Implications of Mesozoic—Recent basin development in the western Inner Moray Firth, UK

I m p l i c a t i o n s of M e s o z o i c - R e c e n t basin d e v e l o p m e n t in t h e w e s t e r n I n n e r M o r a y Firth, U K John R. Underhill D e p a r t m e n t of G e o l o g y and Geophysics, University of Edinburgh, Grant Institute, King's Buildings, West Mains Road, Edinburgh EH9 3JW, UK

Received 10 May 1990; revised 10January 1991; accepted 14January 1991 A new interpretation of a comprehensive seismic database, consisting of over 5000 km of seismic lines, contradicts the notion that strike-slip motion on the Great Glen Fault (GGF) was the dominant control on Mesozoic basin development in the Inner Moray Firth, UK. It is suggested that the GGF, which may have been active during Palaeozoic times, lay dormant during the Mesozoic and that normal fault systems (such as the Helmsdale-Wick system), formed during two extensional events (pre-Jurassic and Late Jurassic), were the dominant controls on Mesozoic basin evolution• The present expression of the GGF as a discrete, sub-vertical through-going fault attests to its subsequent activity as a divergent wrench structure as part of limited oblique-slip motion after the Early Cretaceous• It seems most likely that such movements occurred in the Tertiary and may have been contemporaneous with regional uplift of the Scottish Highlands during Palaeocene-Eocene (Thulean) events, in response to north-east Atlantic rifting or subsequent Oligo-Miocene (Alpine) tectonism. The new model aids the understanding of the area's general hydrocarbon prospectivity. It can now be demonstrated that the most likely kitchen area for oil resident in the Beatrice Field was centred in the Sutherland Terrace area to the north-west when it formed the site of an active half-graben. Maturation in, and direct up-dip migration from, that area probably took place at the time of maximum burial during the Late Jurassic and Cretaceous, below the half-graben depocentre adjacent to the Helmsdale Fault• Subsequent regional uplift and dissection of the Sutherland Terrace may have served to take the kitchen area out of the maturation window and led to reactivation of several pre-existing structures• Breach and remigration of hydrocarbons is likely to have occurred as a result of the later motions. Keywords: basin development; Inner Moray Firth, UK; fault motions

Introduction .

The Inner Moray Firth (IMF) basin forms part of the western arm of the trilete North Sea Basin system and lies to the east of the Sutherland coast, Scotland, UK (Figure 1). It is well defined structurally, being bounded on its northern, north-west and southern margins by the Wick, Helmsdale and Banff fault systems, respectively (Figure 2). It is also dissected by the Great Glen Fault (GGF). Although the IMF has long attracted hydrocarbon exploration, well results have largely been disappointing. Until recently, British Petroleum's Beatrice Field (discovered by Mesa in 1976 and operated by Britoil and BP since 1981; Linsley et al., 1980; Johnson, 1984) remained the only commercial discovery. However, interest in the area has been renewed with the development of Texaco's Captain Field (Block 13/22a), Ultramar's Ross Field (13/28 and 29), recent encouraging discoveries including Kerr-McGee's find in Block 13/22b, the recently announced successful test of a separate closure in Block 11/30a ('Theta Prospect') and advertisement of numerous blocks in recent UK licensing rounds. Despite the renewed interest in the area, many

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Basin development in the Inner Moray Firth: J. R. Underhill .o. /7- ...,,.,;,o;~..° aspects of its structural history remain unclear and a regional synthesis of the basin's prospectivity is lacking. Hence the main aim of this paper is to rationalize existing data and integrate them with new information to determine the main controls on basin development and to discuss their implications for the understanding of the area's hydrocarbon prospectivity. Previous ideas for Mesozoic basin development

Figure2 Inner Moray Firth location map showing features described in the text and figures. The location of the seismic lines and the individual wells used for their correlation are shown; the other wells and seismic lines used in the study are given in Figure 3. Only the main fault zones are depicted

