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Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 Introduction to the development, evolution and petroleum of the

JOHN R. UNDERHILL 1 & ROBERT STONELEY 2

1Department of Geology & Geophysics, The University of Edinburgh, Grant Institute, Kings Buildings, West Mains Road, Edinburgh EH9 3JW, UK (e-maih [email protected]) 21A Pelham Court, 145 Fulham Road, SW3 6SH, UK

Abstract: Despite containing the largest known onshore oilfield in western Europe, the Wessex Basin hydrocarbon province appears to be extremely limited spatially and it currently only consists of three producing oilfields: , Wareham and . The main factor which controls hydrocarbon prospectivity in the area appears to be preservation of oil accumulations originally sited in tilted - blocks. The extensional palaeostructures of Wytch Farm and Wareham are interpreted to have been charged by upwards migration of oil from mature Liassic source rocks situated across the Purbeck- fault in the Channel (Portland-Wight) sub-basin prior to, and unaffected by, either significant effects of intra- (Albian-) easterly tilting or by Tertiary tectonic inversion. To date, only the small Kimmeridge oilfield, which is situated in the core of a periclinal fold created in response to structural inversion, suggests that any hydrocarbon remigration into younger structural inversion structures has taken place.

Basin definition by a fundamental change in subsurface geology running NW along Water, the The Wessex Basin as defined here consists of a WNW-ESE trending Chalk () system of post-Variscan sedimentary depocen- outcrop is included in the descriptions herein tres and intra-basinal highs that developed across central southern and adjacent offshore areas (Figs 1 & 2). Given its limits in age and area, the Wessex Basin may be con- 0 1O0 200km ,~ 1 i I I t I sidered to represent a of extensional sub- 0 50 lOOmiles ~"~" basins that form a component part of a more extensive network of Mesozoic intracratonic basins that covered much of NW Europe (Ziegler 1990). Like many of the other basins around the British Isles (e.g. the Basin, Southern , Cleveland Basin etc.), the Wessex Basin also records the effects of Cenozoic intra- plate contraction and structural inversion of basin-bounding and intra-basinal faults. Onshore the geological boundaries to the Wessex Basin are such that it covers a similar area to the ancient kingdom of the West Saxons and includes the present counties of and , together with parts of East , Somerset and Wiltshire. The basin is bound to the southwest and west by the Armorican and Cornubian Massifs, to the north by the London Platform (otherwise known as the London- Brabant massif) and to the south by the Central Channel High (Fig. 2). Its northeastern and northwestern boundaries are less precisely defined. Although the distinction between the Wessex Basin and the of , Fig. 1. Location map indicating the area covered Surrey and Kent is usually taken to be marked by Fig. 2.

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 WessexBasin, Geological Society, London, Special Publications, 133, 1-18. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

2 J. R. UNDERHILL & R. STONELEY

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INTRODUCTION 3 so as to include the Portsdown in the Purbeck-Isle of Wight fault system, effectively basin (Fig. 2). Although areas to the northwest separates the Channel (or Portland-Wight) Basin of the basin have affinities with the Wessex Basin to the south from the South Dorset Shelf and (e.g. Bristol Channel Basin), its NW limit is Hampshire-Dieppe (or Cranborne-Fording- taken to be marked by a poorly defined bound- bridge) intrabasinal highs. ary running from the Quantock Hills across the Other structural elements are also wholly intra- Central Somerset Trough south of the Mendips basinal. Two E-W-trending extensional faults to the western extension of the London Platform define a narrow South Dorset Basin (otherwise (Fig. 2). known as the Winterborne Kingston Trough or Cerne Basin) within the South Dorset Shelf. The Wardour and Portsdown fault systems repre- Structural components sent important sets of intra-basinal extensional growth faults prior to their reverse reactivation The Wessex Basin itself can be subdivided into in the Tertiary. Finally, the largely subsur- a number of component parts, bounded primar- face NNW-SSE-trending Watchet-Cothelstone- ily by several important exposed or buried tec- Hatch fault system transects the basin (Milior- tonic elements, the most significant of which are izos & Ruffell this volume). given below. During Tertiary times, the main site of The Pewsey fault system and Central Channel deposition differed from those of the impor- High which are taken to define the northern and tant Mesozoic basins. Following the latest southern margins of the Wessex Basin respec- Cretaceous-early Tertiary inversion, sedimenta- tively. tion was mainly restricted to the Hampshire The Purbeck-Isle of Wight Disturbance (Figs Basin which lay above the site of the former 2, 3 & 4) together with the underlying Mesozoic Hampshire-Dieppe High (Plint 1982, 1983,

Fig. 3. Satellite image of the South Dorest area showing the significant topographic effect created by the steep limbs of rtorthward facing monoclinal folds formed in response to Tertiary structural inversion. The approximate location of the satellite image is shown on Fig. 2. Produced with permission of Earth Images Ltd. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

Fig. 4. Sedimentary depocentres and the main structural elements of . (a) Permo- (CT, Crediton Trough); (b) -Cretaceous; (c) Cenozoic. (Modified after Hamblin et al. 1992.) Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

