The Middle North Sea, have simplified to a Jurassic deltaic Brent Group sandstones, northern complicated diagenetic sequence which may be kaolinite Fe,Ca carbonates - High porosities and Ness and Tarbert quartz overgrowths - Fe,Mg carbonates - illite. existed in the Etive, throughout diagenesis. By permeabilities have Formation sandstones contrast, the highly micaceous and r,elatively finer grained Rannach Formation sandstones have good porosities but very poor permeabilities due to burial compaction and carbonate cementation. Oxygen and hydrogen isotope studies indicate that early diagenesis occurred in a dominantly meteoric pore water (6180 = -7%0). The early diagenetic cements of siderite, vermicular kaolinite and calcite started to precipitate at 14, 26 and 32DC respectively. With the onset of burial, below 1.2 km, the Brent sequence eventually became sealed off from the meteoric 'head' by Lower Cretaceous sediments. Blocky kaolinite precipitated and by the end Cretaceous quartz overgrowth formation commenced. Fluid inclusions in the overgrowths indicate formation from a warm, dominantly low salinity water (1 - 5 wt.% eq. NaCl). Homogenisation temperatures range from 73to 131 DC. Illite precipitation is cogenetic with the latest stages of quartz overgrowth precipitation (K/Ar illite date, 58Ma.) and has reduced porosity and permeability markedly in different locations prior to oil migration. The depth of burial at which this last cementation event occurred is 2.3 km. Fluid inclusion microthermometry indicates that quartz overgrowth and latest ankerite precipitation occurred in a geothermal gradient of 70DC/km. After this heat excursion the reservoirs have cooled back to present day temperatures of 85 - 11SDC. Water values computed from the mineral cements indicate precipitation from a porefluid which has gradually evolved isotopically to its present day composition (5180 = -7 to +2%.SMOW) in an isotopic system that has become closed during burial. However the quartz and ankerite suggest one unusual episode of open system hot fluid input.

This paper represents a collective effort by the Supply Analysis Group of the M.I.T. World Oil Project and was partly supported by the U.S. National Science Foundation under Grant no. SIA75-00739. / U.S. National Science Foundation under Grant no. SIA75-00739.

The geometries of the sandstone-hosted calcite concretions of the Jurassic Valtos Sandstone Formation and Bearreraig Sandstone Formation are described and related to the processes operating during concretion growth. As concretionary bodies analogous to those studied form potential permeability barriers within some North Sea petroleum reservoirs; the relationship between the growth processes and permeability barrier formation is examined. The growth times for model spherical concretions are calculated for the complex carbonate-water system. Two growth processes are modelled, solute transport and surface reaction. Growth times for a 1m diameter concretion forming under geologically reasonable conditions are predicted to be 22.3Ma, which reduces to 8.8Ma in porewaters flowing at 1m/year. The depth of formation of the concretions is assessed, through an examination of depth dependent properties of both the host-sediment and the calcite cement, and is found to be less than 500m. Concretion formation preceeded the Paleocene igneous activity which affected the Hebrides. The majority of the concretions examined formed at burial depths which were too great to allow effective contact between the concretions and seawater. The major source of carbonate was the dissolution of aragonitic shell material from within the host sandbody. The nature of the porewaters from which the concretions formed is assessed. The majority were meteoric in origin, though some marine influence is noted. The minor element contents of the cements cannot be used to calculate porewater compositions, as disequilibrium between the porewaters and the cements can be demonstrated. A model is proposed to account for the minor element patterns. Crystal breeding can be demonstrated to have occurred during concretion growth. A hypothesis is presented to explain the petrographic features of a typical Valtos Sandstone Formation concretion.