It has generally been believed that the GGF played a major role in Mesozoic basin development of the IMF (McQuillin et al., 1982; Barr, 1985; Bird et al., 1987; Frostick et al., 1988; Roberts et al., 1990a). Roberts et al. (1990a) have followed earlier workers in their recent, well illustrated paper by stating that they have no reason to disagree with the previous kinematic model which envisages that the basin formed as a trans-tensional feature related to dextral displacement on the GGF. By accepting this model, they were forced to imply 'that only faults trending E S E - W N W (perpendicular to the GGF) will have evolved as dip-slip faults; all others will have evolved with a strike-slip component of displacement'. Assuming the

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Figure 3 Seismic data coverage used to derive the geological interpretation given in this paper. The database consists of GECO's 1977, 1978 and 1986, Merlin Geophysical's (now owned by GECO) 1986, Horizon's 1980 and the Institute of Geological Sciences' 1972 lines. The latter were reprocessed by GECO in 1982. Data from all the wells shown were used to correlate the seismic database; only five are highlighted on seismic sections in this paper (Figure 2). Additional data has been used to tie in wells in BP's Block 12/21 and for the Beatrice Field

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Table 1 Age relationships and seismo-stratigraphic sequence nomenclature used in the western Inner Moray Firth. Other workers' stratigraphic schemes are placed alongside for comparison. Although the subdivision of the J2 megasequence is tabulated for information, it is not referred to directly in this paper. The main reflectors shown on the seismic lines (Foldouts 1 - 5 )are indicated by arrows in the left-hand column• The ornament shown is consistent for all diagrams except Foldout 6, which shows the subdivision of the J2 megasequence

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Basin development in the Inner Moray Firth: J. R. Underhill OFFSHORE- ONSHORE . . . . f - # ' ~ : + + + ~'//I thickening did characterize some areas (e.g. between SEISMIC / OUTCROP . ' . "/ + ~,%,~'<"~,,~.'" z / / f Wells 12/27-1 and 12/27-2 on Figure 6). CORRELA]'ION, 'MF ] " "/ ' ~'e~ + ~ j The Late Oxfordian-Early Berriasian (J2) megasequence shows that much greater sedimentary accumulation and more pronounced spatial thickness variations occurred during the Late Jurassic (Figure 5b). The thickness variation that characterizes the J2 . . . . . . ... S~@ megasequence appears to be controlled by numerous planar, normal faults, the activity of which was penecontemporaneous with deposition (in contrast to Bird et al., 1987). These faults had the effect of subdividing the IMF into a number of half-grabens (Foldouts 1-4 and Figure 5b). ' Thickening of the Late Jurassic megasequences /. ii ~ 7 appears to be unrelated to the line of the GGF. Contours trend obliquely to the strike of the GGF and Figure4 Seismo-stratigraphic sequences of the B r o r a show no abrupt variation across it. Integration of Helmsdale onshore section and its correlative offshore outcrop. The trace of the seismic line illustrated in Foldout 5 is seismo- and sedimentological facies information highlighted, as is the trace of the Great Glen fault (GGF). Note suggests that the GGF had little or no effect on either. that the ornament used is consistent with that given in Table 1 The subdivision of the J2 megasequence into five and shown on the seismic lines in Foldouts 1 - 5 basin-correlative sequences tied to well stratigraphy highlights its extensional wedge geometry and is Rhaetian-Mid-Oxfordian (J1) megasequence down consistent with a half-graben origin (the J2.1-2.5 into five sequences (J1.1-1.5; Table 2) because they lay sequences of Foldout 6 and Table 1). Indeed, on-going below the limits of seismic resolution. Other studies suggest that the internal sequence stratigraphy subsequences may also be identified within the J2 of the J2 megasequence may be best explained as the megasequence but have not been separated out result of syn-sedimentary extensional tectonism and because of their highly localized nature. sedimentary responses within a fully marine system, In a similar manner to Roberts et al. (1990a; their rather than reflecting the effect of global eustatic Figure 12b), it has been possible to correlate reflectors fluctuations as previously believed (Vail and Todd, across the GGF close to its inflection point, enabling 1981; Vail et al., 1984). The recognition of the five mapping of the megasequences and their components regional sequences within the J2 megasequence affords to continue into the area lying between the GGF and the opportunity to assess the timing of extensional the Helmsdale Fault (hereafter termed the Sutherland activity more accurately and suggests that such activity Terrace; Figure 2 and Foldouts 1-4). The validity of peaked during the Kimmeridgian and the Mid-Late the interpretation in this area is supported by the good Volgian (Foldout 6). correlation between the seismic, British Geological Several of the faults that demonstrated growth Survey borehole data (Holes 71/26 and 71/17) and seaduring Jurassic times and inactivity during the Early floor outcrop patterns (British Geological Survey, Cretaceous propagate to the sea bed suggesting that 1977, 1982), the Sutherland-1 land well, the Golspiethey were reactivated. The most severe uplift and fault Brora-Helmsdale onshore exposures and ties to all reactivation (inversion) or nucleation after the Early available strike lines (Figure 4 and Foldout 5). Cretaceous occurs in the north-west of the basin in the vicinity of Blocks 12/16 and 12/17 (Figure 5a, b). ,

