Interpretation of the fault system in a hydrocarbon source rock: The Kimmeridge ClayFormation in the North Sea, UK.

Miguel Guerrero-Muñoz1, 2

1Instituto Mexicano del Petroleo, Programa de Simulacion Molecular, Eje Central Lazaro Cardenas152, Edificio 19 (SIIPI), P. B., Oficina 117 A, Col. San Bartolo Atepehuacan, Deleg. G. A. Madero,C. P. 07730, Mexico, D. F. E-mail:mguerrer@www.imp.mx

2Department of Earth Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK.

ABSTRACT

The Kimmeridge Clay Formation is widely regarded as the predominant source rock of the NorthSea oil. However, in the Wessex Basin it is not the major source to known hydrocarbons, notbecause of its lithology and organic matter content, but because of its low maturity, a function of itsmaximum burial depth. The organic matter in the Kimmeridge Clay Formation of the Wessex Basinis dominantly marine type II kerogen with only a minor contribution of terrestrial material.

The Kimmeridge Bay Field (discovered in 1959) produces from fractures in the middleJurassic Cornbrash limestones and Oxford Clay, being sourced from the Lias. Oil in the field wasbeing generated by and migrating during the Early Cretaceous, with peak generation in the LateCretaceous. The migration of oil from the area of generation to the Kimmeridge Bay Field reservoirmust involve faults as well as a component of lateral migration through carrier bed formations. Thecontinued replenishment of the Kimmeridge Bay Field suggests that some faults are still open. Over25 years this field has produced 2.5 x 106 bbl without decline. This seems more than the fracturesystem in the closure can contain and suggests that the field is being constantly replenished from adeeper reservoir.

Small N-S faults in the Kimmeridge Clay Formation east of Kimmeridge Bay, Dorset,southern England, are all normal. Conjugate faults formed during overpressure generation duringburial in the late Jurassic and early Cretaceous, by a process comparable to fluid pressurehydrofracturing. They are not related to compressional tectonics associated with the formation ofthe Kimmeridge Bay anticline. Faults at Clavell’s Hard were examined in detail, these faults havearrays of smaller displacement faults associated with them that cluster either side of the main fault,defining a damage zone in the mudstones. The extent and symmetry of the damage zone wasinvestigated by measuring the abundance of small faults in a series of traverses across these faults.The faults and associated calcite-filled veins are considered from outcrop evidence to be entirelyenclosed within the Kimmeridge Clay Formation and were the conduits for intraformational fluidsthat were expelled upwards. As such they are ideal for investigating the nature and composition offluids generated in this mudstone sequence during early burial.

Applications to the petroleum industry are obvious, these faults at small scale (even veins)increase permeability in reservoirs. Investigation of this effects are crucial and bring gradually to theprocess of understand the migration pathways, compartmentalization patterns and oil traps.

2

INTRODUCTIONThe succession of sedimentary rocks investigated is entirely marine, deposited relatively rapidly

within an extensional basin setting, is organic matter-rich and contains much carbonate, some of

which is undoubtedly of diagenetic origin (Irwin et al., 1977). The Kimmeridge Clay Formation

succession exposed at Clavell’s Hard on the South Dorset coast, including clay-rich mudstones,

organic matter-rich mudstones and coccolith limestones, is cross cut by a series of small, N-S

oriented normal faults, each containing multiple generations of carbonate cements. On the basis of

structural relationships, fractures propagate upwards (extensional fractures) as a result of excess

pore pressures. In effect, they are hydrofractures (Secor, 1965; Secor and Pollard, 1975; Sibson,

1981; Roberts et al., 1996).

The field-scale faults, have displacements of a few centimetres to 11m (Hunsdale and

Sanderson, 1998) and although the exposure is restricted to the cliff section and the foreshore

between high and low tides, the small size of the faults (a few metres displacement) compared to the

maximum thickness of the Kimmeridge Clay Formation at this locality (approx. 500 m) strongly

suggests that these veins are entirely contained within the Kimmeridge Clay Formation. Detailed

structural arguments to further support this case are presented in Guerrero-Munoz (1999). Given

the thickness of the formation at this locality these faults and associated calcite-filled veins have

therefore most probably trapped fluids that were expelled upwards from within the Kimmeridge

Clay Formation.

