulted and anticlinal highs east of the graben axis may also record limited salt tectonics. (2) Horda Platform . This area comprises a series of eastward-dipping rotated Mesozoic fault blocks. Mesozoic extensional faults developed as splays in the hanging wall of a major westward-dipping deep detachment surface. (3) East Shetland Platform . This area formed a structural high throughout the Mesozoic era. Regional seismic data provide a clear image of the geometry of a Palaeozoic basin, thought to represent the northward continuation of the Orcadian Basin recognized onshore in mainland Scotland, Orkney and Shetland. The sedimentary sequence is probably of Devonian (-Carboniferous?) age and contains major unconformities, which may permit subdivision of this succession into tectono-stratigraphic units. The contact with the underlying basement is characterized by a distinctive change in seismic character, traceable over the entire area. The deep structure of the East Shetland Platform is dominated by westward- or northwestward-dipping low-angle basement faults, interpreted as thrust surfaces. This structural grain is associated with the local development of eastward-vergent folds interpreted as hanging wall anticlines developed in the Upper Palaeozoic cover sequence. These structures provide evidence of a Late Palaeozoic deformation phase. (4) Unst Basin . Lying to the west of the East Shetland Basin and bounded by a faulted horst on its eastern margin, the Unst Basin is underlain by a spectacular low angle, westward-dipping detachment surface. Roll-over into this fault is interpreted as the result of extensional roll-back of an older (?Caledonian) thrust plane. The fault geometry may indicate structural analogies with the West of Shetlands Province. (5) East Shetland Basin . The structure of this area is characterized by the development of extensive, westward-dipping fault blocks underlain by low-angle, eastward-dipping extensional faults traceable to great depth. The seismic data indicate thickening of the sedimentary succession in the footwalls of these faults, consistent with Jurassic faulting of a pre-existing Triassic basin.

Late Jurassic-Early Cretaceous inversion of the northern East Shetland Basin, northern North Sea

Abstract Current research into the structural evolution of the East Shetland Basin, northern North Sea, indicates that a significant phase of structural inversion occurred during the Latest Jurassic-Early Cretaceous. Structural effects of this tectonic phase include the pronounced uplift of pre- and syn-rift sequences along the western margin of the basin and the partial inversion of Mesozoic half-graben along intra-basinal NE-SW controlling faults. In addition, the localized occurrence of flower geometries and pop-ups along NE-SW trending faults suggests a component of strike-slip during the reactivation of these pre-existing structures. Work presented stresses the importance in the role of pre-existing lineaments and the effects of superimposing subsequent non-coaxial extensional events upon them. Lineaments may be prominent reactivated basement shear zones or fault systems from earlier extensional phases. Inversion will be analysed in detail with respect to two areas within the northern East Shetland Basin, the Tern sub-basin and the northern Penguin ridge. A regional tectonic model of northern North Sea rift evolution is presented and its implications to observed Latest Jurassic-Early Cretaceous structure discussed.

Palynostratigraphic Calibration of the Brent Group in the East Shetland Basin, North Sea: ABSTRACT

Palynostratigraphic Calibration of the Brent Group in the East Shetland Basin, North Sea: ABSTRACT

Geochemical modelling of late diagenetic processes in the Brent Sandstone, Alwyn South area (East Shetland Basin, North Sea): 1. Estimation of the circulated fluids composition

This study simulates the third diagenetic stage of the Brent Sandstone in the Alwyn South area through modelling of water-rock interactions at temperatures of 120° and 140°C, as determined by the fluid inclusion method. Simulation results are compared to those from petrographic analyses. The major diagenetic events are recognized petrographically, but data on the chemical speciations of the invading fluid are not available. Thus, the geochemical simulations test both seawater and dilute water. A solid solution model is used in order to test the precipitated complex clay phase and its influence on the chemical composition of the reacting fluid. The best agreement with petrographic analyses is from rock interaction with seawater that is depleted in magnesium.

Tectonic evolution and structural styles of the East Shetland Basin

Geometric and stratigraphic evidence indicates that the East Shetland Basin evolved by a rift mechanism similar to that defined in the East African Rift. Arcuate half-grabens of probable Devonian extensional origin are the fundamental structural units. The stratigraphic data suggest episodic footwall movements during the mid- to late Jurassic as well as alternating regional subsidence patterns, i.e. ‘teeter-totter’, against the Horda Platform. The Northern North Sea rift system, which includes the East Shetland Basin, is believed to have opened primarily by orthogonal movement of facing half-grabens rather than by large-scale strike-slip motion between these half-grabens. Both extensional and compressional (or transpressional) features occur in the East Shetland Basin as syn-rift structures. The extensional structures include the well-known tilted fault blocks and relatively undocumented listric normal faults that occur on the crestal and flanking areas of the larger blocks. The principal compressional stress was associated with oblique shearing at the ends of actively extending half-grabens, which created local reverse faults and anticlinal domes. Some rotated horst blocks, e.g. Murchison Field structure, were also created by the transpressional motion. A broad, four-fold E–W structural zonation has been made for the East Shetland Basin and North Viking Graben based on the degree of half-graben development and structural styles.