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Results The results of regional mapping of the seismostratigraphic packages can only help to build up a picture of meaningful basin geometries when available palaeo-bathymetric information has been included in its assessment (Bertram and Milton, 1989). Although the palaeobathymetric indices indicate that sedimentary thickness variation can be used directly to indicate the Jurassic basin configuration in the IMF, they also indicate that the basin was underfilled during the Early Cretaceous. K1 thickness variations will therefore not be shown because any attempt to make a correlation between thicknesses and syn-sedimentary tectonism in the Early Cretaceous megasequence is likely to be erroneous. Integration of the results of the seismic mapping and well correlations indicate that the RhaetianMid-Oxfordian (J1) megasequence shows a general basin-wide westerly thickening across to the limit of exposure at the Helmsdale Fault (Figures 5a and 6). Although local, syn-sedimentary fault activity does not appear to be highly significant at a seismic scale, well correlation suggests that limited fault-controlled

362

Implications of the seismic interpretation for basin development Although accurate non-decompacted thickness trends have previously been given for the Jurassic of the IMF (Andrews and Brown, 1987; Roberts et al., 1990a), their full significance has not been realized. In the work by Roberts et al. (1990a), this has not been aided by the fact that the thicknesses were not extended into the Sutherland Terrace, but terminated against the GGF, despite the fact that figures in their paper show that seismic extrapolation had been successfully attempted. It is the integration of thickness information from the Sutherland Terrace and field observations from onshore exposures which enables the full effect of seismic interpretation to be appreciated. Such integration shows that the GGF had a negligible effect on Jurassic thicknesses relative to other syn-sedimentary faults, particularly those which define half-grabens in the Late Jurassic (J2) megasequence. This is in direct contradiction to the results which might be expected along a syn-sedimentary strike-slip fault

Marine and Petroleum Geology, 1991, Vol 8, August

Basin development in the Inner Moray Firth: J. R. Underhill

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M a r i n e and P e t r o l e u m G e o l o g y , 1991, Vol 8, A u g u s t

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zone that is meant to have been active for the whole duration of the Jurassic (some 70 million years). If the GGF had been moving, some structural, seismo-stratigraphic or sedimentological expression of the activity would be expected. The removal of major strike-slip motion on the GGF from being the main control on basin development demands that a new mechanism is invoked for IMF basin development. Assessment of seismic geometries and thickness information makes extensional tectonics the most likely causative process. Although a limited degree of strike-slip movement might be envisaged along any of the main half-graben bounding faults, structural and seismo-stratigraphic package geometries are more consistent with a regime dominated by orthogonal (e.g. N W - S E to NNW-SSE directed) extension across the Wick and Helmsdale fault systems (at least during the Late Jurassic). Such a conclusion is consistent with relationships with the other main faults, such as the Smith Bank, Lossiemouth and Banff fault systems. This interpretation is also consistent with the occurrence of footwall uplift adjacent to the syn-sedimentary faults (e.g. the Beatrice Oil Field; Jackson and McKenzie,

