1.2. PLANTEAMIENTO DEL PROBLEMA
1.5.5. La norma ISO/IEC 38500
The Lower Jurassic, consisting o f a broadly transgressive marine argillaceous deposits has the most restricted distribution o f the three Jurassic series in the North Sea basins (Brown, 1990). Lower Jurassic strata are absent, or at best patchily distributed over much o f the northern North Sea, however, such strata attain maximum thicknesses o f 500m in the north Viking Graben, 750m in the Danish Basin and 250-500m in the Central Graben (Ziegler, 1981; 1990). Away from the uplifted areas, the Lower Jurassic succession reaches around 900m in the Sole Pit Basin and includes the organic-rich Posidonia Shale (Brown, 1990).
Within the Inner Moray Firth, the Lower Jurassic is represented by a transgressive sequence, consisting o f shales with minor sandstones which is overlain by a coarsening- upward sandy sequence formed in a broadly deltaic setting capped by the Middle Jurassic Brora Coal Formation (Andrews and Brown, 1987; Brown, 1990). As illustrated in Figure 3.18 these facies do not continue into the Outer Moray Firth/Witch Ground Graben.
Lower Jurassic marine shales above a basal sand are widely distributed throughout the Viking Graben (Brown, 1990). These shales overlie the upper Triassic Statfjord Formation
and show evidence o f a diachronous marine transgression (Anderton et a l., 1985; Brown,
1990).
3 .2 .6 .2 Middle Jurassic (178Ma to 157Ma)
During the Middle Jurassic, crustal extension accelerated in the Arctic-North Atlantic and Tethys-Central Atlantic Rift. This was accompanied by late Aalenian-Bajocian upwarping o f a large rift dome centred over the Central Graben and by the development o f a large volcanic complex at the triple junction between the Viking Graben, Central Graben and Moray Firth-Witch Ground Graben system (Ziegler, 1982; 1990). This volcanic complex
displayed a bimodal mafic-felsic alkaline chemistry typical o f intracontinental rifts (Fall et
a t., 1982; Latin et a l., 1990). The volcanic rocks appear to thicken southwards and reach
a maximum thickness o f 1500m in presumed vent areas (Fall et a l., 1982). Geophysical
modelling o f seismic data reported by Ritchie et a l., (1988) indicates that the thickness o f
the volcanic rocks rarely exceeds 2.5km. The bulk o f the volcanic rocks were extruded by the end o f the Bajocian although radiometric ages support both extrusive and intrusive
The upwarping and development o f the volcanic dome which has been proposed to have had a structural relief o f 2-3km, created a barrier between the Boreal and Tethys oceanic provinces (Ziegler, 1982; Doré and Gage, 1987). More importantly the dome provided a major clastic source for a series o f radially draining fluvio-deltaic systems which followed major structural alignments. One such system is the Brent Group deposited within the proto- Viking Graben. This sequence o f rocks is probably the single most productive reservoir unit in the North Sea (Brown, 1990). It comprises 5 formations: the Broom, Rannoch, Etive, Ness and Tarbert Formations which are composed o f fluvial sands, prograding shoreface- foreshore sandstone and marine mudstones, coastal plain deposits and a transgressive sandstone unit (Eynon, 1981; Hallet, 1981; Brown, 1990).
Within the South Viking Graben, the continually rising sea-level resulted in the deposition o f thick shallow-marine and, eventually, deep-marine sediments (Brown, 1990). Clastic supply in the Callovian is recorded by the barrier shoreline sand bodies o f the Hug in Formation which are overlain by the Late Jurassic Heather Formation (Harris and Fowler, 1987).
In the southern North Sea, sedimentation was continuous throughout the Lower and Middle Jurassic, shallow-marine lagoonal sandstones and shales, and occasional calcareous
sediments conformably overlie the Posidonia Shales (Anderton et a l., 1985; Brown, 1990).
In the Witch Ground Graben, Middle Jurassic strata rest unconformably on Triassic continental sediments (Figure 3.18) and consist o f basaltic lavas and tuffs o f the Rattray
Formation which are thought to be the products o f subaerial eruptions (Fall et a l., 1982;
Harker et a l., 1987). The precise age range o f the lava flows remains uncertain (Ritchie et
a l., 1988). Within the Forties and Piper oilfields, a 740m-thick sequence o f undersaturated
alkali basalts is interbedded with Bajocian-Bathonian volcaniclastic sediments, tuffs, and coal seams o f the Pentland Formation which have been interpreted as being fluvio-deltaic
sediments coeval with the Brent Group in the north Viking Graben (Fall et a l., 1982;
Anderton et a l., 1985; Harker et a l., 1987).
In addition to the mid-Jurassic volcanics to the north o f the Central Graben, the fluvio- deltaic Bryne Formation rests unconformably on either the late Triassic Winterton Formation or directly on continental Triassic deposits (Figure 3.18; Brown, 1990).
