bluuebunny/crossref_metadata_2025_split huggingface dataset:
The structural evolution of the eastern end of the Halibut Horst, Block 15/21, Outer Moray Firth, UK North Sea
Authors: HIBBERT M. J. , MACKERTICH D. S. 
Published: April 2010

The eastern termination of the Halibut Horst occurs in Block 15/21 of the Outer Moray Firth and shows evidence of a complex structural history. The acquisition of high-resolution 3D seismic datasets across the area of study has permitted mapping of key horizons, better imaging of complex faulting within and around the horst, and enhanced understanding of the structural and stratigraphic evolution.
A marked change in active fault trends from NE–SW during the early/mid-Jurassic to WNW–ESE in the late Jurassic/early Cretaceous is evident. Late Jurassic tectonism resulted in significant erosion of the Halibut Horst Spur and syn-tectonic deposition in basinal areas to the north and south. Continuing reactivation and uplift of the horst during the Cretaceous and Tertiary resulted in a complex interrelationship between tectonics and sedimentation.
Comparison of Permo-Triassic and deep structure of the Forties-Montrose and Jaeren highs, Central Graben, UK arid Norwegian North Sea
Authors: PLATT N. H. , PHILIP P. 
Published: April 2010

Recent acquisition of high resolution, deep penetration seismic in the Central North Sea has permitted imaging of the basement structure. The results indicate strong similarities between the deep structures of the Jaeren and Forties–Montrose highs. The Triassic and Zechstein strata show a distinctive relationship on these graben margin and median highs. Interpretation of the pre-Zechstein structure may indicate the distribution of Devonian and Carboniferous intervals in an area where previous imaging of the deep section has not been successful.
Southern parts of both the Jaeren and Forties–Montrose highs show the presence of a sub-Permian reflective package; this (?possible Carboniferous) sequence is absent in the north, where reflectors may be recognized within the deep section. A prominent horizontal reflector may represent a Middle Devonian limestone marker or the Devonian basal unconformity. This reflector may alternatively mark the top of Upper Paleozoic volcanic rocks. A deeper, westward- or northwestward-dipping reflector may represent the top of the crystalline basement or a deeper fault or shear zone of Caledonide origin.
The potential occurrence of Carboniferous strata at depth over extensive areas of the Central Graben and surrounding highs would have important implications for exploration in the area. Carboniferous coal measures might provide a sub-Zechstein gas-prone hydrocarbon source.
Mesozoic structural evolution of the UK Southern North Sea: insights from analysis of fault systems
Authors: ARTHUR T. J. 
Published: April 2010

An understanding of the Mesosoic fault systems in the UK Southern North Sea is important in modelling the burial history of source and reservoir rocks, in determining the time of formation of structural traps in the Permian and older section and in mapping seismic velocity fields in Mesozoic rocks for accurate time to depth conversion.
The evolution of Mesozoic fault systems in the Southern North Sea can be understood by examination of several faults with different amounts of movement across them, and by analysis of fault patterns. Structural analysis has been undertaken in selected areas where complications due to salt movement are minor. Local fault analysis is illustrated by balanced cross-sections and maps from Blocks 49/28,48/23 and 48/11. The faults studied are shown to have been active extensional faults during the Jurassic and early Cretaceous, but have in general undergone only limited movement during later Cretaceous and Tertiary phases of subsidence and uplift.
The Mesozoic faults are detached from basal Permian and Carboniferous rocks by decollement in the Zechstein evaporites and are linked by transfer zones to form a basin-wide system of faults. Mesozoic fault systems overlie zones of weakness in pre-Zechstein rocks, but are not directly connected to them, so their orientation is controlled by Mesozoic forces rather than pre-existing structural grain.
Overpressures in the Central North Sea: implications for trap integrity and drilling safety
Authors: GAARENSTROOM L. , TROMP R. A. J. , de JONG M. C. , BRANDENBURG A. M. 
Published: April 2010