364

1983; Barr et al., 1985) and similar structures drilled by less successful wells, such as 12/27-1 (Foldout 6) and 12/21-3, as well as the unpublished results of the University of Liverpool Fault Analysis Group on normal fault displacement geometries applied in the area (J. Waterson and J. J. Walsh, personal communication). The Helmsdale Fault probably acted as one of the main half-graben, basin-bounding faults during Late Oxfordian-Berriasian times with a depocentre apparently centred in the area to the south-east of Lybster (Figure 5b). The fault appears to have been lined by a proximal submarine fan apron (consisting of the Kintradwell Beds, the Allt na Cuile Sandstone and Helmsdale Boulder Beds). Clasts within the Helmsdale Boulder Beds are consistent with a derivation from a mixed shelf and terrestrial area sited on its immediate footwall (Bailey and Weir, 1932). Each of the units thin and fine rapidly offshore, away from the fault, and distal mud deposition characterizes the same, but reduced, stratigraphic interval offshore. Independent sedimentary and tectonic information support the suggestion that proximal deposition occurred on a palaeoslope which dipped gently away from the

Marine and Petroleum Geology, 1991, Vol 8, August

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Basin development in the Inner Moray Firth: J. R. Underhill If the fault zone was completely dormant, perhaps it Helmsdale Fault (Pickering, 1983, 1984; Roberts, proved impossible for such a vertical feature to 1989). accommodate significant extension perpendicular to its It is interesting to note that mapping of the Triassic strike in a similar way to that argued by Roberts et al. interval into the Sutherland Terrace may also support a (1989, 1990a) in their discussion of Frostick et al. very limited stratigraphic thickening of this interval (1988). An identical mechanism, as a consequence of a towards the north-west (Foldout 1). Although the similar orientation in the stress axes, may also account present database is too inexact to confirm a for the fault's apparent dormancy in the Devonian syn-sedimentary extensional (half-graben) setting for when north-west-south-east directed extension may Triassic sedimentation because there is little evidence also have been prevalent. for significant abrupt thickness changes, the results are consistent with limited extensional activity. It may be Although the GGF may not have played a significant inferred from the seismic interpretation that if any fault role in controlling Jurassic sedimentation patterns, its system could be viewed as an influence on sequence striking appearance on seismic data attests to its later importance in the IMF's basin evolution. The current geometries, then it would be the Helmsdale Fault and expression of the GGF may be interpreted as a not the GGF. Again, the latter appears to have had negligible effect on sedimentation (c.f. Frostick et al., through-going sub-vertical fault from and into which 1988). Any Triassic extensional activity probably numerous faults radiate and converge with dominantly post-dated ?Permo-Triassic rift events following normal displacements. This is consistent with an limited 'Variscan' deformation. interpretation as a divergent wrench fault or negative flower structure (Harding, 1985, 1990; Harding et al., 1985; Foldouts 1 - 4 ) . Further discussion arising from the new An accurate determination of the nature and interpretation magnitude of the displacement is problematic because The occurrence of clasts containing Middle Jurassic of the obliquity of the cut relative to thickness contours (J1) faunas within the Helmsdale Boulder Beds and the local absence of the top of the J2 indicate that the underlying megasequence extended megasequence. However, it is possible to demonstrate further west and was subjected to footwall uplift, that any strike-slip component of post-Jurassic motion erosion and consequent resedimentation in the Late probably had a relatively small net displacement, which Jurassic. It is interesting to speculate that similarities from comparison of seismo-stratigraphic thicknesses, is between the IMF J1 sequences and their counterparts unlikely to have exceeded 10 km. in the Hebridean Basin to the west of Scotland The suggestion of limited amounts of strike-slip (Morton, 1989) may imply a causal link. It seems fairly movement after the Early Cretaceous is consistent with probable that a sedimentary connection existed across other workers' estimates of post-Triassic and what is now the Scottish Highlands. post-Devonian motion (Holgate, 1969; Donovan et al., The apparent increase in the density of clasts of Old 1976; Speight and Mitchell, 1979; Mykura, 1983; Red Sandstone relative to those of Middle Jurassic age, Rogers et al., 1989). None of their figures are on moving up the stratigraphic column within the compromised by the fact that no motion has occurred boulder beds (MacDonald, 1985), may record an during the Triassic to Early Cretaceous. inverted stratigraphy as a result of progressive erosion The accurate timing of movements after the Early into deeper stratigraphic levels as each was successively Cretaceous on the GGF remains problematical. exposed on the footwall to the Helmsdale fault with Although unpublished borehole breakout data and time. neotectonic seismicity show that the fault is still active, Finally, questions must be asked of the existing it is likely that the main phases of uplift experienced by kinematic models for the Late Jurassic development of western areas (e.g. Easter Ross and the Sutherland the North Sea system if the extensional control for IMF Terrace) occurred earlier. An integrated fission-track basin development is accepted. Previous kinematic and vitrinite reflectance study may be the only way to models have been tied to dextral movement on the resolve the timing of events and attempt to quantify GGF (McQuillin et al., 1982; Roberts et al., 1990b, c). relative uplift in areas where suitable rocks have been The suggestion that such movement on the GGF was lifted above the temperatures of annealing of insignificant and instead that approximately susceptible minerals. north-west-south-east extension dominated the IMF requires that new models are invoked to explain the In the absence of other data, it may be speculated development of the tripartite North Sea basin system that the most significant movements occurred in the during the Late Jurassic. Tertiary. Although motion on the GGF and other reactivated faults might conceivably have been later ]perhaps as a consequence of Oligo-Miocene (Alpine) When was the Great Glen Fault active and what events], the timing of regional uplift and the majority of was its role? fault reactivation was most likely to have been At first, it seems remarkable that the GGF, widely Palaeocene-Eocene, in response to Thulean events regarded as having been active prior to Mesozoic during north-east Atlantic rift propagation (Dewey deposition (e. g. Permo- Carboniferous and and Windley, 1988). Both the Scottish Highlands and pre-Devonian; Rathbone and Harris, 1980; Norton et the East Shetland Platform experienced uplift and al., 1987; Hutton, 1988; Rogers et al., 1989) should erosion at that time. Contemporaneous shelf have had such an insignificant role during IMF basin progradation and submarine fan construction in the development in the Mesozoic. Even if the GGF had Outer Moray Firth and Central Graben areas (Parker, been active at depth, it appears to have had little or no 1975; Rochow, 1981) probably occurred in response to surface or sea-floor expression. uplift on the north-west Britain rift shoulder.

