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Home , Chalk Group, Coal Measures, Unconformity, Westphalian (stage)

2013

2010 2011 2012

Carboniferous petroleum systems Promote beneath the Central United Kingdom 2014

1. Abstract

Interpretation of recent long-offset seismic data augmented by potential field modelling has revealed a Devono- basin architecture beneath the Central North Sea that exhibits many features in common with the contemporary basins of northern and southern (Milton-Worssell et al. in press).

Potential source intervals within and -age remote from the traditional Central source kitchen are confirmed by deep Central North Sea wells. The maturity of these potential source rocks is supported by the occurrence of prominent gas chimneys in the and Tertiary strata of southern Quadrant 29, up to 50 km south-west of the Jurassic source kitchen (Hay et al. 2005).

2. Middle to Lower Carboniferous basin architecture

Seismic interpretation has provided indications that many of the basement controls that defined the architecture of the onshore UK Lower Carboniferous basins operated across much of the UK Central North Sea also (see section 5). In the south-east Central North Sea (Fig. 1), Mid North Sea High structural elements have been interpreted that are comparable with the Askrigg Block and Stainmore Trough. The up-thrust Auk-Flora Ridge intra-basinal basement block resembles the inlier within the Midland Valley of Scotland and the Cross Fell inlier in northern England.

SOUTHERN GRANITE-CORED MID-NORTH SEA HIGH CENTRAL NORTH BLOCK CARBONIFEROUS BASIN GRABEN SEA (cf. Askrigg Block) (cf. Stainmore Trough) u/c base Cret. top Hod

Zechstein Gp. Fig. 1 Schematic profile showing generalised structural and strati- Dogger graphic relationships across the Granite Mid North Sea High, from south of ?Fell Sst. the Dogger Block to the Central delta Carb. AUK-FLORA Graben. The Mid North Sea High RIDGE Upper Rotliegend (cf. Pentland Hills, Carboniferous basin includes Base Upper Rotliegend Gp. Cross Fell inliers) c o a l - m e a s u r e s o u r c e - r o c k Lower Rotliegend Group Dinantian Carboniferous intervals equivalent to those in the Base Near base Dinantian basic intrusives Scremerston Coal Group of Late Westph./Steph. Upper Old Red northern England. Kyle Late /Westphalian Lr. Palaeozoic/Lr. Devonian Mid-Carb. unconformity Late Dinantian/Early Namurian Lower crust delta top

0° 3°E Preliminary results of seismic interpretation augmented 58° by potential field modelling are presented in Fig. 2. In addition to Lower Carboniferous basins proven by drilling on the eastern Mid North Sea High and in 19 20 21 22 NJ northern Quadrant 21, undrilled Lower Carboniferous basins have also been interpreted in Quadrants 28 & 29. These are the regions that are the most likely to SJ 23 contain significant volumes of coal-measure source DH 57° rocks within the sub-Permian section. The Quadrant 29 basin occurs adjacent to a region of numerous gas chimneys within the post-Permian section inferred from older 2D seismic data such as that illustrated in Figure 3 Forth Auk- Approaches 27 28 29 Flora - it is the most likely source kitchen for this gas. Ridge Lower Carboniferous basin Lower Carboniferous basement thin block 31 Lower Carboniferous 56° (Dinantian to early Buried granite thick Namurian) thickness Selected fault Upper

Netherlands Middle Devonian and older Selected Jurassic fault 35 F 36 37 38 39 Selected Palaeozoic fault NJ North Jaeren High block SJ South Jaeren High block Selected Variscan syncline D DH Devil’s Hole Granite Selected Variscan anticline 50 km D Dogger Granite Post-Permian gas chimney 55° F Farne Granite (as mapped by Hay et al., 2005) 41 42 43 44

Fig. 2 Lower Carboniferous basin configuration in the Central North Sea, based on the seismic interpretation 2013

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Carboniferous petroleum systems Promote beneath the Central North Sea United Kingdom 2014

SW gas chimney NE 0 TWT (s) Fig. 3 Example of gas chimney 1.0 above Zechstein salt diapir in Cretaceous Quadrant 29. Seismic data courtesy of Zechstein WesternGeco Triassic 2.0

