bluuebunny/crossref_metadata_2025_split huggingface dataset:
Magmatism, deformation and high-
T
, low-
P
regional metamorphism in the Nabitah mobile belt, southern Arabian Shield
Authors: Droop G. T. R. , Al-Filali I. Y. 
Published: September 2007

Summary

This paper examines the temporal and causal relationships between late Proterozoic deformation, plutonism and high-
T
, low-
P
regional metamorphism in a small area in the Nabitah mobile belt (Stoeser et al. 1984). The rocks have island-arc affinities and record the following sequence of events spanning c. 100–150 Ma: (i) calc-alkaline volcanism, (ii) calc-alkaline plutonism, (iii) intense ductile deformation, (iv) thermal peak of metamorphism and formation of mantled gneiss domes, (v) late-stage plutonism.

The
P-T
conditions at the peak of metamorphism are estimated from mineral assemblages in marbles and calc-silicate rocks to have been 620 ± 30°C, 3.2 ± 2 kbar, indicating a transient upper crustal mean thermal gradient of 54 + 64/-19°C km
−1
. No information is yet available on the
P-T
path, but a history of isothermal decompression is unlikely.

The cause of the metamorphism was probably the convective transfer of heat by the emplacement of high-level intrusions in an island-arc complex. The observation that the ductile deformation and the thermal maximum were almost coeval argues against a causal relationship, except in that whatever caused the deformation (an arc-continent collision?) may also have led to a reduction in magmatic activity and hence regional heat flow, thus terminating the metamorphism.
Exploration of the Brent Province
Authors: Bowen J. M. 
Published: September 2007

Abstract
The Brent Province is situated in the central part of the northern North Sea between 60°30′ and 62° North. A few licences were issued in 1965 (Round 2) and again in 1969 (Round 3). Seismic exploration from the mid-1960s onwards showed a thick Tertiary/Cretaceous sequence draped unconformably over tilted fault blocks of unknown age. In 1971, coincident with the opening of the 4th Round, Shell’s 211/29–1 pioneered drilling in the area with a major oil discovery, the Brent Field in Middle Jurassic deltaic sandstones. The 4th Round resulted in allocation of almost all the remaining blocks. Between 1972 and 1978, 32 discoveries were made in the UK sector of which 17 are currently in production and a further 7 in the pre-development stage; thereafter only minor reserves have been added, the area now being relatively mature. The exploration history in Norway has been similar although delayed by a few years; it is also less mature as several blocks have still to be explored. Total reserves for the province now approximate to 15 billion barrels of oil equivalent.
An introduction to the Brent Group: a literature review
Authors: Richards P. C. 
Published: September 2007

Abstract
There is a vast volume of published literature on the Brent Group, and consequently many controversies or differing points of view regarding the age, nature and palaeogeographic evolution of the Group. Brent Group lithostratigraphy is, however, relatively uncontroversial. A five-fold subdivision of the Brent Group into the Broom, Rannoch, Etive, Ness and Tarbert Formations is used by most authors. The Oseberg Formation is used to describe Broom Formation equivalent strata in Norwegian waters. There are, however, problems of applying this simple lithostratigraphy in some parts of the basin, where the artificial subdivision may unduly constrain depositional system correlation. Many companies use their own, field-specific, stratigraphic schemes for reservoir subdivision purposes, and some authors have applied a sequence stratigraphic framework to the Brent Group in preference to the lithostratigraphic scheme. There is, however, discord over the actual stratigraphic position of sequence boundaries and the time equivalence of strata. Brent Group biostratigraphy is less well documented in the literature than lithostratigraphy, and represents the area where most new work is needed if the evolution of the depositional system is to be better understood.
Analyses of the depositional environment of the Brent formations are numerous but more or less similar. The origin of the Broom Formation is the most controversial aspect, with a range of shallow to deep water interpretations; a fan delta interpretation is preferred in recent publications. The Rannoch and Etive Formations are generally interpreted as shoreface to foreshore/channel deposits, and the Ness Formation as delta top sediments. The Tarbert Formation represents an essentially transgressive unit. There are two main published models to explain the regional evolution of the Brent Group, and this is possibly the most active of the controversies in Brent Group geology. The first model considers the system as a northwards prograding delta with a southerly source and a concentric arrangement of facies belts across the basin. The second model envisages a dominantly transverse sediment supply to the basin, with subsequent localized northwards progradation within the basin.
A regional reassessment of the Brent Group, UK sector, North Sea
Authors: Cannon S. J. C. , Giles M. R. , Whitaker M. F. , Please P. M. , Martin S. V. 
Published: September 2007

