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Home , Corallian Group, Great Oolite Group, Portland Group (geology), Wessex Basin, Wytch Farm

2013

2010 2011 2012

The prospectivity of the Promote English Channel United Kingdom 2015

Abstract This poster summarises the hydrocarbon geology of the English Channel from Prawle Point (Devon) in the west to the Straits of Dover (Kent) in the east and as far south as the UK-France median line. This Atlantic area of 23,000 km² forms part of Strategic Environmental Assessment SEA8. Apart from a coastal strip Margin between Portland Bill and the Isle of Wight (Fig. 3), the offshore area is unlicensed. The study area is transected by a sparse grid of 2D seismic data, the most recent of which were shot in the 1990s. A total of 23 wells have been drilled since 1978. strata, which form the only proven system in the area, occur in the offshore continuation of the and in the Central English Channel Basin, which lies south of the major North Purbeck-Isle of Wight structure. Sea Source rocks occur in the Lias, Clay and Clay (all ), while the principal reservoirs comprise the Sherwood Sandstone () and Bridport Sands (Lias) in the Central English Channel Basin and the Great Oolite () in the offshore Weald Basin. At present hydrocarbon discoveries are restricted to the area north of the Purbeck-Isle of Wight structure, and include the offshore extension of the Oil Field (Sherwood and Bridport ScotlandUK Sandstone reservoirs) and the 98/11-2 Sherwood Sandstone discovery. Across the Central English Channel Basin the wells contain numerous minor indications of oil and gas, but a viable structure has yet to be found. English Channel SW The major exploration risk is associated with Tertiary compression and resultant uplift and Approaches reactivation and reversal. Some areas have experienced 6,000 feet of uplift, most of which postdates the primary phase of hydrocarbon generation and migration. Valid trap generation has to rely on immunity from trap breaching (as at the Wytch Farm Oil Field) or re-migration or late migration (as at Fig. 1 Location map the Kimmeridge Oil Field).

105 P Q R S 56 ?

London Platform 0.5

0.5 1 1 Straits of 2.5 Vale of Pewsey Basin Dover 2 Weald Basin 2.5 0.5 51°N 2 1.5 58 1 99 100 96 97 1.5 98

1

Purbeck-Isle3.5 of Wight fault system 3 2 2.5 Central English 1.5 1 2 1.5 Channel Basin UK Central English Channel High France 0.5 Central English Channel fault system Prawle 50 km Point Land well 50°N Offshore well 88 89 Wytch Farm & Kimmeridge fields

3°W 2°W 1°W 0°E 1°E

Fig. 2 The to Jurassic structural framework of the Weald, Wessex and Central English Channel basins. The contours are depths to the top of the pre-Permian surface (in kilometres). The outcrop of and older strata is shown in brown.

3°W 2°W 1°W 0°E ! ! ! ! ! ! !! ! ! ! ! 1. Exploration history ! ! ! ! ! 51°N !! ! ! ! ! ! ! ! ! ! ! ! !!! ! ! Onshore in southern , the first discovery of hydrocarbons dates from ! ! ! ! ! ! ! ! ! ! ! 1896 when gas was found at Heathfield (Kent). Later, oil was discovered at ! ! ! ! ! ! ! ! ! ! ! Kimmeridge () in 1959 and, most importantly, at Wytch Farm (also ! ! ! !! ! ! ! WYTCH FARM! ! Dorset) in 1974. ! ! ! OIL FIELD ! Isle ! 98/07- 2! ! ! ! !! ' ! 99/12- 1 !!! ! !!! ' ' ! of! # ! !!!!! '& ! ! ! ! ! ! & # ! ! KIMMERIDGE! ! ! Wight The first well to be drilled offshore was Lulworth Banks 1 (under an onshore ! ! OIL! FIELD! $+ & !!!! ! 98/13- 1 ! ! 98/12- 1 licence). Completed in 1963 just 3.3 km off the Dorset coast, this well tested a Lulworth Southard 99/18- 1 # Banks 1 Quarry 1 large domed structure and found uncommercial gas in the Kellaways and 97/12- 1 2 # Portland ## Bridport sandstones. 98/16- 1 Bill # 98/18- 1 99/16- 1 97/19- 1 # # The drilling of two wells on the Central English Channel High followed the 5th Offshore Licensing Round in 1978-79. The initial search was for a 2 &# &98/23- 1 Kimmeridge look-alike, but it wasn’t until a later phase of drilling in 1983-84 # 98/22- 1A 97/24- 1A that gas was found in 98/11-2. UK # France Further wells were drilled in the late 1980s and 1990s, with the only success Pointe de Barfleur 97 98 99 being the proving that the Wytch Farm Oilfield extends offshore into 50°N 50 km Nautile # Bay. Gas was also found in Southard Quarry 1 (on the Purbeck peninsula) in 1989, but the well was not tested. Fig. 3 Location map showing wells drilled in southern England and the English Channel. Oil fields and discoveries are shown in red and In total, 23 exploration and appraisal wells have been drilled in the UK sector onshore wells by grey dots. Licensed acreage as of October 2014 of the English Channel. The only wells drilled south of the Central English Channel High have been in French waters (no data from these are available). (onshore and offshore) is shown in yellow. 2013

