Based on 2D seismic surveys covering the entire Norwegian Sea (250 000 km(2)), selected 3D surveys and an extensive well database, the Cenozoic depositional history for the area has been reconstructed. Interpretation of this large database has made possible a regional overview of, and a new insight into the Cenozoic depositional systems. Significant amounts of sediments were fed to the Norwegian Sea during the Cenozoic, while, apart from a thin Quaternary cover, no Cenozoic sediments are preserved onshore. This is interpreted to be the result of several phases of uplift and erosion of the mainland during this period. The sedimentary filling of the basins is interpreted in a sequence stratigraphic context, aiming towards a dynamic understanding of the depositional history. In the Palaeocene, extensional tectonics prevailed and the Norwegian Sea received sediments from uplifted land areas, both to the east and the west. The input of sediments to the deeper parts of the basin were to some degree determined by the intersection of NW-SE trending lineaments intersecting with older structural features on the shelf. With the onset of sea floor spreading in the Eocene, the tectonic regime changed from extensional to compressional. Extrusion of basaltic lavas dominated the western land areas, while a major transgressive event resulted in the deposition of shaly sediments on the eastern continental shelf. Large parts of Scandinavia were probably flooded during this time period. A deltaic system constituting the 'Molo Formation' was deposited all along the eastern Norwegian Sea margin, as a response to regional uplift of the Norwegian mainland. Difficulties in seismic ties and the sparse well control have made the actual age of the Molo Formation a subject for discussion. Both Oligocene and Early Pliocene ages have been suggested. New seismic correlations presented in this chapter suggest that the Molo Formation is Early Pliocene in age. Erosional channels with possible fluvial drainage patterns suggest subaerial exposure over large parts of the continental shelf during the Miocene. Prograding shelf geometries within Middle to Late Miocene sediments support this theory. An unconformity in the Miocene is associated with a strong compressional event leading to flexural doming and inversion of older depocentres on the shelf. Basin scale tectonic movements are the possible causes for both, the unconformity and the compressive movements. An Early Pliocene flooding event shifted the locus of sedimentation in an eastward direction, and the Molo Formation was the first sedimentary unit deposited onto this surface. A marked shift in the prograding style occurred in mid Pliocene, and Late Pliocene/Pleistocene glacial sediments prograded westward as continental ice sheets expanded onto the shelf. Once glacial conditions were established on the shelf, the glacial drainage pattern followed bedrock boundaries and older structural features in the subsurface.
In the Late Permian to Triassic, the present Mid-Norwegian shelf and East Greenland represented an extensional basinal region, c. 400 km wide and 800 km long, composed of several sub-basins. This basinal region was the site of a highly varied sediment infill history during Late Permian to Late Triassic time, controlled by changes and variations in tectonic development, climate and eustasy. In the Late Permian minor movements along some intra-basinal faults resulted in the formation of several sub-basins. This tectonic event is defined as an initial rift phase. In the Mid-Norwegian shelf - East Greenland region the Permian-Triassic transitional interval is represented by a major shift in basin infill style, including erosion of carbonate margins and influx of silisiclastic sediments to marine sub-basins. This combination of processes is controlled by increased fault activity and fault block rotation, which was accompanied by a fall in the relative sea level. The Early Triassic (early Scythian) represents the syn-ritt phase when the dominantly marine sediment infill pattern was controlled by continued fault-block rotation and tectonic activity along several structural lineaments. The basin infill in the late Scythian represents an overall shallowing up of the basinal region, and a marginal marine to continental depositional environment became established. The Middle Triassic post-rift phase I is represented by a dominantly aggrading continental succession on the Mid-Norwegian shelf, probably with some short-lived marine transgressions. In East Greenland, this phase is characterised by a great variability of continental facies with a minor marine incursion. The establishment of a continental depositional environment is caused by a decrease in rate of accommodation relative to rate of sediment input, brought about by cessation in fault activity and reduced tectonic subsidence. However, the Vingleia Fault Zone was still active during deposition of the Middle Triassic succession. This indicates that certain structural elements continued to be tectonically active during the 'post-rift phase'. A second tectonic event influenced the basin infill of the lower part of the Upper Triassic succession in the post-rift phase 2. Thick evaporites formed in isolated marine sub-basins, was triggered by an and climate, oscillation in the relative sea level, and likely, the establishment of a structural threshold to the Borealic open marine seaway. The upper part of the Upper Triassic succession of post-rift phase 3 represents the establishment of a fluvio-lacustrine depositional environment. This facies shift was probably caused by reduced rate of subsidence, tectonic uplift in the hinterland and more humid conditions.