Marine and Petroleum Geology, 1991, Vol 8, August

365

Basin development in the Inner Moray Firth: J. R. Underhill Although quantification of the amount of relative The formation of new structures as a result of uplift experienced by the basin as a whole, or by Cenozoic tectonism, such as those in the north-west of specific parts of the basin, such as the Sutherland the basin (e.g. in Blocks 12/16 and 12/17; Figure 5), is Terrace, is not well defined, circumstantial evidence likely to have post-dated charge. Such structures are from sonic velocity, vitrinite reflectance and clay thus dependent on remigration and spill of mineralogy studies (Hurst, 1980) suggests that Middle hydrocarbon-bearing palaeostructures. Consequently, Jurassic levels have been raised by at least 1 km in the a significant number of the nine potential trap types west (c.f. estimates of about 300 m in western areas, defined by Roberts et al. (1990a) may be the result of given by Roberts et al., 1990a). Elevation of the top post-charge structures and, hence, remain high risk Chalk reflector suggests that the amount of relative plays. Structures which are bound by reactivated faults uplift reduces progressively eastward such that the limit that propagate to the sea bed are likely to have a of uplift approximately coincides with the western significant risk of seal. extent of Tertiary subcrop. The differential movement Whatever the structural history, it naturally remains as a consequence of these Tertiary events has imparted essential to understand the sedimentological control on a 2-3' regional dip to the basin. Jurassic prospectivity because valid structures may not contain units of reservoir quality.

