Moray Firth Basin Research Articles

The Lower Cretaceous plays of the Central and Northern North Sea: Atlantean drainage models and enhanced hydrocarbon potential

In the Central North Sea, the Apto-Albian play appears to be confined to patchy areas within the Moray Firth basins. In this area, it is widely believed reservoir-quality sands were sourced from local residual Late Cimmerian highs, with resultant limited distributions of commercial fairways. Local hydrocarbon modelling proves that most hydrocarbons discovered to date occur within stratigraphic traps, discovered largely by serendipity during appraisal of older structurally defined reservoirs. Re-appraisal of Jurassic to Cretaceous tectonic models shows how tectonic trends within the Moray Firth basins are optimized for discovery of Lower Cretaceous reserves. These models also suggest considerable upside potential in under-explored areas of the Viking Graben and Central Trough, where complex basin geometries and halokinesis conspire to hide potential fairways. The search for Aptian reserves is severely hampered by a widespread inability to see or interpret sand and/or hydrocarbon presence using seismic data alone, resulting in a very high dependence on geological modelling to evaluate potential sweet spots within wide fairway tracts. This style of modelling has been reliant on a ‘Jurassic-upwards’ technique, with limited success. Recent re-modelling of the entire North-West European area has shown that established models of Paleocene sand distribution can be projected downwards to find sweet spots in potential Aptian fairways. Using this approach has resulted in a new plate-wide tectono-sedimentological and drainage pattern model for the Aptian, which suggests that most sand fairways, dominated by deep marine facies, were sourced from an active Atlantic Margin Arch via northwest–southeast-oriented fluvial and shelfal feeder systems. Application of this model to Central and Northern North Sea hydrocarbon fairways suggests a gross current underestimate of Aptian sand volumes and considerable upside potential both within the Moray Firth basins and unappraised exploration territory in the Viking Graben and Central Trough.

MIDDLE AND UPPER JURASSIC (CALLOVIAN TO KIMMERIDGIAN) PALYNOLOGY OF THE ONSHORE MORAY FIRTH BASIN, NORTHEAST SCOTLAND

The Brora Coal, Brora Argillaceous, Brora Arenaceous, Balintore and Kimmeridge Clay formations of the onshore Moray Firth Basin represent an important Middle to Upper Jurassic (Callovian to Lower Kimmeridgian) reference section close to hydrocarbon-rich North Sea basins. This composite succession at Brora and Balintore is c. 233 m thick; it is mudstone/siltstone-dominated and largely rich in zonal and subzonal ammonites. For example, the Callovian succession at Brora is virtually complete, with coverage of all seven ammonite zones. All the five formations examined have yielded abundant palynofloras. The lithostratigraphic units sampled, except the Brora Coal Formation, have yielded rich associations of dinoflagellate cysts. The majority of the Inverbrora Member of the Brora Coal Formation at its type section at Brora is of early Callovian age based on dinoflagellate cysts; this member yielded Meiourogonyaulax caytonensis and Mendicodinium groenlandicum and these species preclude a Bathonian age. This member has been previously attributed to the late Bathonian. Dinoflagellate cysts are diverse and abundant in the overlying Brora Argillaceous to Kimmeridge Clay formations, therefore indicating open marine conditions. The stratigraphic distributions and relative proportions of these Callovian to Lower Kimmeridgian dinoflagellate cyst floras are largely consistent with those reported elsewhere in northern Europe, and the established dinoflagellate cyst biozonations can be readily applied to the Inner Moray Firth Basin. Some taxa, such as Gonyaulacysta dentata, are of distinct Boreal affinity. Furthermore, some minor stratigraphic anomalies were noted, including the range base of Scriniodinium crystallinum being in the early Oxfordian at Balintore. In England and Germany, this bioevent occurs in the late Callovian. Some notable dinoflagellate cyst abundance phenomena were observed. An example of this is the prominence of Korystocysta spp. in the Middle Callovian. This and other quantitative phenomena are of correlative significance. Marine palynomorph diversity increased markedly during the Callovian, stabilizing in the Lower Oxfordian. A suite of characteristic dinoflagellate cysts became extinct in the Middle Oxfordian, and some typically Late Jurassic elements became more prominent at this time. The early Kimmeridgian palynofloras from Ethie are entirely typical of this interval elsewhere in Europe.