Provenance-sensitive heavy mineral criteria, mineral chemistry and detrital zircon age data show that there are strong links between Cretaceous sandstones in the Voring Basin and East Greenland areas. There are marked differences in the age spectra of detrital zircons from wells along the eastern margin of the Voring Basin (sandstone type K1) and those in the centre and west of the basin (sandstone type K2). The K1 sandstones have relatively simple zircon age spectra with largely Mid-Late Proterozoic zircons and a number of Caledonian age zircons. By contrast, the K2 sandstones have complex zircon age spectra, with Archaean, Early Proterozoic, Permo-Triassic and mid-Cretaceous zircons that are absent in the K1 sandstones. Some sandstones of Cenomanian and younger age from East Greenland share mineralogical features with the K2 sandstone type, having overlapping ranges of critical provenance sensitive parameters, such as RuZi, MZi and CZi, and similar types of detrital tourmalines and garnets. Detrital zircon age spectra from East Greenland samples include critical Archaean, Early Proterozoic and Permo-Triassic populations found in K2 sandstones. The zircon age data, therefore, provide support for sourcing of K2 sandstones from East Greenland. However, a source for the K2 sandstones to the east of the Caledonian front in Scandinavia cannot be ruled out, neither can the recycling of older sediment previously transferred across the rift.
3D Palaeobathymetry maps have been constructed for 14 time-steps for Cretaceous and Cenozoic times in the northern North Sea (58-62 degrees N). The restorations have been carried out by integration of relevant information from seismic sequence geometries, zero or shallow water depth indicators, compaction, micropalaeontological interpretations from wells, and isostasy. The high spatial resolution results show that the basin was greatly segmented and locally deep in the early Cretaceous post-rift phase, while it was broadening in the early late Cretaceous. Although during most of the Cretaceous period the underlying Jurassic rift structures controlled the individual depocenters, at the K-T boundary all traces of late Jurassic rift-structures had been levelled out. During the Palaeogene period, the region was characterised by episodic uplift of both western and eastern flanks. Water depths increased dramatically in the basin centre in the latest Paleocene, while shallowing induced by tectonic uplift began in the Oligocene. Subsequently, the late Miocene deep was filled in the late Pliocene to early Quaternary.
Recent deep-water wells drilled in the Voring Basin, situated on the outermost Mid Norway shelf, have revealed new insights into the presence of extensive Upper Cretaceous basin-floor fan deposits, including intervals of stacked, laterally continuous, sheet-like sandstones, up to 1 km thick.The first exploration well aiming for this play was drilled by BP on the Nyk High. BP made a gas discovery in excellent quality reservoir sandstones, thought to represent the middle to proximal part of a Campanian basin-floor system. Correlation of cores, logs and reflection seismic profiles between the wells drilled on the Nyk High and the Verna Dome, 25 km apart, shows a remarkable continuity of intervals of stacked turbidite sandstones separated by mudstones.A Maastrichtian deep-water reservoir section was also drilled by the Nyk well, and was cored by a well on the Gjallar Ridge. Palaeogeographic reconstructions and provenance studies show that the sediments for both the fans were most probably derived from the Greenland craton and its palaeo-shelf, prior to opening of the Norwegian-Greenland Sea.During the Late Cretaceous, the outer Voting Basin was an active rift basin. Bathymetric features controlled by the main bounding faults and transfer zones of the rift basin are interpreted to have had a strong affect on the sediment distribution. The Norwegian-Greenland Sea provides an outstanding example of the progressive evolution of deep-water elastic systems reflecting changes in tectonic style and bathymetry. The Albian-Cenomanian was characterised by erosion of the inherited Jurassic topography and the formation of small immature elastic systems in the pre-existing basins. During Turonian-Coniacian time, a smoother sea-floor topography developed with more regional subsidence and erosion of the large Nordland Ridge during tectonic episodes. The deep-water facies were still heterogeneous but the depositional systems were larger systems and possessed a higher degree of connectivity and reservoir quality in the single-cycle reservoirs. During Campanian-Maastrichtian times, large hinterland areas of the East Greenland mainland became subject to erosion and sourced extensive sheet-like basin-floor fan systems, up to I kin thick, forming multi-cycle reservoirs.
