The prospectivity of the English Channel - Gov.uk

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The prospectivity of the English Channel

United Kingdom

2014

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 area of 23,000 km² forms part of Strategic Environmental Assessment SEA8. Apart from a coastal strip between Portland Bill and the Isle of Wight (Fig. 3), the offshore area is unlicensed.

Atlantic Margin

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. Mesozoic strata, which form the only proven petroleum system in the area, occur in the offshore continuation of the Weald Basin and in the Central English Channel Basin, which lies south of the major Purbeck-Isle of Wight structure.

North Sea

Source rocks occur in the Lias, Oxford Clay and Kimmeridge Clay (all Jurassic), while the principal reservoirs comprise the Sherwood Sandstone (Triassic) and Bridport Sands (Lias) in the Central English Channel Basin and the Great Oolite (Middle Jurassic) 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 Wytch Farm Oil Field (Sherwood and Bridport 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.

Scotland UK English Channel SW Approaches

The major exploration risk is associated with Tertiary compression and resultant uplift and fault 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 the Kimmeridge Oil Field).

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Fig. 1 Location map

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56

S

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London Platform 0.5

0.5 1

1

Straits of Dover

2.5

Vale of Pewsey Basin

Weald Basin

2

2.5

51°N

0.5

96

1

97

1.5

2

99

98

1.5

58

100

Wessex Basin 1

ult system

fa sle of Wight Purbeck-I3.5 3

Central English Channel Basin

2.5 1

2

1.5

2 1.5

UK ce Fran

Central English Channel High 0.5

Central English Channel fault system

Prawle Point

50°N

88

50 km Land well Offshore well Wytch Farm & Kimmeridge fields

89 3°W

2°W

1°W

0°E

1°E

Fig. 2 The Permian 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 Carboniferous and older strata is shown in brown. 3°W

1. Exploration history

2°W

1°W !

51°N

Onshore in southern England, the first discovery of hydrocarbons dates from 1896 when gas was found at Heathfield (Kent). Later, oil was discovered at Kimmeridge (Dorset) in 1959 and, most importantly, at Wytch Farm (also Dorset) in 1974.

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The first well to be drilled offshore was Lulworth Banks 1 (under an onshore licence). Completed in 1963 just 3.3 km off the Dorset coast, this well tested a large domed structure and found uncommercial gas in the Kellaways and Bridport sandstones.

# 97/12- 1

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! WYTCH FARM

97/24- 1A#

In total, 23 exploration and appraisal wells have been drilled in the UK sector 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).

50°N

97

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99/12- 1 # 99/18- 1

# 98/18- 1 #

2 & # 98/22- 1A

98

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98/07- 2 ! !! !! of! '' ! '' & & # Wight $ & + 98/13- 1 98/12- 1

2 # # 98/16- 1

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Lulworth Southard Banks 1 Quarry 1

Portland Bill #

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99/16- 1 #

98/23- 1 &

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Further wells were drilled in the late 1980s and 1990s, with the only success being the proving that the Wytch Farm Oilfield extends offshore into Bournemouth Bay. Gas was also found in Southard Quarry 1 (on the Purbeck peninsula) in 1989, but the well was not tested.

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! OIL FIELD ! ! !! ! ! ! !!!! ! ! !! ! !KIMMERIDGE ! ! ! ! ! !! ! !FIELD OIL ! !!! ! ! ! !

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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 Kimmeridge look-alike, but it wasn’t until a later phase of drilling in 1983-84 that gas was found in 98/11-2.

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Pointe de Barﬂeur 50 km

UK France 99

Nautile #

Fig. 3 Location map showing wells drilled in southern England and the English Channel. Oil fields and discoveries are shown in red and onshore wells by grey dots. Licensed acreage as of November 2013 (onshore and offshore) is shown in yellow. The blocks shown in orange indicate licence applications in the 27th UK Offshore Licensing Round.