The Central North Sea Graben is, at pre-Cretaceous levels, a relatively under-explored province of the United Kingdom Continental Shelf. The hydrocarbon potential was recognized in the early 1970s, but both technology and economics prohibited any serious exploration efforts at that time. Exploration for deep targets increased significantly during the second half of the 1980s. This resulted in the discovery and appraisal of several fields, such as the moderately overpressured Marnock and Skua fields and the highly overpressured Erskine, Puffin, Franklin and Heron fields.
From a regional viewpoint, the overpressures observed in the pre-Cretaceous reservoirs in the Central North Sea Graben follow a well-defined trend, increasing with depth. Similarly, the minimum effective stresses derived from leak-off pressures measured in the overlying seals follow a comparable trend related to the present-day depth of burial. This indicates that the present distribution of overpressures is controlled by retention and that the understanding of overpressure generation is of second-order importance.
A more detailed analysis combining the areal distribution of overpressures with the structural framework suggests that the Central North Sea Graben can be subdivided into several pressure cells. The boundaries between the cells are controlled by faults which, in many instances, show signs of repeated reactivation. This reactivation has led in some cases to failure of otherwise well-defined structural traps, whereas in other cases it may have enhanced the trapping integrity. Examples of intermediate cases can also be recognized, which clearly show that the pressure cells form part of a dynamic system, controlled primarily by the structural evolution of the Central North Sea.
A sound understanding of the structural framework and its evolution is therefore of paramount importance in predicting trap integrity and overpressure regimes prior to drilling. The drilling of deep high-pressure/high-temperature wells and the acquisition of modern 3D seismic data permit a continuous improvement in the understanding of overpressures and their distribution.
Silicate mineral authigenesis in the Hutton and NW Hutton fields: implications for sub-surface porosity development
Authors: McAULAY G. E. , BURLEY S. D. , JOHNES L. H. 
Published: April 2010

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.
Clair appraisal: the benefits of a co-operative approach
Authors: CONEY D. , FYFE T. B. , RETAIL P. , SMITH P. J. 
Published: April 2010

The Clair discovery lies 75 km west of the Shetlands, and comprises an elongate ridge of Lewisian basement and an associated roll-over of variable quality Devonian–Carboniferous continental red beds.
Clair was discovered in 1977, when 206/8-1A tested 25° API oil at 1500 BOPD from the red beds at the crest of the roll-over. Well 206/7-1 followed, producing 960 BOPD from fractured basement on the ridge. Ten further wells drilled between 1977 and 1985 indicated an oil in place measurable in billions of barrels, but test results were disappointing. The success of the discovery well was never repeated and commercial test production rates were never achieved.
In 1990, the companies with licence interests over Clair agreed to pool their resources and expertise and collaborate in a joint appraisal programme. A series of reservoir studies quantified the importance of different fracture types to productivity and predicted that the right combination of fractures and matrix could yield much higher flow rates than had previously been achieved. A 3D seismic survey over the central area of the discovery led to a significant improvement in understanding and visualization of the structure.
Two joint appraisal wells were drilled in 1991. The first, a horizontal well in fractured basement, tested at 2100 BOPD after acid wash stimulation.
The second well tested the red beds on the flank of the roll-over and achieved sustained flow rates of over 3000 BOPD from two zones (one of which had been hydraulically fractured) with a significant contribution from previously unidentified open natural fractures. The orientation of these open fractures suggests they are related to a late Cretaceous episode of faulting.
Two further appraisal wells and an extension to the 3D seismic coverage are planned for 1992.
There are still many uncertainties associated with Clair, but the 1991 wells have finally demonstrated that significant and potentially commercial flow rates are achievable. The behavioural change associated with the joint approach has been the catalyst for the success of this new initiative on Clair.
The Embla Field
Authors: KNIGHT I. A. , ALLEN L. R. , COIPEL J. , JACOBS L. , SCANLAN M. J. 
Published: April 2010