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The prospectivity of the Promote English Channel United Kingdom 2015

Chronostratigraphy 2. Oligocene Solent Barton Group Paleogene The stratigraphic succession beneath the English Channel bears many similarities with that found onshore in the Bracklesham Group Weald and Wessex basins, particularly that found along the Dorset coast, although there are considerable facies Thames Group Paleocene Lambeth Group changes across these basins which result in a complex and variable stratigraphy. Alpine Upper Three major subdivide the succession into four major sequences: (a) the Devono-Carboniferous Chalk Group sequence followed by the Variscan unconformity, (b) the Permian to Lower Cretaceous interval followed by the Upper Greensand Fm Late Cimmerian unconformity, (c) the Lower Greensand to Chalk interval followed by the Alpine unconformity Gault Fm and finally (d) the Paleogene. Lower Greensand Group Cretaceous Late Cimmerian Lower unconformity The major controls on sedimentation are those of intermittent synsedimentary faulting and subsidence, and also Cretaceous successive phases of relative sea-level rise and fall. During the Jurassic, for instance, five depositional sequences are recognised, each of which displays a “shallowing upwards” profile. On a smaller scale, each sequence itself contains evidence of “shallowing upwards” cycles (e.g. within the ). Chrono- Lithostratigraphy stratigraphy Upper Fm Ampthill Ringstead Jurassic “Upper Late Corallian Clay Sandsfoot Subgroup” equiv. Trigonia clav. Fm “Lower Osmington Oolite Kellaways Fm Mid Jurassic Middle Corallian Subgp” Redcliff Great Oolite Group Jurassic Nothe Grit Generalised lithologies Group Early Bridport Lower Late Sands Oxford Clay Mixed lithologies Jurassic Liassic Group Mid Sandstones Penarth Group Early Kellaways and marls Mercia Mudstone Group Abbotsbury Cornbrash Triassic Claystones Late Forest Marble Great Great Frome Clay Sherwood Sdst Group Oolite Bathonian Oolite Mid Group Fuller’s Earth

Early Aylesbeare Upper Mudstone 98/11-2 Permian Group Permian 3. Reservoirs and plays 0 Gamma 200 140 Sonic 40 Mercia Five reservoirs define five distinct hydrocarbon plays in Mudstone Group Lower Breccias the English Channel area. These are the same plays Permian that are present onshore, where they all have proven Variscan unconformity reserves in fields and discoveries. Only two plays have Carbonif- so far proved successful offshore (marked * below). erous Coal Measures, limestones Sherwood and shales in Kent Sandstone 3.1 Sherwood Sandstone Play* Core 4 Group 3.2 Bridport Sandstone Play* 3.3 Great Oolite Play DST 2 3.4 Corallian Carbonate Play Core 5 3.5 Portland Carbonate Play Fig. 4 Lithostratigraphy DST 1 3.1 Sherwood Sandstone Play 6200 P Q R S 56 WORCESTER 105 BASIN

Sherwood Sandstone TRIASSIC tight? Sherwood Sandstone thin and silty? 51°N 100 58 96 97 98 99

Outcrop Wytch Farm

98/11-2 97/12-1

99/16-1 UK France Sherwood Sandstone tight?