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The prospectivity of the English Channel Chronostratigraphy Oligocene Paleogene

Lithostratigraphy Barton Group

The stratigraphic succession beneath the English Channel bears many similarities with that found onshore in the Weald and Wessex basins, particularly that found along the Dorset coast, although there are considerable facies changes across these basins which result in a complex and variable stratigraphy.

Bracklesham Group Thames Group Lambeth Group Alpine unconformity Chalk Group

Paleocene Upper Cretaceous

Three major unconformities subdivide the succession into four major sequences: (a) the Devono-Carboniferous sequence followed by the Variscan unconformity, (b) the Permian to Lower Cretaceous interval followed by the Late Cimmerian unconformity, (c) the Lower Greensand to Chalk interval followed by the Alpine unconformity and finally (d) the Paleogene.

Upper Greensand Fm Gault Fm Lower Greensand Group Cretaceous

Late Cimmerian unconformity

Lower Cretaceous

The major controls on sedimentation are those of intermittent synsedimentary faulting and subsidence, and also 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 Great Oolite Group).

Wealden Group

Purbeck Group Portland Group

Jurassic

Chronostratigraphy

Upper Jurassic

Kimmeridge Clay Fm

Middle Jurassic

Corallian Group Oxford Clay Fm Kellaways Fm Great Oolite Group Inferior Oolite Group Bridport Sands Liassic Group

Lithostratigraphy

Ampthill Ringstead Sandsfoot Clay equiv. Trigonia clav. Oxfordian Osmington Oolite Mid “Lower Corallian Subgp” Redcliff Nothe Grit Early Late

Lower Jurassic

“Upper Corallian Subgroup”

Late Callovian

Penarth Group

Mixed lithologies

Oxford Clay

Sandstones

Early Late

Sherwood Sdst Group

Generalised lithologies

Mid

Mercia Mudstone Group

Triassic

2014

2. Stratigraphy

Solent Group

Eocene

United Kingdom

Bathonian Mid

Great Oolite Group

Limestones and marls

Kellaways Abbotsbury Cornbrash Forest Marble Great Frome Clay Oolite

Claystones

Fuller’s Earth

Early Aylesbeare Mudstone Group

Upper Permian Permian

Lower Permian

98/11-2

3. Reservoirs and plays

Variscan unconformity Coal Measures, limestones and shales in Kent

Devonian

Fig. 4 Lithostratigraphy

Core 4

51°N

96

Outcrop

DST 1 6200

R

S

99

100

56

Sherwood Sandstone thin and silty?

98

97

Sherwood Sandstone Group

TRIASSIC

Sherwood Sandstone tight?

40

DST 2

WORCESTER BASIN

105

Sonic

Core 5

3.1 Sherwood Sandstone Play Q

140

Mercia Mudstone Group

3.1 Sherwood Sandstone Play* 3.2 Bridport Sandstone Play* 3.3 Great Oolite Play 3.4 Corallian Carbonate Play 3.5 Portland Carbonate Play

P

200

Five reservoirs define five distinct hydrocarbon plays in the English Channel area. These are the same plays that are present onshore, where they all have proven reserves in fields and discoveries. Only two plays have so far proved successful offshore (marked * below).

Breccias

Carboniferous

0

Gamma

58

Wytch Farm

98/11-2

97/12-1

UK ce Fran

99/16-1 Sherwood Sandstone tight?

50°N

88

89 3°W

50 km 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)

Deposition took place in a continental, braid-plain and playa-margin environment. The heterogeneous facies contain some highly porous zones. Despite the presence of 754 million bbl STOIIP in the Wytch Farm onshore oil field (Underhill & Stoneley 1998), the offshore drilling has yet to yield any noteworthy hydrocarbon shows other than those in well 98/11-2. This is largely due to the special structural situation required to allow oil migration from Jurassic source rocks into the stratigraphically much lower Sherwood Sandstone.