2 km Lr Permian 3.0

2°W 1°W 0° 1°E 2°E 3°E 58°N 3. Highlights of deep Central North Sea wells Norway 50 km W 2 Devil’s Hole High wells and 30/16-5 drilled Lower 19 est 20 21 22 Paleozoic basement of Scottish Southern Uplands affinity Central Jaeren West Central Shelf High 6 Auk-Argyll area wells drilled Middle Devonian, Kyle Gp. 27/3-1 shallow-marine , evaporites and mudstones 26/7-1 57°N Devil’s Graben Hole 76 wells proved Upper Devonian continental red beds 26/8-1 27/10-1 High 30 directly beneath a regional base Permian unconformity 26/14-1 27 28 29 30/16-5 5 Mid North Sea High wells drilled Lower Carboniferous 26 Auk Argyll strata of Northumberland Trough (N England) affinity - the Flora High was not a uniformly positive pre-Permian feature 56°N 26/7-1 drilled 1,090 m of Lower Carboniferous coal

Mid North Sea High measures of Midland Valley of Scotland affinity Netherlands

37/12-1 39 These coal measures are overlain by red beds ascribed to 38/16-1 UK Mesozoic34 structural elements 35 36 the Westphalian - if so confirming an extension of an intra- Basin 37 38 Basin/graben depocentre Namurian Midland Valley of Scotland unconformity into the Platform/intrabasinal high 55°N offshore area Late Westphalian red beds Ur Dev/Lr Carboniferous red beds 6 Flora area wells proved Late Carboniferous sandstone Westphalian coal measures Middle Devonian units separated by volcanics, unconformably overlain by Upper Dinantian/Namurian Lower Devonian Lower Rotliegend Gp argillaceous and volcanic strata Dinantian coal measures Lower Palaeozoic basement Cementstone equivalent Another unconformity separates Lower Rotliegend and Fig. 4 Pre-Permian stratigraphy recorded in Central Upper Rotliegend Group strata at Flora North Sea wells

Field Upper Zechstein Group Base Zechstein (seismic pick) Permian Reservoir U Rotliegend Gp Lower Rotliegend Auk,Innes, SNS L Rotliegend Gp Gas source Flora succession Coal measures Coal measures Flora, East Midlands Westphalian- oil province, SNS Oil source East Midlands oil province, SNS Namurian Carboniferous Fell Sandstone Yoredale Group Scremerston Coal Scremerstone Carb. Limestone E Midlands province Coal Group Oil shales Dinantian Lower Oil Shale Scottish Midland Valley Fell Sst Gp Sub-group Cementstone Gp Near base Dinantian

Upper Old (seismic pick) Buchan, Argyll / Upper Buchan Formation Ardmore, Auk Red Sandstone Kyle Limestone Devonian Middle Kyle Group (deepest seismic pick) Lacustrine Lower proven in Moray Firth

Silurian Deepest well penetration: Lower Palaeozoic Lower mildly metamorphosed strata Palaeozoic

Cambrian

Fig. 5 Summary of Permian and pre-Permian and of potential source rock and reservoir intervals in the region 2013

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Carboniferous petroleum systems Promote beneath the Central North Sea United Kingdom 2014

4. Potential field modelling of sub-Zechstein basin architecture

Interpretation of the grids of long-offset seismic data has provided a much improved first-pass image of sub-Permian basin architecture beneath the Central North Sea. Nevertheless, there remain areas where the basin architecture is poorly resolved beneath complex structure in the Mesozoic strata, or where the long-offset data coverage is currently sparse. A programme of potential field modelling was therefore initiated to improve resolution of the sub-Permian structure in these areas, and to provide QC in other areas of ambiguous seismic interpretation.

A 3D-forward and inversion modelling technique has been used to separate the gravity response of the crystalline (sub- Lower Palaeozoic) basement from the gravity responses of its overlying Lower Palaeozoic, Devono-Carboniferous, Permian and post-Permian sedimentary basins. The principal aim for the 3D modelling has been to resolve a depth surface to high-density or crystalline basement - this surface then defines the base of the accommodation space to be filled by the Lower Palaeozoic and overlying sedimentary basins.