Two large submarine slides, The Storegga and the Traenadjupet Slides, occurred on the Mid-Norwegian margin during the Holocene. The Ormen Lange gas field is located within the scar of the Storegga Slide. This gigantic submarine slide occurred about 8200 years ago, and caused large waves (tsunamis) that reached the coasts of Norway, Scotland, Shetland and the Faroe Islands. The objectives of this chapter are to present the challenges and the slide risk assessment related to the development the Ormen Lange gas field. The risk evaluation is based on a qualitative approach for large natural slides, and a quantitative approach for new small slides in the vicinity of the development area.The work programme includes extensive, regional multi-disciplinary studies, carried out jointly by academia, industry and research institutions. The database includes an extensive grid of seismic data, detailed sea-floor morphology and sediment properties from a number of 'geoborings' (combined geological and geotechnical borings to sub-bottom depths of 200-400 m). Stability of the steepest slopes in the vicinity of the development area is calculated. Effects of excess pore pressures, earthquakes, reservoir compaction during depletion and underground gas blowouts into possible permeable layers have all been included in the stability calculations. To understand the recent slide history in the area and to find the frequency of the sliding, extensive sea-floor mapping and coring to date slide events are also included.A geological model for the Plio-Pleistocene of the area explains the large-scale sliding as a response to climatic variability. Over long periods, marine deposition prevailed with focused deposition due to current effects in the locations of the Storegga and the Traenadjupet Slides. During short intervals of peak glacial conditions, till and glacial debris flow sediments were deposited at high rates directly on the continental slope. This created excess pore pressures in the thick marine deposits. The most likely triggering mechanism of the slides is a strong earthquake following the onshore uplift after the glaciation. This explains why the slides take place after a glacial period. Since all the soft unstable clays were removed from the Storegga Margin during the last slide, it is concluded that a new cycle with sedimentation of soft clays and deposition of glacial sediments in the upper slopes are needed, to create a new unstable situation in the Storegga area. At present, the slopes in the Ormen Lange area have high safety factors, and the likelihood of new slides, both local and regional, is considered very low.