50°N 88 89 50 km

3°W 2°W 1°W 0°E 1°E 6500 Fig. 5 Distribution of the Sherwood Sandstone Play in southern England and the English Channel (based on Penn et al. 1987) Sherwood Sandstone Offshore, the Sherwood Sandstone Group thickens from 56 ft (of conglomerate) in Group in 98/11-2: Aylesbeare well 99/16-1 to 993 ft of interbedded sandstone, siltstone and claystone in well Mudstone 97/12-1, off the Dorset coast. This compares with c. 560 ft at the Wytch Farm Oil Gross thickness: 464 ft Group Field. Net:Gross: 0.72 Average porosity: 8%

Deposition took place in a continental, braid-plain and playa-margin environment. PERMIAN The heterogeneous facies contain some highly porous zones. DST 2 flowed 9.6 mmscf/d and 170 BCPD Despite the presence of 754 million bbl STOIIP in the Wytch Farm onshore oil field (48/64” choke) (Underhill & Stoneley 1998), the offshore drilling has yet to yield any noteworthy DST 1 flowed 23.5 bbl Sandstone Claystone & siltstone oil in 1 hour hydrocarbon shows other than those in well 98/11-2. This is largely due to the All depths are in feet below KB special structural situation required to allow oil migration from Jurassic source rocks into the stratigraphically much lower Sherwood Sandstone. Fig. 6 Well log across the Sherwood Sandstone Group in well 98/11-2 2013

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The prospectivity of the Promote English Channel United Kingdom 2015

3.2 Bridport Sands Play 98/12-1 0 Gamma 150 140 Sonic 40 P Q R S 56 Fuller’s 105 4200 Earth Fm

Bridport Sands pass laterally M JUR arenaceousinto ferruginous siltstones limestones and mudstones and Limit of Upper Lias Inferior Ferruginous facies Oolite Group

58 51°N 99 100 Bridport 96 97 98 Sands Fm

98/12-1

Calcareous facies

Outcrop Lead 98/12A into marls UK France Bridport Sands pass laterally 50 km 4400

50°N 88 89

3°W 2°W 1°W 0°E 1°E Fig. 7 Distribution of the Bridport Sands Play in southern England and the English Channel (based on Penn et al. 1987). See key below

The Bridport Sands are well developed in the proximity of the Dorset coast. At the Wytch Bridport Sands Farm Oil Field the unit is 150-240 ft thick (Colter & Havard 1981) and south of the Formation in 98/12-1: Purbeck Disturbance, an active Liassic fault, 280 ft was drilled in well 98/16-2, 308 ft in well Southard Quarry 1 on the (Dorset) and 370 ft in well 98/12-1. The Gross thickness: 369 ft unit thins rapidly to the south (114 ft in well 97/19-1 and 66 ft in well 97/24-1A) and is Net:Gross: 0.9 LOWER JURASSIC represented by a calcareous facies further to the east and south-east (e.g. well 98/18-1) Porosity: unknown and a ferruginous facies fringing the London Platform (both areas being starved of clastic 4600 input).

The Bridport sands were deposited in a shallow-marine, migrating barrier bar Downcliff environment. Although heavily bioturbated, local small-scale cross-bedding indicates Clay Fm the presence of currents and some authors suggest that the abundant nodular beds (formed as a consequence of a high calcareous bioclastic component) indicate the influence of storms.