Implications for prospectivity

Conclusions

One further aspect of the new model for IMF basin development is that it may aid our understanding of the area's prospectivity, particularly in terms of trap formation and hydrocarbon sourcing. Sequential palinspastic restoration of a section across the GGF highlights IMF basin development and affords a better understanding of sequence geometries through time

The Mesozoic basin development of the IMF occurred under an extensional and not a strike-slip regime. Strike-slip on the Great Glen Fault (GGF) played a negligible role. Evidence that the RhaetianMid-Oxfordian (Jl) and Late Oxfordian-Berriasian (J2) megasequences and their component sequences all thicken westwards across the restored GGF onto the Sutherland Terrace towards the Helmsdale Fault suggests that the GGF lay dormant during extensional activity. This conclusion is supported by the apparent lack of syn-sedimentary tectonism or facies control exerted by the GGF during deposition of either megasequence. The Triassic and Rhaetian-Mid-Oxfordian (J1) intervals are characterized by limited basin-wide extension, perhaps following an earlier (?PermoTriassic) rift event. However, the Late OxfordianBerriasian (J2) megasequence demonstrates the importance of a syn-sedimentary, intra-basinal halfgraben as a consequence of significant extensional activity. Early Cretaceous sediments record onlap and gentle regional subsidence in an underfilled basin. The inference that the Helmsdale Fault, rather than the GGF, acted as the main basin-bounding fault in the Late Jurassic is consistent with onshore exposures and offshore well correlations. Late Jurassic sequences, including the Helmsdale Boulder Beds, which shale out rapidly away from the coastal outcrop, are interpreted to represent proximal submarine deposition along an actively-eroding extensional fault scarp. The present seismic geometry of the GGF suggests that it moved as a limited strike-slip fault after the deposition of Early Cretaceous sequences. Timing of movement is likely to have been in the Tertiary. It may have been associated with [Palaeocene-Eocene (Thulean)] regional uplift or be the result of subsequent events. Numerous faults also show evidence of post-Jurassic reactivation, which is likely to be associated with movement on the GGF or to accommodate regional uplift. The location of a major Late Jurassic-Cretaceous half-graben depocentre in the Sutherland Terrace may help to explain the occurrence of hydrocarbons in the Beatrice Field. The area probably formed a kitchen area from which oil migrated up-dip into closures (such as Beatrice) formed as the result of footwall uplift associated with Late Jurassic syn-sedimentary activity. Subsequent events in the IMF have served to uplift and

(Figure 7). It seems likely that most of the potential hydrocarbon traps formed during the Late Jurassic (Late Oxfordian-Berriasian) extensional events as a result of fault block rotation and associated footwall uplift adjacent to active normal faults. Those traps lying between the Helmsdale Fault and Central Ridge (Figure 2) were probably charged by mixed Jurassic and/or Devonian sources (Duncan and Hamilton, 1988; Peters et al., 1989; Trewin, 1989; Bailey et al., in press) from beneath the Sutherland Terrace depocentre at the time of its maximum burial during the Late Jurassic and Cretaceous (Figure 7b, c). Migration of hydrocarbons up the Helmsdale Fault at this time may account for the local occurrence of bitumen and associated hydrocarbon filled multi-mineral veins in the footwall to the fault (Tweedie, 1979; Parnell, 1982). Regional uplift and limited motion after the Early Cretaceous on the GGF has served to dissect the former half-graben depocentre (and potential kitchen area), and has imparted a regional easterly tilt and caused local reactivation of some pre-existing structures. Although the Sutherland Terrace has probably been elevated above the maturation window as a consequence of the uplift, it is possible that some Jurassic or, more likely, Devonian potential source levels may still be mature and expelling hydrocarbons in the areas to the east of the Helmsdale Fault, where they remain deeply buried along the GGF and Wick Fault systems. If present day maturation of source intervals is lacking, it becomes important to detect those structures which existed during the Late Jurassic-Cretaceous when hydrocarbon migration probably took place. Such detection would be possible using palinspastic cross-section restoration techniques. In areas where these palaeostructures may be identified, it becomes important to determine whether subsequent movements could have induced them to tilt to cause rotation of palaeo oil-water contacts, limited spill or be completely breached.