Depth-related reactions based on oxidation of organic matter within the mudstones

potentially have profound effects on the composition of the pore fluids, particularly during early

diagenesis (Scotchman, 1993). Oxic and post-oxic reaction zones occur beneath the sediment-water

interface and conditions rapidly became anoxic where sulphate reduction predominates. With anoxic

depositional waters, the sulphate reduction zone occurs directly beneath the sediment-water

3

interface and generally prevails down to depths of a few centimetres to several tens of metres

(Scotchman, 1989). Beneath, a transition occurs into the methanogenesis zone, with reactions

persisting to depths of about 1km where temperatures become too high for most bacteria to survive.

Thereafter, a transition to decarboxylation zone take place (approx. 1.5 km), where catagenic

hydrocarbon-generating reactions occur, a function of the temperature the sediments attain and their

residence time at any particular temperature. The oil ‘window’ is generally recognised as occurring

at temperatures of around 70° whilst gas generation dominates at temperatures exceeding 110°C

(Tissot and Welte, 1984). More generally, as moderate burial processes overlap with hydrocarbon

generation, and the Kimmeridge Clay Formation is both the major source rock and regional caprock

in many Mesozoic sedimentary basins around the United Kingdom Continental Shelf (UKCS), the

results may be relevant to both primary hydrocarbon migration processes and seal integrity.

Conjugate normal fault systems are important in hydrocarbon exploration and production in

different range of scales, they may produce barriers to lateral and vertical flow or increase

permeability during deformation processes, including hydrofracturing during palaeofluid expulsion

from hydrocarbon source rocks. Set of fractures may control migration and the interpretation is very

important in correlation of source(s), migration pathways, reservoir and seal. Small displacements in

faults may originate compartmentalization of a reservoir, seminal factor to be consider in order to

optimise a field development. Ferril et al. (2000) have cited the effects of crossing normal faults in

permeability anisotropy, they found greatest permeability parallel with the line of fault intersection.

GEOGRAPHICAL LOCATION OF THE STUDY SECTION

Field work was undertaken at the Kimmeridge Clay Formation of Late Kimmeridgian to Tithonian

age, east of Kimmeridge Bay along the south Dorset coast, UK. (Figure 1), where some 40 small

scale normal faults cut the upper part of the succession, here approximately 500m in total thickness

4

(Arkell, 1947), although the exposed upper part of Kimmeridge Clay Formation is approximately

only 80m thick. Extensive mudstone succesions are exposed, the mudstones are cut by faults

containing multiple generations of fractures and veins.

Figure 1. Location of outcrop section of the Kimmeridge Clay Formation, east of Kimmeridge Bayat Clavell’s Hard, Dorset coast, UK.

GEOLOGICAL BACKGROUND TO THE KIMMERIDGE CLAY FORMATION

Geological setting

The Dorset coast is located in the south-western part of the petroliferous Wessex Basin. Coastal

exposures provide an opportunity to analyse the petroleum geology of this basin, including source

rocks, carrier bed systems, a variety of reservoirs, live oil seeps, seal rocks and petroleum-trapping

structures (Underhill and Stoneley, 1998; Buchanan, 1998; Butler, 1998). The Wessex Basin, a post-

Variscan extensional structure initiated in the Permian, formed in response to a north-south

extension along east-west Hercynian thrust faults, and now occupies 80 000 km2 across southern

England extending into northern France (Hamblin et al., 1992; Hawkes et al., 1998). The Mesozoic

geological history of the area was dominated by extensional faulting, with Permian and Triassic

5

continental desert deposition being widespread across the basin. Up to 3 km of Permian to Triassic

sediments were deposited in the basin during the immediate post-Carboniferous subsidence

(Chadwick, 1986; Underhill and Stoneley, 1998). Basin subsidence continued throughout the

Jurassic and Early Cretaceous with deposition of an estuarine, coastal system, the Wealdan. In the

Jurassic, sedimentation changed from continental to shallow marine with cycles of shale, fine grained

sandstone and mudstone defined by Arkell (1947). Continental sedimentation returned to the north

in the Early Cretaceous. The Kimmeridge Clay Formation comprises a transgressive, cyclic sequence

of organic-rich and calcareous mudstone deposited during a period of maximum eustatic rise and

tectonic subsidence (Scotchman, 1991). Although deposition took place in a relatively uniform

environment, mudstone facies changes along the outcrop from dominantly organic-rich mudstones to

calcareous mudstones and thickness variations within the beds are apparent. The sedimentation rate

and the oxygenation of the water influenced the preservation of organic matter within the sediments

and therefore influenced the course and duration of the early diagenetic reactions within the

mudstones as well as any future hydrocarbon potential (Didyk et al. 1978).