The Scott Field, Blocks 15/21a, 15/22, UK North Sea

Abstract The Scott Field straddles Blocks 15/21 and 15/22 on the southern flanks of the Witch Ground Graben in the Outer Moray Firth Basin, UKCS. The oil field is developed in the highly productive Upper Jurassic Humber Group sandstones of Oxfordian to Kimmeridgian age. The field was discovered in 1983, sanctioned in 1990, and produced first oil in 1993. The field structure, effectively a large southwards tilted fault block, is compartmentalized into a series of four main pressure isolated fault blocks by mid to late Jurassic faulting. The Kimmeridge Clay Formation provides both the top seal and the source of the trapped hydrocarbons. Fluid contact, overpressure and compositional trends suggest that the trap was filled primarily from the north. Some trap-defining faults were already active during the deposition of the reservoir intervals. Well data indicate that the development of accommodation space was technically controlled during this period, with subsidence occurring more rapidly in the western areas of the field. The Scott Field reservoir consists of two major sand packages, the Scott Sandstone Member and the Piper Sandstone Member, bounded above and below by marine flooding surfaces. The late Oxfordian Scott Sandstone Member consists of a westwards prograding marine shoreface sandstone overlain by aggradational and retrogradational back-barrier deposits. Above this, the Mid Shale is a regionally extensive flooding event separating the Scott Sandstone Member from the overlying Piper Sandstone Member. The early Kimmeridgian Piper Sandstone Member consists of stacked mass flow sandstones, overlain by a shoreface/back-barrier system. Lateral facies changes and thickness variations significantly affect reservoir distribution in both Scott and Piper intervals. The best reservoir quality occurs within the coarsest grained, highest energy facies, particularly the shoreface and proximal washover deposits. At the crest of the field, 10400 ft TVDss, multi-Darcy permeabilities and porosities of 20% are common. However, reservoir quality declines progressively downflank due to increased quartz cementation and compaction. The Scott Field currently produces from 23 wells supported by 20 water injectors. Current modelling is aimed at targeting bypassed oil to increase ultimate recovery. The field has presently produced 300 MMSTB of oil from forecast reserves of 440 MMSTB with an estimated ultimate recovery factor of c. 46%.

Part 6: Central Graben Fields Field map

Abstract Map depicting the position and names of the main oil and gas producing fields located in the South Viking Graben, Inner and Outer Moray Firth Basins and Central Graben areas of the Central North Sea. The international border (median line) between the UK and Norway is highlighted as are the producing fields in the Norwegian sector. The boxed areas show the extent of licensed acreage in the region.