The structural evolution of the Snorre Field and surrounding areas

The Snorre Field is located on the Tampen Spur, Northern Viking Graben between 61°N and 62°N. Main structural elements are large, westerly tilted fault blocks. The Snorre Structure, one of the largest fault blocks in the East Shetland Basin, extends across Block 34/7 into Block 34/4. The Snorre Fault Block includes the Snorre Field, Tordis Field and the Vigdis Field oil discoveries. Major oil discoveries have so far been made in the Brent Group, the Statfjord Formation and the Lunde Formation. Discoveries have also been made in the Upper Jurassic and Paleocene. Upper Jurassic stratigraphic traps are the main target for further exploration in the area. The westerly rotated fault blocks on the Tampen Spur are delineated by NNE–SSW- to NE–SW-trending faults. From west to east, these are the Murchison Fault, the Outer Snorre Fault, the Southern Snorre Fault and the Inner Snorre Fault. Other faults recognized are the E–W- and SE–NW-striking cross-faults dividing the elongated fault blocks into minor compartments. The N–S-striking faults are common in the Snorre area. The number of these faults decreases towards the north. The Snorre Escarpment is the surface expression of the major Inner Snorre Fault representing the eastern border fault of the Snorre Fault Block. Two major rifting episodes are recognized in the area. In the thermal subsidence phase following the late Permian to early Triassic rifting, large amounts of sediments of the Teist-, Lomvi-, Lunde-, Statfjord Formation, Dunlin- and Lower Brent Group were accumulated. A second major rifting episode resulted in intense fault activity from the early Bathonian to the late Kimmeridgian. Two major pulses of increased tectonic activity are recognized in the second rift episode. In the north, in the Zeta area, there is indication of late Jurassic lateral movements. Late Jurassic faulting, block rotation and footwall uplift, combined with Jurassic to Cretaceous subsidence, make the Snorre Fault Block of the Tampen Spur a very pronounced structural high in the area.

Silicate mineral authigenesis in the Hutton and NW Hutton fields: implications for sub-surface porosity development

The Hutton and NW Hutton fields occur in highly faulted, rotated block structures in Block 211/27 of the East Shetland Basin, Northern North Sea. Fourteen wells that core Brent Group reservoir from these fields have been studied in order to characterize the relationship between clay mineral authigenesis and porosity modification in the sub-surface. Reservoir sands are present in the depth interval 9000–13 000 ft; the shallowest sandstones are subarkoses, contrasting with the deepest sandstones which are mostly feldspar-deficient quartz arenites. The change in composition is a result of diagenetic feldspar dissolution that takes place over the depth interval of study. Authigenic kaolin group minerals and illite are present over the depth range studied, and both exhibit depth-related increases. Three habits of authigenic kaolin group minerals are observed: ‘expanded-mica’, vermiform kaolinite and blocky dickite. The occurrence of vermiform kaolinite is enhanced in Hutton and the adjacent water zone, and does not occur in abundance below 10 500 ft. This habit formed as a result of syn-depositional meteoric water ingress. Structural models suggest that neither field underwent subsequent meteoric water flushing. At the deeper levels of its occurrence range, vermiform kaolinite shows signs of alteration to blocky dickite. Dickite occurs in both intergranular and grain dissolution pores. A depth-related trend of increasing dickite cementation is strongly linked to increasing K-feldspar dissolution. Enhanced dickite cementation is found adjacent to bed contacts in sandstones within mudstone sequences and in regressive sandbodies close to thick mudstone units. Illite shows a more pronounced depth-related increase across the studied interval. Illite precipitation is also genetically linked with detrital feldspar dissolution and is favoured by diminished pore water flow during deep burial. Authigenic clay distribution is partly controlled by local features such as detrital clay coats, in addition to larger-scale parameters such as primary lithology, facies associations, present burial depth and inferred palaeofluid flow. Intergranular porosity declines with increasing burial depth, while grain dissolution porosity increases with depth from Hutton to the shallow and intermediate depth wells of NW Hutton. Generation of enhanced porosity is not related to the unconformities overlying the Brent Group, but to sub-surface fluid migration pathways. Compaction is not significantly operative over the depth interval 10 000–13 000 ft. By 10 000 ft burial depth, up to 50% of the intergranular volume has been lost by mechanical compaction.

Mesozoic extension in the North Sea: constraints from flexural backstripping, forward modelling and fault populations

The magnitude and distribuion of late Jurassic extension in the Northern Viking Graben has been investigated by (i) syn-rift forward modelling using the flexural-cantilever model of continental rifting; (ii) post-rift flexural backstripping of a series of cross-sections; and (iii) the analysis of fault-population statistics. Application of these three techniques indicates that Jurassic extension on the tilted fault-block terrains of the East Shetland Basin, Tampen Spur and western Horda Platform is on average c. 15% ( β = 1.15). In the graben axis the regional value of β rises to c. 1.3, perhaps locally rising to 1.4. On the eastern Horda Platform Jurassic extension is low, β = c. 1.05. Flexural backstripping of the post-rift part of a cross-section through the Central Graben yields similar estimates of extension. At the flanks of the Jurassic basin β is estimated to be 1.2, rising to a likely maximum of c. 1.3 in the basin centre. These estimates of extension lie at, or towards, the low end of the previously published range of estimates. The principal reasons for this are (i) the incorporation of the thick sequence of Triassic and Lower Jurassic sediments in the backstripping; and (ii) the use of flexural isostasy (rather than Airy isostasy) in both the backstripping and forward modelling. The estimates of Jurassic extension obtained in this study do not account for the observed crustal thinning and, therefore, point to there having been a significant pre-Jurassic extensional event in the North Sea, of probable Triassic age. While a residual thermal anomaly from this event may have made a small contribution to post-Jurassic subsidence, compaction of the Triassic-Middle Jurassic sequence has been a significant contribution of the Triassic event to the thickness of the Cretaceous/Tertiary basin.