The Mesozoic sequence of the area comprises a predominantly marine Jurassic series, and

includes a varied succession of mudstones, limestones and subordinate sandstones. Below the

Jurassic, in the Lyme Regis borehole (Warrington and Scrivener, 1980) encountered a sequence of

continental Triassic deposits beneath an unconformity. The Triassic sequence is basically non-

marine with exception of the marine Rhaetic beds near the top. The Sherwood Sandstone Group

belongs to this period and it is economically important because it is the reservoir rock for the lower

trap in the Wytch Farm oilfield in Dorset. The boundary between the Triassic and Jurassic periods

is defined by the ammonite Psiloceras. Jurassic and Cretaceous rocks are predominantly marine

excepting the Purbeck and Wealden beds (Figure 2). Ammonites from the marine Rhaetic beds

indicate a late Triassic age (Arkell, 1956) and mark the beginning of the Mesozoic transgression.

6

The whole formation has been subdivided into 48 beds on palaentological and lithological

grounds (Cox and Gallois, 1981) that can be correlated over wide areas based on the ammonite

content. The macro-fossil fauna is dominated by ammonites and bivalves.

Figure 2. Schematic representation of the Mesozoic stratigraphic sequence along Dorset coast.Modified from House, 1993; Scotchman, 1987 and 1989 with additions.

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Sedimentation appears to have been closely controlled by subsidence as indicated by the

close relationship between tectonic structure, the lithological character of the sediments and their

lateral persistence. In the tectonically stable areas, the carbonate-rich nature of the mudstones

suggest that the sedimentation rate generally equalled the subsidence rate, the organic-rich

mudstones of the basins indicate water deepening when subsidence rate considerably exceeded

sedimentation rate.

Source Rock Potential of the Kimmeridge Clay in the Wessex Basin

The Kimmeridge Clay Formation is widely regarded as the predominant source rock of the North

Sea oil (Barnard and Cooper, 1981). However, in the Wessex Basin it is not the major source to

known hydrocarbons, not because of its lithology and organic matter content, but because of its low

maturity, a function of its maximum burial depth.

The organic matter in the Kimmeridge Clay Formation of the Wessex Basin is dominantly

marine type II kerogen with only a minor contribution of terrestrial material (Farrimond et al.,

1984). Extracts from the Kimmeridge Clay Formation of the Wessex Basin show typical alkane

populations, although the lipid concentrations are unusual (Farrimond et al., 1984). Total organic

carbon (TOC) contents range from 0.9 to 57.2%, and are generally high, being highest in the

laminated mudstones (the so-called oil shales). The coccolith limestones and diagenetic cementstones

have the lowest amounts of TOC. The Kimmeridge Clay Formation exhibits a wide range of organic

maturity levels in north-west Europe, ranging from immature in UK onshore section (Gallois, 1979a;

Williams and Douglas, 1981) to the dry-gas generating stage in the central parts of the North Sea

Viking and Central grabens (Baird, 1986). According to Scotchman (1987), the vitrinite reflectance of

the onshore Kimmeridge Clay Formation ranges from 0.3% to 0.6%, while the Rock-Eval pyrolysis

T max. values lie between 412 and 437 ˚ C, indicating the samples to be immature and to lie within

the incipient zone of oil generation, as defined by Lewan (1985).

8

Organic geochemical studies along the Kimmeridge Bay outcrop show that along the Dorset

coast the Kimmeridge Clay Formation is immature (VR% 0.36% for the Whitestone Band and

0.31% for the Blackstone Band; Farrimond et al., 1984) although the Whitestone Band contains an

in situ partially degraded mature oil (Farrimond et al., 1984). There are several seeps (Mupe Bay,

Lulworth Cove, Osmington Mills and Durdle Door; Miles et al., 1993; Bigge and Farrimond, 1998)

and oil fields (Wytch Farm, Wareham, Kimmeridge Bay, Humbly Grove) in the Wessex Basin, and

most of these appear to have been sourced from the Lower Liassic mudstones, although possible

minor additions to the seeps and charges may have been derived from the Oxford Clay (Selley and