The Captain Field, Block 13/22a, UK North Sea

Abstract The Captain Field is located in Block 13/22a in the Western Moray Firth Basin of the UK North Sea, 80 miles NE of Aberdeen in a water depth of 340 ft. Hydrocarbons are trapped in two geographical regions, the Main and Eastern closures, both with a significant stratigraphic pinchout component. The principal reservoirs consist of turbidite sandstones of Lower Cretaceous age which have been informally subdivided into two stratigraphic units comprising the Upper and Lower Captain Sandstones. At the base of the preserved Jurassic section the Heather Sandstone, Oxfordian in age, provides a secondary reservoir. Reservoir quality is uniformly excellent in the Lower Cretaceous with in situ, Klinkenberg corrected permeability averaging 7 Darcies and porosity in the range 28-34%. The reservoir is generally poorly consolidated sandstone with the depth to the crest of the field at -2700 ft TVDss. The reservoirs contain a total oil-in-place of 1000 MMBO. The Upper Captain Sandstone has a small associated gas cap containing 16 BCF gas-in-place. The oil is heavy, by North Sea standards, with oil gravity ranging from 19° to 21° API and has high in situ viscosity, 150 to 47 cP, at the mean reservoir temperature of 87°F. The fluid properties and offshore location necessitate the employment of innovative horizontal drilling methods, completion design and artificial lift technology in order to achieve an economically viable field development. Extended reach horizontal wells, with reservoir completion lengths of up to 8000 ft, are drilled for all oil producers and water injectors. Development risks were significantly reduced following two appraisal drilling campaigns in 1990 and 1993 culminating with the successful drilling and extended testing of a prototype horizontal field development well (13/22a-10). The field is being developed in two phases, Area A and Area B. First oil production commenced from the Captain platform in March 1997 from Area A and the field now produces at between 50000 and 70000 BOPD. Area B development is now underway with first oil planned for December 2000. Completion of this phase of the development will increase the plateau production rate to 85000 BOPD.

Field map

Abstract Map depicting the position and names of the main oil and gas producing fields located in the South Viking Graben, Inner and Outer Moray Firth Basins and Central Graben areas of the Central North Sea. The international border (median line) between the UK and Norway is highlighted as are the producing fields in the Norwegian sector. The boxed areas show the extent of licensed acreage in the region.

The biostratigraphic calibration of the Scottish and Outer Moray Firth Upper Jurassic successions: a new basis for the correlation of Late Oxfordian–Early Kimmeridgian Humber Group reservoirs in the North Sea Basin

The combined results of palynological sampling of Scottish onshore sections and offshore wells from the Outer Moray Firth and the re-appraisal of ammonites extracted from subsurface cores enable a revised, unified macropalaeontological–palynological correlation scheme to be proposed for the Late Oxfordian–Early Kimmeridgian (Regulare–Mutabilis ammonite zones) of the North Sea. The results contradict previous, controversial correlations, which were based upon reported ammonite recovery from boreholes in the Witch Ground Graben, a component part of the Outer Moray Firth Basin. It can now be shown that the Upper Jurassic biostratigraphy of Scotland and the North Sea is essentially comparable to that of the English counterpart, suggesting that no faunal provincialism persisted across the former North Sea Dome from uppermost Oxfordian times onwards. Having clarified and resolved previous conflicting views, the application of the revised scheme should aid exploration and production in the Moray Firth Basin through the construction of more accurate reservoir correlations and robust palaeogeographic (play fairway) maps.

Scotland’s denudational history: an integrated view of erosion and sedimentation at an uplifted passive margin

Abstract Denudational history is commonly reconstructed from basin sediments derived from the denuded source area, and less frequently from the source area itself. Northern Britain is an important source area for the surrounding sedimentary basins and this paper reviews the erosional history of Scotland from Devonian time to the present using evidence both from onshore geology and geomorphology and from patterns of sedimentation in surrounding basins. Cover rocks were extensive in Scotland during late Palaeozoic time but the persistence of sediment source areas within the upland areas of Scotland makes it unlikely that basement highs were ever completely buried, and depths of post-Devonian erosion of basement have been correspondingly modest (< 1–2 km). During Mesozoic time, Scotland experienced several major erosional cycles, beginning with uplift, reactivation of relief and stripping of cover rocks, followed by progressive reduction of relief through etchplanation and culminating in extensive marine transgressions in Late Triassic, Late Jurassic and Late Cretaceous time. Mid-Paleocene pulses of coarse sediment to the Moray Firth Basin coincided with major uplift. This uplift was associated with major differential tectonics within the Highlands, with warping and faulting along the margins of the Minch and the inner Moray Firth Basins. Tectonic activity was renewed on a lesser scale in late Oligocene time and continued into Late Neogene time. Differential weathering and erosion under the warm to temperature humid climates of Neogene time created the major elements of the preglacial relief, with formation of valleys, basins, scarps and inselbergs, features often closely adjusted to lithostructural controls and, in some cases, with precursors that can be traced back to Devonian time. The history that can be ‘read’ from the onshore region complements the source area history interpreted from sedimentary basins derived from these areas.