INTRODUCTION 5

1988). Although temporally and spatially dis- sedimentological studies. For example, Ains- tinct from the main sites of Mesozoic basin worth et al. (this volume a & b) provide a development, the is still con- detailed biostratigraphic calibration and lithos- sidered an integral part of the Wessex Basin tratigraphic subdivision of the megasequence since it records the syn- and post-contractional and its component parts. Although some deformation and sedimentation history of the sequence stratigraphic studies of the Jurassic region. section have focused upon the recognition and correlation of sequence boundaries (e.g. Coe 1995, 1996; Hesselbo & Jenkyns 1995), more Stratigraphic framework recent efforts have attempted to define max- imum flooding surface-bound, genetic strati- The basic stratigraphy and structure of the graphic sequences or depositional episodes Wessex Basin are well displayed in the coastal (sensu Galloway 1989; Underhill & Partington cliffs and inland districts of South Dorset, east 1993, 1994). In particular, Cole & Harding (this Devon and the Isle of Wight. These outcrops, volume) use palynofacies to define genetic and deductions made from them, enable predic- stratigraphic sequences in the Lower Jurassic tions to be made about the possible occurrence with a view to enabling comparison with those of oil and gas in the subsurface, which can then defined in the North Sea and adjacent areas be tested by reference to information now (e.g. Partington et al. 1993). available from exploratory wells and from The acquisition of significant subsurface data seismic data (e.g. Stoneley, 1982; Chadwick together with the recent advances in sequence 1986; Penn et al. 1987; Selley & Stoneley 1987). stratigraphic methods support earlier interpreta- Temporally, the sedimentary history of a tions that subdivided the -Lower Cre- distinctive Wessex Basin post-dates the develop- taceous megasequence into three component ment and closure of the Devono-Carboniferous parts (Fig. 5). Proto-Tethys or Rheic Ocean (Glennie & The lowest division consists of Permian and Underhill 1998). The occurrence of major Triassic continental (red bed) sediments, in thrust faults, intense folding, regional meta- which all desert environments are represented. morphism and intrusion of major granitic bath- Deposition was intially restricted to intramon- oliths (such as the Dartmoor Granite) all attest tane basins such as the Crediton Trough that to the severity of Variscan collisional processes. developed due to extensional collapse of the The deformed Devono-Carboniferous sediments former Variscan mountain belt. Although there lie beneath a marked unconformity which is much alternation, the sequence is charac- represents the effective lower limit to reservoir terised by two large-scale fining upward trends: potential in the Wessex Basin (Smith 1993). conglomerates are confined to the Permian and Extensional basin development and its com- the supposed lowermost Triassic, (both of which ponent sedimentary fill history began in the were deposited in more or less restricted inter- Permian within the Variscan fold-and-thrust montane depressions) and pass up into mud- belt hinterland and continued until the Late stones ascribed to the Aylesbeare and Mercia Cretaceous in the Wessex Basin (sensu stricto; Mudstone Groups respectively (Fig. 5). Fig. 4a & b). The sedimentary fill of the suc- The Permian, Exmouth and Dawlish of cessor Hampshire Basin is entirely Tertiary with the coast pass up into mudstones of the youngest sediments being of age the Aylesbeare which are in turn sharply (Fig. 4c). Except for localized volcanics close overlain by Early Triassic to the base (Exeter Volcanic Series), the basin Pebble Beds (Fig. 5). The latter alternate with, is wholly devoid of igneous rocks. In general and pass up into, sandstones ascribed to the terms, the Permian-Oligocene succession may Otter Sandstone Formation. Together the Bud- be separated into three internally conform- leigh Salterton Pebble Beds and the Otter - able but unconformity bound mega-sequences stone comprise the widespread and important (e.g. Hawkes et al. this volume): the Permian to Triassic Sherwood Sandstone Group (Fig. 5). Lower Cretaceous, Upper Cretaceous and Ter- The upper part of the sequence is formed by the tiary megasequences. extensive argillaceous Mercia Mudstone Group, known in the subsurface to include localised evaporites. The at the top of the Permian-Lower Cretaceous megasequence Triassic succession heralds the effects of wide- spread Liassic that led to The Permian to Lower Creataceous interval has the re-establishment of marine waters in the area been the subject of intensive stratigraphic and for the first time since the Carboniferous. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

Fig. 5. Generalized stratigraphical column for the Wessex Basin illustrating the main megasequences and stratigraphic nomenclature currently used in the basin (JB, Junction Bed; CB, Cinder Bed; GAB, Green Ammonite Beds; PG, Penarth Group; BSPB, Budleigh Salterton Pebble Beds). Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

INTRODUCTION 7

The middle division contains of the domi- extension or, more likely, began to show the nantly marine sediments of Jurassic age. It con- effects of contractional reactivation during the sists largely of a broadly cyclic repetition of late Cretaceous. shallow marine mudrocks, sandstones and lime- stones. Many formations have been defined and mapped but, with the exception of local facies Tertiary megasequence variations, particularly in some of the carbo- nates, all appear to be remarkably widespread in The Tertiary succession is separated from the the basin. The Jurassic contains all of the underlying megasequence by an important, but potential source rocks in the region, and one subtle, regional disconformity. The stratigraphic major reservoir (the Sands at the top of break covers the Maastrichtian and most of the the Lower and base of the Middle Jurassic) and . The overlying sediments that com- a number of minor potential reservoir forma- prise the Tertiary megasequence consist of near- tions including the Corallian Bencliff Grit (Allen shore marine and non-marine sediments and are & Underhill 1989, 1990; Goldring et al. this largely confined to the area east of Dorchester volume), Frome Clay and Cornbrash. The top (Fig. 4c). The megasequence reaches a maximum of the succession records a major marine thickness of over 600 m in the north of the Isle of regression and the highest of the limestones, Wight as proven by the Sandhills-1 borehole. the Portland Limestone, passes up into sabkha Depositional facies analysis of Upper and brackish water sediments (the Purbeck), and (Priabonian) sections demonstrates that the thence into the entirely non-marine Lower basin was dominated by lacustrine and brackish Cretaceous Wealden Group. lagoonal environments which were recharged by The sediments of the Wealden Group have marine waters through a restricted inlet in the a distribution in Dorset apparently localised east Solent area (Hamblin et al. 1992). along the strike south of major syn-sedimentary faults. They are essentially of fluvial origin, although lacustrine environments are well repre- Development and evolution of sented in the considerably thicker succession in structural styles the Isle of Wight. Evidence exists for continued extensional movement on several of the E-W- Cross section geometries trending faults during the . For example, Ruffell & Garden (1997) document Surface outcrops highlight the importance of contemporaneous tectonic control by the Isle of several important zones of disturbance affecting Wight Fault on sediment dispersal in the Lower the Chalk (Arkell 1947; e.g. landsat image of Group. Fig. 3). Integration of seismic data with bore- hole data and field observations enable the construction of representative structural cross- Upper Cretaceous megasequence sections (e.g. Figs 6 & 7; Stoneley 1982). The sections not only demonstrate the occurrence of The Upper Cretaceous megasequence is sepa- east-west trending extensional growth faults rated from the underlying Permian-Lower beneath the present zones of disturbance but Cretaceous megasequence by an important also in other areas of the basin. The sections also Albian-Aptian unconformity, which is marked serve to illustrate the role that many of these by the progressive westerly truncation of Meso- extensional faults had on depositional thick- zoic and Permian strata. The lowest part of the nesses and structural geometries during the Upper Cretaceous megasequence consists of Triassic, Jurassic and Early Cretaceous. Exten- westerly-onlapping, diachronous, marine, and sion not only controlled structural geometries commonly glauconitic sandstones which pass up at a basin-scale but also appears to have been into the familiar Chalk which shows evidence of important at more local outcrop scales too thinning in the far west. (e.g. Hunsdale & Sanderson this volume). Lateral variation in thickness and Chalk Of the major E-W zones of Mesozoic exten- lithofacies, including the development of sion, by far the most significant are the system slumps, slip scars and local lacunae have all of structures that comprise the intra-basinal been recorded in the Wessex Basin (Gale 1980; Purbeck-Isle of Wight fault system. Variation in Mortimore & Pomerol 1997). Their close spatial structural styles seen along the length of the association with areas that subsequently became individual, en-echelon fault segments that col- axes of inversion suggests that the E-W-trending lectively form the Purbeck-Isle of Wight fault buried faults that either remained active in system may in part be due to the presence of Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