Construction of the potential field model has been an iterative process, with successive iterations being designed to improve fit with the geological interpretation in areas of good well control and seismic data quality. Initial modelling assumed a uniform lower crustal density beneath the Central North Sea, but later models included low density granitic bodies within the basement.

SW NE

Fig. 6 Long-offset 2D seismic profile superimposed on a 3D view of the homogeneous crystalline basement model (with only 3 granite bodies added). The seismic profile traverses the West Central Shelf, the West Central Graben, the Forties-Montrose High, the East Central Graben, and ends on the Jaeren High with its Devono-Carboniferous core. 2D seismic data courtesy of TGS NOPEC Geophysical Co. (UK) Ltd.

The next phase of modelling will introduce even more lateral density variations into the basement body and these will be based on known and refraction seismic studies which have helped to define the different basement terrains within the project area. Figures 6 and 7 show two end-member results from the current modelling. Figures 6 shows a long offset seismic line superimposed on the homogenous basement model (with only three granite bodies added) and figure 7 shows the same seismic line, but this time superimposed on a basement surface which includes significant lateral density variation based on a regional seismic refraction interpretation (Lyngsie, 2007). The variable density basement has helped to modify the 3D modelled basement surface, for example, the basement beneath the West Central Graben on Figure 7 is Modelled crystalline basement surface, gas a few hundred metres lower than that seen on Figure 6 which may indicate that Palaeozoic sediments are preserved even chimneys in the Mesozoic strata (white) and in the deepest parts of the graben. Further iterations of lower crustal density variation are under way to provide a basis for Dogger Bank Round 3 Zone Iteration III (blue modelling the Devono-Carboniferous basin architecture in areas of the North Sea that contain sparse or no long-offset polygon) seismic data coverage. The impact of each iteration can be assessed visually, as on Figure 7, by comparing the geological interpretation with the resultant top crystalline basement surface on key seismic profiles across the study area. SW NE

Fig. 7 Long-offset 2D seismic profile superimposed on a 3D view of the crystalline basement model which includes significant lateral density variation based on a regional seismic refraction interpretation. The seismic profile traverses the West Central Shelf, the West Central Graben, the Forties-Montrose High, the East Central Graben, and ends on the Jaeren High with its Devono-Carboniferous core. 2D seismic data courtesy of TGS NOPEC Geophysical Co. (UK) Ltd. 2013

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Carboniferous petroleum systems Promote beneath the Central North Sea United Kingdom 2014

5. Seismic data and interpretation

Long-offset seismic data offers improved imaging of the deep structure of the Central North Sea. Imaging of the pre- Zechstein section is strongly influenced by structural complexity of the Zechstein to Cenozoic overburden; thus seismic imaging is worst beneath the Central Graben. Elsewhere, seismic interpretation of Lower Carboniferous units relies heavily on comparison with equivalent drilled strata on the southern flank of the Mid North Sea High.

2°W 1°W 0° 1°E 2°E 3°E 5.1 Sub-Zechstein isopachs S N 58°N 50 km The Middle Devonian Kyle Limestone forms a Base Zechstein conspicuous, high-amplitude and continuous 19 20 21 22 reflector in the Auk-Argyll area. Beyond the limit of the Near base Kyle Limestone event, the deepest reliable event is Kyle Lmst Dinantian taken as representing the transition from late Derivation of isopach Devonian continental red-bed deposits to early 57°N Dinantian Cementstone Group-equivalent strata and Norway has been labelled as near base Dinantian. The areas in which Kyle Limestone and intra-Dinantian Auk- Flora Ridge 26 27 reflectors have been picked are not mutually 28 30 exclusive. 29

The isopachs from the base of the Zechstein to the 56°N Kyle Limestone reflector (or near base Dinantian

reflector where the former cannot be picked) provide 0.5 km Netherlands a first impression of the potential thickness of Middle 39