Stoneley, 1987). The Kimmeridge Bay Field (discovered in 1959) produces from fractures in the

middle Jurassic Cornbrash limestones and Oxford Clay, being sourced from the Lias (Evans et al.,

1998). Oil in the field was being generated by and migrating during the Early Cretaceous, with peak

generation in the Late Cretaceous (Selley and Stoneley, 1987). The migration of oil from the area of

generation to the Kimmeridge Bay Field reservoir must involve faults as well as a component of

lateral migration through carrier bed formations. The change in the faults from open migration

pathways to seals may reflect the onset of compression at the beginning of the Tertiary Period. The

oil was therefore in place at that time. However, the oil migrating up these pre-Albian faults would

have escaped to the surface, were they not covered and sealed by Upper Cretaceous strata at that

time. The continued replenishment of the Kimmeridge Bay Field suggests that some faults are still

open. The reservoir is provided by fractures in the Cornbrash limestone and Oxford Clay. Over 25

years this field has produced 2.5 x 106 bbl without decline. This seems more than the fracture

system in the closure can contain and suggests that the field is being constantly replenished from a

deeper reservoir (Selley and Stoneley, 1987).

9

THE KIMMERIDGE CLAY FORMATION AND ASSOCIATED FAULTS AT CLAVELL’S

HARD, DORSET COAST, UK.

The Kimmeridge Clay Formation along this section of coastline, in common with the general nature

of the Kimmeridge Clay Formation, comprises a cyclical combination of interbedded silty

mudstones, calcareous mudstones, coccolith limestones, kerogen-rich mudstones, nodular

cementstone horizons and continuous cementstone horizons, known locally as ‘stone bands’ , on

account of their hardness, a function of the intense diagenetic cementation (Irwin et al. 1977,

Scotchman, 1989). These beds are laterally continuous along the length of the outcrop, except where

interrupted by faulting or removed by erosion (Figure 3).

Figure 3. General view of Kimmeridge Bay showing the form of the Kimmeridge Bay anticline andthe type of exposure along the coast. Notice on top of the cliff (close to the crest of the anticline)the nodding donkey pumping crude oil from fractured Cornbrash in the Kimmeridge Bay oilfield; thewell has produced more than 2 million barrels since the 1960’s, more than the volume of thereservoir (Corallian Group) as it was originally mapped (Fleet et al., 1987; Stoneley and Selley,1986; Evans et al., 1998).

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The section investigated is exposed approximately for 1km east of Kimmeridge Bay between

the low tide ridges (known locally as ‘ledges’) at Yellow Ledge and Clavell’s Hard. Based on its

ammonite fauna, the succession has been divided into two major units: the Upper and Lower

Kimmeridge Clay (Blake, 1875; Arkell, 1947; House, 1993). The sequence exposed extends from the

Yellow Ledge Stone Band (Bed 37 of Cox and Gallois, 1981; Pectinatites scitulis zone) to the

Freshwater Steps Stone Band (Bed 48 of Cox and Gallois, 1981; Pectinatites pectinatus zone), all

within the Upper Kimmeridgian (Figure 4). The upper part of Kimmeridge Clay Formation is

approximately 80m thick in the study area. The sequence is dominated by various silty and

calcareous shales, typically comprising masive grey to dark grey mudstones, with compacted, well

laminated, (10-20 cm.) interbedded calcareous mudstones with abundant pyrite. A brownish-grey

bituminous mudstone horizon in the upper part of the sequence, known as the Blackstone Band or

Kimmeridge oil shale, varies laterally in thickness from 80 cm to 85 cm, is the major source rock

within the Kimmeridge Clay Formation. Thin bands of muddy dolomitic limestone (called

cementstone) occur at many levels, usually enclosed within the pale grey calcareous mudstone. The

sequence of Kimmeridge Clay Formation sediments exposed at Clavell’s Hard is summarised in

Figure 4 and is illustrated in Figure 5.

Five types of sediments are recognised on the basis of their outcrop characteristics and

known petrographic lithological fabrics (Macquaker and Gawthorpe, 1993):

1. Clay-rich mudstones, which are generally massive, rarely displaying sedimentary structures

except for occasional shell-lag deposits;

2. Silt-rich mudstones are characteristic by relic laminations and bioturbated fabrics, and are

typically pale grey in colour, and contain a low diversity fauna of ammonites and benthic bivalves;

11

3. Nannoplankton-rich mudstones dominated by coccoliths and form the well cemented ledges that

stand out from the adjacent mudstones. These are the so called ‘cementstone bands’ and are the

most competent mudstones in the succession;

4. Laminated mudstones are easily recognised on the basis of their dark brown colour, well

developed millimetric laminations and low density, as result of the extremely high organic matter

contents. Pyrite is also abundant.