RESPONSE OF FORAMINIFERAL FACIES TO TRANSGRESSIVE-REGRESSIVE CYCLES IN THE CALLOVIAN OF NORTHEAST SCOTLAND

At Brora on the northwestern margin of the Inner Moray Firth Basin, an Upper Bathonian to Middle Oxfordian (Jurassic) sedimentary succession of marine shales, siltstones and sandstones is subdivided into four 4th order transgressive–regressive (T-R) cycles. The middle two of these cycles, B2 and B3, comprise the Callovian Brora Argillaceous Formation which is the subject of this study, integrating foraminiferal, sedimentological and stratigraphical data. The principal factor acting on the foraminiferal distribution pattern both in cycle B2 and B3 is oxygen supply to the benthic environment. The oxygenation is controlled mainly by the changing water depth during transgressive and regressive episodes. In the studied faunal succession these changes are reflected by two main biofacies trends: 1) Diversity decrease, accompanied by increasing dominance and decreasing proportion of calcareous taxa. This faunal trend reflects increasing stagnation during transgression. 2) Diversity increase, coupled with reduction in dominance and expansion in proportion of calcareous taxa. In typical cases, this development reflects improving oxygenation during regression. The intervals which include the maximum flooding surface, in both cycle B2 and B3, are typified by significantly increased organic carbon content and strongly reduced faunal diversity, developed in dark shales representing severe oxygen depletion. The transgressive phase of cycle B2 shows a faunal diversity decrease, while that of cycle B3 reveals a diversity expansion in its lower part. The regressive phase of both cycles is characterized by a diversity increase. In addition to these major faunal trends, the distribution of species and genera appears useful for “fingerprinting” individual components of T-R developments.

Controls on the Lower Cretaceous Punt Sandstone Member, a massive deep-water clastic deposystem, Inner Moray Firth, UK North Sea

Interpretation of high-quality seismic data, constrained by exploration wells, provides insights into controls on the stratigraphic architecture and deep-water sedimentary processes that governed deposition of the Lower Cretaceous Punt Sandstone Member in the Inner Moray Firth Basin. We suggest a model of deposition in which sediment provenance from the north and west progressively filled depositional accommodation in proximal depocentres before spilling into more distal areas via linear, confined and incised channel complexes. As well as giving important clues into post-rift depositional processes in the basin, and a well-imaged ancient analogue for the deposition of massive deep-water sands, the seismic and stratigraphic data may also provide important insights into factors governing the poorly imaged Lower Cretaceous sands in neighbouring basins of the North Sea.

Giant irregular pockmark craters in the Palaeogene of the Outer Moray Firth Basin, UK North Sea

A cluster of irregular pockmark craters of Palaeogene age in block 15/18 in the Outer Moray Firth, UK North Sea have been mapped on 3D seismic data with well control. The craters range from 0.5 to 4 km in diameter, between 50 and 200 m in depth, and have been buried by Tertiary subsidence and sedimentation to present-day depths of between 1000 and 1400 m sub-sea. They are circular to elliptical in plan view, and are located in a delta front and upper pro-deltaic slope position on the Dornoch delta. The structures are confined to the top of the Lower Eocene Balder Formation, and regional mapping across the Moray Firth Basin shows they are restricted to an area of ca. 15 × 15 km 2. Well data show that the craters are sand-filled, and have excavated an argillaceous and tuffaceous unit. The crater-filling sands are sealed by overlying mudstone units. Planar wing-like features emanate from the margins of the craters. Well control indicates that these seismically identified ‘wings’ consist of sandstone, suggesting that the craters have been modified by an episode of sand remobilisation and injection. The proposed mechanism for the formation of these structures is large-scale fluid eruption (probably shallow gas) at the Early Eocene seafloor. These craters are significantly larger in size than those typically observed on contemporary sea floors. We argue that the gas is more likely to have been sourced from deeper Jurassic or Carboniferous sediments within the Witch Ground Graben, than from shallow biogenic origins. These structures rank in scale alongside the largest known pockmarks and demonstrate that natural fluid and hydrocarbon seeps have a long ancestry in this basin.