In 200 1, a high-resolution seismic survey was conducted for the detailed study of the distribution, both spatially and vertically, of gas hydrate and free gas accumulations west of Svalbard, as part of the HYDRATECH and INGGAS projects.High-resolution single-channel seismic reflection and the 4-component ocean-bottom seismometer (OBS) data illustrate the widespread nature of gas hydrates and free gas accumulations north of the Knipovich Ridge off Western Svalbard, by the presence of a nearly-continuous polarity-reversed bottom-simulating reflection (BSR) on down-slope seismic profiles. In the absence of a distinct and/ or a continuous BSR, it is the sudden change in reflection amplitude and frequency content that marks the base of the hydrate zone. The BSR coincides with the top of the free gas zone. Compressional wave velocity analyses and modelling reveal increased velocities above the BSR attributed to a gradual increase of partial hydrate saturation (6 - 10% of pore volume). A sharp drop in compressional-wave velocity across the BSR is due to free gas accumulation. The sub-bottom depth of the BSR closely matches the calculated stability limit for methane hydrates.To the east of the Knipovich Ridge, mud diapirism is observed in a deeper basin (similar to 2250 m water depths). The domes rise from an extensive chaotic source zone buried under a 200-400 ms thick sediment drape, and are more pronounced in the south. At some places, there is evidence of stratigraphically-controlled shallow gas accumulations (bright spots) and short cross-cutting BSR-like features that might point towards the presence of hydrate and/or free gas. The diapiric movement is believed to be a recent and still ongoing process of mass mobilisation.In both the cases, the nearby and tectonically-active slow-spreading, Knipovich Ridge is assumed to play an important role in the generation of elevated heat and methane fluxes as well as faulting and subsequent fluid migration. As a result, shallow subsurface hydrates (< 10% of pore volume) may form and mud diapirs may develop.
The Jan Mayen microcontinent lies between the active Kolbeinsey Ridge spreading centre and the extinct Aegir Ridge spreading centre in post-Paleocene oceanic crust to the north of Iceland. Uncertainties concerning the age of seafloor magnetic anomalies and the precise extent of oceanic crust in this segment of the northern North Atlantic have hindered attempts to model the spreading history. Here, we propose a new, geometrically self-consistent spreading model that uses a single set of rotation poles for the entire northern North Atlantic.In our model, the Jan Mayen microcontinent separated sequentially from the East Greenland margin during Oligocene time as a consequence of stepwise northward propagation of the Kolbeinsey Ridge and simultaneous northward retreat of the Aegir Ridge. The ridge tips were linked by a fracture zone that was periodically replaced by a new fracture zone to the north, resulting in balanced propagation/retreat of the spreading ridges and segmentation of intervening oceanic and microcontinent lithosphere. Spreading azimuths remained parallel with the West Jan Mayen Fracture Zone through the propagation/retreat phase. A number of possible fracture zones of the appropriate orientation can be identified that cut both the microcontinent and the oceanic crust to the cast. Systematic sinistral offset across these fracture zones produces an apparent counterclockwise rotation of the microcontinent with respect to the adjacent continental margins, whereas structural trends within the Jan Mayen microcontinent are not rotated appreciably.At least two factors appear to have been important in initiating the Kolbeinsey Ridge, and thus creating the Jan Mayen microcontinent: (1) the geometry of the plate boundary generated between Europe and Greenland at continental break-up (chron 24R), with the Aegir Ridge significantly offset to the east with respect to the Mohns and Reykjanes ridges; (2) a change of spreading azimuth, which acted to lock the transform system that had previously connected the southern tip of the Aegir Ridge with the northern end of the Reykjanes Ridge. The thermal effect of the Iceland plume on the overlying plates probably played little part in microcontinent generation, although the gravitational effect of the plume may have been significant.
From the Palaeozoic to the Cretaceous, crustal thinning in the Mid Norway area was associated with the denudation of gneiss-cored culminations and metamorphic core complexes in the footwalls of major extensional faults. The development of the culminations led to warping and deactivation of early detachments, to the nucleation of new faults in more distal positions and to the exhumation of high-grade metamorphic rocks to more shallow levels in the crust. Some of the culminations and core complexes became part of the erosional template in Mid-Late Palaeozoic time, some were probably exhumed in the Mesozoic, whereas some may never have reached the surface. We present an overview of five types of gneiss-cored culminations and core complexes that have been identified in the field, through the interpretation of offshore, long-offset seismic reflection data. We furthermore address their mechanism(s) of formation, and their role in the progressive evolution of the Mid-Norwegian margin.
