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Marine and Petroleum Geology

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Research paper

Mapping hydrocarbon charge-points in the Wessex Basin using seismic,geochemistry and mineral magneticsR. Abubakara,b,∗, A.R. Muxworthya, A. Frasera, M.A. Sephtona, J.S. Watsona, D. Heslopc,G.A. Patersond,e, P. Southernf

a Department of Earth Science and Engineering, Imperial College London, London, UKb Department of Geology, Ahmadu Bello University, Zaria, Nigeriac Research School of Earth Sciences, Australian National University, Canberra, 2601, Australiad Department of Earth, Ocean and Ecological Sciences, University of Liverpool, Liverpool, UKe Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, Chinaf Department of Medical Physics and Biomedical Engineering, University College London, London, UK

A R T I C L E I N F O

Keywords:MagneticsGeochemistrySeismicPetroleum migration

A B S T R A C T

This study reports a multidisciplinary approach to determining hydrocarbon charge-points and migration in theWessex Basin, southern England. Geochemical analysis of reservoir core material (Bridport Sandstone andInferior Oolite) using gas chromatography-mass spectrometry (GC-MS), suggests that the oil in the Wessex Basinis from a single source, and that small variations in environmentally sensitive biomarkers are likely due to smalldifferences in maturity or depositional conditions during the formation of the oil over millions of years. Usingseismic data, basin modelling revealed two potential hydrocarbon migration pathways from the hanging wall ofthe Purbeck fault into the Sherwood Sandstone reservoir at Wytch Farm. One of these potential pathways isrepresented by cores termed Creech and the other Bushey Farm. To try to distinguish between the two potentialpathways, cores were studied using mineral magnetic techniques. The magnetic signature was characterisedusing low-temperature (< 50 K) magnetic measurements; this is because much of the magnetic signature wasdominated by nanoparticles < 30 nm, which are thermally activated at room temperature and magnetically“transparent”. Wells that contained considerable amounts of hydrocarbons were dominated by nanometricmagnetite (< 30 nm). Such particles are small enough to migrate with the oil, through pore spaces, which are ofthe order ~100 nm. Wells located at the fringes of large hydrocarbon accumulation had enhanced pyrrhotite-dominated magnetic signals. Of the two potential migration pathways, the mineral magnetic results suggest thatthe oil migrated through Creech rather than through Bushey Farm.

1. Introduction

The Wessex Basin located in Dorset, southern England (Fig. 1), wasformed during the Late Permian and evolved throughout subsequentextensional and compressional tectonic events in the Mesozoic andTertiary (e.g., Butler, 1998; Hawkes et al., 1998; Underhill andStoneley, 1998). The Wessex Basin is an established hydrocarbon pro-vince (Underhill and Stoneley, 1998).

One of the key unresolved questions regarding the petroleum systemof the Wessex Basin is the presence of over one billion barrels in theSherwood Sandstone reservoir at Wytch Farm. The SherwoodSandstone reservoir sits on the footwall of the Purbeck fault in an areathat has been shown by geochemical and basin modelling to be tooimmature for oil generation in the Liassic source interval (Fig. 2). The

apparent inconsistency is derived from the reorganisation of the basinduring Tertiary inversion and an associated complex hydrocarbon mi-gration history (Hawkes et al., 1998).

To resolve these complexities, this study presents burial historyreconstructions developed using PetroMod™ and incorporating welldata and Rock-Eval pyrolysis data from selected wells from the WessexBasin. Geochemical and mineral magnetic analyses on drill-core ex-tracts from nine wells in the Wessex Basin (Fig. 1) are used to com-plement these reconstructions. While geochemical analysis is not un-common, the use of mineral magnetic proxies to help map hydrocarbonmigration is novel. Mineral magnetic techniques have an advantageover geochemical ones, as they are relatively fast and inexpensive, al-lowing us to study many samples. They can, however, suffer fromambiguity in interpretation.

https://doi.org/10.1016/j.marpetgeo.2019.08.042Received 13 September 2018; Received in revised form 19 August 2019; Accepted 21 August 2019

∗ Corresponding author. Department of Earth Science and Engineering, Imperial College London, London, UK.E-mail address: [email protected] (R. Abubakar).

Marine and Petroleum Geology 111 (2020) 510–528

Available online 27 August 20190264-8172/ Crown Copyright © 2019 Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).

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There are known links between hydrocarbon presence and themagnetic minerals found in drill-cores (Machel, 1995; Aldana et al.,2003; Emmerton et al., 2012; Zhang et al., 2018). These magnetic mi-nerals are usually iron oxides and iron sulphides; with speciation sen-sitive to the formation conditions (Elmore et al., 2012). Direct labora-tory pyrolysis of immature source rock, coincidentally from the WessexBasin, has been shown to generate significant quantities of nanometric(< 100 nm) magnetic particles that are easily identifiable (Abubakaret al., 2015). Whilst the size of the particles might be slightly larger innature than those produced on laboratory timescales, the generalfinding holds: the conditions that generate hydrocarbons also generateferromagnetic minerals. However, this apparently simple relationshipbetween ferromagnetic mineral abundance and hydrocarbon presenceis complicated by a number of mechanisms: (1) In the laboratory pyr-olysis, both iron oxides and iron sulphides were produced, the exactspeciation depending on temperature (Abubakar et al., 2015). Conse-quently, the magnetic signature is highly dependent on the mineralogy;therefore, simply measuring the total magnetisation without identifyingthe mineralogy can be problematic. (2) Host rocks can themselves behighly magnetic, making the isolation of the hydrocarbon signal fromthe background signal challenging (McCabe and Elmore, 1989;Emmerton et al., 2013a). (3) Hydrocarbon presence can change ther-modynamic equilibrium states, leading to alteration of the host rock'smagnetic mineralogy, which, depending on the exact temperature,pressure and redox potential, can lead to both enhancement or reduc-tion in the total magnetisation (Burton et al., 1993; Machel, 1995;Emmerton et al., 2013a). (4) Microbial biodegradation of oil can alsocause both dissolution and precipitation of magnetic minerals (Machel,1995; Emmerton et al., 2013a), though dissolution is thought to begenerally more common. These factors clearly can cause problems ininterpreting magnetic mineral signatures, however, with careful un-derstanding it can be used to constrain hydrocarbon-related processes(e.g., Emmerton et al., 2013b).

In this study, the mineral magnetic signal of Wessex Basin core ischaracterised and compared to geochemical analysis on the same core.To characterise the magnetic mineralogy a suite of mineral magnetictechniques were employed including high- and low-temperature mag-netometry to identify magnetic mineralogy and grain size distributionswithin the samples, and magnetic hysteresis measurements to further

constrain magnetic grain size. The samples were geochemically studiedusing gas chromatography-mass spectrometry. These data were com-bined and the mineral magnetic signal attributed to various stages ofhydrocarbon migration. Subsequently, the mineral and geochemicalanalysis were used to constrain basin models of the Wessex Basin.

2. Geology of the Wessex Basin

The Wessex Basin has two recognised primary hydrocarbon plays:the Triassic Sherwood Sandstone Group and the Jurassic BridportSandstone Formation (Hawkes et al., 1998). The Sherwood Sandstonewas deposited in the Early Triassic (Fig. 2) and it consists mainly ofcontinuous arkosic sandstone bodies with minor amounts of mudstone.Facies present include continental braided alluvial deposits with aeo-lian interbeds (Underhill and Stoneley, 1998). Exposures around Bu-dleigh Salterton, Otterton and Sidmouth in Devon (Fig. 1) are estimatedto be around 120 m thick (Lorsong and Atkinson, 1995). The SherwoodSandstone forms an excellent reservoir at the Wytch Farm oil field,especially the alluvial deposits, which have very little mud/calcareoushorizons (Bowman et al., 1993), but the unit has poor reservoir quali-ties elsewhere in the basin, because of the presence of interbeddedmuds, poorly sorted conglomerates and widespread calcretes and as-sociated carbonate cementation (Buchanan, 1998). The SherwoodSandstone is overlain and sealed by the Murcia Mudstone Group(Fig. 2), which is a thick unit (100–600 m) of playa mudstones andevaporites, extensively developed throughout the Wessex Basin andprovides an excellent topseal at Wytch Farm (Lott et al., 1982).

The Bridport Sandstone consists of clean, very fine to fine grainedshallow marine sands of about ~70 m thickness, deposited during theEarly Jurassic (Fig. 2) (Buchanan, 1998; Underhill and Stoneley, 1998).It forms a diachronous unit and its deposition is structurally controlledin some areas within the Wessex Basin by the pre-existing extensionalfault system, with main depocenters located south of the Purbeck Dis-turbance (Jenkyns and Senior, 1991; Hawkes et al., 1998). Buchanan(1998) reported good reservoir properties for the Bridport Sandstone inthe western part of the basin, where the unit is mainly composed ofsiliciclastic sediments, whereas in the eastern part of the Purbeck-Isle ofWight Basin (PWB), the unit contains abundant calcite cement andmudstones (Bjorkum and Walderhaug, 1993). Overall, the Bridport

Fig. 1. Map of Wessex Basin, England, with the locations of the nine sampled wells.

R. Abubakar, et al. Marine and Petroleum Geology 111 (2020) 510–528

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Sandstone is generally less permeable than the Sherwood SandstoneGroup, due to the presence of cemented layers even within the silici-clastic units, which limits the flow of hydrocarbons (Bjorkum andWalderhaug, 1993).