Mid-Jurassic volcanic structures in the Outer Moray Firth Basin, UK

3D seismic mapping in the southern part of Quadrant 15 in the Outer Moray Firth Basin reveals an array of circular to oval downthrown blocks bound by steep ring faults. These lows affect Triassic and older strata, are approximately 1.5–2 km in diameter and 0.5–1 km deep. The footwall crests of the structures now lie at a depth of c. 2.5 km. Several alternative causal mechanisms for these circular fault blocks are examined including salt dissolution, strike-slip pull-apart and impact cratering, but in terms of geological and geometrical criteria, the most likely mechanism appears to be small calderas or diatreme-conduit collapse. Seismic data indicates that the circular structures cut Triassic, Permian and Devono-Carboniferous strata and are infilled with pre-Upper Oxfordian sediments, therefore they are broadly synchronous with Mid-Jurassic volcanism–local well control demonstrates the presence of several thousand feet of basalt tuffs and lavas. The model proposed here to account for the circular structures consists of an initial phase of diatreme/maar cratering as magmas associated with the Rattray Volcanics Member intruded shallow aquifers in the Pentland Formation fluviodeltaic rocks. These maars enlarged and deepened via conduit wall collapse, supplemented by downsag-faulting due to magma withdrawal. The post-volcanic evolution of the structures included (1) differential erosion creating accommodation space which may have been infilled with Oxfordian reservoir sands and (2) local reactivation by underlying basement faults in the Kimmeridgian.

The Captain Field, UK North Sea; appraisal and development of a viscous oil accumulation

The Captain Field is located in Block 13/22a in the Western Moray Firth Basin of the UK North Sea, 80 miles northeast of Aberdeen in a water depth of 340 ft. Hydrocarbons are trapped in two geographical regions, the Main and Eastern closures, both with a significant stratigraphic pinchout component. The principal reservoirs are the Lower Cretaceous Upper and Lower Captain Sands, which are predominantly of turbiditic origin. Reservoir quality is uniformly excellent with in situ permeability averaging 7 D and porosity in the range 28-34%. The reservoir is largely unconsolidated sandstone with the depth to the crest of the field at "2700 ft TVDSS. The reservoirs contain a total oil-in-place of 1500 MMBO. The Upper Captain Sand has a small associated gas cap containing 29 BCF gas-in-place. The oil is heavy, by North Sea standards, with oil gravity ranging from 19 degrees and 21 degrees API and in situ viscosity between 88 and 47 cP, at the mean reservoir temperature of 87 degrees F. The fluid properties and offshore location necessitate the employment of innovative horizontal drilling methods, completion design and artificial lift technology in order to achieve an economically viable field development. Extended reach horizontal wells, with reservoir completion lengths of up to 6500 ft, are proposed for all oil producers and water injectors. Development risks were significantly reduced following two appraisal drilling campaigns in 1990 and 1993 culminating with the successful drilling and extended testing of a prototype horizontal field development well (13/22a-10). First oil production commenced from the Captain platform in early 1997.