Two-dimensional seismic data from the Mid-Norwegian margin provide evidence for sediment liquefaction and fluid mobilisation within the sediments that were located at the base of the hydrate stability zone before the Storegga Slide occurred. The disturbed subsurface sediments are overlain by a prominent roll-over structure and sea-floor collapse. This indicates fluid escape from the formerly hydrated sediment and suggests that the landslide caused a pressure drop strong enough to dissociate the gas hydrates. We calculate that this fluid escape must have taken place within less than 250 years after the slide, as the effect of pressure decrease on hydrate stability was later compensated by a temperature decrease, related to the slumping process. The volume of expelled fluids from the collapse structure exceeds the volume of the gas hydrate dissociation products, implying that gas hydrate dissociation significantly affected the surrounding sediments.
The tectonostratigraphic framework of the Oligocene-Miocene succession in the northern North Sea Basin (58-62 degrees N) is closely linked to the large-scale structural evolution of the NW European passive margin. Fairly contemporaneous with the structural doming on the Mid-Norwegian margin uplift activity also affected the Shetland Platform and southern Fennoscandia, including the sedimentary basin of the northern North Sea. This uplift caused a gradual shallowing-upward trend of the northern North Sea Basin, which culminated in severe submarine and possibly also subaerial erosion during middle Miocene, creating a northward increasing stratigraphic break (20 million years in northernmost North Sea), which is visible as a distinct seismic unconformity. Uplift of the East Shetland Platform caused three major phases of sand influx to the basin (1) an early Oligocene phase, resulting in deposition of gravity flow sands in the northern Viking Graben (Statfjord Tampen area); (2) an early Miocene phase, resulting in deposition of turbiditic sands (Skade Formation) in southern Viking Graben; and (3) a late Miocene-early Pliocene phase, resulting in deposition of shelfal sands (Utsira Formation). During the latter phase, the northern North Sea Basin formed a relatively shallow marine, shelfal strait between deeper marine settings to the north and south. The Utsira Formation sands accumulated in this narrow strait in a high-energy, possibly tidal-current controlled regime.This chapter also presents an improved lithostratigraphic and chronostratigraphic subdivision of the Oligocene-Miocene including redefinitions of the Skade and Utsira formations. The Oligocene-Miocene succession in the northern North Sea has been subdivided into two megasequences, separated by a seismically distinct unconformity (mid-Miocene break). The age diagnostic Bolboforma assemblages, known from ODP/DSDP boreholes in the North Atlantic and on the Voting Plateau, have aided in correlation between wells and have been important in resolving the basin history.
This review discusses regions of vertical movements of southern Norway and its continental shelf from the Present to the Devonian. The processes examined are distinguished on the basis of their effect into long-wavelength and short-wavelength.On the mega-scale, two main configurations are identified. The older configuration relates to the late- to post-Caledonian stage, which was dominated by orogenic denudation processes that followed the Caledonian Orogeny. This probably included thermal as well as isostatic effects, which contributed to the development of an asymmetrical long-wavelength uplift area with a low-relief eastern flank towards the Baltic countries, and a western hinterland region of high relief, especially above exhumed gneissic regions. The hinterland probably had a rugged topography, similar to that of the present Himalaya.The younger configuration is mirrored by smoother highs and lows of events that were probably, to a large extent, thermally controlled. They include Carbo-Permian, Permian and Jurassic rifts, of which the latter particularly affected the flanks of a south Norwegian high, or dome. From the earliest Tertiary, this feature seeiris to have been stabilised and supported by a horizontal transfer of hot material associated with the Icelandic plume. Pluming may be superimposed by more recent glacial rebound, which presently interferes with the long-term effects of the North Atlantic asthenospheric plume.