8 J. R. UNDERHILL & R. STONELEY

Fig. 6. North-south cross-section through Kimmeridge, Stoborough and Wareham illustrating the nature and extent of the structural inversion related folding in the hangingwall and Mesozoic extensional tilted fault blocks in the footwall of the Purbeck Fault.

Fig. 7. North-south seismic line and interpreted cross-section through the offshore extension of the Wytch Farm oilfield. The presence of a second monoclinal fold at the base of the Late Cretaceous demonstrates that at least one tilted fault block was affected by the structural inversion process in addition to the more obvious presence of the major structural inversion-related hangingwall anticline. (OC, Oxford Clay; JB, Junction Bed; SSG, Sherwood Sandstone Group.) Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

INTRODUCTION 9

easy slip (decollement) horizons in the Triassic Purbeck-Isle of Wight fault system led to in western parts of the basin (Stewart et al. 1996; relative uplift of the Channel (Portland-Wight) Harvey & Stewart this volume). Several workers Basin. The amount of uplift has been estimated believe that there is evidence that syn-sedimen- to be approximately 1.5 km from outcrop geol- tary roll-over developed in the hang- ogy which is consistent with estimates derived ing walls to many of the extensional faults, from sonic velocity data (Law this volume) and particularly those that have a more listric apatite fission track data (Bray et al. this geometry due to the presence of a salt decolle- volume). The inversion had the effect of modify- ment at depth (e.g. Selley & Stoneley 1987) ing former extensional structures along the Extension on component fault systems of the whole length of the basin (e.g. Butler this Wessex Basin appears to have largely ceased volume; Smith & Hatton this volume), with the during Early Cretaceous times, and except for creation of major northwards-verging mono- those characterised by unusual Chalk lithofa- clinal folds above the reactivated faults (e.g. the cies, most were essentially inactive during Late Purbeck-Isle of Wight Disturbance), modifica- Cretaceous deposition which is generally con- tion of any pre-existing roll-over anticlines and sidered to represent the period of post-rift the formation of periclinal folds in hangingwall sedimentation. Total displacement on parts of locations and the initiation of local thrusting in the Purbeck-Isle of Wight fault system are now the post-rift sediments (Underhill & Paterson known to have had a cumulative pre-Tertiary 1998). Outwith the areas affected by Tertiary displacement in excess of 2 km. fault reactivation, the dips in the Upper Cretac- Towards the end of the Cretaceous and early eous are for the most part very gentle. in the Tertiary, initiation or continuation of south to north compression led to pronounced Strike-section geometries reversal of movement and structural inversion on many of the formerly extensional Wessex In marked contrast to the N-S-trending dip sec- Basin fault segments including those bound- tions, east-west (strike-parallel) cross-sections ing the Central Channel High (Beeley & Norton demonstrate a much simpler structural pic- this volume). Structural inversion along the ture. There is little evidence of fault-controlled

Fig. 8. Diagrammatic representation of the east-west effects of, and progressive westerly subcrop of, Jurassic- Permian strata beneath the intra-Cretaceous unconformity across the Wessex Basin. The occurrence of this particular unconformity is interpreted to have been detrimental to the area's hydrocarbon prospectivity since it probably inhibited source rock maturation in western areas as well as emparting a pronounced easterly tilt to previously formed extensional tilted fault blocks. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