The Embla Field is the first development of a pre-Jurassic reservoir in the Norwegian sector of the Central Graben. The field is located 5 km south of the Eldfisk Alpha platform, near the Greater Ekofisk production complex, which provides an infrastructure where development of marginal fields becomes commercially viable.
Although the field was initially discovered in 1974 with the 2/7-09 well, inconclusive drill stem test (DST) and geological data left the field potential uncertain. In 1988 a second well, 2/7-20, successfully tested oil and gas from a thick sandstone of indeterminate age. A third well, 2/7-21S, drilled in 1989, gave even better test results from two separate sandstones. Core, log and DST data from these two wells indicate that the reservoir is a faulted and highly fractured sequence of conglomeratic and finer-grained sandstones deposited in a continental environment of probable Paleozoic age.
A 3D seismic programme in 1989 also demonstrated the structural complexity of the field. Situated as it is on the Grensen Nose and bounded on its eastern flank by the Skrubbe Fault/Lindesnes Ridge inversion, it was evident the reservoir had experienced a complex burial and tectonic history. Poor seismic definition of the pre-Cretaceous stratigraphy further dictated the need for a flexible approach to field development.
A phased development programme was initiated, justified by the reserves attributable to the 2/7-20 and 2/7-21S wells. Expendable delineation wells were avoided. Instead, deviated step-out drilling from a central template was favoured, with the intent to subsequently tie the wells back to a remotely controlled surface production facility. First production is scheduled for early 1993.
The evolution of a fractured chalk reservoir: Machar Oilfield, UK North Sea
Authors: FOSTER P. T. , RATTEY P. R. 
Published: April 2010

Machar is the first of the ‘Diapir’ fields discovered in the eastern trough of the Central North Sea, in BP sole interest Block 23/26a. It comprises a fractured Cretaceous chalk and Paleocene sand reservoir overlying a steeply dipping salt structure. The oil column is greater than 1100 m and reserve estimates are in the region of 100 million barrels of oil equivalent.
The history of the Machar reservoir represents a complex interplay of salt movement, oil migration and burial diagenesis. Structural studies have shown that the Permian Zechstein salt has been in a state of buoyant equilibrium for most of its geological history, with only one phase of true diapirism identified in the mid-Miocene. The buoyancy resulted in the deposition of a condensed (<300 m) Triassic–Eocene section over the salt high. In the surrounding area, sediment loading and related salt withdrawal caused ‘downbuilding’ and the deposition of a thick (3000 m) sediment package. A 50–300 m section of chalk was thus deposited over the diapir with a mature Jurassic oil source in the adjacent depocentre. Paleocene sands are thin or absent over the crest of the structure, but thicken to about 100 m downflank.

Regional subsidence and oil migration commenced in the early Oligocene, the trap filling from the top down. Here, shallow burial depth (
c
. 100 m) and early oil emplacement ensured porosity was preserved in the chalk matrix. At the same time, on the flanks of the structure greater depth of burial and diagenesis were destroying chalk porosity from the bottom up. These competing influences resulted in a diagenetic front marking the limit of pay in the chalk. That front lies in the depth range 2000–2500 m.

Passive movement of the chalk during periods of buoyancy, plus regional tectonic compression in the mid-Miocene, created a widespread fracture system. Studies have identified a continuous process of fracture, stylolite and cement formation across a variety of brittle and ductile deformation styles. Average spacing of open fractures in 200 m of chalk core from Well 23/26a-13 is 7.5 cm. About a third of these are open.
The burial history, oil emplacement and fracturing resulted in a reservoir with a dual porosity system. The fine-grained pelagic chalk matrix has primary porosity in the range 12–35% with permeability generally less than 1 mD. Secondary fracture porosity, although difficult to measure, is probably less than 1%. Production test data over fractured intervals, however, show permeability in excess of 1000 mD. The fracture system is the key to developing the chalk reservoir as it provides effective drainage of the tight matrix. In a wider context the recognition of buoyancy, not piercement, as the dominant structural process provides a model for exploration elsewhere.
Estimation of recoverable reserves: the geologist’s job
Authors: CORRIGAN A. F. 
Published: April 2010