The Early Triassic Wordie Creek Formation at Kap Stosch, Hold with Hope, represents sediments deposited during the rift climax stage in a Late Palaeozoic-Early Mesozoic rift episode in NE Greenland. The Wordie Creek Formation comprises over 750 m of elastic marine sediments deposited in less than four million years. The formation is dominated by mudstones. However, in the lower part, four coarse grained units have been identified from the Kap Stosch sections. These units are interpreted to represent southwards progradation of coarse elastic deltas filling the half-graben basin from the north. Foresets and cross-stratification measurement from the coarse elastic units demonstrate that the sediments were deposited axially into one or more N-S trending marine half-grabens. The intra-basinal faults controlling these half-grabens, on the basis of sedimentological data, are interpreted to have been active during deposition of the Wordie Creek Formation. The Kap Stosch area appears to be positioned on a large relay ramp linking two major N-S striking fault zones. Progradation of coarse elastic deltas into the half-grabens are suggested to be associated with contemporaneous movement along these two faults, resulting in rotation of the relay ramp. This led to increased uplift and erosion of the upper part of the ramp, and local subsidence and progradation of coarse elastic deltas in the lower parts of the ramp. The coarse elastic units show a back-stepping trend, related to a gradual increase in water depth in the half grabens. We link this to an overall transgression in the region controlled by rapid tectonic subsidence of the entire rift basin.
Large, up to 45 m high and at least 8600 year old coral reefs composed of the reef-building, stony coral Lophelia pertusa (L.) occur off Mid Norway. Their locations have been documented by side-scan sonar, multi-beam echosounder, and ROV (remotely operated vehicle) surveys carried out by the hydrocarbon industry and by authorities (Foss and Mortensen, 1998; Hovland and Mortensen, 1999; Jung et al., 2001; Freiwald et al., 2002). So far, the following large, continental shelf- and continental slope-based reefs have been found and mapped off Mid Norway: the Sula Ridge reef (including the Haltenpipe reef cluster), the Horse-Shoe Ridge reefs (Hesteskoen), the Traena Deep reefs, the Rost Bank reefs, and the Storegga escarpment reefs (Fig. 1). There are also numerous smaller coral structures of 1-3 m height, which include colonies of Lophelia pertusa. These are scattered on the general seafloor and tend to be located on bathymetric highs, such as moraine ridges, on glacial flutes and on iceberg plough marks. Both the Sula Ridge reef and the Traena Deep reefs are located on top of sub-cropping Mesozoic sedimentary rocks that dip towards the west. The Horse-Shoe reefs are located on top of a moraine ridge, which is located over dipping Mesozoic strata. The Storegga escarpment reefs are located on top of Quaternary marine and glacimarine sediments. Thus, all these large reefs seem to have at least two conditions in common: They are located on top of a firm sea floor, and are on local heights. Because of their importance as feeding and breeding grounds for some of the fish species, and also because of their obvious importance as biological resources, the large coral reefs, off Mid Norway must be carefully respected by the hydrocarbon industry. Large reefs are also known to occur along the coast and in some of the fjords of Mid Norway (Mortensen and Fossa, 2001). This means that the existence of reefs will have to be considered in all aspects of hydrocarbon exploration and exploitation, off Mid and Northern Norway, i.e., prior to and during exploration drilling, field development, and hydrocarbon transportation.