Abstract
The deltaic Brent Group sequence consists of three formations. The Rannoch is a shoreface section; the Etive is a barrier facies. The Ness contains three members: the Enrick, representing a marginal marine back barrier environment; the Oich, an organic-rich open lagoonal shale and the Foyers, a largely fluvially-dominated delta-top or coastal plain deposit. During the Bajocian, facies belts rapidly prograded northwards until the northern part of the area was reached. Here progradation stalled and thick delta front sediments were deposited. Shortly before the delta began to retreat, a temporary relative sea-level rise pushed the delta front southwards resulting in the deposition of an intra-Ness barrier. The progradational sequence is underlain by the Aalenian to lowermost Bajocian Broom Formation fan delta and overlain by the transgressive Tarbert sequence of largely Bathonian age.
Advance and retreat of the Brent delta: recent contributions to the depositional model
Authors: Helland-Hansen W. , Ashton M. , Lømo L. , Steel R. 
Published: September 2007

Abstract
The behaviour and the development of the Brent Group in the Northern North Sea is documented by cross-sections and paleogeographic maps. Although this work is based on a previously published Norsk Hydro study, we present recent progress in the understanding of the Brent Group which has been achieved by a better timing of depositional events, by integrating new well information, and by generating new models based on the new information.
The early (Aalenian) lateral infill of the Oseberg Formation was deposited as a response to a relative fall of sea-level, probably generated by a tectonic uplift. Fan deltaic sediments built out towards the west and northwest and backfilled the previously emergent areas during the subsequent relative sea-level rise.
The progradation of the Brent Delta (Rannoch, Etive and lower Ness Formations) took place in Late Aalenian to Early Bajocian. The thickness of the Etive relative to the Rannoch Formation indicates thickening due to bathymetric deepening in the area south of the maximum regression whereas subsidence controlled the thickening in the area of the maximum extent of the delta.
During the retrogradational part of the delta development (the Tarbert and upper Ness Formations, Early Bajocian to Early Bathonian), increasing tectonic activity can be documented, both in terms of enhanced differential subsidence across faults and by early rotational uplift on some fault blocks.
The palaeoenvironments and dynamics of the Rannoch—Etive nearshore and coastal succession, Brent Group, northern North Sea
Authors: Scott Elaine S. 
Published: September 2007

Abstract
This paper investigates the palaeoenvironments and dynamics of the Rannoch Formation in the East Shetland Basin and the sedimentology of the Etive Formation in one particular part of the basin, the southern Cormorant area. The study utilizes core and well data from the North Sea as well as comparative outcrop data from Utah, western USA.
Rannoch Formation facies, coupled with a slight coarsening and cleaning upward of sand-dominated profiles above offshore shales and below channelized delta top sediments, are indicative of a prograding shoreface. The middle shoreface is dominated by hummocky cross-stratified fine sand characterised by horizontal to very low angle, planar to undulatory laminations, often cut by steeper scour features.
The Etive Formation is characterized in the southern Cormorant area by development of a microtidal, wave-dominated barrier cut by associated inlet channels.
These associations are consistent with an interpretation of the Rannoch Formation as a high energy, storm-dominated environment, with strong wave oscillatory motion accompanied by weak translatory currents as the most likely depositional setting. In the overlying Etive Formation, shoreline characteristics are those of a dissipative coastal system with a complex three-dimensional inshore topography and at least one nearshore bar system developed during fairweather periods.
Samarium-neodymium isotopic constraints on the provenance of the Brent Group
Authors: Mearns Euan W. 
Published: September 2007