There are three potential source rocks for the hydrocarbons gener-ated in the Wessex Basin: the Liassic Mudstone Group (Lower Jurassic),the Oxford Clay Formation (Middle Jurassic) and the Kimmeridge ClayFormation (Upper Jurassic) (Underhill and Stoneley, 1998), though, theLiassic Mudstone Group is generally thought to be the main source ofthe hydrocarbon accumulations because of its stratigraphic position(e.g., Ebukanson and Kinghorn, 1986b; Cornford et al., 1988). TheSherwood Sandstone Group lies stratigraphically below the LiassicMudrock Group (Fig. 2) (Ebukanson and Kinghorn, 1986b; Butler,1998; Hawkes et al., 1998). The main depocenter in the Wessex Basin islocated to the south of the Purbeck Disturbance, where the Lias shalesare thought to be mature and have generated most of the oil found inthe Wessex Basin (Ebukanson and Kinghorn, 1986a; Underhill and

Stoneley, 1998).The generation of hydrocarbons in the Wessex and other adjacent

basins in southern England was halted by inversion events that tookplace at various intervals (Hawkes et al., 1998). Hawkes et al. (1998)argued that the main axis of the inversion lies south of the PurbeckDisturbance (Fig. 1) in the Portland-South Wight Basin, but that theinversion was not restricted to this area. The Late Cretaceous-Tertiaryinversion of the pre-existing east-west trending extensional faults hasbeen identified throughout the Wessex Basin (Drummond, 1970; Law,1998; Underhill and Paterson, 1998), and there is evidence that theinversion may have started as early as the Campanian (Drummond,1970; Westhead and Woods, 1994). Plint (1982) reported significantuplift and erosion in the basin during the Eocene, but the most ex-tensive inversion movement is thought to have occurred during theMiocene (Butler and Pullan, 1990; Chadwick, 1993). It is generallythought that maturation and generation of hydrocarbons occurred onthe hanging wall of the Abbotsbury-Ridgeway and the Purbeck-Isle of

Fig. 2. Generalized stratigraphic column for the Wessex Basin showing the three mega-sequences (JB = Junction Bed; CB= Cinder Bed; GAB = Green AmmoniteBeds; PG= Penarth Group; BSPB= Budleigh Salterton Pebble Beds). Modified from Underhill and Stoneley (1998).


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Wight fault systems during the Late Cretaceous maximum burial priorto the Tertiary inversion (Hawkes et al., 1998). The generated petro-leum is thought to have migrated northwards into the footwall, eithervertically along fault planes (e.g., Ebukanson and Kinghorn, 1986a;Penn et al., 1986) or laterally across the fault plane where sand-sandjuxtaposition occurred (Hawkes et al., 1998). However, others haveargued that there was some maturation of the Lias mudstones at WytchFarm, even though that might not have resulted in any significantgeneration of hydrocarbons (Bray et al., 1998; McMahon and Turner,1998).

In order to estimate the amount of shortening and uplift caused bythe inversion, several theories have been proposed: (1) Based on in-terval velocity mapping, Butler (1998) suggested there was about900 m of shortening across the Portland – South Wight Trough, due tocompression, and up to 1300 m of reverse displacement along thePurbeck – Isle of Wight Disturbance. Butler (1998) argued that footwallshort-cut thrusts reported by Hawkes et al. (1998), could be have beenovercome by a strike-slip component of inversion. (2) Using seismicvelocities, Law (1998) estimated about 1200 m of missing section atKimmeridge 5 (Fig. 1), whilst (3) Beeley and Norton (1998), usingseismic velocity forward models argued for about 1500 m of missingsection for the along the Central Channel Anticline. In contrast, (4) Bray

et al. (1998) carried out burial reconstruction models and estimated2–3 km of missing section around the Kimmeridge area and areasaround Wytch Farm. They argued the Lias mudstones entered the oilgeneration window briefly at Wytch Farm during the Late Jurassic/Early Cretaceous, but did not generate a significant amount of oil beforecooling on uplift.

3. Methods and sampling

3.1. Seismic

One of the aims of this study was to produce an uplift map thatwould resolve hydrocarbon generation and migration in the WessexBasin. To achieve this, present day maps of the Top Chalk, InferiorOolite and White Lias (Rheatic) and the base of the Wealden Groupwere determined using 63 2D seismic lines.

The interpretation of the 2D seismic lines was carried out using welldata and depth and two-way travel time pairs obtained from the UKOnshore Geophysical Library (UKOGL) database (https://www.ukogl.org.uk). A composite of seismic sections (sections B90-32 and B90-37)were depth converted and decompacted prior to the Albian-Aptianerosional event using Midland Valley Move 2015™ software. Present

Table 1Core samples analysed during this study, showing sample depths, formation and lithology. The relative locations of the wells is shown in Fig. 1.

Sample Name Sample code Depth (ft.) Formation Lithology

Bushey Fm SSK27620 BS1 3464 Bridport Sandstone Fm SandstoneBushey Fm SSK27621 BS2 3468 Bridport Sandstone Fm SandstoneBushey Fm SSK27622 BS3 3477.3 Bridport Sandstone Fm SandstoneBushey Fm SSK27623 BS4 3485.1 Bridport Sandstone Fm SandstoneBushey Fm SSK27624 BS5 3486 Bridport Sandstone Fm SandstoneBushey Fm SSK27625 BS6 3504 Bridport Sandstone Fm SandstoneBushey Fm SSK27626 BS7 3504.8 Bridport Sandstone Fm SandstoneBushey Fm SSK27627 BS8 3518.1 Bridport Sandstone Fm SandstoneBushey Fm SSK27628 BS9 3524 Bridport Sandstone Fm SandstoneBushey Fm SSK27629 BS10 3526 Bridport Sandstone Fm SandstoneCoombe K. SSK35431 COOMB1 3443 Bridport Sandstone Fm SandstoneCoombe K. SSK35432 COOMB2 3475 Bridport Sandstone Fm SandstoneCoombe K. SSK36413 COOMB3 3498.75 Bridport Sandstone Fm SandstoneCoombe K. SSK36414 COOMB4 3522.9 Bridport Sandstone Fm SandstoneMartinstown 1 SSK35425 MT1 1514.6 Bridport Sandstone Fm SandstoneMartinstown 1 SSK35426 MT2 1534.12 Bridport Sandstone Fm SandstoneMartinstown 1 SSK35427 MT3 1516.83 Bridport Sandstone Fm SandstoneMartinstown 1 SSK35428 MT4 1534.12 Bridport Sandstone Fm SandstoneStoborough 2 SSK36416 ST1 3536.75 Bridport Sandstone Fm SandstoneStoborough 2 SSK36417 ST2 3561.3 Bridport Sandstone Fm SandstoneStoborough 2 SSK36418 ST3 3562 Bridport Sandstone Fm SandstoneStoborough 2 SSK36419 ST4 3569.23 Bridport Sandstone Fm SandstoneWaddock Cross SSK36415 WX1 2197.1 Bridport Sandstone Fm SandstoneWaddock Cross SSK35430 WX2 2203 Bridport Sandstone Fm SandstoneWaddock Cross SSK36415 WX3 2285 Bridport Sandstone Fm SandstoneChickerell SSK27638 CH1 860.89 Bridport Sandstone Fm SandstoneChickerell SSK27639 CH2 863.85 Bridport Sandstone Fm SandstoneChickerell SSK27640 CH3 874.84 Bridport Sandstone Fm SandstoneChickerell SSK27641 CH4 876.64 Bridport Sandstone Fm SandstoneChickerell SSK27642 CH5 879.76 Bridport Sandstone Fm SandstoneChickerell SSK27648 CH6 900.26 Bridport Sandstone Fm SandstoneChickerell SSK27649 CH7 905.2 Bridport Sandstone Fm SandstoneCreech SSK27629 CR1 3451.4 Inferior Oolite Fm LimestoneCreech SSK27630 CR2 3456 Inferior Oolite Fm LimestoneCreech SSK27631 CR3 3452.76 Inferior Oolite Fm LimestoneCreech SSK27632 CR4 3456.04 Bridport Sandstone Fm SandstoneCreech SSK27633 CR5 3456.92 Bridport Sandstone Fm SandstoneCreech SSK27634 CR6 3463.75 Bridport Sandstone Fm SandstoneCreech SSK27635 CR7 3470 Bridport Sandstone Fm SandstoneKimmeridge SSK27636 KM1 2828.08 Bridport Sandstone Fm SandstoneKimmeridge SSK27637 KM2 2854.66 Bridport Sandstone Fm SandstoneKimmeridge SSK27643 KM3 2861.71 Bridport Sandstone Fm SandstoneKimmeridge SSK27644 KM4 2870.57 Bridport Sandstone Fm SandstoneWytch Fm SSK27645 WC1 2996 Bridport Sandstone Fm SandstoneWytch Fm SSK27646 WC2 2997.25 Bridport Sandstone Fm SandstoneWytch Fm SSK27647 WC3 2998.15 Bridport Sandstone Fm Sandstone


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https://www.ukogl.org.uk

https://www.ukogl.org.uk

day to top Inferior Oolite and White Lias surfaces were generated in thetime domain and were converted to depth using calibrated time/depthpairs from over a hundred wells obtained from the UKOGL website.Similarly, for rock properties such as composition, porosity, velocity,grain size and density, well logs were acquired from IHS Global for thenine wells and the velocities calculated from the well top data.