Maps of the depth to the crystalline basement and to the Base Cretaceous, and of the pre-Cretaceous sedimentary thickness have been constructed. These maps illustrate the deep geology of the study area. In addition to showing the total thickness of sedimentary basins (> 12 km in the More and Voring basins), the maps also show a lucid picture of structural relief, both at basement depth levels and in the pre-Cretaceous sediment thickness map. Buried rift valleys are seen in the maps. In the platform area, a picture of horsts and grabens emerges that, in most areas, is probably related to the combined late Palaeozoic to early-middle Mesozoic structuring of the area.The geometry of structural highs and lows shown in our maps is the combined result of several rifting events that were superimposed on the Devono-Carboniferous, late- to post-orogenic extensional collapse structures. We think that the NW SE structural grain on the shelf reflects the offshore continuations of Devono-Carboniferous shear zones and detachments that are observed onshore. Locally, these detachments were reactivated as transfer zones. Movements along these shear zones have determined the position of boundaries between the magnetic basement terranes (primarily Precambrian high-grade rocks) and the less magnetic Caledonian allochthons.From our maps, it can be seen that 'old' sediments must occur within Permo-Triassic basins and Jurassic basins, not only on the Trondelag Platform (previously well documented), but also farther west, beneath the Cretaceous and Cenozoic sediments of the More and Voring basins. The basement topography reveals a narrow, deeply buried, rift-related relief (Jurassic rift valleys), and a wide basinal area recording prolonged subsidence in the mid and late Cretaceous, as well as the Tertiary and Quaternary subsidence. The difference between the basement map and the Base Cretaceous map supports the idea that the Cretaceous sedimentation represents a post-rift thermal and isostatic subsidence stage resulting in the infill of a pre-existing rift topography.Comparison of the Base Cretaceous and the basement maps shows a close correspondence of trend features, demonstrating a basement influence through to the Cretaceous. The topography and the offshore basement map demonstrate similar tectonic trends, probably indicating that a basement similar to that in western Scandinavia underlies the marine areas, and that several tectonic events have affected both the land and the sea areas.Highly magnetic, granulite-facies, felsic rocks give rise to strong magnetic anomalies along the Precambrian gneiss terranes of the coastal zone in central-northern Norway. The occurrence of these magnetic rocks is structurally related to the Devonian shear zones and the basement antiforms. We recognise these antiforms and synforms in the offshore, thereby providing a model for the distribution of low-magnetic Caledonian rocks and high-grade Precambrian intermediate rocks in the shelf areas.The rhomboid-shaped geometry seen in the basement map is partly due to the Devonian extensional collapse structures and partly to N-S and NE-SW oriented faults active during post-Devonian rifting. Major trends responsible for the rhomboid-shaped geometry (NW-SE, N-S and ENE-WSW to NE SW) are also found in the topography of the Precambrian and the Cambro-Silurian basement units of western Scandinavia, suggesting that Precambrian faults have been reactivated both on land and in the offshore. The More-Trondelag Fault Complex (MTFC) is suggested to consist of two branches in the offshore. One branch is the extensional fault, located east of the Slorebotn Sub-basin sub-parallel to the coastline, and the other one is the fault alignment from Hitra towards Shetland.
At the 1996 NPF Conference on Hydrocarbon Seals we gave the first presentation of results of a fault-seal study using the Shale Gouge Ratio algorithm, describing a project undertaken in 1994 by Badleys and Norsk Hydro on the Oseberg Syd field. Over subsequent years the methodology has been applied to many tens of data sets in both exploration and production environments. This Special Publication represents an opportunity to review the performance of this fault-seal predictor. Shale Gouge Ratio, or SGR, is an estimate of the proportion of shaly material in the fault zone. This parameter is of direct importance in fault-seal prediction because the very fine-grained nature of phyllosilicates results in very small pore-throats, giving high capillary entry pressures and low permeabilities for the fault-zone material. Measurements on fault-gouge samples show that phyllosilicate content is the first-order control on their fluid-flow properties. It is used to define the fault-gouge type in mixed clastic sequences (e.g. cataclasites/framework-phyllosilicate fault rocks/clay smears). The basic assumption in the SGR algorithm is that the fault-gouge composition is governed by the bulk composition of the wall rocks that have slipped past that point on the fault. Faulting through clean sandstones generates cataclasites, whereas dragging clay beds along the fault generates clay smear. Analysis of outcrop and experimental observations suggests that the algorithm does indeed make a fair estimate of the fault-zone composition. The Oseberg Syd study suggested that an SGR value between 15 and 20% represented a threshold value between non-sealing and sealing faults, in an appraisal context. This value also represents the maximum clay content of cataclastic gouge, implying that in this field cataclasites do not form significant seals whereas more clay-rich gouges do. This threshold has proven to be surprisingly robust, not only in the Brent Province but also in other basins with mixed clastic reservoirs. Compilation of many SGR analyses with in situ pore-pressure data has allowed a better definition of the relationship between calculated SGR and maximum trapped hydrocarbon column height, i.e. the ‘fault-seal failure envelope’, for different geological histories (e.g. depth of burial). An advantage of the SGR method over others (e.g. ‘clay smear potential’) is that it predicts a physically measurable parameter (composition) and can therefore be used to predict other properties that are compositionally controlled. The most significant of these is fault-zone permeability, which may vary by many orders of magnitude between cataclasites and clay smears. If correctly calibrated, the SGR distribution on a fault plane can therefore be used as a map of fault-zone permeability, which can in turn be used to provide fault transmissibility multipliers for reservoir simulations. Case studies (e.g. as described here on the Scott Field) show that the SGR methodology can provide a very quick (and yet geologically based) route to a high-quality history match. The experience gained over the last six years shows, not surprisingly, that high-quality input data are essential to quantitative fault-seal studies, in particular good fault mapping and well-prepared Vshale (volumetric shale fraction) data. Nevertheless, Shale Gouge Ratio has proven to be a robust and quantitative predictor of fault seal in mixed clastic sequences.