Abstract
Samarium-neodymium data for the Brent Group of Gullfaks oilfield provide a detailed record of provenance. The Broom Formation, yielding relatively high Sm-Nd ages in the range 1700–1800 Ma, is interpreted as being derived from a proximal southwesterly source on the Shetland Platform. The Rannoch and lower Etive Formations, having lower Sm-Nd provenance ages (1550–1650 Ma) than the Broom, are interpreted in terms of easterly provenance involving transport westwards to Gullfaks by marine currents. The upper Etive Formation returns to the proximal southwesterly source. This source area grew southwards during initial deposition of the Ness Formation to encompass the Forties volcanic centre resulting in young Sm-Nd provenance ages in some Ness strata. Provenance ages in the upper Etive-Ness section thus have a large range from 1300 Ma to 1800 Ma. It is therefore suggested that the Brent Group was deposited by at least two major river systems: one draining an easterly source in southern Norway and the other draining a proximal southwesterly and southerly source in the Shetland Platform and the central North Sea.
Provenance of Brent Group sandstones: heavy mineral constraints
Authors: Morton A. C. 
Published: September 2007

Abstract
Heavy mineral suites in Brent Group sandstones have been heavily influenced by hydraulic and diagenetic processes in addition to provenance. Geochemical studies of detrital garnet assemblages have shown a great complexity in provenance that argues against a dominant sediment supply northward from an uplifted central North Sea dome. Much of the sediment was fed laterally, particularly in the Broom and Oseberg Formations. The Rannoch, Etive and Tarbert sequences are dominated by similar suites to those in the Oseberg Formation, implying that most of the shoreface and barrier sands were transported longshore from the east. Ness mineralogy is extremely complex. It has a broadly southerly source (possibilities including the southern Shetland Platform and Horda Platform). Close to the western margin of the basin there are strong northern Shetland Platform influences. The lateral and stratigraphic variation in Ness assemblages argues for deposition by several small-scale river systems rather than one single major river. The strong mineralogical variations provide a basis for reservoir subdivision in most of the studied fields, enable the identification of Ness channels that occur at the Etive level, and identify deposits within the Ness sequence that were derived from the barrier sequence into the Ness lagoonal sequence.
Statistical analysis of garnet compositions and lithostratigraphic correlation: Brent Group sandstones of the Oseberg Field, northern North Sea
Authors: Stattegger K. , Morton A. C. 
Published: September 2007

Abstract
Garnet compositions from sandstones of the Middle Jurassic Brent Group of the Oseberg Field, northern North Sea, have been treated statistically in order to assess objectively their value in stratigraphic classification and grouping and for provenance analysis. The data set consists of the compositions of 1550 garnet grains from three wells, as determined by electron microprobe analysis. The statistical techniques used include summary statistics, exploratory data analysis, analysis of correlation structure and of subcompositions, discriminant analysis and fuzzy clustering. General univariate statistics provide the characteristic properties of the data distribution pattern. Almandine is the prevailing component, followed by pyrope, grossular and spessartine. Variation in the bulk data is generally low except for many high spessartine outliers. Stepwise linear discriminant analysis is the first step in stratigraphic grouping of the samples and in end-member modelling. Using all 1550 garnet grains, group classification and separation is efficient for the Tarbert Formation and the Etive-Oseberg interval, but not for the Ness Formation. End-member analysis by fuzzy c-means clustering generates similar stratigraphic garnet logs to those generated by discriminant analysis, the advantage of the method being that fuzzy clustering allows a dynamic interpretation by assuming that the calculated cluster centres represent the garnet compositions of source rocks or source areas. There is an overall increase in almandine with time: the Etive-Oseberg interval is characterized by high pyrope contents: the Ness Formation has high grossular and spessartine contents and depletion of pyrope: and the Tarbert Formation is depleted in grossular. Therefore multivariate end-member modelling of garnet compositions has provided objective criteria for classifying and differentiating samples. The resulting grain and sample distribution patterns provide a clearer picture of the stratigraphic variations than those obtained by using the geochemically-defined garnet end members.
Diagenetic processes in the Brent Group (Middle Jurassic) reservoirs of the North Sea: an overview
Authors: Bjørlykke Knut , Nedkvitne Tor , Ramm Mogens , Saigal Girish C. 
Published: September 2007