3.2. Core analysis

Cores from nine wells stored at the British Geological Survey core-store at Keyworth, England, were sampled: Martinstown (MT),Chickerell (CH), Waddock Cross (WX), Coombe Keynes (Coomb),Stoborough (ST), Creech (CR), Kimmeridge (KM), Wytch Farm (WC)and Bushey Farm (BS) (Fig. 1). The sampling was restricted to BridportSandstone and the Inferior Oolite formations of the Upper Lias (Fig. 2),which represent the reservoir and cap rocks respectively (Hawkes et al.,1998).

3.2.1. GeochemistryForty-five samples were collected in total for geochemical char-

acterisation (Table 1). These samples were powdered with a mortar andpestle and then solvent extracted (ca. 1.5 g). Internal standards (squa-lane and p-terphenyl) were added to each sample prior to extraction.The powdered samples were placed in test tubes and 4 ml of di-chloromethane (DCM): methanol (DCM: MeOH; 93:7 v/v) added. Themixtures were then sonicated for 3 min, centrifuged, and the super-natant removed; the extraction process was repeated three times. Afterextraction, activated copper turnings were added to each sample toremove elemental sulphur. The concentrated extracts were fractionatedby column chromatography using activated alumina powder in whichthe total hydrocarbon fraction was eluted using hexane with DCM-hexane ratio of 1:1 (v/v).

The hydrocarbon fraction was concentrated (ca. 0.5 ml) using drynitrogen prior to analysis by gas chromatography-mass spectrometry(GC-MS). GC-MS analysis was carried out on aliphatic and aromaticfractions of the samples using an Agilent Technologies 7890. A gaschromatograph (GC) coupled to a 5975C mass spectrometer. The GC

injector was operated in a splitless mode (1 μl) with a column flow rateof 1.1 ml min−1. AJ &W Scientific DB-5MSUI capillary column (30 m,250 μm i.d., 0.25 μm film thickness) was used for separation and heliumemployed as a carrier gas. The GC oven temperature was initially heldat 40 °C for 2 min, and then raised to a 310 °C at a rate of 5 °C min−1

where it was held for 14 min. Mass spectra were acquired in electronimpact mode (70 eV) in the scan range of 50–500 amu and also in se-lective ion monitoring (mass-to-charge ratios, m/z, of 177, 191, 205,217, 218).

3.2.2. MagneticsIn the core store, an in-house low-field susceptibility logger was

used for initial core selection, measuring the magnetic susceptibility at1 cm spacing. The rocks generally had weak magnetic signals; therefore,sections with a reasonably high magnetic susceptibility were sampledfor further analysis. Not all sampled wells were measured for magneticsusceptibility: Chickerell (CH) and Kimmeridge (KM) were not.

Low-temperature magnetic measurements of both the potentialsource rocks and the well core samples were measured using QuantumDesign Magnetic Property Measurement Systems (MPMS) at three lo-calities: (1) an MPMS-VSM at the Royal Institute of Great Britain, (2) anMPMS2 at the Institute of Geology and Geophysics, Chinese Academy ofScience, and (3) an MPMS2 at the Australian National University. Low-temperature measurements allow for the identification of nanoparticles(< 30 nm) that are magnetically ‘transparent’ at room-temperature dueto thermal instability, and the identification of mineral phases throughwell known-transitions. Two types of low-temperature measurementswere carried out: zero-field cooled/field cooled (ZFC/FC) measure-ments and hysteresis measurements. The zero-field cooled (ZFC) mea-surements were carried out with the samples cooled down from roomtemperature (RT) to 5 K in zero magnetic field. At 5 K, the samples wereinduced with a saturation isothermal remanent magnetisation (SIRM) at7 T and the magnetic moment of the samples measured duringwarming. The field cooled (FC) measurements, were similar except thesamples were initially cooled in a 7 T field. RT-SIRM experiments werealso carried out on some of the samples; the samples where inducedwith a saturating field (7 T) at room temperature and their magnetic

Fig. 3. Composite sections running from the offshore Portland – South Wight Basin to the Wytch Farm area showing: a) the current the structures and the strati-graphy, and b) a geometrically restored section with hanging wall restored prior to Tertiary inversion.


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moments measured as they cooled to 20 K. Magnetic hysteresis wasmeasured at low-temperature (LT, 5–15 K) and at room temperature.

Room temperature magnetic hysteresis measurements were carriedusing a Princeton vibrating sample magnetometer (VSM) at ImperialCollege London. Approximately 300 mg of the powdered sample wastightly packed and placed into a gel cap for hysteresis measurement.

4. Results

4.1. Seismic interpretation

To generate depth-converted decompacted surfaces, stratigraphicdata, such as, rock type, age and thickness, were sourced from theUKOGL well database and the literature (Chadwick et al., 2005). Duringthe decompaction process, all Tertiary to Recent sediments were re-moved and horizons imported and appended in Petrel™ (Schlum-berger). After the decompaction there was an average increase inthickness of about 150 m from Top Chalk to Top Inferior Oolite, withsimilar increases in thickness observed from Top Inferior Oolite to TopWhite Lias. In contrast, only a 100 m increase in thickness was observedon line B90-37 after decompaction in layers between Top Chalk andTop Inferior Oolite, while there was no observable increase in thickness

in layers between Top Oolite and Top White Lias. The reason for thedifferences in the thicknesses in the model is likely because seismicsection B90-32 lies across the Abbotsbury-Ridgeway Fault System,whereas seismic section B90-37 is located on the hanging wall of thefault system making it the most affected by the basin inversion as re-ported by Hawkes et al. (1998).

Geometrical restoration was carried out on a selection of 2D seismicsections (Fig. 3). Some missing sections were estimated from seismiclines B92-40 and a composite of BP-83-060L and 80-268, which passthrough the Kimmeridge area. PetroMod™ (Schlumberger) was used tomodel the timing of organic matter maturation, hydrocarbon genera-tion and expulsion from the penitential source rocks, using well andRock Eval data. These models (results not shown here) reveal the timingof organic matter maturation for Wytch Farm, Waddock Cross, Radipoleand Kimmeridge.

Fig. 4 is a composite map, called an upheaval map, which consists ofpresent day Inferior Oolite surface on the footwall of the Abbotsbury-Ridgeway and Purbeck faults, with the restored Inferior Oolite in thehanging wall superimposed on it (inset). The red arrows indicate po-tential hydrocarbon migration areas from the hanging wall into thefootwall. The upheaval map for the White Lias with the restored In-ferior Oolite superimposed (Fig. 5), was used to approximate the

Fig. 4. Upheaval map, consisting of present day Inferior Oolite surface on the footwall of the Abbotsbury-Ridgeway and Purbeck faults, with the restored InferiorOolite in the hanging wall superimposed on in the lower section. Potential migration trends are marked.


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juxtaposition between Bridport Sandstone Formation in the hangingwall and Sherwood Sandstone Group on the footwall. Mapping theSherwood Sandstone Group directly was difficult due to very low-am-plitude resolution in most of the sections used in this study with in-creasing depth. Instead, the top White Lias (Rhaetic/Penarth Group)was mapped and depths of 300–400 m were used to approximate depthsto the Top of the Sherwood Sandstone Group, based on well data fromthe Wytch Farm area. The red arrows in Figs. 4 and 5 also indicatepotential migration directions as indicated from the seismic analysisaround Kimmeridge-Creech, across the Purbeck-Isle of Wight Fault andRadipole.

4.2. Geochemical characterisation

Not all the core samples (Table 2) contained sufficient concentrationof petroleum hydrocarbon for geochemical extraction and analysis, assampling was carried out based on high magnetic susceptibility read-ings, and/or visible oil staining, with a small number of randomly se-lected samples. It was, however, difficult to unambiguously classify thesamples as ‘oil-stained’ and truly ‘oil-free’, because the inability to de-tect oil, does not mean that it was never present. Therefore, sampleswith what appears to be only contamination from drilling mud

additives are marked ‘poorly stained’ denoted by ‘P’ (Table 2), whereassamples with a measureable amount of crude oil in them (i.e., enoughfor biomarker analysis) are marked ‘oil-stained’ denoted by ‘O’(Table 2). Therefore, ‘poorly stained’ is used to define cores that haveeither never experienced oil or have so little oil in them, that the oilcould not be detected.

4.2.1. Environment of depositionSeveral parameters that yield information about the depositional

environment, e.g., m/z 57 and m/z 191, were measured (Peters andMoldowan, 1991). Pristane (Pr) and phytane (Ph) relative responseswere measured to investigate the redox conditions at the time of sourcerock deposition. Pr/Ph ratios < 1 indicate anoxic environment, Pr/Ph > 1, implying an oxic environment and Pr/Ph > 3 indicate terri-genous organic matter deposited in an oxic environment, but only highand low values can be reliably used (< 0.5, > 1.5) (Didyk et al., 1978).Within the analysed samples, the Pr/Ph ratios are relatively consistent,but the n-alkane distribution varies. Some samples (e.g., BS1 and KM3)have a unimodal distribution with a restricted range of n-alkanes (C15 toC26) and a relatively low molecular weight mode (n-C18) (Fig. 6a and b);others (e.g., CH1 and WC3) have a unimodal distribution with a widerange (C15 to C37) of compounds and a relatively high mode (n-C19 to n-

Fig. 5. Upheaval map, consisting of present-day Sherwood Group Surface (White Lias + 300 m) surface on the footwall of the Abbotsbury-Ridgeway and Purbeckfaults, with the restored Inferior Oolite in the hanging wall superimposed. Potential migration trends from Bridport Sandstone to Sherwood Sandstone via Creech aremarked.