The accurate prediction of recoverable reserves in the North Sea has proved to be a difficult task for many operators. This paper summarizes the performance of a number of fields by tracking the way in which reserves have changed with time, compared to the volume of reserves thought to exist at the time that development approval was given. In many fields reserves have changed by between 20 and 70%, with the areal distribution also exhibiting considerable variation from that in the field development plan. The changes have been both positive and negative. In the latter case, the economic consequences of a significant reduction in reserves are obvious. However, increases in reserves also bring financial penalties in poorly sited and sized facilities, additional well requirements and inefficient use of funds.
Although this work has shown that there are a number of reasons for poor estimation, the underlying cause is usually insufficient recognition of reservoir complexity and thus the use of simple or inappropriate models to predict reserves. It is also clear that there are geological factors that are common to fields that have exhibited large losses or increases in reserves. In the latter category are fields that are structurally simple, often deposited in submarine fan environments, where the volume of movable oil was underestimated. Fields suffering large drops in reserves are often structurally complex, highly compartmented and contain high permeability zones.
It is proposed that geologists should take a much more active role in reserve estimation than has generally been the case in the past. In particular, at the appraisal stage, early recognition of complexity is vital, and understanding of the complexity should become one of the major goals of the appraisal wells. In addition, the geologist needs to become closely involved in the construction and operation of the simulation models that are used by most reservoir engineering groups to predict reserves. This is to ensure that geological features which affect drainage are correctly modelled in the simulators.
Meillon–Saint Faust gas field, Aquitaine basin: structural re-evaluation aids understanding of water invasion
Authors: HALLER D. , HAMON G. 
Published: April 2010

The case history of the Meillon–Saint Faust gas field is an example of how a structural interpretation based on well data can help in the understanding of the dynamic behaviour of the reservoir.
This field was initially developed with clustered wells at the structural culminations of the trap, relying on good fluid communication in the field. After production of 50–65% of the initial gas-in-place, the productivity had drastically decreased, due to overall water breakthrough at the wells, and the field was apparently approaching the end of its economic life.
Detailed structural analysis and reservoir simulation have now led to an improved understanding of field behaviour. Since the southern border of the field is understood to be a major E–W normal fault, it is apparent that most of the production wells are located in a highly fractured zone and that good fluid circulation is restricted to this southern fringe.
This structural model is taken into account for numerical reservoir simulation. The pressure predictions of these simulations match the pressure history and thus validate the structural model. The main conclusion of the simulation is that water has only invaded the fracture network and that the total water volume entering the reservoirs is low.
The origin and genesis of major Jurassic unconformities within the triple junction area of the North Sea, UK
Authors: DAVIES R. J. , O’DONNELL D. , BENTHAM P. N. , GIBSON J. P. C. , CURRY M. R. , DUNAY R. E. , MAYNARD J. R. 
Published: April 2010

Three Jurassic unconformities of intra-Aallenian, intra-Callovian and intra-middle Oxfordian age are consistently identified within the literature for the North Sea region but despite their common recognition these unconformities remain poorly understood and they are essentially spatially and temporally unconstrained. The recent development of a sequence stratigraphic framework for the Jurassic of the North Sea allows for a thorough investigation of the spatial and temporal characteristics of the intra-Callovian and intra-Oxfordian unconformities and these data have important implications for Middle and Late Jurassic rifting within the North Sea region.
The intra-Aalenian unconformity is believed to have been generated as a result of an increase in geothermal gradient within the area and subsequent thermal uplift that preceded rifting. The stratigraphic gap associated with this unconformity increases towards the proposed location of a Middle Jurassic, triple junction centred dome. In contrast the stratigraphic break associated with the intra-Callovian and the intra-middle Oxfordian unconformities is found to increase onto what are now intra-rift highs and rift margin areas. These unconformities occur at the same time as distinct changes in the geometry of the fill of the triple junction and also occur at the same time as seismic scale extension. They do not appear to correlate to any supposed global fall in relative sea-level. Stratigraphic data indicate that three phases of extension occurred during the Callovian, middle and late Oxfordian and lastly during the Kimmeridgian and Volgian. Although the intra-Callovian and intra-middle Oxfordian unconformities can be confused with the effects of the Middle Jurassic North Sea dome unconformity, they are in fact the result of extension during the initial Callovian and Middle/late Oxfordian rift phases. The recognition of these regional unconformities suggests that the Middle and Late Jurassic rifting process was punctuated rather than being continuous.