3 .2 .6.3 Upper Jurassic (157Ma to 145Ma)
During the Upper Jurassic, it is thought that subsidence rates within the North Sea greatly exceeded those o f sedimentation (Ziegler, 1981). The development o f extensive marine basins and a restriction o f water circulation, coupled with minimal clastic input, led to the formation o f anoxic bottom conditions and the accumulation o f the organic-rich Kimmeridge Clay Formation (Ziegler, 1981). The Kimmeridge Clay Formation forms the principal source rocks within the northern North Sea area (Brown, 1990).
Tectonic activity in the Upper Jurassic was concentrated around the Viking, Central and Moray Firth-Witch Ground Graben systems, normal faulting in these areas is proposed to have been accompanied by local dextral wrench deformations (Ziegler, 1990). By mid- Kimmeridgian time marine connections had been established between the central and southern North Sea across the Mid-North Sea High (Ziegler, 1988, 1990).
Along the margins o f the Central Graben, sand bodies accumulated contemporaneously with the shales adjacent to a number intra basinal ‘highs’. These form important reservoir units such as the Fulmar and Ula Formations and are thought to result from a complex interplay
o f local tectonic activity and halokinesis (Armstrong et a l., 1987; Brown, 1990).
In the Moray Firth area. Upper Jurassic strata consist o f shales and minor beds o f sandstone with pronounced spatial thickness variations which are overlain by shales equivalent to the Kimmeridge Clay Formation (Brown, 1990; Underhill, 1991). The thickness variations in this area are thought to be controlled by a number o f planar, normal faults which were active during the deposition o f the sediments (Underhill, 1991).
The Upper Jurassic sequence o f the Outer Moray Firth and Witch Ground Graben comprises three widely distributed units: the deltaic to shallow-marine heterolithic deposits
o f the Sgiath Formation, the shallow-marine sandstones o f the Piper Formation (Marker et
a l., 1987) and the overlying Kimmeridge Clay Formation (Figure 3.18). In places, shales
o f the Kimmeridge Clay Formation are replaced by Turbiditic sands derived from fault- controlled ‘highs’ (Maher and Marker, 1987).
The Upper Jurassic sequence within the Viking Graben consists o f marine shales and turbiditic sandstones o f the Heather Formation. These sediments are overlain by submarine-
fan sandstones and conglomerates o f the Brae Formation (Figure 3.18) which is in turn blanketed by the Kimmeridge Clay Formation and its Norwegian equivalent, the Draupne Formation (Cayley, 1987; Harris and Fowler, 1987; Brown, 1990).
Along the western graben-margin fault, in the South Viking Graben, coarse, proximal submarine fan sediments originating from the Fladen Ground Spur were deposited whilst distal sands were deposited in the East Shetland Basin (Brown, 1990). The coarse clastic sediments are overlain by a thin veneer o f Kimmeridge Clay which passes laterally into intercalations o f sandy siltstones and shales (Brown, 1990).
Within the literature, the identification o f a ‘base Cretaceous’ unconformity is commonplace (see Brown, 1990). It has been proposed that a major ‘Late Cimmerian’ rifting pulse during the earliest Cretaceous produced a regional unconformity (Ziegler, 1981) however, increasingly refined palynological age determinations (Rawson and Riley, 1982) have illustrated that the hypothesis o f a regional unconformity is a gross over-simplification. An unconformity between Jurassic and Cretaceous strata is documented from marginal and intra-basinal ‘highs’ (see, for example, Boote and Gustav, 1987; Brown, 1990), but, for the most part, the Jurassic-Cretaceous transition is a conformable one.
3 .2 .7 Cretaceous (145Ma to 65Ma)
The Cretaceous sediments within the North Sea area document the transition from tectonically-controlled sedimentation with extensive stable massifs, to much quieter, fully marine conditions o f regional subsidence over the axial graben system (Ziegler, 1981; Doré and Gage, 1987; Hancock, 1990).
During the Lower Cretaceous, deep-water sedimentation, with half-grabens being infilled by shales and minor pelagic carbonates, is thought to have kept pace with subsidence (Ziegler, 1990). The area was then blanketed by more uniform Albian and Upper Cretaceous sediments (Hancock, 1990). In some basins, however, the rate o f subsidence was faster than that o f clastic supply, and water depth increased (Hancock, 1990).
Not all structural changes during the Cretaceous were simply a matter o f broad crustal downwarping, in some areas such as the Sole Pit and Broad Fourteens Basins the process was reversed, the basins being uplifted along inversion axes (Doré and Gage, 1987;
Hancock, 1990). The change from basin to Inversion axis is thought to have been episodic and there is no unique pattern of timing (Hancock, 1990).