5. Concretionary carbonate horizons composed of iron-rich carbonates (ferroan calcite and ankerite)

that are diagenetic in origin. These also form competent horizons in the mudstone sequence.

These are all mudstones, but differ in the proportions of clay, quartz silt and carbonate they

contain. The clay-rich and silty mudstones are typically massive and unlaminated, with a variable

content of shell material and are the softest mudstones in the succession. Organic matter-rich

laminated mudstones conspicuous by their dark grey to black or brown colour, and obvious

presence of organic matter and pyrite in hand specimen, form an intermediate degree of hardness.

The carbonate cemented nannoplankton mudstones and concretionary carbonate horizons are both

very hard and brittle with little or no remaining porosity, and are laterally continuous.

12

Figure 4. Schematic graphic sedimentary log of the Kimmeridge Clay Formation exposed along thecliffs east of Kimmeridge Bay towards Clavell’s Hard, based on field description and Scotchman(1989).

13

Figure 5. Photograph of the Kimmeridge Clay Formation exposed at Clavell’s Hard showing thecyclical sequence of interbedded calcareous mudstones and cementstone bands. The BlackstoneBand is the prominent dark grey mudstone horizon that intersects the cliff top at the base of thevalley to the left of the photograph.

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The faults

The section for study selected at Clavell’s Hard is cut by several small normal faults, each

associated with extensive calcite veining (Figure 6), 7 normal faults, with little or no evidence for

reverse reactivation, are well exposed. Displacements on these faults range from 0.05-2.5m.

Figure 6. Map of the normal faults present in the cliffs and foreshore at Clavell’s Hard.

All the faults are normal and the dominant direction of displacement is parallel to the dip of

the fault plane (i.e. dip-slip faults). The faults all trend essentially north-south and dip to the west,

with dips typically of 45-80˚, averaging 60˚. Faults 3 and 4 form a conjugate set, formed of two

intersecting, opposed-dipping normal faults developed by incidental intersection (Nicol et al., 1995).

Only two faults (Fault 4 and Fault 5) dip to the east. Results of measurements and displacement of

studied faults are listed in Table 1.

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Table 1. List of measurements and displacement of studied faults.

Fault Number Dip Strike Displacement (m)

F1 52˚ W 2˚ N 2.50F2 78˚ W 356˚ N 0.05F3 74˚ W 358˚ N 1.04F4 46˚ E 358˚ N 0.35F5 82˚ E 8˚ N 0.10F6 56˚ W 358˚ N 0.10F7 52˚ W 12˚ N 2.80

Fault morphology as observed at outcrop is strongly dependent on the nature of the host

Kimmeridge Clay Formation lithology that the faults cross-cut. Dependent on the competence of

the host mudstone, a variety of deformation features are recognised. These features are summarised

in Figure 7.

Within the clay and silt-rich mudstones the fault trace is always simple, with a single or

multiple slip surface often (but not always) associated with single or few carbonate vein fills. In

these weak mudstone horizons, carbonate vein fills are sometimes not present and the fault may be

represented by only a single slip plane surface. In the laminated mudstones and as the mudstones

become more cemented with carbonate (calcareous mudstones to cementstones) the complexity of

the fault zone increases. Calcareous mudstones and cementstones commonly display a complex

fault zone, several cm wide, with multiple slip surfaces, and several generations of carbonate veining.

Where significant competence contrasts are developed, large dilational jogs and pull-aparts are

present, up to 10’s of cm wide, with significant vuggy porosity and brecciation of both the host

mudstones and earlier vein material. Extension in the cemented layers is produced by inclined tensile

fractures, that refract with changing mudstone competence, to produce complex dilatational jogs and

pull-aparts.

16

Figure 7. Schematic illustration of the fault-geometry and its relationship to host Kimmeridge Claylithologies (based partly on Hunsdale and Sanderson, 1998), showing the development of veining,damage zones and dilational jogs.