Biomarker maturity profiles in the Inner Moray Firth Basin and implications for inversion estimates

Abstract The thermal stress (maturity) of Oxfordian to Kimmeridgian rocks in 13 Inner Moray Firth wells and at outcrop has been assessed using sterane and hopane isomer ratios. Significant depth profile displacements between the maturity trends for the Beatrice suite, other groups of Inner Moray Firth wells and a Viking Graben calibrant well are considered to be partly due to differences in uplift during Tertiary inversion and partly to varying palaeo-thermal gradients. Present day temperature plots are used to measure cooling from biomarker-derived maximum temperatures and hence to obtain approximate estimates of inversion. Consideration of errors in cooling estimates allows the limits of uplift to be constrained. Relative uplift between Inner Moray Firth fault blocks is likely to be subject to fewer estimation errors than absolute uplift (inversion) with respect to the Viking Graben. 12/26-1, the most uplifted well on the Central Ridge, is estimated to have experienced between 400 and 700 m of uplift relative to 12/24-1, the least uplifted in the Smith Bank Graben, although for other wells relative uplift is not always related to present-day structural elevation. Inversion (absolute uplift) of the Smith Bank Graben is probably at least 150 m although the error in this estimate is large. Inversion of well sections in the Beatrice Field is thus estimated to have been around 400 m.

Forward and reverse stratigraphic modelling of Cretaceous-Tertiary post-rift subsidence and Paleogene uplift in the Outer Moray Firth Basin, central North Sea

Abstract The Paleogene stratigraphy in the Outer Moray Firth is part of the North Sea post-rift sedimentary sequence that was deposited in a thermally subsiding basin following Late Jurassic rifting. Previous studies have drawn attention to anomalous uplift and departure from the McKenzie (1978, Earth Planetary Science Letters , 40 , 25–32) post-rift subsidence trend in the early Paleocene followed by an accelerated phase of basin subsidence in the early Eocene before returning to the normal post-rift subsidence. 2D forward and reverse stratigraphic modelling incorporating first-order global sea-level variations have been used to determine the timing and magnitude of departures from McKenzie post-rift thermal subsidence in the early Paleogene for the Outer Moray Firth. Reverse post-rift modelling, consisting of flexural backstripping, decompaction and reverse thermal subsidence, has been used to produce restored post-rift sections from present-day stratigraphy, which have been constrained by palaeobathymetric data. Forward syn-rift and post-rift structural and stratigraphic modelling uses the flexural cantilever model of rift basin formation and has been constrained by syn-rift structural data, present-day stratigraphy and palaeobathymetric markers. Forward and reverse modelling show that observed Cretaceous and Tertiary stratigraphy of the Outer Moray Firth was generated by the combined effects of inherited Late Jurassic syn-rift accommodation space, post-Jurassic rift thermal subsidence, sediment supply, long-term eustasy and an additional transient uplift event in the Paleocene. The much thicker Tertiary compared with Cretaceous can be explained by sediment infilling of starved Cretaceous palaeobathymetry. Reverse and forward post-rift modelling predict regional Paleocene uplift in the Outer Moray Firth of the order of 375–390 m (375 m reverse modelling, 390 m forward modelling), followed by c. 160 m of rapid Eocene subsidence, both superimposed on post-rift thermal subsidence following Late Jurassic rifting. Regional Paleocene uplift in the Outer Moray Firth is attributed to dynamic uplift associated with the development of the Iceland plume.

Pre-Mesozoic structure of the Inner Moray Firth Basin: constrains from gravity and magnetic data

The Moray Firth Basin is located off the north-east coast of Scotland and forms the western arm of the North Sea rift system. The basin is divided into two parts: the Outer Basin east of l°E and the Inner Basin to the west (e.g. Ziegier 1982): The distribution of the Mesozoic sequence within the Inner Moray Firth Basin is well known from seismic reflection data which are capable of imaging well-defined reflectors down to the Permo-Triassic level (Chesher and Bacon 1975).