In addition to the 120 million bbl STOIIP reservoired in the Wytch Farm Oilfield (Underhill Sandstone Claystone & siltstone & Stoneley 1998), Lulworth Banks 1 and Southard Quarry 1 had gas shows in the Bridport Sands and these may be commercially viable. Well 97/19-1 had oil shows and All depths are in feet below KB core porosities up to 23% at the top of a coarsening-upwards sandstone unit. Fig. 8 Well log across the Bridport Sands Formation in 98/12-1 3.3 Great Oolite Play P Q R S 56 ? 105 WORCESTER BASIN 99/12-1 0 Gamma 100 140 Sonic 40

Outcrop Great Oolite limestones pass Oxford Only thin porosity developments 3000 are occasionally present in the Great Oolite Clay Fm limestones in this area BEST 51°N 58 RESER 99 100 96 97 98 QUALITY Kellaways VOIR Fm ? ? CB & FM * no data available in this area 98/13-1 99/12-1 laterally into Frome Clay 99/18-1B Great 99/16-1 Oolite Fm 98/18-1 ? UK France 50 km

50°N 88 89 MIDDLE JURASSIC 3200 3°W 2°W 1°W 0°E 1°E Fuller’s Fig. 9 Distribution of the Great Oolite Play in southern England and the English Channel Earth Fm (based on Penn et al. 1987). See key below

Limestones of the Great Oolite Formation, which in an oolitic shoal facies form a key reservoir in the western CB & FM* Cornbrash & Forest Marble Formations Weald Basin, are only present in the three north-easternmost English Channel wells: with 129 ft present in 99/12-1, 120 ft in 99/18-1B and 97 ft of cryptocrystalline limestone with minor oil shows in well 98/13-1 off the south-west coast of the Isle of Wight. Carbonates are also predicted in the eastern English Channel where Claystone & siltstone Great Oolite there are currently no wells. Formation in well Limestone 99/12-1: To the west the limestones grade into the argillaceous facies of the Frome Clay. Well 99/16-1 contains a Gross thickness: transitional facies, with thin limestone units in an otherwise argillaceous succession. A poor show is recorded All depths are in feet below KB 119 ft from some argillaceous limestone and marl in well 98/18-1. Net:Gross: 1.0 Porosity: unknown Sandstone reservoir (high risk play) Carbonate reservoir (high risk play) Sandstone reservoir (low risk play) Fig. 10 Well log across the Great Carbonate reservoir (low risk play) Oolite Formation in well 99/12-1 Non-reservoir facies Land well Offshore well Wytch Farm & Kimmeridge fields 2013

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The prospectivity of the Promote English Channel United Kingdom 2015

3.4 Corallian Carbonate Play 97/19-1 0 Gamma 150 140 Sonic 40 Kimmeridge P Q R S 56 Clay Fm

105 Subcrop

Palmer’s Wood Outcrop wells

20 20 10 1500 0 0 10 51°N 20 58 10 99 100 96 97 98 0 Osmington Oolite Fm JURASSIC

10 98/12-1 98/12-1 Oxford 0 Clay Fm 98/18-1 1600 UK 97/19-1 Osmington Oolite facies France 98/23-1

50°N 88 89 50 km Claystone & siltstone 3°W 2°W 1°W 0°E 1°E Limestone Fig. 11 Distribution of the Corallian Carbonate and Sandstone Play in southern England and the English Channel (based on Penn et al. 1987). Isopachs are gross thicknesses (in All depths are in feet below KB metres) of Corallian Sandstones (from Hawkes et al. 1998). See below for key Osmington Oolite Formation in 97/19-1:

Gross thickness: 97 ft The Corallian Group contains potential reservoirs in shallow-marine sandstones (e.g. onshore oil in the Net:Gross: 0.65 Palmer's Wood wells) and inner shelf carbonates (e.g. the Osmington Oolite of the Dorset coast outcrops). Porosity: unknown

No significant sandstones are present offshore, but in this area the Osmington Oolite is best developed in a Fig. 12 Well log across the Osmington north-west to south-east zone sampled by wells 98/12-1, 98/18-1 and 98/23-1. Minor shows are present in all three of these wells. The thickness of the unit varies from 69 ft to 104 ft. Oolite Formation (Corallian Group) in well 97/19-1 The Corallian Group has been eroded from the structural highs in the Dorset area, including the Wytch Farm Oil Field, and the Hampshire-Dieppe High.