516

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52.5

48.7

1.5

COM

B2O

Coom

beKe

ynes

11.

10.

70.

8…

……

0.5

……

……

0.4

……

1.2

COM

B3O

Coom

beKe

ynes

11.

00.

91.

0…

……

0.8

……

……

0.3

30.0

8.0

1.2

COM

B4O

Coom

beKe

ynes

11.

00.

91.

0…

…3.

3…

3.6

0.2

……

0.4

90.2

79.1

0.9

MT1

PM

artin

stow

n1

1.1

0.5

0.6

……

…0.

4…

……

…0.

5…

…0.

9M

T2O

Mar

tinst

own

11.

70.

60.

5…

……

…1.

21.

0…

-…

……

……

MT3

OM

artin

stow

n1

0.4

0.1

0.5

……

……

……

……

0.6

……

0.8

MT4

PM

artin

stow

n1

1.3

0.8

0.8

……

……

……

……

……

……

ST1

PSt

obor

ough

11.

00.

60.

7…

……

0.8

……

……

…40

.421

.61.

1ST

2P

Stob

orou

gh1

1.0

0.7

0.7

……

…0.

8…

……

…0.

352

.077

.21.

1ST

3O

Stob

orou

gh1

0.3

0.5

0.8

……

6.5

0.3

……

0.4

0.3

0.3

62.2

49.9

2.1

ST4

OSt

obor

ough

10.

90.

70.

6…

…5.

10.

8…

……

…0.

333

.340

.41.

2W

X1O

Wad

dock

Cros

s1

0.7

1.0

1.5

……

5.8

0.3

……

0.6

0.5

0.3

57.6

38.3

2.0

WX2

OW

addo

ckCr

oss

11.

30.

91.

13.

71.

4…

0.5

……

……

0.4

82.7

36.3

0.9

WX3

OW

addo

ckCr

oss

10.

61.

11.

5…

…5.

90.

3…

…0.

60.

50.

352

.657

.22.

3CH

1O

Chic

kere

ll1

0.8

1.4

1.6

2.4

0.9

3.9

0.3

3.6

0.6

……

0.4

68.1

22.4

3.2

CH2

OCh

icke

rell

11.

01.

51.

62.

50.

84.

00.

33.

70.

6…

…0.

456

.227

.13.

1CH

3O

Chic

kere

ll1

0.8

1.0

1.0

2.3

0.8

3.6

0.3

3.3

0.6

……

0.4

38.9

20.8

3.8

CH4

OCh

icke

rell

10.

71.

21.

12.

30.

83.

70.

23.

50.

6…

…0.

468

.157

.93.

2CH

5O

Chic

kere

ll1

0.9

1.2

1.3

2.5

0.9

3.8

0.3

3.5

0.6

……

0.4

82.8

75.1

3.5

CH6

OCh

icke

rell

10.

10.

21.

22.

50.

94.

00.

23.

50.

6…

…0.

463

.463

.53.

1CH

7O

Chic

kere

ll1

1.4

0.8

0.7

……

3.8

0.5

……

……

0.5

……

0.9

CR1

OCr

eech

11.

60.

60.

6…

…1.

51.

30.

6…

…0.

689

.972

.40.

8CR

2P

Cree

ch1

1.0

0.6

0.5

……

……

……

……

63.3

41.5

1.1

CR3

OCr

eech

10.

80.

80.

62.

10.

77.

40.

34.

60.

50.

50.

20.

449

.131

.10.

7CR

4O

Cree

ch1

1.0

0.8

0.7

2.0

0.6

7.9

0.3

5.0

0.5

0.6

0.3

0.4

49.0

42.2

1.3

CR5

OCr

eech

11.

00.

70.

62.

20.

77.

40.

34.

60.

50.

40.

30.

455

.251

.81.

5CR

6O

Cree

ch1

0.8

0.8

0.6

1.9

0.6

7.8

0.3

4.8

0.5

0.7

0.3

0.4

41.0

41.0

1.2

CR7

PCr

eech

12.

30.

60.

5…

……

……

……

……

……

…KM

1O

Kim

mer

idge

51.

00.

80.

9…

……

……

……

…0.

4…

…0.

9KM

2P

Kim

mer

idge

50.

70.

71.

0…

……

0.5

……

……

0.3

……

0.9

KM3

PKi

mm

erid

ge5

1.2

0.7

0.7

……

…0.

4…

……

…0.

3…

…0.

7KM

4O

Kim

mer

idge

51.

10.

60.

6…

……

0.4

……

……

0.2

……

1.0

(continuedon

next

page

)


517

C23) (Fig. 6c and d). The alkane distribution observed in Fig. 6a–b ischaracteristic of diesel-based drilling fluids.

Most samples have Pr/Ph ratios were between 0.75 and 1.5, butCoomb1 and CH6 had low values of 0.09 and 0.11 respectively, andCR7 had a Pr/Ph ratio of 2.32 (Table 2). In addition, a plot of phytane/n-C18 against pristane/n-C17 that provides evidence for biodegradation,organic matter source and environment of deposition is presented inFig. 7. Nearly all the samples plot in the Type II algal marine organicmatter field, indicating a reducing depositional environment. The ex-ceptions are Coomb1, MT3 and CH6, which have anomalously low Pr/C17 values (Fig. 7).

The homohopanes can also be used as indicators for redox potentialof the oil source rock during the time of deposition (Seifert et al., 1980;Moldowan et al., 1985; Kolaczkowska et al., 1990; Wenger et al., 2002).They are usually visualised using the m/z 191 mass chromatogram(values given in Table 2). Samples BS9, ST4, CR3-CR6 and all the WCsamples have values between 0.2 and 0.35, while WX1 and WX3 haveratios > 0.45; this indicates a reducing depositional environment(Seifert et al., 1980). Analysis of C31/C30 hopane ratios was used toseparate marine from lacustrine deposited source rock oils (Moldowanet al., 1985), with most samples ranging between 0.3 and 0.45. Somegrouping is evident, with CH, CR and WC samples mostly having similarvalues around 0.36, whereas ST and WX samples had values of ~0.33(Table 2). These values are indicative of marine source rocks(Kolaczkowska et al., 1990; Moldowan et al., 1985).

4.2.2. Maturity parametersA plot of Ts/Tm vs C30 hopane (concentration of C30H in arbitrary

units as it was semi-quantified using the ISTD squalane) was used toinvestigate thermal maturity in oils from a common source (Fig. 8 andTable 2). Oil samples with low Ts/Tm ratios (< 1) and high C30H valuesare characterised as immature (Clark and Philp, 1989; Wenger et al.,2002). Oil samples from Stoborough (ST), Kimmeridge (KM) andMartinstown have Ts/Tm ratios of ~1 and extremely low C30H con-centration; most samples from Creech (CR) have slightly higher C30Hconcentrations (Fig. 8). Oils from Wytch Farm (WC) and Waddock Cross(WX) have similar Ts/Tm ratios of ~1.5–2.0, with two samples fromWaddock Cross (WX) having the highest C30H values. Oil samples fromChickerell (CH) have the highest Ts/Tm ratios, but relatively low C30Hconcentrations (Fig. 8), indicating that Chickerell is the most mature oilstudied. Ts/Tm parameter could be influenced by facies, but using it C30

mitigates the facies dependence of the Ts/Tm parameter.The C29 epimer ratios αββ/(αββ+ααα) and (S/(S + R)) were also

measured to investigate further maturity variations (Fig. 9). Ratios >0.4 for both biomarkers are considered to be in the oil window(Moldowan et al., 1985; Wenger et al., 2002). Most of the oil samplesanalysed have values between 0.3 and 0.7 for both biomarkers (C29

epimers), with few exceptions from three CH samples, which have C29

(αββ/(αββ+ααα)) ratios of < 0.3 (Fig. 9).

4.3. Magnetic characterisation

All the core samples were subjected to magnetic measurements tocharacterise their magnetic mineralogy, and the grain size and abun-dance of the magnetic minerals.