Devonian to Lower Permian strata in the region, as UK 34 derived from the seismic interpretation (Fig. 8). 35 36 37 38 5.5 km 55°N Northern limit of near base Dinantian seismic pick Fig. 8 Approximate thickness of pre-Zechstein Upper Palaeozoic (base Zechstein Gp to near base Dinantian, or where not interpretable base Zechstein Gp to Middle Devonian Kyle Limestone); isochrons converted to 5.2 Auk-Flora Ridge area S N 2 All of the onshore examples of Lower Palaeozoic basement blocks are bounded by prominent oblique-slip, reverse or normal fault systems. The 3 northern flank of the Auk-Flora Ridge is Inverted West Central Graben interpreted to be a major reverse fault . The Dinantian and older beds at 4 the centre of the profile appear to be folded, and are underlain by high- amplitude dipping events that are tentatively interpreted as igneous Auk -Flora Ridge 5 intrusives.

Carboniferous basement block 6 Buried TWT granite (secs) AF

7 5 km MV SU

Top Group Rotliegend groups Base Chalk Group Base Permian unconformity Central North Cromer Knoll Group Dinantian to Namurian Boundary fault Sea seismic Base Cretaceous Mid Dinantian (Mesozoic) of profile Dinantian W Central Graben Top Zechstein Group Near base Dinantian Comparable Zechstein Group Carboniferous intrusives geological Base Zechstein Group section onshore Fig. 9 Interpretation of example seismic profile crossing the northern flank of the Auk-Flora Ridge (AF) (on inset map MV Midland Valley of Scotland; SU Southern Uplands). Seismic data courtesy of CGGVeritas 2013

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Carboniferous petroleum systems Promote beneath the Central North Sea United Kingdom 2014

5.3 Mid North Sea High area SW Mid North Sea High NE 0 TWT Carboniferous 100 km (s) basement 1 block Buried granite 2 AF

3

4 ST Central North KB Sea seismic 5 profile

Comparable 10 km 6 geological section onshore Top Chalk Group Namurian to Westphalian/Stephanian Mid-Carboniferous unconformity Base Chalk Group Dinantian to Namurian Fig. 10 Interpretation of example seismic profile Mid Dinantian from the Dogger Granite to the southern flank of Dinantian the Auk-Flora Ridge (AF Auk-Flora Ridge; KB Top Zechstein Near base Dinantian Askrigg Block; ST Stainmore Trough) Zechstein Group Upper Old Red Sandstone Base Zechstein Kyle Limestone Rotliegend groups Seismic data courtesy of TGS Base Permian Base Devonian Dogger Granite

South of the Auk-Flora Ridge, there is compelling evidence for the preservation of up to 5.5 km of Upper Devonian to Dinantian strata in a half-graben feature within what is conventionally regarded as the Mid North Sea High (Fig. 10). Notably, this basin’s fill could potentially include Scremerston Coal Group-equivalent gas-prone source rocks, although thermal modelling (Hay et al. 2005) suggests that such may be only marginally mature here. The southern flank of this Upper Devonian to Dinantian basin is constrained by a prominent Lower Palaeozoic basement block that was persistently buoyant through Carboniferous times. Potential field modelling predicts that this Lower Palaeozoic basement block contains a Caledonoid granite core (Dogger Granite). Comparable granite-cored Lower Palaeozoic basement blocks in England include the Askrigg Block, and the half-graben on the profile is thus analogous to the early Carboniferous Stainmore Trough of northern England. 5.4 South Jaeren High area N south Jaeren High East Central Graben S

Carboniferous 100 km TWT basement (s) block SJ Buried 3 granite

4

Central North Sea seismic profile 5 CB Comparable geological section onshore

2 km CB Craven Basin SJ south Jaeren High 6

Upper Rotliegend Group The seismostratigraphic relationships Top Chalk Group Top Lower Rotliegend Group established in the Auk-Flora Ridge are Lower Rotliegend Group Base Chalk Group Base Permian unconformity replicated on the Jaeren High. The southern part Namurian to Westphalian/Stephanian of the Jaeren High Mesozoic footwall block is Base Cretaceous Mid-Carboniferous unconformity interpreted to overlie another pre-Carboniferous Dinantian to Namurian Top Zechstein Group Near base Dinantian basement block. Both a uniformly thick Upper Zechstein Group Upper Old Red Sandstone Rotliegend and northward-thinning Lower Base Zechstein Group Kyle Limestone Rotliegend unit are also clearly developed Fig. 11 Interpretation of example seismic profile from the south Jaeren High. above a base Permian truncation surface. Seismic data courtesy of CGGVeritas 2013