Middle Triassic deposition in Northwest Europe occurred in the enclosed Germanic Basin, which extended from its connection with Tethys in eastern Poland to eastern England. A detailed lithostratigraphic subdivision of the middle Triassic Dowsing Dolomitic Formation of the U.K. Southern North Sea and subsurface eastern England is presented, based on petrophysical logs, well cuttings and limited core. This subsurface lithostratigraphy is linked to the established outcrop lithostratigraphy of eastern England by means of a surface gamma-ray profile at Kirton Brickworks, Nottinghamshire. The outcrop lithostratigraphy is thereby accurately correlated with that of the basin centre in north Germany. The major tectonic elements active during middle Triassic deposition in the Southern North Sea area are identified, including previously unreported differential subsidence in an extension of the Dutch Broad Fourteens Basin in the U.K. Sector. The basinwide Hardegsen unconformity marks the base of the middle Triassic. A transgressive phase resulted first in the deposition of lacustrine pro-delta mudstones towards the basin centre, followed by the deposition of a more extensive thin marine mudstone which formed the base of the middle Triassic over most of the U.K. Southern North Sea area. Two regressive cycles followed. Each started with deposition of the cyclic, hypersaline-marine halites (the Main and Upper Röt Halites), followed by sabkhas, then playa mudflat/lagoonal deposits. At the start of the Muschelkalk deposition, restricted marine conditions were established in the Southern North Sea area. These were followed by a further regressive sequence of hypersaline-marine halites (the Muschelkalk Halite), sabkhas, and finally playa mudstones. The establishment of continental sedimentation across the whole of the Germanic Basin marked the end of the middle Triassic. The cyclicity within the Germanic Basin correlates with the published coastal onlap charts, indicating its origin is eustatic fluctuations in Tethys.

This study focuses on the main depocenter of the Andrew Formation in the Moray Firth Basin, located at the junction of the Central and Viking Grabens, in the central North Sea. The objectives of this report are to (1) define the sequence stratigraphic framework of the Andrew Formation, (2) describe and characterize the depositional systems associated to the Andrew slope to basin system, and (3) interpret the depositional processes that have dominated sediment emplacement. Specific facies association of the Andrew Formation are determined by the nature (point source or linear source) and caliber (volume, grain size, sand:mud) of sediment supply to the slope environment. Genetic interpretation of the Andrew Formation focuses on understanding depositional processes which dominated sediment emplacement. Seven depositional facies have been identified for the Andrew slope and basinal system; turbidite channel-fills, turbidite lobes, mounded turbidite lobes, sheet turbidites, debris flows and slumps, low density turbidites and hemipelagic drapes. Seven depositional processes collectively create the above mentioned Andrew depositional facies; turbidity currents, cohesive mud flows, sandy debris flows, muddy debris flows, slumping, low density turbidity currents and suspension settling. The Andrew Formation consists of upper and lower depositional units identified on seismic by bounding downlap terminations and on well logs by high-gamma marker beds. The lower Andrew displays three distinct sand-rich lobes, delineated by isopach and sand percent maps and log motif characteristics. Proximal, mounded, sand-rich units disperse into unchannelized sheet turbidites in the basin plain areas. The upper Andrew downlaps the lower unit, and a single, linear sediment source was centered in the Witch Ground Graben. The sediment dispersal pattern and internal facies character suggest the upper unit is a proximal slope-apron downlapping and filling inter-lobe bathymetric lows of the underlying unit. The lower Andrew is interpreted to be a structurally focused, sand-rich lobe complex, without associated incised canyons. The Andrew system evolved as the delta platform expanded onto the proximal fan, resulting in a linear sediment source spilling over the slope as a fringing slope-apron. The Andrew depositional system in the slope and basin environment is characterized by a high degree of facies disorganization composed of a wide array of gravity-flow deposits. / text

No description available.

No description available.