98/13-1 0 Gamma 150 140 Sonic 40 Lower 3.5 Portland Carbonate Play Purbeck Beds Purbeck Anhydrite P Q R S 56 105

Subcrop Portland Group 2500 Portland Sands pass laterally 51°N into100 mudstones 58 96 97 98 99

no data

98/13-1

Outcrop 98/16-1 UPPER JURASSIC Portland Limestone facies UK France 50 km Kimmeridge Clay Fm 50°N 88 89 2700 3°W 2°W 1°W 0°E 1°E Fig. 13 Distribution of the Portland Carbonate and Sandstone Play in southern England and the English Channel (based on Penn et al. 1987). See below for key Anhydrite

Claystone & siltstone Beneath the evaporites of the lower Purbeck Group, the Portland Group of the offshore area is composed of limestone and dolomite grading to claystone, and lacks the arenaceous lithologies Limestone that form the locally productive reservoirs in the Weald Basin. Limestone thicknesses vary from 48 ft to 188 ft. Minor oil-stained siltstones in well 98/16-1 may be related to the well crossing a major All depths are in feet below KB fault at this level. Portland Group in well 98/13-1:

Gross thickness: 179 ft Net:Gross: 0.6 Sandstone reservoir (high risk play) Carbonate reservoir (high risk play) Porosity: unknown

Sandstone reservoir (low risk play) Carbonate reservoir (low risk play) Fig. 14 Well log across the Portland Group Non-reservoir facies Land well Offshore well in well 98/13-1. Wytch Farm & Kimmeridge fields 2013

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The prospectivity of the Promote English Channel United Kingdom 2015

(b) Reservoirs in areas that have been uplifted retain reservoir 4. Basin development, and properties, particularly porosity, associated with their pre-uplift location. consequences In the centre of the Weald Basin, Great Oolite porosities are lower than would initially have be expected. Surface outcrops, wells and seismic data reveal several key structural features in the onshore and offshore area: a series of monoclinal disturbances running east-west adjacent to major extensional growth 5. Trap types faults. The faults controlled depositional thicknesses through the Triassic, Jurassic and Early Cretaceous; after which time stresses Two trap types are recognised: reversed into a compressional regime resulting in a pronounced reversal of movement on these faults and structural inversion of the basins. (a) Mesozoic tilted fault blocks and horsts which pre-date hydrocarbon generation. To succeed, these structures should be within migration The Central English Channel Basin is flanked to the north by the down-to- distance of a source kitchen and importantly should not have been the-south Purbeck-Isle of Wight Fault, and to the south by the down-to- breached by Tertiary inversion. In the Wytch Farm area, the faults north of the-north Central English Channel Fault. Both have been reactivated to the disturbance remain in net extension and appear to have been little form a mirror image of each other on either side of the basin (Beeley & affected by the inversion process. Norton 1998). (b) Tertiary inversion which predominantly post-date Estimates of the amount of uplift range from 1,500 feet (500 m) or less hydrocarbon generation. These were the first structures to be targeted by north of the Purbeck-Isle of Wight Fault (which is assumed to have exploration onshore, and with the exception of the Kimmeridge Oil Field, remained relatively stable during inversion) to 3,000-6,000 feet (1-2 km) have all proved unsuccessful. Kimmeridge is possibly ”the sole survivor south of that fault (well 98/11-2) and north of the Central English Channel of a much more significant play which was wiped out during inversion” Fault (well 98/23-1) (Law 1998). (Evans et al. 1998).

Tertiary (Alpine) inversion has had several effects on prospectivity: (c) There is currently no evidence for stratigraphic traps in this area. Basin-scale facies changes are prevalent and no channelised or (a) Late, reactivation of faults could have beached old hydrocarbon mounded sandstones have been identified in the generally tabular accumulations and also structural realignment could have caused geometry of the seismo-stratigraphic units. previous accumulations to remigrate into new structures or to seep to the surface.