Facies variations across the Middle-Upper Jurassic boundary in Western Europe and the relationship to sea-level changes

Abstract A facies analysis of Callovian-Oxfordian boundary beds has been carried out on numerous sections across western Europe, with the aim of constructing marine bathymetric curves for each geographic area. Onshore outcrops in northwest and northeast Scotland and borehole cores from the Beatrice Field in the Inner Moray Firth Basin were studied; virtually all the sediments are siliciclastic and often thick. An Early Callovian transgression is recorded from Skye and northeast Scotland, but the influence of local tectonics limits the lateral extent of distinctive lithological units. Within England the Callovian succession differs considerably from that found elsewhere. The most interesting unit is the Upper Callovian Hackness Rock, a condensed bed with chamositic ooids, which is thought to signify deposition at a time of rising relative sea level. The basal Oxfordian contains an organic-rich clay that may reflect a period of maximum flooding associated with anoxic conditions, and the clays above are likely to have been deposited during the ensuing regression. In central and southern England the Callovian-Oxfordian boundary occurs within the Oxford Clay. The change from the Lower to the Middle Oxford Clay (Middle-Upper Callovian boundary) marks a regional shallowing of the sea, signified by higher silt and kaolinite content, while the topmost Callovian, the Lamberti Limestone, is condensed and marks a time of relatively high sea level, with a fall in the proportion of kaolinite. Four regions in France were studied—Normandy, Poitou, Burgundy and Ardeche. The Normandy succession is very similar to that in Dorset, with the stage boundary occurring at the junction of two clay formations. A period of maximum flooding occurred at this boundary, before initial regression in the Early Oxfordian resulted in a renewed terrigenous input. The other regions generally have a very condensed, calcareous Callovian, often with a pelagic character. The condensed, iron ooidal limestones of the Spanish Iberian Chain were deposited in a relatively deep-water offshore setting. The majority of sections record a phase of severe sediment starvation during the latest Callovian and earliest Oxfordian zones, which are likely to reflect a period of maximum flooding. In the Swiss Jura the most significant period of negligible sedimentation rates occurred in the two zones across the stage boundary. Clear deepening or shallowing trends cannot readily be recognised in the offshore succession of the Swabian Alb. The comparison of a series of bathymetric curves has led to the recognition of regional sea-level events in western Europe during the Callovian-Early Oxfordian interval. A regional sea-level rise occurred during the Middle and Late Callovian, reaching its greatest extent in the latest Callovian ( lamberti Zone) and highstand conditions and incipient sea-level fall characterised the Early Oxfordian. A literature review of boundary beds in seven further regions from five continents has shown that a Middle Callovian sea-level rise and late Callovian-Early Oxfordian phase of high sea level can be recognised on a global scale. The European regional sea-level changes are therefore considered to be eustatic in origin, and probably related to sea-floor spreading in the Atlantic. A eustatic curve has been constructed and compared with two other such curves. It differs from that of Hallam, also based on the facies analysis technique, but is quite similar to that of Exxon workers, based on sequence stratigraphic concepts.

Tectonic controls on Upper Jurassic to Lower Cretaceous reservoir architecture in the Jeanne d’Arc Basin, with some comparisons from the Porcupine and Moray Firth Basins

Abstract Seismic, wireline log and core data from three geographically separate basins in the North Atlantic provide an understanding of the regional dynamics of the basin system, and quantify the sedimentary geometry response to a composite rift episode. The response to the Late Cimmerian rift episode in the Jeanne d’Arc, Porcupine and Moray Firth Basins is broadly comparable, with an initial episode of clastic reservoir facies deposition associated with active growth faulting. This was followed by a mid-rift period where subsidence outstripped sedimentation, resulting in organic-rich source rocks and culminating in a mappable maximum flooding surface. During the final phase of the rift episode, the decrease in subsidence rate resulted in the return of reservoir-quality clastics. The reservoir-scale response to the initial rifting is illustrated in cored sequences from the Jeanne d’Arc Basin. These indicate the development of axial and lateral sandy and conglomeratic braided fluvial and alluvial fan systems, with periods of muddy lacustrine deposition. Pedogenic surfaces offer much potential for detailed correlation and the analysis of sandbody geometry. The onset of anoxic conditions conducive to source rock deposition during the mid-rift phase of Late Cimmerian tectonism, and the preferential development of reservoir facies at the base and top of syn-rift sediment fill, are characteristics of considerable importance to exploration of the North Atlantic borderland rift basins.