4.3.1. Magnetic hysteresis measurementsHysteresis parameters indicative of the abundance of magnetic

materials, e.g., the saturation magnetisation Ms, were measured on allsamples at room-temperature (RT) and low-temperature (LT; 10 K, 15 Kor 20 K). Samples from the Kimmeridge (KM), which is thought to bethe closest to the charge point (Hawkes et al., 1998) show variation inthe RT-Ms for the ‘O’ and the ‘P’ samples (Table 3) – the ‘P’ samples havehigher values. For example, KM2 and KM3, have 1.96 × 10−3 and5.04 × 10−3 Am2/kg, respectively, while the ‘O’ samples, KM1 andKM4, have 5.2 × 10−4 and 1.27 × 10−3 Am2/kg, respectivelyTa

ble2

(continued)

Sam

ple

Stai

nLo

calit

yPr

/Ph

Pr/C

17Ph

/C18

10G

/(G

+C 3

0)G

/C31

RC 3

0/Ts

C 29H

/C30

HC 3

0/29

Ts29

Ts/(

29H

+29

Ts)

C 35/

C 34

C 35/

(C31

–C35

)C 3

1/C 3

0C 2

9ββ

/(ββ

+αα

)20

S/(2

0S+

20R)

Ts/T

m

WC1

OW

ytch

Farm

A1

1.2

0.6

0.5

2.1

1.0

4.5

0.3

4.5

0.6

0.4

0.2

0.3

63.0

30.8

1.7

WC2

OW

ytch

Farm

A1

1.1

0.6

0.5

2.1

0.7

4.4

0.3

4.4

0.5

0.7

0.3

0.4

96.2

59.1

1.8

WC3

OW

ytch

Farm

A1

1.2

0.6

0.5

2.5

1.0

4.2

0.3

4.2

0.5

0.5

0.2

0.3

59.9

55.7

1.7

Pr:P

rist

ane.

Ph:P

hyta

ne.

10x

G/(

G+

C 30)

:Gam

mac

eran

e/(g

amm

acer

ane

+C 3

0αβ

hopa

ne)

x10

.G

/C31

R:G

amm

acer

ane/

C 31

hopa

ne.

C 30/

Ts:C

30αβ

hopa

ne/C

2718

αtr

isno

rhop

ane

C 30/

C 29T

s:C 3

0ho

pane

/C29

18α

norn

eoho

pane

.C 2

9Ts/

(C29

αβ+

C 29T

s):C

2918

αno

rneo

hopa

ne/(

C 29α

βho

pane

+C 2

918

αno

rneo

hopa

ne).

C 35/

C 34:

C 35

hopa

ne/C

34ho

pane

.C 3

5/(C

31–C

35):

C 35

hopa

ne/(

C 31

hopa

ne-C

35ho

pane

).C 3

1/C 3

0:C 3

1ho

pane

/C30

αβho

pane

.C 2

9ββ

/(ββ

+αα

):C 2

9ste

rane

epim

ers

ratio

.20

S/(2

0S+

20R)

:C29

ster

anee

pim

ers

ratio

.O

:oil-

stai

ned,

P:po

orly

stai

ned.


518

(Table 3). In contrast, LT-Ms of the two ‘O’ samples is higher than thecorresponding ‘P’ samples, indicating that the ‘O’ samples have anabundance of nanometric magnetic particles, that are thermally acti-vated at RT (superparamagnetic) and are not detected via hysteresis(Table 3). In addition, high-field susceptibility (χhf) values (determinedfrom the hysteresis curves), which measures the contribution made by

paramagnetic and diamagnetic minerals, are shown in Table 3. For boththe LT- and RT-χhf, the ‘O’ samples have higher values than the ‘P’samples (Table 3), which further supports the presence of nanometricmagnetic particles in the oil stained particles.

All Bushey Farm (BS) samples except for BS9 are oil poor (‘P’;Table 2). The LT- and RT-Ms values for the ‘P’ samples are higher LT-Ms

Fig. 6. Mass chromatogram (m/z 57) of aliphatic fractions of a) and b) Bushey Farm A1 (3464) Kimmeridge 5 (872) respectively, stained probably by the drillingmud, these samples and others with similar geochemical signature are termed poorly-stained (P); c) and d) Chickerell 1 (262) and Wytch Farm A12 (2998) exhibitingtypical n-alkane distribution from crude oils these samples and others with similar geochemical signature are termed oil-stained (O).

Fig. 7. Bi-plot of Ph/C18 and Pr/C17 ratios to investigate organic matter type, and environments of deposition (Lijmbach, 1975). Nearly all the samples plotted withinthe Type II kerogen field, from algal derived organic matter in a strongly reducing environment.


519

than the ‘O’ sample, with an overall decrease in the LT-Ms values withdepth (Table 3). All ‘O’ samples from Creech have LT-Ms values boundby the two ‘P’ samples (CR2 at the lower bound and CR7 at the upperbound, with LT-Ms values of 0.26 and 0.89 Am2/kg, respectively;Table 3). No similar trend was observed in either the RT-Ms or LT- andRT-χhf data (Table 3). LT- and RT-hysteresis parameters for the WytchFarm (WC) samples, all of which were oil stained, show that WC1 has aslightly lower LT-Ms value of 0.64 Am2/kg compared to WC2 and WC3(0.65 and 0.82 Am2/kg, respectively; Table 3). This indicates a smallincrease in LT-Ms values with depth. Conversely, RT-Ms decreases withdepth (Table 3). Both the LT- and RT-χhf indicate increasing para-magnetic contribution with sample depth. For example, WC1 and WC2have nearly identical LT-χhf values, while WC3 has a slightly higher

paramagnetic contribution of 7.1 × 10−7 m3/kg (Table 3). Similarly,the RT-χhf data indicate that WC1 and WC2 have similar values of4.3 × 10−8 and 4.13 × 10−8, respectively, with WC3 having a slightlyhigher value of 5.7 × 10−8 (Table 3).

LT-Ms values for all the samples are given in Table 3. There is amixed pattern in LT-Ms values from the Kimmeridge to Wytch Farm. Allthe Creech samples, except for CR7, have lower LT-Ms values thanKimmeridge samples, while the Bushey Farm samples mostly havehigher LT-Ms values than the Kimmeridge samples. However, the WytchFarm, the Waddock Cross and all the ‘O’ Stoborough samples havehigher LT-Ms values than the Kimmeridge samples (Table 3).

Hysteresis parameters sensitive to magnetic grain size such ascoercive force (Bc) and the ratios of saturation remanence

Fig. 8. Plot of maturity biomarkers Ts/Tm versus C30H for samples from all the wells.

Fig. 9. Plot of C29 sterane epimers, %αββ/(αββ+ααα) versus %(S/(S + R)) for samples from all the wells.


520

Table3

Low

-tem

pera

ture

(LT;

10–2

0K)

and

room

-tem

pera

ture

(RT)

hyst

eres

ispa

ram

eter

sfor

alla

naly

sed

sam

ples

.See

Tabl

e1

ford

epth

and

litho

logi

calv

aria

tions

.Msis

the

satu

ratio

nm

agne

tisat

ion,

Mrs

the

rem

anen

tsat

urat

ion

mag

netis

atio

n,B c

isth

eco

erci

vefo

rce

and

χ hf

the

high

-fiel

dsu

scep

tibili

ty(i

.e.,

the

para

mag

netic

slop

eco

rrec

tion

grad

ient

).

Sam

ple

Nam

eLT

-Ms

(Am

2 /kg)

LT-M

rs×

10−

3 (Am

2 /kg)

LT-M

rs/M

s×

10−

2LT

-Bc

(mT)

LT-χ

hf×

10−

7 (m3 /k

g)RT

-Ms×

10−

4 (Am

2 /kg)

RT-M

rs×

10−

5 (Am

2 /kg)

RT-M

rs/M

sRT

-Bc

(mT)

RT-χ

hf×

10−

8 (m3 /k

g)

BS1-

P0.

324.

71.

54.

66.

49.

018

.00.

190.

43.

8BS

2-P

0.67

13.0

1.9

86.

09.

016

.00.

1712

.13.

6BS

3-P

0.71

20.0

2.8

10.9

6.0

7.0

16.0

0.22

10.9

3.6

BS4-

P0.

488.

61.

84.

39.

05.

811

.00.

20.

76.

3BS

5-P

0.63

4.0

0.1

0.4

12.0

6.3

……

…8.

0BS

6-P

0.5

6.0

1.2

3.4

9.0

20.0

28.0

0.15

46.

1BS

7-P

0.51

6.8

1.4

29.

75.

124

.00.

4811

.17.

8BS

8-P

0.23

7.5

3.2

8.8

5.0

7.1

10.0

0.15

2.5

4.0

BS9-

O0.

166.

44.

010

.83.

013

.015

.00.

115.

51.

8BS

10-P

0.3

14.0

4.5

14.6

2.9

11.0

17.0

0.16

12.3

1.6

COO

MB1

-O0.

226.

02.

74.

75.

960

.019

.00.

314

3.4

COO

MB2

-O0.

122.

01.

72.

34.

17.

110

.00.

144.

32.

1CO

OM

B3-O

0.35

2.1

0.6

1.7

8.7

7.7

……

…5.

4CO

OM

B4-O

0.35

2.3

0.7

1.9

8.3

6.9

6.4

0.09

7.4

4.9

MT1

-P0.

193.

92.

04

6.1

13.0

24.0

0.19

5.8

3.7

MT-

O0.

173.

62.

14.

54.

936

.062

.00.

1712

.12.

9M

T3-O

0.4

6.4

1.6

55.

512

.019

.00.

165.

43.

5M

T4-P

0.11

2.2

2.0

4.2

3.8

9.7

16.0

0.16

6.8

1.9

ST1-

P0.

332.

70.

81.

97.

213

.011

.00.

812.

44.

3ST

2-P

0.98

2.3

2.3

3.8

4.0

7.3

13.0

0.18

5.6

2.0

ST3-

O0.

235.

12.

26

5.4

49.0

37.0

0.75

6.6

3.1

ST4-

O0.