10 J. R. UNDERHILL & R. STONELEY

depositional thickness changes along strike main reservoir potential is provided by silici- during deposition of the Jurassic-Early Cretac- clastic units. However, some minor carbonate eous. However, marked stratigraphic onlap and reservoirs also exist. pinch-out of Permian and Triassic sediment- In the lower part of the outcropping sequence, ary units (including the Sherwood Sandstone the best potential would be provided by the Group; Fig. 3a) is recorded towards the basin Permian aeolian sands of East Devon. They have margin (e.g. Butler this volume). surface porosity up to 40% and are possibly E-W-trending structural cross-sections do, equivalent to the Rotliegend sandstones of the however, demonstrate that an important easterly Southern North Sea. The Permian sandstones, tilt was emparted to the basin during the Early however, are not generally considered to have a Cretaceous (Fig. 8). The underlying Permian- high reservoir potential because it is believed that Jurassic sequence was subjected to and they either do not extend sufficiently far to the shows progressive truncation towards the west east in the subsurface or lie below structural (e.g. McMahon & Underhill 1996; McMahon & closure to be of significance in the basin. Turner this volume), the derived sediment prob- Sandstones are frequent in the Lower Triassic ably mostly passing eastwards into the Weald succession. They reach their optimum develop- Basin. Subsequent westerly transgression during ment in the Triassic Sherwood Sandstone Group. the Aptian and the Cenomanian eventually led The formation is extensively exposed between to cover of the truncated Jurassic, Triassic and Budleigh Salterton, Otterton and , Permian above the megasequence bounding where its thickness approaches 120m (Lorsong unconformity. & Atkinson 1995). The unit comprises an almost continuous, high net:gross arkosic sandstone body with limited mudstone lenses. Facies Hydrocarbon habitat analysis suggests that the Sherwood Sandstone Group consists of braided alluvial deposits and For any area to become a successful hydro- perennial sheetflood sandstones, with the local carbon province, a number of factors must be development of distributary channels and a satisfied. There must be one or more suitable couple of aeolian sandstones interbedded with reservoirs capped by coherent seals'. There must subsidiary playa lake and floodplain mudstone be a candidate source rock with sufficiently high lenses (e.g. McKie et al. this volume and total organic carbon content. Well-defined trap references therein). The latter occasionally con- configurations must exist, be they obvious tain palaeosols (e.g. Purvis & Wright 1991; structural closures or more subtle stratigraphic Wright et al. 1991). At Wytch Farm, the Sher- features. Even given all the of the above, an wood Sandstone Group is interpreted to have area would remain unprospective without source been deposited in a mixed fluvial and lacustrine rock maturation due to burial, the existence proximal braided alluvial plain setting (Dranfield of suitable migration routes from source to et al. 1987; Bowman et al. 1993; McKie et al. this trap and, most importantly, appropriate timing volume). Significantly, the Sherwood Sandstone of reservoir-bearing trap formation relative to Group forms the main reservoir unit (c. 150m hydrocarbon migration. Finally, preservation thick) in the Wytch Farm Field (McKie et al. this of any accumulation must be maintained. volume), where it is generally similar to the That the Wessex Basin contains producing outcrops, even though the facies associations oilfields attests to the fact that all the factors appear not to be continuous in the subsurface in controlling hydrocarbon prospectivity have been between. Porosities up to nearly 30% have been met, at least locally. Integration of onshore measured with permeabilities in the darcy range. outcrops with subsurface (seismic and borehole) The stratigraphically lowest potential reser- data enables consideration of each of the voir in the Lower Jurassic interval is the essential requirements which have combined to Thorncombe Sands which lie towards the top make selected parts of the Wessex Basin highly of the Lias Group and are exposed at Watton prospective. Each of these essential requirements Cliff near Bridport. Since they are relatively thin will now be reviewed in turn. (23m) and very fine-grained sandstones with porosity only in the range 7-10% and perme- Reservoirs ability of approximately 25-30 mD, the Thorn- combe Sands have not excited as much interest Potential reservoirs are present in many parts as either the underlying Sherwood Sandstone of the succession, some of major and some of Group or the overlying, Bridport Sandstone. apparently more minor significance. Review However, they are believed to contain oil in the of onshore exposures suggests that the basin's vicinity of Wytch Farm. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

INTRODUCTION 11

Of more significance are the diachronous 41 m Seals thick Bridport Sands which lie at the top of the Lias Group and extend up into the Aalenian. All of the major potential reservoir sandstones The sands are fairly uniformly very fine to fine- are each covered by a substantial thickness of grained, clean with outcrop porosities up to mud rocks which have good sealing potential. 15% and permeability up to 250mD (and up to The Permian sandstones are overlain by mud- 32% and 800mD at Wytch Farm). They are stones of the Aylesbeare Group. The Sherwood interrupted by numerous 0.33m thick cemented Sandstone Group is overlain by the red silty beds which could be barriers to cross-forma- mudstones and local evaporites of the Upper tional fluid flow. The sands are believed to be Triassic Mercia Mudstone Group, which is over shallow marine, whilst the origin of the cemen- 350 m thick near Sidmouth. The Bridport Sands ted layers is still debatable (Bryant et al. 1988). and are overlain by 190 m of the They are capped by a thin (3m) limestone bentonitic clays of the Fuller's Earth. representative of the Inferior Oolite which, where fractured, has yielded oil in the vicinity of Wareham. The Bridport Sands (c. 70 m thick) Source rocks form the upper reservoir in the Wytch Farm Potential source rocks are confined to the Field (Colter & Harvard 1981). Jurassic and occur at three main levels. In the Thin limestones in the Middle Jurassic, the vicinity of and , and also Forest Marble and the Cornbrash, are similar to in the subsurface to the east, black shales of the the Inferior Oolite in that they too have little Lower Lias reach some 100m in thickness natural porosity. However, the Frome Clay (House 1993). In the lower part of the section Member does contain oil at Wytch Farm and they alternate with pale, very fine-grained lime- the Cornbrash has been proven to act as a minor stones approximately 30-40 cm thick. Although reservoir in the Kimmeridge oilfieId where it has marginally immature at outcrop, a total organic been extensively fractured (Evans et al. this carbon content (TOC) of up to some 8% has volume). been measured (Ebukanson & Kinghorn 1985): In the Upper Jurassic, exposures of the mixed the organic matter is predominantly Type II siliciclastic-carbonate sequence of the Corallian algal material. Group suggest that it could offer some reservoir Potential source rock horizons also occur in potential in the subsurface, particularly the the Middle and Upper Jurassic. Some source approximately 3 m thick fine-grained sandstones rock potential has been proven to occur in the known as the Bencliff Grit. Interestingly, at lower part (Upper Callovian) of the Oxford outcrop at , the sandstones Clay, on the shore of the Fleet Lagoon and in show signs of having been extensively impreg- abandoned brickpits in the neighbourhood of nated with oil and still provide a live seepage some 7 km WNW of Weymouth, as (Allen & Underhill 1989). Despite this, however, well as in the subsurface to the east. there are no known occurrences of Corallian oil- The Kimmeridge Clay in the Upper Jurassic bearing reservoirs in the subsurface to date. reaches a combined total of some 520 m in out- The Portland Limestone at the top of the crops at Burning Cliff and at Jurassic is porous in two facies: there is still itself. The succession consists of interbedded some primary porosity in oolitic grainstones and black anoxic shales and fine dolomitic limestones. secondary dissolution occurs in bioclastic pack- TOCs up to about 20% have been measured stones. It could conceivably provide a target (Farrimond et al. 1984), although some 70% has beneath the Upper Cretaceous unconformity to been recorded from the 1 m thick Kimmeridge Oil the north of the Purbeck-Isle of Wight Dis- Shale just above the middle of the section. Like turbance if it could be located seismically, the Liassic source rock intervals, Type II organic although the carbonate develops a more chalky matter is marginally immature at outcrop (vitri- facies inland from the coast. nite reflectance equivalent 0.48%; Ebukanson & Restricted fluvial sands in the continental Kinghorn 1985). Wealden Group might conceivably provide minor reservoirs, but it is unlikely that they are sealed in the subsurface in southern Dorset. The Traps Upper Cretaceous and Tertiary do not have any significant reservoir potential in Dorset since Two main possible trap types predominate in carbonates of the usually lack the Wessex Basin: periclinal closures mainly porosity and effective permeability and the related to structural inversion and buried, tilted Tertiary clastics occur at or near surface. extensional fault-blocks. To date, no evidence Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