Wessex Basin Central English Channel Basin Portland-Isle of Central English Channel High N Vale of Pewsey Wardour-Portsdown Wight structure S fault structure 0 V ariscan Front Wardour Front 10

20 Depth (km)

30 Not to scale Fig. 15 Schematic cross-section across the Wessex and Central English Channel basins showing the proposed deep fault geometry of Beeley & Norton (1998)

N S

Chalk N S Gault L. Greensand Wealden MSL Tertiary Wealden Tertiary Portland BedsKCF Chalk Kimmeridge- 3000 Purbeck Inferior Oolite - Corallian Lias Triassic Oxford Clay - Corallian 6000 Middle Variscan Jurassic Lias Depth (feet) 1 km 9000 Mercia Mudstone Not to scale Sherwood Sandstone

N S Top Lower Greensand MSL L. Greensand N S Wealden 3000 Kimmeridge- Purbeck

6000 Oxford Clay - Corallian Middle Depth (feet) Jurassic Lias 9000

Mercia Mudstone Sherwood Sandstone 1 km 12000

Fig. 16 Schematic cross-section across the Portland-Isle Fig. 17 Schematic cross-section across the Central of Wight structure (Butler 1998) (a) restored to Albian time, English Channel High (Beeley & Norton 1998) (a) restored (b) present day with eroded section added. to Top Wealden, (b) present day with eroded section added. 2013

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6. Seismic examples 2°W 1°W 0°E

Two regional seismic profiles illustrate the regional structure and 51°N stratigraphic thickness variations across the English Channel. 98 99 Profile A traverses north-west to south-east across the main Central English Channel Basin and across the Central Channel High. Stratigraphic thickening towards the north-west is discernable, especially in the Permo- 8 Triassic and Liassic intervals. The outcrop pattern provides evidence for 79 98/07- 2 99/12- 1 uplift to the north-west and the south-east, with a syncline of Chalk 98/11-1 3 2 98/13- 1 preserved in the middle of the basin. Uplift caused by reversal of the 4Z 98/12- 1 Central Channel Fault is sufficient to expose the Kimmeridge Clay 2 99/18- 1B Formation at seabed. This structure was drilled by well 98/23-1. The 98/16- 1 97/19- 1 succession found south of this major feature is speculative (the results of Profile 98/18- 1 99/16- 1 the French well Pointe de Barfleur are not available). A Profile B Profile B crosses the southern pat of the Central English Channel Basin 2 98/23- 1 97/24- 1A Central English Channel and continues to the north-east across the continuation of the Purbeck- 98/22- 1B Fault Zone Isle of Wight Fault, also known as the Wight-Bray line or the Bembridge-St. UK

Valery line. In the SW there is evidence for sedimentary thickening toward Pointe de Barfleur well France the Central English Channel Fault. To the north-east, on the Hampshire- 50 km Dieppe High, the effects of basin inversion in the mid-Cretaceous are 50°N evident with a major unconformity separating Middle Jurassic strata from the Lower Cretaceous. The uplifted area is overlain by a basin of Cretaceous and Tertiary age. The Permo-Triassic is thin in this eastern part Fig. 18 Location map for seismic profiles A and B of the study area.

NW 98/16- 1 Central English Central English SE 98/16- 2 Central English Channel Basin Channel High Channel Fault Zone 0

TWT Base Aptian (s) Unconformity Tertiary

Top Purbeck Chalk 0.5 Intra KCF marker Lower Greensand, Gault and Upper Greensand Top Corallian Lmst Wealden Group Top Cornbrash Kimmeridge Clay Fm - Purbeck Group Thin Permo-Triassic 1.0 Oxford Clay Fm - Corallian Group Top Inferior Oolite Great Oolite Group - Cornbrash Fm

Liassic Group - Inferior Oolite Group Top Penarth Group Permo-Triassic 1.5 Top Basement Variscan basement 10 km

Seismic data courtesy of IHS Energy 2.0 Fig. 19 Profile A: NW-SE-trending representative seismic line and geological interpretation, English Channel

Purbeck-Isle of Wight Fault SW Central English Central English Channel Basin Hampshire-Dieppe High NE Channel Fault Zone 99/18- 1B (inverted to a basin) 0 TWT (s)

Base Aptian unconformity Top Chalk 0.5

Top Purbeck Base Chalk Intra KCF marker Top Corallian Lmst

Top Cornbrash 1.0 Top Inferior Oolite

Top Basement Thin Permo-Triassic 10 km

Seismic data courtesy of IHS Energy 1.5 Fig. 20 Profile B: SW-NE-trending representative seismic line and geological interpretation, English Channel 2013

