The Wessex Basin is a petroleum system composed of post-Variscan sedimentary deposits and intra-basinal highs, in the South of England (Fig. 1) (Underhill and Stoneley, 1998). The basin system represents a series of extensional sub-basins, associated with Mesozoic intracratonic basin networks through North West Europe (Ziegler, 1990). Effects of Cenozoic intraplate contraction and structural inversion associated with basin bounding faults are also displayed within the Wessex Basin system (Underhill and Stoneley, 1998). The South Western basinal limits are the Armorcian and Cornubian Massifs, the London Brabant massif limiting the basin to the North, with the Central Channel High bounding the southern extent of the Wessex Basin. Hydrocarbon reserves from this onshore basin can be associated with the Permian-Lower Cretaceous Megasequnce containing strata of sandstones, mudstones and limestones (Buchanan, 1998).
1.2 Petroleum systems
A petroleum system incorporates all geological evolutional processes that allow for the accumulation of natural gases and crude oil; source rock, maturity, migration pathways, reservoir rocks, cap rock and trap structures. Petroleum defines a compound that includes high concentrations of natural bitumen’s, crude oils, condensates and biological hydrocarbon gas found in reservoirs (Magoon, 1991). Whilst a system is associated with the interdependent elements that form the functional unit to generate hydrocarbon accumulates (Magoon, 1991). For these compounds to be generated a specific environment must occur associated with heat and pressure and interaction with surrounding strata (Veress and Szigethy, 2009).
2. Structural Geology & Evolution
The Wessex Basin (Fig. 2) was initiated in the late Carboniferous – early Permian during the climax of the Variscan Orogeny. Wessex Basin deposition occurred during Permo-Triassic rifting, the extensional movement continued throughout the Mesozoic (Zanella, Cobbold and Boassen, 2015). The basin comprises of four sub-basins which share characteristic geometries and stratigraphic relationships. The Channel Basin, Vale of Pewsey Basin and the Weald Basin developed as a series of half-grabens whilst the Winterborne Kingston Trough developed as a symmetric graben system.
4.0 Petroleum System Components
4.1 Source Rocks
Wessex Basin source rocks are confided to the Jurassic Period (199.6 Mya – 145.5 Mya BGS, 2011). Three intervals within the Jurassic package record sufficient total organic contents (TOC). These units are the source of all hydrocarbons with the Wessex Basin, namely the Kimmeridge Clays, Oxford Clay (Upper Callovian) and the Liassic Mudstones (E. J. Ebukanson and R. R. F. Kingh, 1986). The Mudstones of the Liassic Group (Blue Lias) are interbedded with very fine-grained, pale limestones, TOC of 8%. The dominate source of Kerogen with is Type ll hydrogen-rich algal material. The presence of the organic facies, Kerogen Type ll indicates a high biological productivity and restricted oxygenation. Furthermore, Liassic Mudstones contain minor quantities of Type lll Kerogen indicating sufficient hydrogen levels have been produced to be gas generative. Composed of vitrinite and has insufficient hydrogen to be oil prone (Schidlowski, 1981). Rock units from the middle to upper Jurassic, Oxford Clays and the Kimmeridge Clays, display source rock properties. Predominantly the interbedded black anoxic shales of the Kimmeridge Clays. These shales are interbedded with fine grained dolomitic limestones. The Kimmeridge Clay TOC averages at 20% but has been recorded at 70% in parts of the Kimmeridge Oil shales (Farrimond et al., 1984). Comparable to the Liassic Mudstones, the Kimmeridge Clay’s organic facies is Kerogen Type ll.
Outcrop/ Formation Location Toc %
Blue Lias (1) Lyme Regis 8.14
Kimmeridge Clay (2) Kimmeridge Bay 3.22
Kimmeridge Clay (3) Clavell’s Hard 10.98
Gault Clay (4) Lulworth Cove beach 0.59
Nothe Clay (5) Osmington Mills 1.72
Oxford Clay (6) Chickerell, Weymouth 2.7
Oxford Clay (7) East Fleet Beach, Weymouth 8.11
Purbeck Black Shale (8) Durdle Door Beach 5.36
Figure 5, a modified line graph of Akande, 2012, displaying two rock units with higher TOC (%), Kimmeridge Clay and Oxford Clay. Two potential source rocks for the Wessex petroleum basin.