12 J. R. UNDERHILL & R. STONELEY exists for the presence of any stratigraphic plays. The expectation that maturation has occurred The periclinal closures are seen running parallel is supported not only by the presence of pro- to, on the south side of, and up to 2 km from ducing oilfields but also the occurrence and the Purbeck-Isle of Wight line of inversion distribution of seepages. There are a number of (e.g. Kimmeridge). These are limited in length biodegraded oil seeps and impregnations in Jur- and largely correspond to the centres of indivi- assic and Wealden beds along the coast between dual fault segments. Whilst the southerly and Osmington Mills and east of along plunge dips are very gentle, the north (Bigge & Farrimond this vol- flanks of the periclines commonly steepen into ume), and gas seepages have been reported from the main fault. Although these periclines were the sea-floor south of (Stoneley 1992). clearly uplifted and no doubt modified, possibly The oil seep in the Wealden at has even breached, by the Tertiary structural inver- proved to be a particular focus of research, and sion and uplift, it has been argued that some its genesis and evolution have been subject to may have developed as hanging wall roll- considerable debate (e.g. Selley & Stoneley overs during extension at least in the late 1987; Hesselbo & Allen 1991; Miles et al. Jurassic-early Cretaceous (Stoneley 1982). How- 1993, 1994; Kinghorn et al. 1994; Wimbledon ever, most smaller anticlines close to the et al. 1996; Bigge & Farrimond this volume; disturbance to the west are now believed to Parfitt & Farrimond this volume; Hesselbo this have formed largely during Tertiary inversion volume). A channel sandstone contains boulders (including the Anticline; Mottram & of mineralogically similar sandstones bound House 1956). together by black residual oil. These have been Although early exploration was directed taken by some to be evidence for the existence of towards the periclinal traps (Buchanan this eroded and reworked sediment impregnated by a volume), the advent of seismic data led to the nearby Early Cretaceous palaeo-oil seepage initial recognition and ultimate successful test (Selley & Stoneley 1987; Kinghorn et al. 1994; for hydrocarbons in buried tilted extensional Wimbledon et al. 1996). As the channel sand- fault blocks lying to the north of the Purbeck- stone also contains a live, light oil, it has been Isle of Wight fault system in the Wareham area. argued that the outcrop marks the site not only It is the occurrence of these tilted fault blocks Early Cretaceous but also of recent seepage. adjacent to the active kitchen area to the south Irrespective of its exact evolution, however, the that provides the main structural plays to the Mupe Bay exposures together with the other basin including the Wytch Farm and Wareham known seeps have been successfully typed to a oilfields (Fig. 8; Colter & Harvard 1981). A Lower Lias source (Cornford et al. 1988). series of such tilted fault-blocks and terraces have been defined in the basin. In almost all cases, faults defining the blocks north of the Migration Purbeck-Isle of Wight Disturbance remain in net extension and appear to have been little Given the likelihood that the proven Lower Lias affected by the structural inversion process other source rock only reached maturity in the kitchen than possibly by converting former channels of area south of the Purbeck-Isle of Wight migration into seals. disturbance, charge into the South Dorset Shelf was reliant upon leakage across the Purbeck-Isle of Wight fault system and upwards Maturity migration to backfill tilted reservoirs in the footwall to the extensional fault segments At outcrop, all of the potential source rocks are (Fig. 9). Such a mechanism is envisaged for the immature for oil generation. However, there is filling history both of the Sherwood Sandstone ample evidence for source rock maturation Group and Bridport Sands together with other having occurred prior to tectonic inversion in minor reservoirs, within the tilted fault blocks the Tertiary. Basin modelling supported by well located to the north of the kitchen area. Support data suggest that the Lower Lias has reached for such a mechanism comes from the occur- peak oil generation throughout the area to the rence of the numerous oil seeps in locations south of the -Purbeck-Isle of Wight where they could be fed by migration up the disturbance and that, in the east, the Oxford plane of faults. BP's Kimmeridge oilfield may Clay just entered the oil window (e.g. Penn et al. be an important exception to this rule, but 1987, fig. 4). To the east of Swanage, the Lower it remains the only producing oilfield in the Lias has been buried deeply enough to raise the hangingwall to the reactivated Purbeck fault possibility of significant gas generation. (Fig. 6; Evans et al. this volume). Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