Sherwood Sandstone Group in 98/11-2:

Gross thickness: 464 ft Net:Gross: 0.72 Average porosity: 8% DST 2 flowed 9.6 mmscf/d and 170 BCPD (48/64” choke) DST 1 flowed 23.5 bbl oil in 1 hour

Aylesbeare Mudstone Group

PERMIAN

Offshore, the Sherwood Sandstone Group thickens from 56 ft (of conglomerate) in well 99/16-1 to 993 ft of interbedded sandstone, siltstone and claystone in well 97/12-1, off the Dorset coast. This compares with c. 560 ft at the Wytch Farm Oil Field.

Sandstone

Claystone & siltstone

All depths are in feet below KB

Fig. 6 Well log across the Sherwood Sandstone Group in well 98/11-2

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United Kingdom

3.2 Bridport Sands Play

98/12-1 R

56

S

105 Br en into idp ac fe ort eo rru S us gin an sil ou ds ts s pa to lim ss ne e l s sto ate an n ra d es lly m a ud nd st on es

ar

96

Outcrop

UK ce Fran

idp

50 km

4400

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 Farm Oil Field the unit is 150-240 ft thick (Colter & Havard 1981) and south of the Purbeck Disturbance, an active Liassic fault, 280 ft was drilled in well 98/16-2, 308 ft in well Southard Quarry 1 on the Isle of Purbeck (Dorset) and 370 ft in well 98/12-1. The unit thins rapidly to the south (114 ft in well 97/19-1 and 66 ft in well 97/24-1A) and is represented by a calcareous facies further to the east and south-east (e.g. well 98/18-1) and a ferruginous facies fringing the London Platform (both areas being starved of clastic input).

Bridport Sands Formation in 98/12-1: Gross thickness: 369 ft Net:Gross: 0.9 Porosity: unknown

LOWER JURASSIC

88

Bridport Sands Fm

Calcareous facies

Br

50°N

4600

The Bridport sands were deposited in a shallow-marine, migrating barrier bar environment. Although heavily bioturbated, local small-scale cross-bedding indicates 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.

Downcliff Clay Fm

In addition to the 120 million bbl STOIIP reservoired in the Wytch Farm Oilfield (Underhill & 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 core porosities up to 23% at the top of a coarsening-upwards sandstone unit.

Sandstone

Fig. 8 Well log across the Bridport Sands Formation in 98/12-1 Q

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99/12-1 0

Gr BE

lim

ton

es

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p es

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s

as

99/18-1B

y in

98/18-1

to F

rom

eC

lay

99/16-1

UK ce Fran

?

50 km 50°N

88

89 3°W

40

Oxford Clay Fm

CB & FM *

no data available in this area

99/12-1

98/13-1 rall

Sonic

Kellaways Fm

? late

140

58

100

ST R QU ESE AL RV ITY OI R

98

100

MIDDLE JURASSIC

lite

Oo

97

Gamma

3000

Only thin porosity developments are occasionally present in the Great Oolite limestones in this area

t ea

96

56

S

WORCESTER BASIN

Outcrop

105


Limestone All depths are in feet below KB

3.3 Great Oolite Play

51°N

40

Fuller’s Earth Fm

4200

58

100

ort

Lead 98/12A

Sonic

140

Inferior Oolite Group

99

Sa int nds o m pa arl ss la s te

98/12-1

150

Ferruginous facies

98

97

Gamma

ral ly

51°N

r Lias

f Uppe

Limit o

M JUR

0

Q

P

2014

Great Oolite Fm

3200 2°W

1°W

0°E

1°E

Fuller’s Earth Fm

Fig. 9 Distribution of the Great Oolite Play in southern England and the English Channel (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 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 there are currently no wells. To the west the limestones grade into the argillaceous facies of the Frome Clay. Well 99/16-1 contains a transitional facies, with thin limestone units in an otherwise argillaceous succession. A poor show is recorded from some argillaceous limestone and marl in well 98/18-1. Sandstone reservoir (high risk play)

Carbonate reservoir (high risk play)

Sandstone reservoir (low risk play)

Carbonate reservoir (low risk play)