4.2 Reservoir Rocks
The Wessex Basin consists of two facies of reservoir rocks. The major type, siliclastic units whilst some minor facies containing Carbonates. Although Permian Aeolian sands of East Devon indicate porosities up to 40%, they are not reservoir potentials due to the lack of subsurface coverage in the East or do not lie beneath key basin structures.
Lower Triassic Sandstones reach optimum development within the Sherwood Sandstone Group, a succession with a thickness of 100m-300m (McKie, Aggett and Hogg, 1998). The Sherwood Sandstones (Fig. 8) are extensively exposed in Otterton and Sidmouth where outcrops display continuous red sandstone interbedded with mudstone lenses (Buchanan, 1998). Analysis of the Sherwood suggest a braided alluvial depositional environment and perennial sheetflow sandstones with mudstone injections from floodplain deposits (McKie, Aggett and Hogg, 1998). At Wytch Farm interpretations suggest a mixed lacustrine and fluvial environment and Sherwood Sandstones, 150m thick, at Wytch Farm form the main reservoir rock. Accurate porosity analysis has been undertaken in order to determine the quality of the reservoir depending on porosity characteristics and quantitative data. jPOR: ImageJ was used to acquire this data.
The stratigraphically lowest potential reservoir rock in the Lower Jurassic is the Thorncombe Sands which lie in the middle if the Lias Group. This unit comprises very fine-grained sandstones, at a thickness of 23m (BGS, 2011), with a maximum porosity of 10% and permeability ranging between 25 and 30 mD (millidarcy). The Thorncombe Sands are oil bearing in the Wytch Farm area. A unit with the highest reservoir potential lies at the top of the Lias Group, the Bridport Sands (Fig. 10).
The Bridport Sands (25-100m thick) are uniformly very fine up to medium grained sands with a permiability ranging from 10% to plus 30%, interbedded by well cemneted units, these interbeds act as barriers to the hydrocarbons whilst the siliclastic sediments act as a the upper resevoir rock in the Wytch Oil field (Buchanan, 1998). The Bridport Sands is a diachronous unit with deposition controlled by faulting, deposited in a shallow marine setting (Underhill and Stoneley, 1998). Overlaying the Bridport Sands is the Inferior Oolite, a thin discontinuous belt of limestone with a porosity ranging from 12% to 34%. Intense fracturing of the Inferior Oolite and high porosity enabled oil extraction from within this unit at the Wytch Farm Field (Mudge, 1978).
Minor reservoirs have been located within the middle Jurassic, such as the Forest Marble and the Cornbrash. These units contain little natural primary porosity but stages of fracturing have allowed minor extraction of oil in the Kimmeridge oilfields (Evans, Jenkins and Gluyas, 1998). The Upper Jurassic exposed siliciclastic-carbonate sequence of the Corallian Group despite being extensively impregnated with hydrocarbons; no current sub-surface reservoirs have been located in the Wessex Basin. The remaining units ranging from the Portland Limestones in the Jurassic to the Wealden Group in the Cretaceous do not have any significant reservoir potential due to a lack of natural porosity and low permeability from a more Chalk facies (Underhill and Stoneley, 1998).
4.2.1 Petrographic Analysis: Sherwood Sandstone Group
SAMPLE MINERALS AND % OVERALL POROSITY % POROSITY AND % PORE TYPE
-Plagioclase Feldspar (12%)
-Accessory Minerals – lithics, clay crud (3%) -Estimated: 10%
-Actual: 10.92% -Intergranular- 80%
-Intragranular – 15%
-Mouldic – 5% Mesopore
4.2.2 Petrographic Analysis: Bridport Sands
Sample Minerals and % Overall Porosity % Porosity and % Pore Type
Bridport Sandstone – B11 (low) -Quartz (79%)
-Plagioclase and Orthoclase Feldspars (evidence of striped and tartan twinning) (15%),
-Muscovite (low concentrations – around 5%)
-Accessory Minerals – clay crud, lithics, and skeletal grains (less than 1%) -Estimated: 15%
-Intragranular- 5% Micropores to Mesopore
Bridport Sandstone – B2 (high) -Quartz (80%)
-Plagioclase Feldspars (evidence of striped twinning) (10%),
-Muscovite (low concentrations – ~ 5%)
-Accessory Minerals – clay crud, lithics, and skeletal grains (5%) -Estimated: 30%
-Actual: 26.8% -Intergranular- 95%
-Intragranular- 5% Micropores to Mesopores
4.3 Cap/Seal rocks
Mudstone groups overlay many of the potential reservoir rocks within the Wessex Basin and act as a lateral top seal. Sandstones of the upper Permian are overlain of muds of the reddish- brown Aylesbeare Group. The undivided (~500m thick) mudstone to clayey siltstone form the low permeability characteristics required to cap a reservoir rock.