INTRODUCTION 13

Fig. 9. North-south present-day and Late Cretaceous (restored) cross-sections though Wytch Farm depicting the main controls on hydrocarbon maturation and migration in the area. Modified after Colter & Harvard (1981). (KCF, Kimmeridge Clay Fro; OCF, Oxford Clay Fm.) Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

14 J. R. UNDERHILL & R. STONELEY

Timing of generation, migration appears to have been particularly severe in the and entrapment immediate vicinity of the Purbeck-Wight Dis- turbance, large areas to the north seem to have Generation of hydrocarbons from the Channel remained largely unaffected (e.g. the fault block (Portland-Wight) kitchen area occurred prior to containing the Wytch Farm oilfield) suggesting Tertiary inversion. Analysis of burial histories that the footwall to the Purbeck-Wight fault for wells located in the Channel (Portland system acted as a rigid buttress with most of the Wight) depocentre, and the supposed fossil seep- compressional strain taken up on it or within the age at Mupe Bay, suggest that the Lower Lias Channel Basin (Underhill & Paterson 1998). entered the oil window by Early Cretaceous Intense fracturing in the Chalk, exposed along times with peak generation in the Middle to Late the Purbeck-Isle of Wight Disturbance (e.g. at Cretaceous (Fig. 9). Whilst source rock matura- Lulworth Cove) might have created a seal risk tion is likely to have been inhibited or stopped for the fault block immediately to the north, in western areas affected by Albian-Aptian tilt- which has indeed been found to be water- ing (i.e. west of Lulworth), the main Channel bearing although with residual oil staining. (Portland-Wight) kitchen probably continued to More than a few hundred yards away to the generate hydrocarbons. That kitchen area was north, however, where the Chalk remains probably only switched off when uplift south undeformed, hydrocarbon accumulations have of the Purbeck-Isle of Wight fault system com- remained unbreached as is evident at Wareham menced some time between the Campanian and and Wytch Farm. the Late Palaeocene (Fig. 9). The disposition of known accumulations Limits to the hydrocarbon play fairway requires that the faults that acted as channels of migration were sealed over by the Upper Assessment of the above factors leads to the Cretaceous at the time of maximum migration to conclusion that there may be effective limits to prevent escape to surface, and that they sub- the main hydrocarbon play fairway. The inter- sequently became sealing probably due to the preted limits to the main Sherwood Sandstone onset of compression (e.g. Selley & Stoneley Group and Bridport Sands plays are governed 1987). As in many sedimentary basins affected by the respective pinch-out of the lower, and by tectonic inversion, structures like the peri- shale-out of the upper reservoir interval towards clines that formed during the tectonic inversion the east, the line to the west of which either the process (e.g. Poxwell) would have been reliant source rock never reached maturity or was upon late- remigration for them to be affected by intra-Cretaceous uplift, and to the successful plays. As the relative disappointment south by the faults and the tilted fault blocks of exploratory well results sited on such peri- that they define that show the effects of clines suggests that such re-migration was of contractional reactivation. The resultant play relatively minor importance in the Wessex Basin. fairway thus appears to be limited to the area Finally, understanding of the structural inver- covered by the (north of the sion history, superimposed on a very gentle Purbeck Disturbance), Harbour and overall eastwards plunge towards the centre of Bay, but appears not to extend the sub-basin, is critical to the interpretation of to Solent or to northern areas of the Isle of the petroleum geology because it implies that Wight. The northerly limit to the play fairway, long-established unbreached palaeostructures however, is not well defined but is probably are likely to be restricted to the eastern parts controlled by the distance that oil could migrate of the basin lying north of the principal axes of from the kitchen area and hence, is dependant inversion. upon the availability of migration routes. Although the presence of oil at Stockbridge Preservation of accumulation (Fig. 2) might, however, be taken as evidence that some oil was able to migrate long distances Once an accumulation is in place, it must be to the north of the Channel (Portland-Wight) preserved intact and not permitted to escape. depocentre, or from the more local Pewsey Although many remain undeformed, some basin, it is more likely that the field is sourced palaeostructures that existed at the end of the from the Weald Basin (Butler & Pullan 1990). Cretaceous appear to have been affected by the structural inversion process and either partially Oil production and the Wytch Farm oilfield or totally reconfigured by the effects of fault reactivation and basin inversion during the Despite the profound bias resultant from oil Tertiary. Indeed, whilst Tertiary deformation production rates from one field, Wytch Farm, Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