Facies distribution of Cretaceous strata in the North Sea was controlled by tectonic setting, climate and availability of source materials (Hancock, 1990). The sea-level reached a maximum in the late Campanian-mid Maastrictian, during which time pelagic chalk facies dominated the continental shelf (Ziegler, 1981; Rawson and Riley, 1982). Only in the Viking Graben and the northern part of the Shetland Platform do fine-grained clastic sediments predominate (Hancock, 1990).
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AB SENT I S O P A C H MAP ON T HE L. C R E T A C E O U S IN T HE N O R T H S E A R E G I O NFigure 3.20: A map of Cretaceous isopachs and a possible compressional tectonic model. Contour interval = 250m. (After Olsen, 1987). Note the NNW-SSE orientation of maximum horizontal compression in the Southern North Sea Basin and the proposed change from NW-SE compressive stress in the Lower Cretaceous to NNE-SSW in the Upper Cretaceous.
The NW-SE trending stress regime o f the Upper Jurassic discussed in section 3 .2 .6 is proposed to have continued into the Lower Cretaceous (Olsen, 1987) resulting in the deposition o f Lower Cretaceous sediments in areas with thick Jurassic deposits (Figure 3.20). The orientation o f maximum tectonic stress is proposed by Olsen to have changed at the end o f the Lower Cretaceous to a NNE-SSW orientation resulting in reactivation o f shear zones in the Central Graben and compression leading to inversion in the Southern North Sea.
3.2.7.1 Lower Cretaceous
During the Lower Cretaceous, the North Sea was cut o ff from the southern European Ocean by the London-Brabant M assif (Figure 3.21) and its extension across the Rhenish Massif (Ziegler, 1981). Marine connections existed to the east through Germany and to the north via the Viking Graben into the early Atlantic (Hancock, 1990). The major transgressions o f the Aptian and Albian more than doubled the submerged area o f the North Sea (Hancock, 1990). Outside the main depositional basins, Aptian and Albian sediments dominate the Cretaceous system (Hancock, 1990).
In the Moray Firth Basin, the thickest sequence o f arenaceous Cretaceous sediments occurs. This 1150m thick pile consists o f submarine fan conglomerates and turbidites at the base o f fault scarps (Hancock, 1990). These sediments are associated with gravity-collapse on the upthrown sides o f contemporaneous faults and deposition in a deep topographic low
(Harker et a /., 1987; Hancock, 1990). Marls, shaly sandstones, and sandstones were
deposited towards the centre o f the basin (Harker et a l., 1987; Hancock, 1990).
On the uplifted horst blocks in the Outer Moray Firth and Witch Ground Graben, a thin veneer o f pelagic marls and limestones were deposited (Boote and Gustav, 1987). In the basins, on the other hand, debris flows and turbidites are again apparent (Hancock, 1990). These sediments are considerably better sorted than the primary mass-flow deposits,
consequently enhancing the reservoir quality (Boote and Gustav, 1987; Harker et a l., 1987;
Maher and Harker, 1987). These deposits are capped and sealed by Aptian marls and marly limestones which complete the transition to a low energy sedimentary basin (Rawson and
Riley, 1982; Harker et a l., 1987). At the eastern end o f the Witch Ground Graben, pale
tuffs o f Aptian-Albian age occur, these are thought to have originated from a volcanic centre at the southern end o f the Viking Graben (Hancock, 1990).
Within the Viking Graben, deposition was continuous from the Jurassic, and hundreds of metres of shales of the Cromer Knoll Group (Table 3.2) were deposited in relatively deep water (Ziegler, 1981; Hancock, 1990).
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Figure 3.21: The thickness and structural setting of the Lower Cretaceous. (After Hancock, 1990).
In the Central Graben, the Lower Cretaceous succession of grey shales and occasional marls such as the Plenus Marl (which form important marker horizons), reach thicknesses of up to 800m (Hancock, 1990). Sedimentation in the northern and central parts of the graben was continuous from the Jurassic, whilst in the Dutch Sector a distinct break has been observed (Hancock, 1990).
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Table 3.2: Approximate correlation of Cretaceous formations in the North Sea region. (After Hancock, 1990).
The Lower Cretaceous succession in the Sole Pit Basin consists o f basal sandstones o f the Spilsby Formation, which are overlain by pyritic clays o f the Speeton Clay Formation, and a ferruginous marlstone o f Aptian-Albian age, known as the Red Chalk Formation (Hancock, 1990),
3 .2 .7 .2 Upper Cretaceous
The Upper Cretaceous was characterised by a major rise in global sea level (Hancock, 1990). Most o f the London-Brabant Platform, and the Mid-North Sea and Ringkobing-Fyn High became submerged during the Albian (Figure 3.22). Submergence o f the highs was completed during the Campanian, when the sea spread over the Fladen Ground Spur and the Halibut Horst (Hancock, 1990). It is clear from Figure 3.22 that, in the trenches away from the relatively uplifted areas, large thicknesses o f Upper Cretaceous sediments were deposited: 1800m in the north Viking Graben, 1200m in the Central Graben, and 1000m in the South Halibut Basin and Witch Ground Graben (Hancock, 1990).