Abstract
The Brent Group is extensively cored in the northern Viking Graben and East Shetland Basin in the range between 1.8 and 4.5 km. It therefore offers the opportunity to study diagenetic processes arrested at different burial depths and temperatures. The sandstones of the Brent Group remain poorly indurated and contain only small amounts of quartz cement down to burial depths of about 2.5–3.5 km. The quartz cementation increases with depth reflecting progressive silica cementation sourced by pressure solution. Dissolved feldspar and authigenic kaolinite occur in the shallowest of the Brent Group reservoirs and secondary porosity shows no significant increase with depth. Petrographic analyses show a strong depletion of K-feldspar with depth. This can be explained by albitization of K-feldspar at intermediate burial depths (2.5–3.5 km) and dissolution of K-feldspar during illitization of kaolinite on deeper burial (below 3.7–4.0 km). Extensive quartz cementation and formation of illite are the main causes of an observed rapid permeability reduction below 4 km burial depth. K-Ar dates of authigenic illite indicate that illite formed mostly in Early Tertiary at rather shallow burial depth. Illite formed by alteration of kaolinite and K-feldspar seems, however, to increase very significantly at present burial depths of 3.5–4.0 km, suggesting a more recent formation. Emplacement of oil probably does not stop all diagenetic reactions in the remaining residual pore water, and precipitation of illite and quartz may continue in a closed system.
The reservoir properties and diagenesis of the Brent Group: a regional perspective
Authors: Giles M. R. , Stevenson S. , Martin S. V. , Cannon S. J. C. , Hamilton P. J. , Marshall J. D. , Samways G. M. 
Published: September 2007

Abstract
A regional study of Brent Group diagenesis and reservoir properties has been undertaken in order to determine the main controls on porosity and permeability in the sandstone reservoirs. Data from 44 wells from block 211/7 in the north to block 3/8 in the south and spanning current depths from 6700 to 13 400 ft include 9000 porosity, permeability and grain density determinations, quantitative petrographic information from 850 thin sections, and stable isotopic and K/Ar analyses of authigenic phases.
The diagenesis of the sediments is similar across the study area and most of the diagenetic phases occur in all formations. The sequence of precipitates and dissolution events reflects early porewater evolution in shallow burial environments and later reactions which were essentially isochemical and controlled largely by increasing burial depth and therefore temperature. Early diagenetic products include siderite, calcite, chlorite and vermicular kaolinite. Only where the calcite cements form concretions or cemented horizons have they a significant effect on reservoir properties. Local dissolution of feldspars and carbonate cements took place on the crest of some fault blocks inferred to have been emergent during the Jurassic and this has caused local enhancement of porosity in some crestal wells.
In general, the porosity of each of the reservoir facies decreases systematically with depth but permeability only starts to decrease significantly at depths greater than 10 200 ft (3109.0 m). The general decrease in porosity can be attributed to compaction, together with burial cementation by quartz and iron-rich carbonates. Secondary porosity resulting from feldspar dissolution is increasingly common at depth but there is no net increase in porosity as much of the dissolution was evidently accompanied by the precipitation of authigenic quartz initially with kaolinite but at greater depths with illite. The systematic changes in porosity and the decrease in permeability are compatible with thermally driven dissolution of feldspar and the local reprecipitation of the authigenic silicates; the decrease in permeability corresponds to the presence of increased quantities of illite at depth.
K-Ar dating of illites in Brent Group reservoirs: a regional perspective
Authors: Hamilton P. J. , Giles M. R. , Ainsworth P. 
Published: September 2007