INTRODUCTION 15

volumetrically oil production from the Wessex currently not included with the Wytch Farm basin has now overtaken that from the East statistics. Midlands and Weald Basin, thus making it Development of Wytch Farm, which initially Britain's most prolific hydrocarbon area. took place in the 1980's, has more recently been To date three producing fields have been extended to include that part of the stucture discovered in the Wessex Basin: Wytch Farm, that runs offshore beneath and Wareham and Kimmeridge. The first two of Bournemouth Bay (see McClure et al. 1995, these occur within the Sherwood Sandstone play fig. 2). Innovative drilling technology including fairway defined above. It is worth noting that the use of highly deviated extended reach wells although often shown as a separate accumula- has been used to drain the field (McClure et al. tion, the Arne discovery is now considered to 1995; Hogg et al. 1996; McKie et al. this volume) form an integral part of the Wytch Farm oilfield. whilst at the same time avoiding the need to put Furthermore, the offshore 98/7-2 discovery may rigs or other infrastructure in such an envir- also represent an easterly extension of the field onmentally sensitive area. rather than a separate accumulation since it has the same oil-water contact. Of the discoveries to date, however, by far the Conclusions most significant is the Wytch Farm oilfield which is the largest onshore field in western (1) The Wessex Basin and the successor Europe. Although the field was discovered when Hampshire Basin contain a Permian-Oligocene the oil-bearing Bridport Sands were penetrated sedimentary fill which may be subdivided into in 1974, its full potential was not realised until three unconformity-bound megasequences, each 1977 when an appraisal well was deepened to of which record important phases in the devel- test for possible hydrocarbons in the Sherwood opment and evolution of the basin. Sandstone Group (Fig. 7; Colter & Harvard (2) The lower, Permian-Lower Cretaceous 1981; Buchanan this volume). megasequence records several phases of exten- Volumetrically the Wytch Farm oilfield lar- sional faulting which led to the creation of gely comprises two main reservoir units: an numerous intra-basinal depocentres, tilted fault- upper, Bridport Sands reservoir and a lower blocks and terraces. It also contains at least two Sherwood Sandstone Group reservoir. A minor main sealed reservoir intervals in the Sherwood contribution also comes from the Middle Sandstone Group and the Bridport Sands and at Jurassic Frome Clay Member. The field is now least three potential source rock intervals in the thought to have had a stock tank oil initially in Lias Group, the Oxford Clay and Kimmeridge place (STOIIP) of 924 MMbbls and contain oil Clay Formation. reserves slightly in excess of 428 MMbbls of (3) Burial of the Liassic souce intervals in which just under one-half remain to be pro- areas south of the Purbeck-Wight intra-basinal duced (239MMbbls have been produced up segmented, extensional fault system led to mat- until 31/12/97). The main Sherwood Sand- uration and migration of hydrocarbons from stone Group reservoir had a STOIIP estimate Early Cretaceous times. Whilst hydrocarbon of 754MMbbls of which 397MMbbls were generation was probably arrested in western thought to be recoverable (52% recovery effi- areas that experienced intra-Cretaceous (Albian- ciency). The Bridport Sandstone is believed to Aptian) uplift, maturation and migration con- have contained a STOIIP of 120MMbbls of tinued in eastern areas until Late Cretaceous which 27MMbbls (23%) is thought to be re- times. Neither the Kimmeridge Clay nor Oxford coverable, and the Frome Clay Member had an Clay formations appear to have been matured estimated STOIIP of 50MMbbls and approx- for hydrocarbon generation anywhere in the imate reserves of 4MMbbls (8% recovery basin at any time. efficiency). (4) Late Cretaceous and Tertiary compres- STOIIP and reserve estimates for Wytch sion led to contractional reactivation and Farm dwarf those of the other two producing structural inversion along the line of many of fields and other discoveries in the area. Current the former extensional structures (e.g. Purbeck- estimates suggest that the Wareham field had a Isle of Wight fault system), uplift of the Channel STOIIP of 21 MMbbls of which 5 MMbbls are (Portland-Wight) sub-basin and formation of thought to be recoverable. Kimmeridge has a north-facing and numerous peri- STOIIP of 10 MMbbls, reserves of 3.2 MMbbls clinal folds. of which 3 MMbbls have been produced to date. (5) An understanding of the tectono-strati- The 98/7-2 discovery is believed to contain an graphic development and evolution of the additional 20MMbbls in reserves which are Wessex Basin helps determine why the basin's Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