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Carboniferous petroleum systems Promote beneath the Central North Sea United Kingdom 2014

5.5 West Central Shelf area

Late Carboniferous (?Namurian) W channel on West Central Shelf E 1 Carboniferous 100 km basement TWT block (s) Buried DHH granite 2 WCS 5 MV

3

Central North Sea seismic profile

4 Comparable geological section onshore

1 km Fig. 12 Interpretation of example seismic Upper Rotliegend Group profile from the West Central Shelf Top Chalk Group Top Lower Rotliegend Group (WCS); (DHH Devil’s Hole Horst; MV Lower Rotliegend Group Midland Valley of Scotland) Base Chalk Group Base Permian unconformity Namurian to Westphalian/Stephanian Seismic data courtesy of CGGVeritas Base Cretaceous Mid-Carboniferous unconformity Dinantian to Namurian Top Zechstein Group Near base Dinantian Zechstein Group Upper Old Red Sandstone Base Zechstein Group Kyle Limestone

The Lower Permian, Rotliegend Group strata are divisible into two discrete seismostratigraphic units acrioss part of the West Central Shelf (Fig 12). Locally, a strongly erosional intra-Carboniferous unconformity dissects into a seismic facies of discontinuous high-amplitude reflectors that may be indicative of coal measures. A lenticular body imaged directly overlying the deepest notch in the unconformity surface is interpreted to represent the fill of an erosional channel. In three dimensions, both the channel and its infill body trend roughly W-E and are up to 2 km wide. Their scale and trend are comparable with mapped mid-Namurian channel features associated with an influx of coarse detritus in the Midland Valley of Scotland

W Northern part of West Central Shelf E 0 Carboniferous 100 km TWT basement (s) block WCS 1 Buried granite DHH 2 MS

3

4 Central North 5 km Sea seismic profile 5 Top Chalk Group Top Zechstein Group Comparable Zechstein Group geological Base Chalk Group Base Permian unconformity section onshore Rotliegend groups Base Cretaceous Base Permian unconformity Fig. 13 Interpretation of example seismic profile Dinantian to Namurian from the West Central Shelf (WCS); (DHH Mid Dinantian (near base Asbian) Devil's Hole Horst; MS Midlothian Syncline, Dinantian Near base Dinantian Midland Valley of Scotland). Seismic data courtesy of TGS

Across the northern part of the West Central Shelf lies a simple pre-Permian syncline, bounded to the west by a westerly- dipping reverse fault (Fig. 13), and comparable in style and orientation if not in scale to the Midlothian Syncline in the Midland Valley of Scotland. The latter developed in the late Carboniferous as syn-sedimentary folding superseded extension in front of a basement block. The two profiles in Figs 12 and 13 suggest that large parts of the Carboniferous tectonic style beneath the West Central Shelf has more in common with the Midland Valley of Scotland than with the tectonic style of the Auk-Flora area and the eastern part of the Mid North Sea High. 2013

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Carboniferous petroleum systems Promote beneath the Central North Sea United Kingdom 2014