693.

60.

51.

713

.032

.033

.00.

13.

39.

0W

X1-O

0.56

4.2

0.7

2.1

8.3

55.0

52.0

0.94

6.5

4.8

WX2

-O0.

124.

00.

30.

621

.021

.018

.00.

861.

613

.0W

X3-O

0.35

6.7

1.9

5.1

6.8

14.0

27.0

0.2

6.3

4.0

CH1-

O0.

43.

00.

75.

18.

74.

78.

80.

1912

.73.

6CH

2-O

0.5

3.5

0.7

4.8

6.2

7.3

9.8

0.13

12.3

3.2

CH3-

O0.

473.

50.

84.

77.

37.

111

.00.

1510

.93.

9CH

4-O

0.57

4.1

0.7

4.8

8.1

5.4

9.2

0.17

11.8

4.4

CH5-

O0.

473.

50.

74.

56.

26.

111

.00.

1713

.43.

2CH

6-O

0.48

5.1

1.1

5.9

7.5

5.2

11.0

0.22

11.8

4.3

CH7-

P0.

466.

81.

27

9.8

7.8

78.0

0.1

10.7

5.7

CR1-

O0.

512.

30.

53.

47.

29.

514

.00.

158.

94.

1CR

2-P

0.26

2.7

1.0

7.9

6.2

−9.

57.

0−

0.73

21.6

13.0

CR3-

O0.

3315

.04.

515

3.8

17.0

25.0

0.15

11.2

1.9

CR4-

O0.

4320

.04.

615

.14.

615

.025

.00.

1714

.12.

6CR

5-O

0.35

18.0

5.1

186.

64.

321

.00.

4916

.912

.0CR

6-O

0.27

15.0

5.6

15.2

5.3

−3.

916

.0−

0.39

14.7

14.0

CR7-

P0.

8918

.02.

07.

78.

09.

319

.00.

2113

.65.

6KM

1-O

0.67

1.0

0.2

16.

74.

38.

50.

29

3.6

KM2-

P0.

441.

30.

32.

26.

420

.027

.00.

149.

73.

3KM

3-P

0.45

1.1

0.2

1.6

5.0

57.0

28.0

0.48

5.6

2.3

KM4-

O0.

841.

50.

21.

17.

913

.020

.00.

165.

24.

6W

C1-O

0.64

19.0

3.0

11.5

5.7

14.0

20.0

0.13

8.9

4.3

WC2

-O0.

6518

.02.

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521

magnetisation to saturation magnetisation (Mrs/Ms) were considered fora few of the individual wells followed by a combined result for all thewells presented here. Whereas, Mrs alone is affected by abundance,mineralogy and grain size, Mrs/Ms is predominantly sensitive to grainsize.

RT-Mrs for the Kimmeridge (KM) samples show the oil poor (‘P’)samples have higher remanence than the ‘O’ samples. For example,KM2 and KM3 have RT-Mrs values of 2.7 × 10−4 and 2.8 × 10−4 Am2/kg respectively, whereas KM1 and KM4 have RT-Mrs values of8.5 × 10−5 and 2.0 × 10−4 Am2/kg respectively (Table 3). In contrast,LT-Mrs/Ms values have the opposite trend, with the oil stained (‘O’)samples having lower ratios than the ‘P’ samples (Table 3). The LT-Bc

values for the KM ‘P’ samples are higher compared to their ‘O’ coun-terparts (e.g., KM2 and KM3 have LT-Bc values of 2.2 and 1.6 mT re-spectively, whereas KM1 and KM4 had LT-Bc values of 1.0 and 1.1 mTrespectively). However, the RT-Bc shows no clear trend.

For the other cores trends were also present, for example, LT-Mrs

increases for all the samples from Kimmeridge to Wytch Farm throughBushey Farm and Creech, with Waddock Cross having higher averagevalues than wells north of the Abbotsbury Ridgeway Fault. However, in

the case of grain-size dependent parameters (e.g., LT- Mrs/Ms and Bc),the data trends are less apparent (Table 3).

4.3.2. Low-temperature ZFC/FC and cycling measurementsZero-field cooled (ZFC) and field cooled (FC) warming experiments

were carried out in order to investigate contributions from super-paramagnetic and paramagnetic minerals and to identify crystal-lographic transitions, such as the Verwey Transition, which occurs at~120 K and is indicative of magnetite (Verwey, 1939; Muxworthy andMcClelland, 2000), and the Besnus transition, which occurs at ~34 Kand is indicative of pyrrhotite (Besnus and Meyer, 1964). In addition,the ZFC/FC data were reduced and quantified, by taking two ratios ofmagnetisation: 1) 30 K divided by that at 15 K (30 K/15 K) to look at thepresence of ‘fine’ ferromagnetic particles (~5–15 nm) and 2) at 300 Kdivided by 50 K (300 K/50 K) to look at the presence of ‘larger’ ferro-magnetic particles (~ < 30 nm) (Table 4). The 30 K/15 K data could beinfluenced not only by the abundance of superparamagnetic minerals,but also room-temperature paramagnetic minerals that become ferro-magnetic at low temperatures, e.g., siderite.

All the Kimmeridge (KM) ZFC data exhibit a Verwey transition inaddition to a sharp decrease in magnetisation observed in the range10–30 K on warming (Fig. 10). This drop could be either due to un-blocking and/or low-temperature Curie-temperature minerals. How-ever, the ZFC and FC warming curves for the Bushey Farm samplesexhibit a Besnus transition, which is indicative of pyrrhotite (Fig. 11).Moreover, ZFC data for Stoborough (ST), Waddock Cross (WX) CoombeKeynes (Coomb) and Martinstown (MT) all have double crystal-lographic transitions (i.e., Besnus and Verwey transitions; Fig. 12),which indicate these samples contain both pyrrhotite and magnetite.

The FC and ZFC 30 K/15 K ratios show an increase fromKimmeridge to Wytch Farm, through Creech and Bushey Farm(Table 4), and the ratios increase again from Stoborough. The ratiosgradually decrease through Waddock Cross, Coombe Keynes, Chickerellto Martinstown (Table 4). The 300 K/50 K ratios in both the FC and ZFCdo not show consistent variations, though the Kimmeridge sampleshave consistently higher ratios indicating an abundance of nano-particles < 30 nm (Table 4).

5. Discussion

5.1. Basin restoration

The Wessex Basin was restored to its geometry during LateCretaceous maximum burial by plotting the restored hanging wall ofthe Portland-South Wight and Abbotsbury-Ridgeway faults against thepresent-day structure on the footwall (Fig. 3). This is a simplification ofthe footwall geometry at the time of Late Cretaceous maximum burialas there has been some tilting of the basin to the west during the Ter-tiary (e.g., Butler, 1998; Hawkes et al., 1998). This tilt is best observedby a spill of roughly two-thirds of the Wytch Farm field in a westerlydirection during the Tertiary and introduces a degree of uncertainty tothe interpretation of footwall migration trends. Fig. 4 shows a possiblemigration direction for Wytch Farm oil within the Bridport Sandstone.Assuming the oil was generated at deep areas around Kimmeridgewithin the Portland–South Wight Basin, the oil could easily find its wayto Wytch Farm via the Creech conduit, rather than through BusheyFarm (Fig. 4).

In areas around Kimmeridge, the restored Bridport Sandstone isabout 1750–2000 m deep and the Sherwood Sandstone across the faultaround Creech is about 1500–1900 m deep (Fig. 5). This area aroundCreech has the potential to serve as a migration path for the SherwoodSandstone-rated oil found at Wytch Farm. Migration of oil along what isalmost certainly a sealing fault (Underhill and Stoneley, 1998), is notrequired in this new model.

Table 4Field-cooled (FC) and zero-field cooled ratios of magnetisation: 30 K/15 K and300 K/50 K.

Sample Name 30 K/15 K(ZFC)

300 K/50 K(ZFC)

30 K/15 K(FC)

300 K/50 K(FC)

BS1-P 0.08 0.18 0.11 0.15BS2-P 0.27 0.15 0.28 0.13BS3-P 0.11 0.21 0.12 0.19BS4-P 0.04 0.22 0.06 0.20BS5-P 0.17 0.12 0.14 0.11BS6-P 0.1 0.28 0.09 0.27BS7-P 0.06 0.17 0.09 0.15BS8-P 0.06 0.16 0.06 0.17BS9-O 0.1 0.15 0.12 0.14BS10-P 0.10 0.15 0.08 0.12COOMB1-O 0.42 0.15 0.42 0.14COOMB2-O 0.5 0.09 0.48 0.1COOMB3-O 0.27 0.15 0.29 0.14COOMB4-O 0.25 0.05 0.29 0.06MT1-O 0.55 0.12 0.56 0.11MT2-P 0.29 0.09 0.30 0.10MT3-O 0.51 0.13 0.5 0.12MT4-O 0.51 0.13 0.5 0.12ST1-P 0.20 0.16 0.22 0.13ST2-P 0.48 0.10 0.50 0.10ST3-O 0.22 0.29 0.23 0.27ST4-O 0.64 0.12 0.37 0.12WX1-O 0.65 0.17 0.64 0.17WX2-O 0.35 0.12 0.41 0.11WX3-O 0.48 0.09 0.46 0.09CH1-O 0.39 0.18 0.38 0.13CH2-O 0.41 0.17 0.45 0.10CH3-O 0.34 0.18 0.37 0.14CH4-O 0.28 0.24 0.30 0.21CH5-O 0.27 0.20 0.37 0.12CH6-O 0.26 0.25 0.27 0.20CH7-P 0.21 0.28 0.21 0.24CR1-O 0.54 0.22 0.29 0.19CR2-P 0.27 0.28 0.06 0.11CR3-O 0.28 0.16 0.28 0.14CR4-O 0.26 0.11 0.30 0.10CR5-O 0.24 0.11 0.14 0.11CR6-O 0.18 0.15 0.05 0.13CR7-P 0.19 0.13 0.19 0.12KM1-O 0.62 0.31 0.48 0.26KM2-P 0.7 0.43 0.67 0.38KM3-P 0.72 0.56 0.66 0.47KM4-O 0.73 0.23 0.69 0.11WC1-O 0.18 0.15 0.13 0.13WC2-O 0.18 0.13 0.13 0.12WC3-O 0.21 0.15 0.10 0.13