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The prospectivity of the Promote English Channel United Kingdom 2015

7. Source rocks Marls and Green Ammonite Beds contain the more organic-rich strata with Viable source rocks are confined to the Jurassic, and occur at three levels. Black TOC contents of up to 8%. shales within the Lower Liassic Group, the lower part of the Oxford Clay Formation and the Kimmeridge Clay Formation all have higher than average levels of The Oxford Clay consists mainly of bituminous shales in its lower part, and organic matter. calcareous mudstones and limestones in its middle and upper parts. The more organic-rich, lower part contains up to 12% TOC. The Lower Lias contains alternations of laminated, dark grey, organic rich shales deposited in anoxic or dysaerobic bottom conditions and calcareous shales with The Kimmeridge Clay also contains both organic-rich and more calcareous shales. limestones deposited in more aerobic bottom conditions. The Blue Lias, Black Ven The formation records TOC contents of up to 20% and represents the best potential source rocks in the English Channel area.

P Q R S 56 105 London Platform

1997 4059 Vale of Pewsey 5000 PLUS UNKNOWN UPLIFT Basin MAX MATURITY LOWER LIAS ERODED 1099 4388 PLUS c.2000-5000ft UPLIFT (TRIASSIC AND OLDER 5630 c.6800 c.8240 3989 Weald Basin/ AT OUTCROP) 7546 5951 58 51°N 2386 6101 6719 100 96 97 98 99 3826 Hampshire-Dieppe High 3424 4090 4282 5908 NR 4257 Abs 4837 5068 6343 5474 6280 LIASSIC HIGH PLUS UP TO 1500ft UPLIFT 2908 PLUS 3000-6000ft UPLIFT 6407 PLUS 2500-3000ft UPLIFT ?3955 4734 SHALLOW 5512 5789 UK BURIAL PLUS 4500+ft UPLIFT 2250 2953 France 2297 1782 Central English Channel High LOWER LIAS OUTCROP Central English 50°N 88 89 Channel Basin

3°W 2°W 1°W 0°E 1°E Fig. 21 Generalised distribution of potentially mature Lower Liassic source rocks (yellow) in southern England and the UK English Channel. Figures in red are depths to top Triassic (feet subsea) to which late uplift figures (green) should be added to give maximum Liassic burial. Uplift figures are derived from Law (1998) and McMahon & Turner (1998).

8. Maturation 9. Timing of migration relative to trap formation

The three Jurassic source rock intervals are locally The key issue in assessing the chance of success of specific structures within the Central English Channel mature for oil and gas generation, and in general Basin is the relative timing of hydrocarbon generation, migration and trap formation. only the relatively poor organic-rich, deeper horizons are mature for hydrocarbon generation. It Without doubt much oil and gas have been generated, but unfortunately very little has been discovered in is only in areas that received (Wealden Group) viable fields outside the Wytch Farm Oil Field. sediment during the phase of -early Cretaceous uplift and erosion that the Lias was Oil generation from the primary source, the Lower Lias, took place during the early to mid- Cretaceous, and buried to a sufficient depth for hydrocarbons to be the aim is to locate structures which existed at this time and which have not been subsequently disturbed by generated (McMahon & Turner 1998). inversion.

It is feasible for later-formed structures to contain hydrocarbons, but these have to rely on remigration from The Lower Lias has reached the oil window over earlier, transient reservoirs or very late hydrocarbon generation. much of the Channel Basin and has been sufficiently deeply buried in the axial part to raise the possibility of significant gas generation. 10. References Reconstructions of burial history suggest that the onset of oil generation began during the early Ainsworth, N.R., Braham, W., Gregory, F.J., Johnson, B. & King, C. 1998. The lithostratigraphy of the latest Triassic to earliest Cretaceous of the English Channel and its adjacent Cretaceous and peaked at about the mid- areas. In: Underhill, J.R. (ed) Development, Evolution and Petroleum Geology of the Wessex Basin. Geological Society, London, Special Publication 133: 103-164. Beeley, H.S. & Norton, M.G. 1998. The structural development of the Central English Channel High - constraints from section restoration. In: Underhill, J.R. (ed) Development, Cretaceous (Penn et al. 1987). Evolution and Petroleum Geology of the Wessex Basin. Geological Society, London, Special Publication 133: 283-298.