Non-reservoir facies

Offshore well Land well Wytch Farm & Kimmeridge fields

CB & FM* Cornbrash & Forest Marble Formations

Claystone & siltstone Great Oolite Formation in well 99/12-1: Limestone All depths are in feet below KB

Gross thickness: 119 ft Net:Gross: 1.0 Porosity: unknown

Fig. 10 Well log across the Great Oolite Formation in well 99/12-1

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3.4 Corallian Carbonate Play

97/19-1 0

Q

P

R

Ou tcr

40

Kimmeridge Clay Fm

10

99

10 0

58

100

20

Osmington Oolite Fm

20 10 0

JURASSIC

98

97

10 0

Sonic

Palmer’s Wood wells

0

96

140

Subcrop

20

51°N

Gamma 150

56

S

op

105

2014

1500

98/12-1 98/12-1 98/18-1

97/19-1

Oxford Clay Fm

1600 UK e nc Fra

Osmington Oolite facies 98/23-1

50°N

88

89

50 km


3°W

2°W

1°W

0°E

1°E

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 metres) of Corallian Sandstones (from Hawkes et al. 1998). See below for key

Limestone All depths are in feet below KB Osmington Oolite Formation in 97/19-1:

The Corallian Group contains potential reservoirs in shallow-marine sandstones (e.g. onshore oil in the Palmer's Wood wells) and inner shelf carbonates (e.g. the Osmington Oolite of the Dorset coast outcrops). No significant sandstones are present offshore, but in this area the Osmington Oolite is best developed in a 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.


Fig. 12 Well log across the Osmington 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

Q

Sonic

40

Lower Purbeck Beds

3.5 Portland Carbonate Play P

140

R

Purbeck Anhydrite

56

S

105 Subcrop

96

99

98

97

Sands Portland rally te la s s pa es mudston into 100

58 no data

98/13-1 tcr

Ou

98/16-1

Portland Group

UPPER JURASSIC

51°N

2500

UK ce Fran

op

Portland Limestone facies

50 km 50°N

Kimmeridge Clay Fm

88

89 3°W

2700 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 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 fault at this level.

Limestone All depths are in feet below KB Portland Group in well 98/13-1:

Sandstone reservoir (high risk play)

Carbonate reservoir (high risk play)

Sandstone reservoir (low risk play)

Carbonate reservoir (low risk play)

Non-reservoir facies

Offshore well Land well Wytch Farm & Kimmeridge fields


Fig. 14 Well log across the Portland Group in well 98/13-1.


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 faults. The faults controlled depositional thicknesses through the Triassic, Jurassic and Early Cretaceous; after which time stresses reversed into a compressional regime resulting in a pronounced reversal of movement on these faults and structural inversion of the basins. The Central English Channel Basin is flanked to the north by the down-tothe-south Purbeck-Isle of Wight Fault, and to the south by the down-tothe-north Central English Channel Fault. Both have been reactivated to form a mirror image of each other on either side of the basin (Beeley & Norton 1998). Estimates of the amount of uplift range from 1,500 feet (500 m) or less north of the Purbeck-Isle of Wight Fault (which is assumed to have remained relatively stable during inversion) to 3,000-6,000 feet (1-2 km) south of that fault (well 98/11-2) and north of the Central English Channel Fault (well 98/23-1) (Law 1998). Tertiary (Alpine) inversion has had several effects on prospectivity: (a) Late, reactivation of faults could have beached old hydrocarbon accumulations and also structural realignment could have caused previous accumulations to remigrate into new structures or to seep to the surface. Wessex Basin Vale of Pewsey fault

United Kingdom

2014

(b) Reservoirs in areas that have been uplifted retain reservoir properties, particularly porosity, associated with their pre-uplift location. In the centre of the Weald Basin, Great Oolite porosities are lower than would initially have be expected.