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5.2. Oil geochemistry

The analysed samples mostly have Pr/Ph ratios between 0.5 and 1.5(i.e., within the oxic-suboxic region; Fig. 7), but the use of Pr/Ph ratioshas limitations in classifying source depositional environments of oilsamples (Peters and Moldowan, 1991). On the other hand, the Ph/n-C18

and Pr/n-C17 ratio plot can provide a more reliable classification of oilsin terms of environment of deposition, organic matter type, oil maturityand biodegradation (Lijmbach, 1975). Nearly all of the oil samplesanalysed in this study plot within the range of oils derived from Type IIkerogen of algal marine origin and were deposited in a reducing en-vironment (Fig. 7). Furthermore, the C30/C29Ts hopane ratios, whichare an indicator of source type and environment of deposition(Moldowan et al., 1985), indicate that the source rocks that generatedthe Creech oils were deposited in an oxic-suboxic environment, WytchFarm in suboxic-dysoxic environment and Chickerell and CoombeKeynes oils in dysoxic-anoxic environment (Table 2). This suggests atrend in the distribution of the biomarkers, along the Abbotsbury Rid-geway-Purbeck faults.

The C31/C30 homohopane ratios are highly specific for distin-guishing marine from lacustrine depositional conditions; samplesmeasured in this study using this ratio had values > 0.25 (Table 2)(Kolaczkowska et al., 1990; Moldowan et al., 1985). This reaffirms that

the source rock for these oils was deposited in a marine environment(Kolaczkowska et al., 1990; Moldowan et al., 1985). Furthermore, thepresence of gammacerane in some of the analysed oils (Table 2) con-firms that these oils were sourced from hypersaline, semi restricted,stratified water environments (Damsté and De Leeuw, 1995). Bio-marker analysis carried out to investigate maturity differences amongthe oil samples indicates varied maturity levels in the oil samples fromthe same wells and across the wells. The Ts/Tm v C30H plot (Fig. 8)indicates that oil samples from Waddock Cross are generally less maturethan oil samples from Creech, which, in turn, are less mature than oilsamples from Wytch Farm. Interestingly, oil samples from Chickerellseem to be the most mature based on Ts/Tm vs C30H biomarker evi-dence, with oils from Kimmeridge, Stoborough and Martinstown havingsimilar maturities (Fig. 8). Oil samples from Waddock Cross appear tobe less mature than oils from Creech, which are in turn less mature thanoils from Wytch Farm (Fig. 8).

5.3. Changes in magnetic mineralogy

5.3.1. Are there variations in ferromagnetic mineral abundance?The abundance-dependent magnetic data (Table 3), suggest in-

creases in abundance of magnetic materials in oil-stained (O) samplescompared to the poorly-stained (P) ones, in the majority of the samples

Fig. 10. Kimmeridge: ZFC warming curves plot showing: a) KM1 with arrows showing magnetisations at 15, 30, 50 and 300 K whose ratios were used in this study toinvestigate the abundance of fine (< 30 nm) and very fine (5–15 nm) ferromagnetic particles, b) KM2 showing higher magnetisation than KM1 with what appears tobe a Verwey Transition at ~120 K, indicating the presence of magnetite in the sample, c) KM3 showing similar magnetisation to KM2 and possible Verwey Transition,d) KM4 showing a sharp drop in magnetisation around 120 K, which is indicative of magnetite.


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analysed. However, some of the data contradicts this model. For ex-ample, values for the Kimmeridge samples show increases in abundanceof magnetic minerals in the oil-stained samples, based on the LT-Ms andRT-χhf values (Table 3). Similarly, the oil-stained, Creech samples hadhigher concentration of magnetic materials than CR2 (P), but less thanCR7 (P), albeit CR1 and CR2 were sampled in the Inferior Oolite in-tervals (Table 1). In addition, the oil-stained Chickerell samples all havehigher concentrations of magnetic materials based on LT-Ms valuesexcept for CH1 (Table 3). Samples from Wytch Farm, Chickerell andCoombe Keynes have increasing abundance of magnetic materials withdepth of sampling (Table 3), with some variations based on lithologyalso observed. For example, only CH1 was sampled from Inferior Ooliteamong the Chickerell (CH) samples (Table 1) and this sample has thesmallest concentration of magnetic materials compared to otherChickerell samples (Table 3).

The χhf parameter (Table 3) indicates an increase in abundance ofsuperparamagnetic and/or paramagnetic particles from the Kimmer-idge (hydrocarbon ‘kitchen’ area) to Wytch Farm (hydrocarbon re-servoir). This signal is likely due to increased superparamagnetism, i.e.,nanoparticles < 30 nm in size, which suggests that the small ferro-magnetic particles are easier to transport and accumulate in WytchFarm (Abubakar et al., 2015).

Table 3 shows the distribution of LT-Mrs of the sampled wells; thisparameter measures the total ferromagnetic contribution. Significantincreases in LT-Mrs from Kimmeridge to Wytch Farm via Creech wereobserved, but the trend breaks down at Stoborough; this could be due toan increase in the total number of magnetic minerals, or growth ofsmaller magnetic minerals as Mrs is also affected by grain size. Based onaverage LT-Mrs values, there appears to be growth of magnetic grainsfrom the area around the hydrocarbon kitchen (Kimmeridge) to areas

where currently there is an accumulation of oil (Wytch Farm) viaCreech (CR) and Bushey Farm (BS) (Table 3).

5.3.2. Are there variations in ferromagnetic mineral grain sizes?Magnetic grain-size proxies such as Mrs/Ms and Bc indicate grain-

size variations between the samples. For example, Mrs/Ms and Bc valuesfor Chickerell indicate increasing grain sizes with sampling depth. Mrs/Ms and Bc all reaffirm increase in grain-size of magnetic minerals fromKimmeridge via Bushey Farm and Creech to Wytch Farm (Table 3).Moreover, there appears to be an increasing amount of remanencecarrying materials from Stoborough to Coombe Keynes to Chickerell toWaddock Cross to Martinstown (Table 3).

ZFC-FC ratios (300 K/50 K and 30 K/15 K) indicate relative varia-tions in both the ratio of small ferromagnetic particles ferromagneticand/or low-temperature ferromagnetic materials (paramagnetic atroom-temperature); these data are a measure of grain size and magneticmineralogy (Table 4). Overall, the ZFC-FC data supports the χhf data(Table 3). That is, there is evidence for increasing abundance of ‘small’and/or paramagnetic particles from the Kimmeridge to Wytch Farm(Table 3). Even though the average grain-size appears to be increasing,the absolute number of small ferromagnetic particles also appears toincrease.

5.3.3. Are there variations in ferromagnetic mineralogy?The ZFC-FC warming plots (Figs. 10–12) measured to investigate

thermal relaxation and crystallographic transition of certain minerals(e.g., pyrrhotite), reveal the presence of two dominant magnetic phases:magnetite (Fe3O4) and pyrrhotite (Fe7S8). For example, samples fromBushey Farm have a Besnus Transition at ~34 K, which indicates thepresence of pyrrhotite (Fig. 12). Samples from Stoborough, Waddock

Fig. 11. Bushey Farm: Plots of a and b) ZFC warming curves showing pyrrhotite as the dominant magnetic phase and c and d) ratios of ZFC and FC (30 K/15 K and300 K/50 K).


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Cross, Coombe Keynes and Martinstown (Table 3) all exhibited doublecrystallographic transitions (i.e., Besnus and Verwey (at ~ 120 K)transitions), indicating the presence of both pyrrhotite and magnetite.Although other magnetic minerals (e.g., siderite or rhodochrosite) ex-hibit low-temperature crystallographic transitions similar to that ofpyrrhotite (32–37 K), these were not observed during field-cooled (FC)measurements, which should be expected for transitions not associatedwith pyrrhotite (Frederichs et al., 2003). Some samples (e.g., fromCreech, Wytch and Chickerell) did not exhibit any crystallographictransitions. This absence could be due to: (1) lack of magnetite orpyrrhotite, (2) the magnetic grains being too small to exhibit low-temperature crystallographic transitions (Dekkers, 1989), or (3) non-stoichiometry of the phases.