16 J. R. UNDERHILL & R. STONELEY hydrocarbon prospectivity appears to be con- BIGGE, M. A. & FARRIMOND, P. 1998. Biodegradation centrated in the east Purbeck, Poole Harbour of seep oils in the Wessex Basin: a compilation for and Bournemouth Bay areas. It is only there correlation. This volume. that the extensional palaeostructures containing BOWMAN, M. J. B., MCCLIRE, N. M. & WILKINSON, D. W. 1993. Wytch Farm oilfield: deterministic res- Sherwood Sandstone Group and Bridport Sands ervoir description of the Triassic Sherwood Sand- reservoirs have been unaffected by the pro- stone. In: PARKER, J. R. (ed.) Petroleum Geology of nounced effects either Albian-Aptian easterly Northwest Europe: Proceedings of the 4th Confer- tilting or by Tertiary tectonic inversion. ence, 1513-1518. The Geological Society, London. (6) As in other structurally inverted sedimen- BRAY, R. J., GREEN, P. F. & DUDDY, I. R. 1998. tary basins in which the kitchen area has been Multiple heating episodes in the Wessex basin: switched off, hydrocarbon charge into subse- Implications for geological evolution and hydro- quent, inversion-related periclinal folds relies carbon generation. This volume. upon remigration from breached palaeostruc- BRYANT, I. D., KANTOROWICZ, J. D. & LOWE, C. F. 1988. The Origin and Recognition of Laterally tures. Despite numerous exploratory wells, it Continuous Carbonate-Cemented Horizons in the appears that re-migration did not play a major Upper Lias Sands of Southern England. Marine role in the basin. Nevertheless, some periclines and Petroleum Geology, 5, 108-133. modified during the structural inversion episode BUCHANAN, J. G. 1998. The exploration history and may, as at Kimmeridge, have retained from controls on hydrocarbon prospectivity in the Wes- Cretaceous times or received oil by limited re- sex Basins, Southern England, UK. This vohtme. migration. BUTLER, M. 1998. The Geological History of the southern Wessex Basin: a review of new informa- tion from oil exploration. This volume. Both authors are greatly indebted to their many BUTLER, M. & PULLAN, C. P. 1990. Tertiary structures academic and oil company colleagues for input into and hydrocarbon entrapment in the Weald basin their current understanding of the petroleum geology of southern England. In: HARDMAN, R. F. P. & of the Wessex Basin. R.S. would particularly like to BROOKS, J. (eds) Tectonic Events Responsible for thank R. Selley for introducing him to the area and for Britain's Oil and Gas Reserves. Geological Society, his companionship on numerous fieldtrips. S. J. Davies, London, Special Publications, 55, 371-391. J. Evans and A. Ruffell are thanked for providing CHADWICK, R. A. 1986. Extensional tectonics in the advice on the text and G. White is acknowledged for Wessex Basin, southern England. Journal of the drafting the diagrams. Earth Images Ltd are acknowl- Geological Society, London, 143, 465-488. edged for permission to produce the satellite image in COE, A. L. 1995. A comparison of the Oxfordian the paper. successions of Dorset, Oxfordshire and Yorkshire. In: TAYLOR, P. D. (ed.) Field Geology of the British Jurassic. Geological Society, London, 151-172. References --1996. Unconformities within the Portlandian Stage of the Wessex basin and their sequence- AINSWORTH, N. R., BRAHAM, W., GREGORY, F. J., stratigraphical significance. In: HESSELBO, S. P. JOHNSON, B. & K1NG, C. 1998a. The lithostrati- & PARKINSON, D. N. (eds) Sequence Stratigraphy graphy and biostratigraphy of the latest Triassic in British Geology. Geological Society, London, to earliest Cretaceous of the and Special Publications, 103, 109 143. its adjacent areas. This volume. COLE, D. C. & HARDING, I. C. 1998. Use of , , , & 1998b.A proposed latest palynofacies analysis to define Lower Jurassic Triassic to earliest Cretaceous lithostratigraphic ( to stages) genetic classification for the English Channel and its stratigraphic sequences in the Wessex Basin, adjacent areas. This volume. England. This volume. ALLEN, P. A. & UNDERHILL, J. R. 1989. Swaley cross- COLTER, V. S. & HAVARD, D. J. 1981. The Wytch stratification produced by unidirectional flows, Farm Oilfield, Dorset. In: ILLING, L. V. & Bencliff Grit (Upper Jurassic), Dorset, UK. HOBSON, G. D. (eds) Petroleum Geology of the Journal of the Geological Society, London, 146, Continental Shelf of North- West Europe. Hayden 241-252. & Son, London, 494 503. -- & 1990. Discussion on swaley cross-strati- CORNFORD, C., CHRISTIE, O., ENDRESSEN,U., JENSEN, fication produced by unidirectional flows, Bencliff P. & MYHR, M. 1988. Source Rock and Seep Oil Grit (Upper Jurassic), Dorset, UK [reply]. Maturity in Dorset, Southern England. Organic' Journal of the Geological Society, London, 147, Geochemistry, 13, 399-409. 398 -400. DRANFIELD, P., BEGG, S. H. & CARTER, R. R. 1987. ARKELL, W. J. 1947. The Geology of the Country Wytch Farm Oilfietd: reservoir characterisation of around Weymouth, Swanage, Corfe and Lulworth. the Triassic Sherwood Sandstone for input to Memoir of the Geological Survey. reservoir stimulation studies. In: BROOKS, J. & BEELEY, H. S. & NORTON, M. G. 1998. The structural GLENNIE, K. W. (eds) Petroleum Geology of development of the Central Channel High: con- northwest Europe. Graham & Trotman, London, straints from section restoration. This volume. 494 503. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

INTRODUCTION 17

EBUKANSON, E. J. & KINGHORN, R. R. F. 1985. JENKYNS, H. C. & SENIOR, J. R. 1991. Geological Kerogen Facies in the Major Mudrock Forma- Evidence for Intra-Jurassic Faulting in the tions of Southern England and the Implication on Wessex Basin and Its Margins. Journal of the the Depositional Environments of their Precur- Geological Society, London, 148, 245-260. sors. Journal of Petroleum Geology, 8, 435-462. KINGHORN, R. R. F., SELLEY, R. C. & STONELEY, R. EVANS, J., JENKINS, D. & GLUYAS, J. 1998. The 1994. Discussion on the Mupe Bay oil seep Kimmeridge Bay Oilfield: an enigma demystified. demythologized? Marine and Petroleum Geology, This volume. 11, 124. FARRIMOND, P., COMET, P., EGLINGTON, G., LAW, A. 1998. Regional uplift in the English Channel: EVERSHED, R. P., HALL, M. A., PARK, D. W. & quantification using sonic velocity data. This WARDROPER, A. M. K. 1984. Organic Geochem- volume. ical study of the Upper Kimmeridge Clay of the LORSONG, J. A. & ATKINSON, C. D. 1995. 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Upper Cretac- & ALLEN, P. A. 1991. Major erosion surfaces in eous tectonic phases and end Cretaceous inver- the basal Wealden Beds, Lower Cretaceous, south sion in the Chalk of the Anglo-Paris Basin. Dorset. Journal of the Geological Society, London, Proceedings of the Geologists' Association, 108, 148, 105-113. 231-255. --- & JENKYNS, H. C. 1995. A comparison of the MOTTRAM, B. H. & HOUSE, M. R. 1956. The Structure to Bajocian successions of Dorset and of the Northern Margin of the Poxwell Peficline. Yorkshire. In: TAYLOR, P. D. (ed.) Field Geol- Proceedings' of the Dorset Natural History and ogy of the British Jurassic. Geological Society, Archaeological Society, 76, 129-135. London, 105 150. PARFITT, M. & FARRIMOND, P. 1998. The Mupe Bay HOGG, A. J. C., MITCHELL, A. W. & YOUNG, S. 1996. Oil Seep: a detailed organic geochemical study of Predicting well productivity from grain size a controversial outcrop. This volume. analysis and Logging While Drilling. Petroleum PARTINGTON, M. 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18 J. R. UNDERHILL & R. STONELEY

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