Sherwood Sandstone Group, one of the most efficient reservoir rocks, is capped by red silty mudstones with subordinates of halite – bearing units of the Upper Triassic Mercia Mudstone Group. Large accumulations proximal to the Wytch Oilfield (100m – 600+m) (Buchanan, 1998).
The Bridport Sands reservoir rocks and Inferior Oolite are overlain laterally by bentonitic clays of the Bathonian Fuller’s Earth (max 300m) (Buchanan, 1998).
4.4 Traps and Migration
A petroleum trap forms when buoyant migration of hydrocarbons through a permeable rock, such as the Sherwood Sandstones, cannot overcome the capillary forces of a sealing medium (Gluyas and Swarbrick, 2013). The trapping over petroleum reservoirs can be sub dived, structural traps, associated with faults, folds and dome formations. Stratigraphic traps, associated with texture, lithology and porosity. Finally, hydrodynamic traps created via water pressure fluctuation.
The Wessex Basin has developed two main trap types, both associated with structural reservoir traps (Gluyas and Swarbrick, 2013). The most efficient trap is the tilted extensional fault block or horst in the footwall of basin bounding faults (Buchanan, 1998). Substantial extentional dispalacement proximal to the the Wytch Farm oilfields resulted in the juxtaposition of the source rock or resevoir rocks creating a lateral migration pathway into the structualy trap (Underhill and Stoneley, 1998) (Fig. 13).
The second structural petroleum traps were produced during Tertiary compressional events associated to the Alpine Orogeny (Buchanan, 1998). This tectonic event produced periclinal closures, limited in length but correspond to singular fault segments in the Kimmeridge region (Fig. 14 A). Although these structural plays were modified during uplift in the Tertiary inversion events, Stoneley (1982) debates that hanging wall roll-overs were formed during extension in the late Jurassic – early Cretaceous. Timing of migration and formation of the trap controls how effective a pertoleum plays are, the periclinal closures may have formed post – generation or migration, the hanging wall has been uplifted 1.5Km during the compression (Bray, Duddy and Green, 1998). Associated with the structual traps, the Wessex Basin aslo diplays salt dome tectonics, where salt domes and swells can be attritubed to detachment between the Sherwood Sandstone and Jurassic units. The source of salt is from the evaporite units within Meria Mudstone Group (Buchanan, 1998).
Seismic analsysis (Fig. 14) of the structual traps provide evidence that the extenstional fault blocks juxstaposed source and resevoir rocks at the Wytch and Sourthward Quarry area with migration pathways forming along the normal faults. In contrast, the percilinal closures may have been breached during inversion allowing migartion to the surface (Fig. 14 & 15). However, evidence suggest inversion post-dates migration as uplift switches off the hydrocrabonn kitchen.
Moreover, Mudstones of Mesozoic have been analysed by Zanella et al. (2015) identifying bedding-parallel veins of homogeneous fibrous calcite (‘beef’). Hydrocarbons are within the calcite veins, indicating liquid hydrocarbons were present during crystallization of ‘beef’ veins, thus synchronous with the migration into extensional tilted fault blocks (Zanella, Cobbold and Boassen, 2015).