Tectonic control on sedimentary evolution of three North Atlantic borderland Mesozoic basins1

AbstractMultiple episodes of extensional tectonism dominated the formation of Mesozoic fault‐bounded basins on the Grand Banks of Newfoundland, the Irish Continental Shelf and the central North Sea. A range of structural and stratigraphic responses in the Jeanne d'Arc, Porcupine and Moray Firth basins support widespread synchronous tectonic controls on sedimentation during one of these episodes, the Late Cimmerian.Rifting was preceded by a phase of related tectonism during which subsidence rates began to vary across broad areas but without significant fault block rotation. This Late Cimmerian ‘onset warp’ pattern of subsidence is considered to have been essential in the establishment of restricted anoxic basins from latest Oxfordian through Kimmeridgian (sensu gallico) time and the development of one prolific layer of organic‐rich source rocks.The most prominent and widely recognized structural/lithostratigraphic response to Late Cimmerian rifting was the deposition of sediment wedges. Tithonian to early Valanginian strata generally thicken‐ into northerly trending faults in the Jeanne d'Arc and Porcupine basins, indicating that extensional stress was orientated WNW‐ESE across a very broad area. The misalignment of this regional Late Cimmerian extensional stress with local inherited structural fabric may be responsible for transpressional uplift of individual fault blocks in the Outer Moray Firth basin.Sedimentological responses to Late Cimmerian rifting were varied, though a common lithofacies stacking pattern is recognized. Variably thick conglomerates and/or sandstones were widely deposited at the start of rift deformation, while palaeoenvironments ranged from alluvial and braid plain to submarine fan even within individual basins. The relatively coarse basal sediments fine upwards into a second layer of commonly organic‐rich shales and mark The widest variations in palaeoenvironments and sediment thicknesses occurred during the last phase of Late Cimmerian rift tectonism, though all three basins show evidence of decreasing water depths, increasing oxygen levels and increasing grain size. This lithofacies stacking pattern of relatively coarse to fine to coarse (reservoir/source/reservoir) and the development of bounding unconformities are largely attributable to progressive changes in rift‐controlled subsidence. Rift basin subsidence rates are interpreted to increase from a low at initiation of faulting to a mid‐rift peak, followed by slowing subsidence to the end of extension. A number of counteracting crustal mechanisms that may account for progressive variations in rift‐induced subsidence are considered.

Investigating the crustal structure of a strike‐slip “step‐over” zone along the Great Glen fault

Strike‐slip step‐overs and bends are regions of anomalous deformation that may yield clues to the mechanical behavior of the Earth's crust. Experimental reprocessing of a marine deep seismic reflection profile cutting across an ∼35‐km‐wide right‐stepping step‐over developed on the Great Glen fault system north of the Inner Moray Firth basin (east of Northwest Highlands, Scotland) reveals a restricted zone of prominent reflections and diffractions beginning at 7–8 km depth, continuing as deep as the interpreted Moho discontinuity at 26–27 km. Geological interpretation of this zone suggests a concentration of possible diffractors marking sharp structural disruption underlain by a ∼12‐km thick layer of subhorizontal and moderately dipping reflectors that dominates the lower crust in the step‐over region. The known kinematic history of the Great Glen fault system, together with the observed direction of the step‐over, implies that a zone of contraction would have formed within the step‐over during early Caledonian sinistral strike slip. The seismic reflection structure can be interpreted as developing by deformation associated with contraction and possible block rotation between the two fault segments such that the diffractive zone represents steep structure in a near‐vertical zone of strike slip which passes deeper into a layer of low‐angle dipping thrusts or shear zones. The results of this study are consistent with a concept of mechanical detachment in the middle crust as documented for areas of contraction along active strike‐slip faults.