366

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Basin d e v e l o p m e n t in th e I n n e r M o r a y F i r t h : J. R. U n d e r h i l l British Geological Survey Map (1982) Caithness (Sheet 58N 04W) Sofid Edition. Scale 1:250000 Brown, L. F. and Fisher, W. L. (1977) Seismic-stratigraphic interpretation of depositional systems: examples from Brazilian rift and pull-apart basins. In: Seismic Stratigraphy -Applications to Hydrocarbon Exploration (Ed. C. E. Payton) Am. Assoc. Petrol. Geol. Mere. No. 26, pp. 213-248 Dewey, J. F. and Windley, B. F. (1988) Palaeocene-Oligocene tectonics of NW Europe. In: Early Tertiary Volcanism and the Opening of the NE Atlantic (Eds. A. C. Morton and L. M. Acknowledgements Parson) Spec. Pub/. Geol. Soc. London No. 39, pp. 25-31 Donovan, R. N., Archer, R., Turner, P. and Tarling, D. H. (1976) The author thanks GECO, Horizon, Mobil, British Devonian palaeogeography of the Orcadian Basin and the Petroleum, Unocal and Fina for access to their Great Glen Fault Nature (London), 259, 550-551 proprietary data. GECO gave permission to publish the Duncan, A. D. and Hamilton, R. F. M. (1988) Palaeolimnology and organic geochemistry of the Middle Devonian in the seismic data included here. Philip Trayner of GECO Orcadian Basin. In: Lacustrine Petroleum Source Rocks was particularly helpful in facilitating the release and (Eds. A. J. Fleet, K. Kelts and M. R. Talbot) Spec. Publ. GeoL eventual publication of the data. Mike Clutson and Soc. London No. 40, pp. 173-201 Katrine Holdoway (Mobil) are both thanked for help in Frostick, L., Reid, I., Jarvis, J. and Eardley, H. (1988) Triassic sediments of the Inner Moray Firth, Scotland: early rift providing copies of all released data. Shell and Esso deposits J. Geol. Soc. (London) 145, 235-248 Expro are thanked for contributing towards the costs of Harding, T. P. (1985) Seismic characteristics and identification of seismic reproduction and fieldwork expenses. British negative flower structures, positive flower structures and Petroleum and their partners are acknowledged for positive structural inversion Am. Assoc. Petrol. Geol. Bull. access to data over Blocks 11/30 and 12/21 and 69, 582-600 Harding, T. P. (1990) Identification of wrench faults using permission to publish Figure 6 and Foldout 6. A1 subsurface structural data: criteria and pitfalls. Am. Assoc. Fraser, Tom Fyfe and Richard Hedley's assistance, Petrol. GeoL Bull. 74, 1590-1609 constructive criticism and help are particularly Harding, T. P., Vierbuchen, R. C. and Christie-Blick (1985) acknowledged. George Bertram, Mike Bowman, Structural styles, plate-tectonic settings and hydrocarbon traps of divergent (transtensional) wrench faults. In: StrikeStewart Brown, Mike Clutson, John Dixon, George slip Deformation, Basin Formation and Sedimentation (Eds. Farrow, Andrew Hurst, Dave Latin, Ian Main, Nick K. T. Biddle and N. Christie-Blick) Spec. PubL Soc. Econ. Milton, Roger Scrutton, Tony Spencer, Kevin Stephen, PaleontoL Mineral. 37, 51-77 Ken Thomson, Nigel Trewin and Juan Watterson are Holgate, N. (1969) Palaeozoic and Tertiary transcurrent all thanked for their constructive criticisms. Diana Baty movements on the Great Glen Fault Scot. J. GeoL 5, 97-139 Hubbard, R. J. (1988) Age and significance of sequence and Yvonne Cooper are thanked for their assistance in boundaries on Jurassic and Early Cretaceous rifted producing the diagrams. I acknowledge my ex-Shell continental margins Am. Assoc. Petrol Geol. Bull. 72, 49-72 colleagues Marc Alberts, David Bowler, Roelef Hubbard, R. J., Pape, J. J. and Roberts D. G. (1985) Depositional Janssen, John Parker, Gordon Parry, Larry Wakefield sequence mapping as a technique to establish tectonic and stratigraphic framework and evaluate hydrocarbon potential and Ben Reinhardt for their active encouragement of on a passive continental margin. In: Seismic Stratigraphy