Basinal facies were widespread during the Upper Cretaceous. Coccolith lime muds were deposited below wave base, in water depths between 100 and 1000m (Selley, 1975). Subsequent diagenesis resulted in the development o f the micritic limestones o f the Chalk Group (Selley, 1975; Ziegler, 1988). It is thought that much o f the chalk in the trenches was deposited by mass-flows from the flanks (Hancock, 1990).
In the southern North Sea, south o f 57®N, most o f the Upper Cretaceous is represented by chalk. This continues across the Cretaceous-Lower Palaeocene boundary as the Danian or Ekofisk Formation (Hancock, 1990).
Pelagic carbonate deposition gives way northward to calcareous claystone deposition, either as a consequence o f climatic change or o f increased fine clastic input from the Atlantic rift shoulders (Doré and Gage, 1987). Over the Shetland Platform and in the Viking Graben north o f 59®30’, the succession is dominantly clastic and comprises 1000 to 2500 metres o f pelagic marls and clays; only a few metres o f Maastrictian chalk are apparent (Hancock,
Within the Witch Ground Graben, Late Cretaceous carbonate deposition occurred in a stable marine basin, the topography being infilled and buried by pelagic and hemipelagic shales,
marls and chalks (Boote and Gustav, 1987; Harker et al., 1987).
NO RWAY SHETLAND ISLES SHETLAND I PLATFORM , ORKNEY STAVANGER halibut HORST ■ I J / ABERDEEN? . SCOTL AN D I \ \ V 56“nH RINGK0B EDINBUR A ntral GRABEN INVERSI 54 N - - 5 4 ° N W EST GERMANV A M S T E R D A M '^ N O L A N D lc^estort 52 N - V > \ - 5 2 N LON DO SOUTH HOLLAND - EINDHOVEN INVERSION ISOPACH CONTOURS IN METRES ^.''''iSLE OF WIGHT
Figure 3.22: Isopachs of the Chalk Group (Upper Cretaceous plus Lower Palaeocene), and the Shetland Group in the region of the North Sea. (After Hancock, 1990).
3 .2.8 Cenozoic (65Ma to Present)
The North Sea Basin exhibited its present day north-south configuration by Cenozoic times, the axis following the trend o f the rift systems described throughout this chapter. Sediments deposited during this time are largely unfaulted and only slightly deformed, halokinesis o f Zechstein salts again has a marked influence on formation thickness (Ziegler, 1982; Lovell, 1990).
Within the Cenozoic rocks of the North Sea Basin, two main phases o f pyroclastic sedimentation are recognised. The first, dated around 58-57Ma, the other around 55-52Ma. The top o f the latter, the Balder Tuff Formation, is a widespread seismic and stratigraphie marker (Lovell, 1990). These ashes result from contemporaneous igneous activity to the west o f the British Isles that was associated with a particularly active phase in the opening o f the North Atlantic (Ziegler, 1988; Lovell, 1990).
Figure 3.23 illustrates the great changes in the palaeogeography o f the British Isles from the Upper Cretaceous to the Early Tertiary from almost total submergence, to the emergence o f a landmass exhibiting the shape o f the present day British Isles (Lovell, 1986).
3.2.8.1 Palaeocene (65Ma to 56Ma)
During the Lower Palaeocene (Danian), subsidence of the North Sea basin continued. Chalk deposition persisted within the Central Graben, whilst in the Viking Graben, clastic sediments became the dominant lithotype (Ziegler, 1981; Lovell, 1986).
Four major oil- or gas-bearing sandstone units can be recognised within the Palaeocene o f the central North Sea. Two of them were derived from the Scottish Highlands, and the other two were derived from the Shetland Platform (Lovell, 1990). These clastic rocks comprise sands o f deltaic, shelf, and submarine fan origin (Ziegler, W.H. 1975). The large submarine fans are separated in basinal areas by hemipelagic muds (Lovell, 1990).
It has been proposed that a large Palaeocene fluvial delta built out into the Moray Firth and
East Shetland Platform (Rochow, 1981; Whyatt et a l., 1991). The Moray Firth delta
developed a series o f extensive submarine fans which prograded into the Central Graben and were responsible for the deposition o f the major sand bodies o f the Montrose, Forties
and Frigg fields (Figure 3.24). At the end of the Palaeocene, coarse clastic input was halted by a basin-wide rise in sea-level and sedimentation was reduced to hemipelagic fallout of
clays and tuffs into what had become an anoxic basin (Whyatt et al., 1991).
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