BGS. 1983. Portland Sheet 50°N-04°W 1:250 000 Solid Geology Series. The Oxford Clay is considered to fall within the oil BGS. 1995. Wight Sheet 50°N-02°W 1:250 000 Solid Geology Series. Second Edition. generation window in the northern part of the Butler, M. 1998. The geological history of the southern Wessex Basin - a review of new information from oil exploration. In: Underhill, J.R. (ed) Development, Evolution and Petroleum Channel Basin, but the Kimmeridge Clay does so Geology of the Wessex Basin. Geological Society, London, Special Publication 133: 67-86. Colter, V.S. & Harvard, D.J. 1981. The Wytch Farm Oilfield, Dorset. In: Illing, L.V. & Hobson, G.D. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Hayden & only in the deepest, axial parts. Also, whereas the Son, London. 494-503. Oxford Clay may have locally reached peak oil DECC. 2012. The hydrocarbon prospectivity of Britain’s onshore areas. http://og.decc.gov.uk/en/olgs/cms/explorationpro/onshore/onshore.aspx. See also Section 2 generation, the Kimmeridge Clay is not thought to Hamblin , R.J.O., Crosby, A., Balson, P.S., Jones, S.M., Chadwick, R.A., Penn, I.E. & Arthur, M.J. 1992. The Geology of the English Channel. British Geological Survey, United have reached this level of maturation (Penn et al. Kingdom Offshore Regional Report, London, H.M.S.O.

1987). Hawkes, P.W., Fraser, A.J. & Einchcomb, C.C.G. 1998. The tectono-stratigraphic development and exploration history of the Weald and Wessex basins, Southern England, UK. In: Underhill, J.R. (ed) Development, Evolution and Petroleum Geology of the Wessex Basin. Geological Society, London, Special Publication 133: 407-413.

Law, A. 1998. Regional uplift in the English Channel: quantification using sonic velocity. In: Underhill, J.R. (ed) Development, Evolution and Petroleum Geology of the Wessex Basin. In summary, the three principal source rocks have Geological Society, London, Special Publication 133: 187-197. reached varying degrees of maturation in the McMahon, N.A. & Turner, J. 1998. The documentation of a latest Jurassic-earliest Cretaceous uplift throughout southern England and adjacent offshore areas. In: Underhill, J.R. (ed) Development, Evolution and Petroleum Geology of the Wessex Basin. Geological Society, London, Special Publication 133: 215-240. Central English Channel Basin. Being the most Penn, I.E., Chadwick, R.A., Holloway, S., Roberts, G., Paraoh, T.C., Allsop, J.M., Hulbert, A.G. & Burns, I.M. 1987. Principal features of the hydrocarbon prospectivity of the Wessex- deeply buried, the Lower Lias shales provide the Channel Basin, UK. In: Brooks, J. & Glennie, K. (eds) Petroleum Geology of North West Europe. Graham & Trotman. largest volume of source rock which falls within and Underhill, J.R. & Paterson, S. 1998. Genesis of tectonic inversion structures: seismic evidence for the development of key structures along the Purbeck-Isle of Wight Disturbance. even beyond the oil generation window. Away from Journal of the Geological Society, London 155: 975-992. the basin depocentre and particularly in those Underhill, J.R. & Stoneley, R. 1998. Introduction to the development, evolution and petroleum geology of the Wessex Basin. In: Underhill, J.R. (ed) Development, Evolution and areas severely affected by late Cimmerian uplift Petroleum Geology of the Wessex Basin. Geological Society, London, Special Publication 133: 1-18. and erosion, levels are maturation are The material presented in this document is for information only. Whilst every effort has been made to ensure that the information is considerably lower. accurate, it does not constitute legal, technical or professional advice. For more information contact: Toni Harvey Email: [email protected] Joy Gray Email: [email protected]