Fig. 13 shows an interpretive summary of magnetic mineral dis-tribution in the study area; areas with appreciable amount of hydro-carbon in them (either reservoirs or carrier beds; e.g., Kimmeridge,Chickerell and Wytch Farm) have magnetite as the predominant mag-netic mineralogy, while areas at the fringes of hydrocarbon accumu-lation have either mainly pyrrhotite (e.g., Bushey Farm) or a mixture ofpyrrhotite and magnetite (e.g., Martinstown and Waddock Cross).

5.3.4. Are the variations in magnetic mineralogy affected by intrinsicmagnetic contributions or contamination?

Hounslow (1987) reported that a relatively small ferromagneticsignal from the Bridport Sandstone saturated in a field of < 0.3 T.Hounslow (1987) suggested that the ferromagnetic signal is from det-rital ferromagnetic ferrianilmanite (ilmenite with exsolved hematite),

and to a lesser extent Fe-rich titanomagnetite. For the Bridport sand,Hounslow (1987) also reported the presence of paramagnetic chloritesand micas, plus pyrite and, to a lesser extent, goethite (from theweathering of pyrite). There are some significant methodological dif-ferences between the study of Hounslow (1987) and this one: (1)Hounslow (1987) sampled surface outcrops, not drill cuttings; theoutcrops are > 20 km to the west of the drill cuttings studied in thisstudy; (2) Hounslow (1987) did not mention oil staining, and (3) grainsizes were determined using optical microscopy, i.e., he only assessedparticles > 300 nm; in this study low-temperature measurements ac-cessed ferromagnetic particles down to below 30 nm in size. In thisstudy, we found no evidence for hematite, though magnetite and ironsulphide phases were present in both the oil-stained and poorly oil-stained samples, suggesting that these phases are intrinsic to the Brid-port sand. Given the variation of the magnetic mineralogy within thesamples studied in this study, and the distance to the samples ofHounslow (1987), it is difficult to infer a “true” background signal. Weargue that the poorly oil-stained samples from the same wells as the oil-stained samples represent a better “true” background signal based onlocality. It should be noted that the nominal absence of oil does notmean that oil was not formerly present, which makes a unique andrigorous interpretation of a “true” background signal problematic. Inthis sense, the poorly oil-stained samples can only be used as a guide tothe background signal. For reference, Hounslow (1985) also reportedthe presence of titanomagnetite and titanohematite in the Blue Lias Fmand Oxford Clay Fm collected distant from the reservoir.

Fig. 12. Plots of ZFC warming curves for Stoborough (ST), Waddock Cross (WX), Coombe Keynes (Coomb) and Martinstown (MT). All of these samples exhibit doublecrystallographic transitions (Besnus and Verwey) confirming the presence of pyrrhotite and magnetite in the samples. The kink at ~190 K in c) is most likely aninstrument error.


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5.3.5. Proposed model for the mineral magnetic signaturePrevious hydrocarbon-focused magnetic studies of soil or core

samples (e.g., Costanzo-Alvarez et al., 2006) reported enhancement ofmagnetic signals (mainly magnetite), which are thought to have formedin the presence of crude oil. There are two ways through which thepresence of hydrocarbons could enhance magnetic signals either: (1)through changes in the redox conditions (-Eh) within its vicinity, whichcould lead to the reduction of non- or weakly magnetic iron or sulphideminerals, e.g., hematite and pyrite, to strongly magnetic minerals suchas magnetite and pyrrhotite, or (2) through anaerobic microbial bio-degradation of the hydrocarbons and the production of magnetite fromFe2+ and Fe3+ ions present (Burton et al., 1993). Conversely, if theenvironment is aerobic the presence of hydrocarbons can be detri-mental to the magnetic mineralogy, as aerobic microbial biodegrada-tion generally oxidises these minerals thereby decreasing the overallmagnetic signal (Emmerton et al. 2013a,b). All the extracted oil has nosignificant biodegradation as indicated by relevant biomarkers, withthe exception of some of the Chickerell samples, which fall within theslightly biodegraded range (Fig. 7 and Table 2); raw data for all thesamples can be found in Abubakar (2016). Therefore, biodegradationcan be ruled out, which leaves changes in redox conditions as the mostlikely mechanism for the increase in magnetic signal observed fromKimmeridge through Creech and Bushey Farm to Wytch Farm.

This increase in the magnetic signal observed along the proposedoil-migration path from Kimmeridge to Wytch Farm is largely due toenhancement of magnetite nanoparticles < 30 nm, as identifiedthrough the low-temperature remanence and coercivity experiments; asgrains become larger and thermally blocked, i.e., no longer

superparamagnetic, their coercivity increases. This increase in nano-particles could be due to several mechanisms: (1) the conversion ofhematite into magnetite and/or pyrrhotite, which occurs within hy-drocarbon habitats (Machel, 1995), (2) the precipitation of nanometresized ferromagnetic particles along the oil migration path, or (3) ac-cumulation of magnetic nanoparticles towards the end of the migrationpath. Mechanism (1) can be dismissed, as there is no or little evidencefor hematite in the samples, though, as magnetite is magnetically muchstronger than hematite only a small amount of hematite would need tobe converted. It seems likely that mechanisms (2) and/or (3) are thelikely cause; that is magnetic nanoparticles are being formed in thepresence of hydrocarbons and being transported. The predominance ofpyrrhotite in the Bushey Farm samples is likely due to partial conver-sion of magnetite to either pyrrhotite or siderite, which occurs in near-distal plume environments (Machel, 1995). At shallow depths pyr-rhotite is potentially enhanced at the expense of magnetite along theedges distal to the migration path (Fig. 13). Samples from Stoborough,Waddock Cross, Coombe Keynes and Martinstown all had mixed pyr-rhotite and magnetite mineralogies and weaker magnetic signals com-pared to Creech and Wytch Farm (Fig. 13), suggesting there is not alarge accumulation of oil around these areas, nor has migration beensignificant.

5.4. A model for Wessex Basin

Biomarker analysis carried out on the aliphatic hydrocarbons of theoil samples provides no evidence to suggest that oils were not from acommon source rock, i.e., all oils analysed in this study likely had the

Fig. 13. Map of study area showing my interpretation of the mineralogical changes along migration trends; from mainly magnetite in KM area with arrows showingdirection of increasing magnetic grain size and pyrrhotite content; ST, WX and Coomb and MT areas have mixed magnetite and pyrrhotite while there was mainlymagnetite around CH.


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same source rock; (Fig. 6, Table 2). The small variations observed insome of the environmentally sensitive biomarkers, e.g., the C35 homo-hopane, and Ph/n-C18 and Pr/n-C17 (Fig. 7), are probably due to smalldifferences in oil thermal maturity or depositional conditions of theLias; the deposition of the Lias took millions of years, and it is likelyconditions varied during with time. There is also geochemical evidenceto suggest oil maturity varies across the sampled wells, for example, oilsamples from Kimmeridge are more mature than samples from Creechand Waddock Cross, but less mature than Wytch Farm samples. Basedon biomarker analysis it would be possible to argue that oils found atdifferent locations in Wessex Basin may have accumulated from variouscharge points, instead of a series of spill-chains, however, this is notsupported by the basin reconstructions (Figs. 4 and 5) or the mineralmagnetic data (Table 3 and Figs. 10–13).

From the basin modelling, the constructed top surface of the WhiteLias and the upheaval maps for both the Bridport and the Sherwoodreservoir/carrier beds provide insight into potential oil-migrationtrends (Figs. 4 and 5). The models suggest that crude oil found withinthe Bridport Sandstone at Wytch Farm likely migrated through theCreech conduit, but less likely through Bushey Farm. The Creech con-duit migration hypothesis is supported by the mineral magnetic data(Fig. 13), which indicates via a common magnetite nanoparticle signalthat oil migrated through Creech, rather than through Bushey Farm,which has a distinctive iron sulphide signal.

6. Conclusion

In this study a multidisciplinary approach to understanding oil mi-gration paths in the Wessex Basin was taken. Geochemical data suggeststhat the data is from a single source (Table 2), and that small variationsin environmentally sensitive biomarkers are probably due to smalldifferences in oil thermal maturity or depositional conditions during themillions of years it took to form the Lias.

Magnetically the Kimmeridge, Creech, Wytch Farm and Chickerellwells are dominated by the presence of nanometric magnetite. Thesewells contain appreciable amounts of hydrocarbons, or along hydro-carbon migration trends. Wells located at the fringes of large hydro-carbon accumulation had enhanced pyrrhotite-dominated magneticsignals (e.g., Bushey Farm).

The basin models suggest that the hydrocarbons have migrated fromKimmeridge to Wytch Farm likely through Creech rather than BusheyFarm (Figs. 4 and 5). Mineral magnetic measurements were used tohelp constrain this basin model. The magnetic measurements supportthe hypothesis that hydrocarbons have migrated via Creech and notBushey Farm (Fig. 13). This magnetic interpretation is based mainly onthe identified magnetic mineralogies rather than variations in magneticabundance.

Acknowledgements

This work was funded by the Petroleum Technology DevelopmentFund, Nigeria (PTDF) Nigeria to RA, and the Natural EnvironmentResearch Council (NERC) UK (NE/J01334X/1) to ARM and MAS. GAPacknowledges funding from the National Natural Science Foundation ofChina (grant no. 41574063) and a NERC Independent ResearchFellowship (NE/P017266/1).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.marpetgeo.2019.08.042.

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