6 Implications for Central North Sea Carboniferous petroleum system Source rocks Mid-Dinantian coal measures (cf. Scremerston Coal Group of northern England) offer significant volumes of source rocks in most of the Central North Sea’s Lower Carboniferous basins. Non-marine oil shales (cf. Midland Valley of Scotland) may be present locally. Dinantian to early Namurian organic-rich marine oil shales, the principal source rocks for the oil and gas fields of the East Basin and the East Midlands of England, are almost certainly absent from the Central North Sea. Westphalian coal measures, the principal source rocks in the Southern North Sea, have been completely eroded across much (but not all) of the Central North Sea. 0° 3°E Source rock maturity 58° Norway Much of the Midland Valley Carboniferous source rock interval was raised out of the oil window by uplift, but petroleum generation may be 19 20 21 22 NJ continuing in the deepest-buried strata there and beneath the Central North Sea today (Underhill et al. SJ 2008). 23 Mid-Dinantian coal-measure source rocks beneath south- DH 57° central Quadrant 29 have been modelled as mature for oil generation, and may even have entered the Forth Auk- gas window locally (Hay et al. 2005). Up to 28 billion Approaches 27 28 29 Flora barrels of oil and 156 tcf of gas could have been Ridge expelled from this area alone since the . 31 Conversely, Hay et al. (2005) predicted that any Dinantian 56° strata preserved in Mid North Sea High

Carboniferous basins are only marginally mature for Netherlands oil generation. 35 F 36 37 38 39

Plays and traps D Plays are largely dependent on the efficiency of their 50 km regional Zechstein Group evaporite topseal. 55° 41 42 43 44 Where this seal is intact, hydrocarbon charge will be Lower Carboniferous basin limited to sub-Zechstein reservoirs and traps. Lower Carboniferous basement thin Lower Carboniferous block Seal breach (areas of Zechstein Group salt withdrawal) (Dinantian to early Buried granite provides opportunities for charging Triassic to thick Namurian) thickness Selected Cretaceous fault Tertiary reservoirs. Upper Old Red Sandstone Rotliegend aeolian sandstone targets above thermally Middle Devonian and older Selected Jurassic fault Selected Palaeozoic fault mature Carboniferous source-rock basins: structural NJ North Jaeren High block traps, or basin-margin pinch-out traps. SJ South Jaeren High block Selected Variscan anticline Potential sub-Rotliegend targets fall into three categories: DH Devil’s Hole Granite Selected Variscan syncline i) Zechstein subcrop traps beyond the limit of the D Dogger Granite Post-Permian gas chimney Rotliegend fairway, ii) Lower Rotliegend subcrop F Farne Granite (as mapped by Hay et al., 2005) traps, and iii) intra-Carboniferous traps. Fig. 14 Lower Carboniferous basin configuration, Central North Sea 7. Conclusion Seismic interpretation augmented by potential field modelling has revealed a Devono-Carboniferous basin architecture for the Central North Sea that exhibits many features in common with the contemporary basins of northern England and southern Scotland. By analogy with these basins, their Central North Sea equivalents are likely to contain significant volumes of coal-measure and perhaps also oil-shale source rocks. Where thermally mature, these source-rock intervals are key elements of an underexplored Carboniferous petroleum system, potentially extending far to the west of the Jurassic source kitchen that has been the focus of almost all Central North Sea exploration to date. Exploration opportunities associated with this petroleum system occur both in Devonian to Lower Permian plays beneath the regional Zechstein evaporite seal and, where halokinetic thinning has enabled seal breach, in the underexplored Mesozoic and Tertiary plays above.

References Hay, S., Jones, C.M., Barker, F. and He, Z. 2005. Exploration of unproven plays; Mid The material presented in this poster is for information only. North Sea High. CD available on request Whilst every effort has been made to ensure that the Lyngsie, S.B. 2007. Continental sutures and their influence on rifting in the North information provided is accurate, it does not constitute legal, Sea. PhD. Thesis, University of Copenhagen. technical or professional advice. Milton-Worssell, R., Smith, K., McGrandle, A., Watson, J. & Cameron, D. In press. The search for a Carboniferous petroleum system beneath the Central North For more information contact: Sea. In: Vining, B.A. & Pickering S.C. (eds) Petroleum Geology: From Mature Joy Gray Email: [email protected] Basins to New Frontiers - Proceedings of the 7th Petroleum Geology Conference, 57-75. Richard Milton-Worssell Underhill, J. R., Monaghan, A. A., and Browne, M. A. E., 2008. Controls on structural Email: [email protected] styles, basin development and petroleum prospectivity in the Midland Valley of Scotland. Marine and Petroleum Geology, 25, 1000-1022.