Abstract
This paper reviews existing K-Ar dates from diagenetic illites from the Brent Group and presents new age data from 11 wells from across the East Shetland Basin.
K-Ar ages from diagenetic illites record illite growth in various parts of the Brent Group of the East Shetland Basin from about 60 Ma until 17 Ma. Diagenetic illites from the Cormorant field yield ages, which when combined with burial history simulation, suggest substantial illite growth from 75°C. This temperature is believed to be close to the temperature for the start of illite growth in the Brent Group. Other illite dates, when combined with burial history simulations, demonstrate continued illite growth to temperatures in excess of 110°C. Etive Formation samples of wells in which the overlying Kimmeridge Clay Formation source rocks are mature for oil generation, show a strong correlation between the K-Ar age of the finest size fraction of diagenetic illite and the calculated time at which the overlying source rocks reached a vitrinite reflectance of 0.62%. Data from other formations do not follow this trend and frequently give ages which are considerably older than those from the Etive Formation. It is considered likely that many of these older dates may arise as a result of contamination by detrital illites which are likely to be less common in the highly reworked sands of the Etive Formation barrier complex.
No recent (< 10 Ma) K-Ar dates have been obtained from diagenetic illites of the water zone of any wells so far studied in the Brent province. It is also apparent from many wells where diagenetic illites from both the hydrocarbon and water zones have been dated, that illite growth apparently stops in the water zone relatively soon after hydrocarbon emplacement. Furthermore, the time span for illitization, suggested by dating different illite size fractions of the same sample, is much shorter than that suggested by comparison of the range of temperatures illite apparently grew at and burial histories of Brent Group wells. Consideration of these points has led the authors to conclude that illite begins to form at a temperature of about 75°C probably at quite a low reaction rate. The process of oil migration at somewhat higher temperature perturbs the static aqueous pore fluid medium and increases effective water-to-rock ratio by fluid transport. This results in increased rates of illite growth. Subsequent to the peak of oil generation, fluid flow rates decrease, with a concomitant decrease in illite-forming reaction rates. Any attempt to isolate illite formed at lower or higher temperatures will inevitably also sample illite from the most rapid period of growth and so yield a false and reduced time span of illite formation and apparently old ages for most recently formed illite in the water zone.
Cathodoluminescence of quartz cements in Brent Group sandstones, Alwyn South, UK North Sea
Authors: Hogg A. J. C. , Sellier E. , Jourdan A. J. 
Published: September 2007

Abstract
Quartz cements can, at best, enhance sandstone reservoir quality through strengthening the clastic framework against compaction or, at worst, degrade poroperm properties by occluding pore spaces and constricting fluid pathways. An understanding of the controls on distribution and timing of quartz cementation can therefore aid exploration and production strategy.
In the Middle Jurassic Brent Group sandstones in Alwyn South authigenic quartz, kaolinite and illite development has resulted in severe deterioration of reservoir properties. Further resolution is obtained using a scanning electron microscope (SEM) adapted for cathodoluminescence (CL) mode observations. CL images show a clear distinction between detrial and authigenic quartz. In detail quartz overgrowths are complex and multiply zoned and a recurrent sequence of three major and two minor CL zones is distinguished. Zones become increasingly euhedral from core to rim and reflect variation in crystal growth rates and silica supply during several pulses of silicification. Heterogeneous zones and dissolution-boundary development shows that quartz precipitation is flux-controlled. Correlation of CL zones with clay mineral inclusions allow cement paragenesis to be refined and correlations with fluid inclusion data allow the conditions of quartz precipitation to be constrained.
Migration of petroleum into Brent Group reservoirs: some observations from the Gullfaks field, Tampen Spur area North Sea
Authors: Larter Stephen , Horstad Idar 
Published: September 2007

Abstract
Whereas the processes of petroleum generation and primary migration are beginning to be understood from a quantitative viewpoint, the process of secondary migration is still incompletely quantitatively understood. We estimate that typically the carrier systems feeding Brent Group reservoirs contain an average of around 3% residual oil saturation, the petroleum flowing in high saturation petroleum rivers through the most permeable carrier bed zones. Most of the carrier porosity contains no petroleum. The filling of the Brent Group reservoir in the Gullfaks field (Tampen Spur) is elucidated by petroleum geochemistry and this study indicates that the field has filled from more than one source basin and has been biodegraded post oil-emplacement. The complex filling and degradation history suggests that geochemical study of reservoir petroleum columns is a useful exercise providing information on the fillpoints of these complex reservoir systems and potentially information on the palacohydrogeology of the area.