Fm Wytch (ft) Wytch (m) TWTT (s)
Oxford Clay -2041 -622.1 0.520
Cornbrash -2406 -733.3 0.600
Inferior Oolite -2971 -905.6 0.690
Penarth Grp -3951 -1204.3 0.828
Sherwood -5119 -1560.3 1.050
The generation of mature hydrocarbons within a source rock depend on the temperature and burial depth. In turn with deposition within anoxic, extensional environments. Basin models have been utilised to indicate that the Blue Lias, a source rock, has reached optimum oil generation conditions towards the South of the Wessex Basin. Furthermore, studies by Brooks, (1983) propose the Oxford Clay had also reached the maturation window. A subsidence model (Fig. 16) can be analysed to determine the maturation window and economic evaluations using borehole data.
4.6.1 Subsidence Model
Borehole well data at Southard Quarry (Appendix 8) has been plotted to establish the absolute ages of the Wessex stratigraphy at varying depths. Geothermal figures from Underhill and Stoneley, (1998) were used with a surface temperature at 10oC, this heat flow model indicates sub-surface temperature increase by increments of 25oC every 1000m.
Borehole data, combined with a heat flow model are then compared to a simplified diagram illustrating hydrocarbon generative phases versus increasing burial depth and temperature (Bjørlykke 1989) (Fig. 17). The Southard Quarry data has been overlain on Bjørlykke’s model, where the Blue Lias is situated within the maturation window, below the 60oC isotherm.
Blue Lias Thickness Source
100m Underhill and Stoneley, (1998)
87m Bushey Farm Data
102m Buchanan, (1998)
The generation of the oil from the Portland-Wight ‘kitchen area’ is associated with the extensional graben system which buries the source rocks (Underhill and Stoneley, 1998). The mature hydrocarbons migrate, backfilling into tilted reservoir rocks within the footwall of normal faults. However, oil generation depends on the thickness of the source rock. Fig 17 B shows the range of published data. A figure of 100m for the Blue Lias was plotted on the subsidence graph. Borehole data suggests the Blue Lias was buried, at a depth greater than 2000m for 61Ma, ranging from the Early Cretaceous to Late Cretaceous, compression during peak migration sealed the hydrocarbon accumulates (Underhill and Stoneley, 1998).
Oil generation temporarily ceases towards to West of the Wessex Basin due to tilting of the Albian-Aptian (Underhill and Stoneley, 1998). Tertiary inversion, the reactivation of the Purbeck-Isle of Wight fault system uplifted the Blue Lias above the oil generating window.
4.6.2 Economic Evaluation
The estimated economic value of the crude oil at the Southard Quarry is $27,810,474,053.92. The evaluation was created using the Kim and Sanderson, (2005) fault displacement model (Fig. 18). The Wessex Basin Blue Lias has a displacement ratio of 25:1. The levels of recoverable accumulates depend on the volume of loss and the TOC% of the buried source rock. The Wytch Farm output of oil in 1996 was 57.3 million tonnes (Evans, Gunn and Bloodworth, 2011), estimated recoverable reserves are 70 million barrels of oil (BGS, 2011).
1) The Wessex Basin was formed through extension graben systems with sedimentary infilling from Permian-Oligocene, unconformity-bound megasequences express three evolutionary phases.
2) The Permian-Lower Cretaceous megasequnce contains sandstone reservoir rocks, the Bridport and the Sherwood, in addition to potential source rocks such as the Blue Lias, Oxford Clay and Kimmeridge Clay, these are subsequently capped by mudstones and the Inferior Oolite Group.
3) Subsidence History Modelling of Southard Quarry indicates the Blue Lias was buried to depth of 2.3Km, and within the maturation window for 61Ma, correlating with burial analysis by Stoneley and Buchanan.
4) Migration occurred along extensional fault systems from the early-Cretaceous and oil backfilled into reservoir rocks within the footwall of the extensional tilted fault blocks.
5) Late Cretaceous and Tertiary compressional events forced normal extensional faults to be reactivated creating periclinal closures towards the South of the basin.
6) Inversion proximal to the Purbeck- Isle of Wight fault system post-dates migration an uplift switches off the hydrocrabon kitchen in the Portland-Wight sub-basin.
7) Borehole analysis has allowed accurate economic calculations using fault displacement models to indicate the Wessex Basin has of $27.8 Billion of recoverable oil proximal to the Wytch Farm, correlating with studies from the BGS and confirming it as being one of the largest onshore oilfields in Europe.
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