I1: the work and my new University of Edinburgh staff, An Integrated Approach to Hydrocarbon Exploration (Eds. students and friends for creating such a stimulating O. R. Berg and D. Woolverton) Am. Assoc. Petrol GeoL Mem. working environment in which to research. This paper 39, pp. 79-91 is Petroleum Science and Technology Institute Hurst, A. (1980) The diagenesis of Jurassic rocks of the Moray Firth, NE Scotland Unpublished PhD Thesis University of Publication No. 1. Reading, UK Hutton, D. H. W. (1988) Igneous emplacement in a shear-zone termination: the biotite granite at Strontian, Scotland Bull. GeoL Soc. Am. 100, 1392-1399 Jackson, J. and McKenzie, D. P. (1983) The geometrical References evolution of normal fault systems J. Struc. GeoL 5, 471-482 Allen, P. A. and Allen, J. R. (1990) Basin Analysis Blackwell Johnson, R. W. (1984) The development of the Beatrice Field Scientific, Oxford, 451 pp Inst. Petrol. 1984Ann. Conf. Rep. 19 pp Andrews, I. J. and Brown, S. (1987) Stratigraphic evolution of Linsley, P. N., Potter, H. C., McNab, G. and Racher, D. (1980) The the Jurassic, Moray Firth. In: Petroleum Geology of NorthBeatrice Field, Inner Moray Firth, UK North Sea. In: Giant Oil West Europe (Eds. J. Brooks and K. Glennie) pp. 785-795 Fields of the Decade (Ed. M. T. Halbouty) Am. Assoc. Petrol Bailey, E. B. and Weir, J. (1932) Submarine faulting in Geol. Mere. pp. 117-129 Kimmeridgian times: East Sutherland Trans. R. Soc. MacDonald, A. C. (1985) Kimmeridgian and Volgian fault margin Edinburgh 62, 429-467 sedimentation in the northern North Sea Unpublished PhD Bailey, N. J. L., Burwood, R. and Harriman G. E. Application of Thesis University of Aberdeen, UK pyrolysate carbon isotope and biomarker technology to McQuillin, R., Donato, J. A. and Tulstrup, J. (1982) Development organofacies definition and oil correlation problems in North of basins in the Inner Moray Firth and the North Sea by Sea basins Org. Geochem., in press crustal extension and dextral displacement of the Great Glen Barr, D. (1985) 3-D Palinspastic reconstruction of normal faults fault Earth Planet. Sci. Lett. 4, 25-32 in the Inner Moray Firth: implications for extensional basin Mitchum, R. M., Vail, P. R. and Thompson, S. (1977) Seismic development Earth Planet. Sci. Lett. 75, 191-203 stratigraphy and global changes of sea-level, part 2: the Barr, D., McQuillin, R. and Donato, J. A. (1985) Footwall uplift in depositional sequence as a basic unit for stratigraphic the Inner Moray Firth basin, offshore Scotland J. Struct. Geol. analysis. In: Seismic Stratigraphy- Applications to Hydro7, 267-268 carbon Exploration (Ed. C. E. Payton) Am. Assoc. Petrol. Bertram, G. T. and Milton, N. (1989) Reconstructing basin Geol. Mere. 26, 53-62 evolution from sedimentary thickness: the importance of Morton, N. (1989) Jurassic sequence stratigraphy in the palaeobathymetric control with reference to the North Sea Hebrides Basin, NW Scotland Mar. Petrol GeoL 6, 243-260 Basin Res. 2, 247-257 Mykura, W. (1983) Old Red Sandstone. In: Geology of Scotland Bird, T. J., Bell, A., Gibbs, A. D. and Nicholson, J. (1987) Aspects (Ed. G. Y. Craig), Second edition, Scottish Academic Press, of strike-slip tectonics in the Inner Moray Firth basin, Edinburgh, pp. 205-251 offshore Scotland Norsk Geol. Tiddskr. 67, 353-369 Norton, M. G., McClay, K. R. and Way, N. A. (1987) Tectonic British Geological Survey Map (1977) Moray-Buchan (Sheet evolution of Devonian Basins in northern Scotland and 57N 04W) Solid Edition. Scale 1:250000 southern Norway Norsk GeoL Tidssk. 67, 323-338

dissect western parts of the basin such that the Sutherland Terrace was largely lifted above the hydrocarbon-generating window. Tertiary uplift is also thought to have caused tilting, spill and/or breach of pre-existing structures and reorganization of migration pathways.

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