4. Basin development, inversion and consequences

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5. Trap types Two trap types are recognised: (a) Mesozoic tilted fault blocks and horsts which pre-date hydrocarbon generation. To succeed, these structures should be within migration distance of a source kitchen and importantly should not have been breached by Tertiary inversion. In the Wytch Farm area, the faults north of the disturbance remain in net extension and appear to have been little affected by the inversion process. (b) Tertiary inversion anticlines which predominantly post-date hydrocarbon generation. These were the first structures to be targeted by exploration onshore, and with the exception of the Kimmeridge Oil Field, have all proved unsuccessful. Kimmeridge is possibly ”the sole survivor of a much more significant play which was wiped out during inversion” (Evans et al. 1998). (c) There is currently no evidence for stratigraphic traps in this area. Basin-scale facies changes are prevalent and no channelised or mounded sandstones have been identified in the generally tabular geometry of the seismo-stratigraphic units.

Central English Channel Basin Central English Portland-Isle of Channel High Wight structure

Wardour-Portsdown structure

S

0 Depth (km)

Var is

can

10

War do

ur F

Fro

nt

ront

20 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 Gault

N

S Wealden

L. Greensand MSL

Tertiary

Wealden Portland

Oxford Clay Corallian

1 km

Middle Jurassic

n KCF Corallia Oolite o ri Infe r Lias Triassic

3000

6000

Lias

Depth (feet)

KimmeridgePurbeck

Beds

Tertiary Chalk

Variscan Basement

9000

Mercia Mudstone

Not to scale

Sherwood Sandstone

N

S Top Lower Greensand

L. Greensand

MSL

N

S

Wealden

Oxford Clay Corallian Middle Jurassic

Lias

6000

Depth (feet)

3000

KimmeridgePurbeck

9000

Mercia Mudstone Sherwood Sandstone

1 km

12000

Fig. 16 Schematic cross-section across the Portland-Isle of Wight structure (Butler 1998) (a) restored to Albian time, (b) present day with eroded section added.

Fig. 17 Schematic cross-section across the Central English Channel High (Beeley & Norton 1998) (a) restored to Top Wealden, (b) present day with eroded section added.

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2°W

6. Seismic examples Two regional seismic profiles illustrate the regional structure and stratigraphic thickness variations across the English Channel.

51°N

Profile B crosses the southern pat of the Central English Channel Basin and continues to the north-east across the continuation of the PurbeckIsle of Wight Fault, also known as the Wight-Bray line or the Bembridge-St. Valery line. In the SW there is evidence for sedimentary thickening toward the Central English Channel Fault. To the north-east, on the HampshireDieppe High, the effects of basin inversion in the mid-Cretaceous are 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 of the study area.

98/16- 1 98/16- 2

0 TWT (s)

79

8

2

0°E

99/12- 1

98/07- 2 3 98/12- 1

4Z

97/19- 1

2014

99

98/11-1 2

98/13- 1

99/18- 1B

98/16- 1

Pr

98/18- 1

ofi

le

97/24- 1A

99/16- 1

ile of

B

Pr

A

annel Central English Ch Fault Zone

98/23- 1

2 98/22- 1B

Pointe de Barfleur well

UK France

50 km

50°N

Fig. 18 Location map for seismic profiles A and B

Central English Channel High


1°W

98

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 PermoTriassic and Liassic intervals. The outcrop pattern provides evidence for uplift to the north-west and the south-east, with a syncline of Chalk preserved in the middle of the basin. Uplift caused by reversal of the Central Channel Fault is sufficient to expose the Kimmeridge Clay Formation at seabed. This structure was drilled by well 98/23-1. The succession found south of this major feature is speculative (the results of the French well Pointe de Barfleur are not available).