The North Sea is a marginal, epeirogenic sea lying in an intra-plate setting on the NW European continental shelf in the northern hemisphere at about 54 degrees north. It is a shallow, elongate, saucer-shaped depression that is more than 970 km long and 560 km wide; with an area of around 570,000 km2 and connected with the Atlantic Ocean both to the north through the Norwegian Sea, and to the south through the Dover Straits and English Channel. However, it has not always had this form nor existed in such a state, the rocks buried beneath the sea-floor indicate a long history of tectonic activity and markedly different environmental conditions from those present today. The North Sea is an important hydrocarbon province and it is essential to understand how it has evolved into its present form for successful exploration. Additionally, the large amounts of data gathered for use in hydrocarbon exploration provides a unique opportunity to investigate the structural and stratigraphic history of the area which may then provide analogues for understanding tectonic, stratigraphic, sedimentological and climatic responses through time in areas of the world where data is more limited. Climate is also recognized as playing a vital role in the stratigraphic development of the basin, influencing sedimentary settings and depositional facies. Extreme climate events such as hyperthermals and ice-ages are therefore important to study as these will have the most measurable effect on basin evolution. Additionally, studying hyperthermal events can provide information on the causes and consequences of global warming which is particularly relevant to the present day. The accepted understanding of the geology of the Central North Sea is that the current structural configuration arose from a period of rifting during the Jurassic. This extension formed a trilete extensional system consisting of three rift arms, the Moray Firth Basin, Viking Graben and Central Graben. The underlying structure of the Central North Sea is dominated by the influence of the Central Graben which itself is split into two arms, the Eastern and Western Troughs. This dominance highlights one of the central problems in interpreting regional geological histories as the most recent tectonic events tend to overprint and often obscure critical features of older geological events, leading to erroneous and confusing tectonic reconstructions. Additionally, the Jurassic rifting episode created many of the structural traps which today are exploited for hydrocarbons and has therefore been the subject of many of the previous geological studies undertaken within the Central North Sea.

No description available.

This study is based on a regional grid of seismic data acquired by Merlin Geophysical. The main emphasis of this thesis was on the detailed structural mapping of the Danish Central Graben, since this area has the highest density of well and seismic data. The principal objective of the study was to explain why the North Sea Rift changes strike in the Danish Sector, from the N-S trending Dutch Central Graben, to the NW-SE trending U.K/Norwegian Central Graben. The Danish Central Graben opened initially in the Late Carboniferous, as part of a regional dextral transtensile deformation that affected much of Europe. The initial extensional structures developed by reactivation of a pre-existing basement fabric. The NNW trending Coffee Soil Fault bounding the rift, is interpreted as a planar structure transecting the entire crust, and is thought to have developed by extensional reactivation of an east-verging Caledonian thrust. Basement shear zones identified on the rift shoulders on the Ringkobing-Fyn High are interpreted as the along-strike continuation of the Caledonides of Southern Norway, offset to the east by syn-orogenic transform motion along fracture zone elements of the Tornquist Zone. The WNW trending fault zones that dominate the structural grain in the Danish Central Graben are shown to be closely related to WNW trending fracture zones on the Ringkobing-Fyn High, which are regarded as splay shears of the Tornquist Zone. The WNW trending transverse fault zones segment the Danish Central Graben. The segmentation exerted a fundamental influence on the structural and stratigraphic development of the rift, in that individual segments were free to subside at different rates, in different styles, at different times. Two main phases of rifting are recognised, Permo-Triassic, and Middle and Upper Jurassic. These two phases have contrasting patterns of subsidence, and contrasting structural expression, particularly as regards the extent of the involvement of the transverse fault zones. The Permo-Triassic phase is characterised by parallelism of stratal configurations, and horizontal subsidence of the graben floor, whereas the Jurassic phase is characterised by strongly divergent configurations and asymmetric, rotational subsidence directed towards and controlled by the Coffee Soil Fault. Active rifting ceased at the end of the Jurassic, and Lower Cretaceous sediments are found to onlap extant fault scarps in a passive infill of the rift basin. The major bounding structures of the rift change strike abruptly in several incremental steps across the points of intersection with the transverse segment boundaries. The re-orientation of the rift is therefore explained as a consequence of the underlying presence of the earlier transform dominated basement fabric. A model for the formation of the North Sea Rift is proposed, which draws heavily on observations of the fracture patterns in continental rifts such as the Oslo Rift and the Rio Grande Rift, and is a development of the Megashear-Rhombochasm concept of S.W.Carey.