We have developed a model of fluid flow and pressure development in sedimentary basins that incorporates pore volume loss due to mechanical compaction and to chemical diagenesis (quartz cementation, grain contact quartz dissolution and illitization). Mechanical compaction is modeled to be a function of effective stress. In this model, pore volume loss due to mechanical compaction will be retarded when overpressure develops. The diagenetic processes are modeled as being kinetically controlled and the reaction progress depends only on the temperature history. Hence pore volume loss due to chemical compaction is not retarded by overpressure. By including diagenetic effects on overpressure development, the pressure model should be more generally applicable than models that consider mechanical compaction to be the sole process that reduces porosity. To demonstrate the potential importance of chemical compaction in the formation of fluid overpressures in different settings, we calibrated our model with data obtained from the Halten Terrace offshore mid-Norway and from the Gulf of Mexico. In both cases, the diagenetic processes have the potential to control on the timing and magnitude of overpressuring. From 25% and up to 80% of the present-day overpressure may be caused by pore volume loss resulting from diagenetic reactions. Pressure build-up from diagenetic processes also potentially controls the timing of hydraulic fracturing. If diagenetic processes are actively contributing to overpressure generation, then unrealistically low shale permeabilities are not needed to retain overpressures for geologic time periods (>10 My).
One of the main mechanisms for generating fault seal is by shale smearing or shaley gouge formation in the fault zone and fault sealing potential is therefore generally considered to be related to the amount of shale within the faulted sequence (Yielding et al. 1997).
Empirical observation indicates that hydrocarbon accumulations in exhumed basins of the Atlantic Margin are commonly characterised by underfilled traps and hydrostatically pressured or modestly overpressured reservoirs. These observations are reviewed in the context of the generic mechanisms by which top-seals leak, the properties of cap-rocks and the physical processes which occur during exhumation. The fluid retention capacity of any cap-rock lithology during exhumation is dependent upon the physical and mechanical characteristics of the cap-rock at the time of exhumation and the timing and conditions of the associated deformation relative to the timing of hydrocarbon emplacement. The permeability and deformational characteristics of halite renders it an excellent cap-rock with a high retention capacity, even under conditions of exhumation. However, mudrocks may also form effective cap-rocks in exhumed basins when the deformation associated with exhumation occurs prior to embrittlement and the shale cap-rock exhibits ductile behaviour. Shale and evaporite cap-rocks form the main regional seals to hydrocarbon accumulations in exhumed basins of the Atlantic Margin and borderlands. Syn-exhumation top-seal efficiency (fluid retention capacity) is a major exploration risk in these basins, though post-exhumation top-seal integrity in these basins may be relatively high under certain conditions. Consequently, a major exploration risk factor in exhumed basin settings pertains to the limited hydrocarbon budget available post-regional uplift and the efficiency of the re-migration process.