NW

United Kingdom

Central English Channel Fault Zone

SE

Base Aptian Unconformity

Tertiary Top P

urbe

0.5

ck

Chalk

er

ark

Fm

C ra K

Lower Greensand, Gault and Upper Greensand

Int

Top Coral

lian Lmst

Wealden Group

Top Cornbra

sh

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 Variscan basement

Top 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

SW 0

Central English Channel Fault Zone

Purbeck-Isle of Wight Fault 99/18- 1B


Hampshire-Dieppe High (inverted to a basin)

NE

TWT (s) Base Aptian unconformity Top Chalk

0.5 Top Purbeck

Base Chalk

ker Intra KCF mar st Lm lian ral Co Top

Top Cornbrash olite Top Inferior O nt aseme Top B

1.0

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

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United Kingdom

2014

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.

Q

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56

S

London Platform 1997

51°N

LOWER LIAS ERODED (TRIASSIC AND OLDER AT OUTCROP)

96

1099

5630

98 3826

6101

Hamp 4090

4257 4837

t UPLIFT 00-6000f PLUS 30 6407 4734

99

6719

5908

ieppeNR

Weald Basin/Anticline 5951

100

2250 2297

LOWER LIAS OUTCROP

89 3°W

58

4282

High Abs

5068 6343

LIASSIC HIGH PLUS UP TO 1500ft UPLIFT

PLUS 2500-3000ft UPLIFT 5512 5789

?3955

UK ce Fran

PLUS 4500+ft UPLIFT

1782

88

c.8240

shire-D

5474 6280

SHALLOW BURIAL

50°N

c.6800

7546

3424

2908

5000 MAX MATURITY PLUS c.2000-5000ft UPLIFT

3989 2386

97

4059

Vale of Pewsey PLUS UNKNOWN UPLIFT 4388 Basin

2953

Central English Channel High

Central English Channel Basin 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 mature for oil and gas generation, and in general only the relatively poor organic-rich, deeper horizons are mature for hydrocarbon generation. It is only in areas that received (Wealden Group) sediment during the phase of late Jurassic-early Cretaceous uplift and erosion that the Lias was buried to a sufficient depth for hydrocarbons to be generated (McMahon & Turner 1998).

The key issue in assessing the chance of success of specific structures within the Central English Channel Basin is the relative timing of hydrocarbon generation, migration and trap formation.

The Lower Lias has reached the oil window over much of the Channel Basin and has been sufficiently deeply buried in the axial part to raise the possibility of significant gas generation. Reconstructions of burial history suggest that the onset of oil generation began during the early Cretaceous and peaked at about the midCretaceous (Penn et al. 1987).

Without doubt much oil and gas have been generated, but unfortunately very little has been discovered in viable fields outside the Wytch Farm Oil Field. Oil generation from the primary source, the Lower Lias, took place during the early to mid- Cretaceous, and the aim is to locate structures which existed at this time and which have not been subsequently disturbed by inversion. It is feasible for later-formed structures to contain hydrocarbons, but these have to rely on remigration from earlier, transient reservoirs or very late stage hydrocarbon generation.

10. References 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 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, 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 generation window in the northern part of the Channel Basin, but the Kimmeridge Clay does so only in the deepest, axial parts. Also, whereas the Oxford Clay may have locally reached peak oil generation, the Kimmeridge Clay is not thought to have reached this level of maturation (Penn et al. 1987). In summary, the three principal source rocks have reached varying degrees of maturation in the Central English Channel Basin. Being the most deeply buried, the Lower Lias shales provide the largest volume of source rock which falls within and even beyond the oil generation window. Away from the basin depocentre and particularly in those areas severely affected by late Cimmerian uplift and erosion, levels are maturation are considerably lower.

BGS. 1995. Wight Sheet 50°N-02°W 1:250 000 Solid Geology Series. Second Edition. 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 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 & Son, London. 494-503. 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 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 Kingdom Offshore Regional Report, London, H.M.S.O. 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. Geological Society, London, Special Publication 133: 187-197. 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. 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 WessexChannel Basin, UK. In: Brooks, J. & Glennie, K. (eds) Petroleum Geology of North West Europe. Graham & Trotman. 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. Journal of the Geological Society, London 155: 975-992. 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 Petroleum Geology of the Wessex Basin. Geological Society, London, Special Publication 133: 1-18.

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