The Prospectivity of the English Channel
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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. 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 North Purbeck-Isle of Wight structure. 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 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 fault 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 Wessex Basin 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 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 2°W 1°W 0°E ! ! ! ! ! ! !! ! ! ! ! 1. Exploration history ! ! ! ! ! 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 ! ! ! !! ! ! ! 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 # Bournemouth 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 2010 2011 2012 The prospectivity of the Promote English Channel United Kingdom 2015 Chronostratigraphy Lithostratigraphy 2. Stratigraphy Oligocene Solent Group Barton Group Paleogene Eocene 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 unconformity Upper Three major unconformities subdivide the succession into four major sequences: (a) the Devono-Carboniferous Cretaceous 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 Wealden Group 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). Purbeck Group Portland Group Chrono- Lithostratigraphy stratigraphy Upper Kimmeridge Clay Fm Ampthill Ringstead Jurassic “Upper Late Corallian Clay Sandsfoot Subgroup” equiv. Corallian Group Oxfordian Trigonia clav. Oxford Clay Fm “Lower Osmington Oolite Kellaways Fm Mid Jurassic Middle Corallian Subgp” Redcliff Great Oolite Group Jurassic Nothe Grit Generalised lithologies Inferior Oolite Group Early Bridport Lower Late Sands Oxford Clay Mixed lithologies Jurassic Liassic Group Callovian Mid Sandstones Penarth Group Early Kellaways Limestones 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* Devonian 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 2010 2011 2012 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.