A damage zone, composed of disturbed and faulted mudstone, with abundant tensile splay

fractures, is developed adjacent to the main fault slip surface. These splay fractures curve away

from the main fault surface at angles of 20-30°, and are commonly also carbonate cemented. This

damage zone also varies in thickness depending upon the lithology of the host sediment. In the

more massive, less competent clay rich and silty mudstones, the damage zone is absent or poorly

developed, and comprises only few sub-parallel splay fractures. In the more competent horizons

(such as the cementstones), the damage zone extends for many cm beyond the fault zone and

significant fracturing and brecciation of these layers is present outside of the fault zone, commonly

with zones of hydro-brecciation and veining. Where well developed, these splay fractures typically

bifurcate at their tips into several smaller strands. Examination of these structures in thin section

indicates that they propagate as very discrete planar fractures with finite widths of ca. 30um. A

17

common feature of the fault geometry and damage zone development is the concentration of veining,

brecciation and other damage zone features into the hangingwall of the faults.

Displacements on all the faults are small, ranging from a few millimetres up to 2.8 metres

(Figure 8). On some faults displacement decreases up the cliff from a maximum near the base,

suggesting that the faults are contained completely within the Kimmeridge Clay Formation, although

the displacement variation with increasing depth is not known. Further details of the fault

displacements and scaling relationships are given by Hunsdale and Sanderson (1998).

Figure 8. Sketch map of the displacements of the normal faults at Clavell’s Hard.

The abundance of small faults is clearly asymmetric. In most of the cases the number of

small faults is greater in the hanging-wall nearest to the main fault plane but falls away very rapidly.

This “clustering” effect results in narrow bands of intense deformation in the foot-wall very close to

the fault plane (within 20 cm). In the hanging-wall, small faults are less clustered and more spread

out than in the foot-wall. Therefore, examining in detail these deformation zones, the broader

damaged zone is in the hanging-wall and there is only one symmetric arrange across Fault 4. A

second series of small faults trend approximately east-west, displacement in these faults is minor,

typically less than 1 cm.

Faults F1 to F7 (Figure 6) were examined in detail. These faults have arrays of smaller

displacement faults associated with them that cluster either side of the main fault, defining a damage

18

zone in the mudstones. The extent and symmetry of the damage zone was investigated by measuring

the abundance of small faults in a series of traverses across these faults (Described as T in Figure 6).

The results of these traverses are listed in Table 2.

Table 2. Distance of extent of damage zone in faults examined

Fault and Deformation in Deformation in TotalTraverse foot-wall (cm) hanging-wall (cm) Deformation (cm)

F1(T4) 160 160 320F2(T1) 40 80 120F3(T2) 100 140 240F4(T2) 100 140 240F5(T5) 40 60 100F7(T3) 60 100 160

In order to define the width of the deformation zone, vein populations were counted in the

foot-wall and hanging-wall in a series of traverses at right angles to the fractures, results of these

traverses are reported in tables, and plotted in figures as histograms (See Table 3 and Figure 9 as

examples).

In all cases, the width of the damage zone does not exceed 4-10cm, although discrete clusters

of splay faults related but distinct from the damage zone may be developed up to 20cm away from

the fault slip surface. Variation in width of the damage zone is primarily a function of of lithology,

with the widest damage zones being developed in the most competent lithologies.

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Table 3. Results of vein populations counted in foot-wall and hanging-wall. Traverses at right angle to fault number 1.

Number of interval Vein populations Vein populations (every 20 cm) in foot-wall in hanging-wall

1 (0-20) 31 132 (20-40) 3 23 (40-60) 4 134 (60-80) 2 125 (80-100) 2 76 (100-120) 2 67 (120-140) 3 78 (140-160) 3 89 (160-180) 1 310 (180-200) 3 211 (200-220) 2 212 (220-240) 1 213 (240-260) 1 114 (260-280) 1 215 (280-300) 0 0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 150

5

10

15

20

25

30

35

Frequency

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Number of interval (every 20 cm)

Vein populations infoot-wall

Vein populations inhanging-wall

Figure 9. Fault No 1, histogram of vein populations.

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The calcite cemented vein systems in the faults

All faults examined are pervasively cemented with white coloured, sparry calcite, that is largely

undeformed; breccias are rarely developed, indicating that for the majority of faults, displacement on

any single surface took place before calcite cementation. The calcite cement forms large, elongate,

euhedral crystals that nucleate on fracture walls and grow towards the fault plane centres (Figure

10). In most cases, calcite fills the fracture; larger fractures maintain vuggy porosity in their centres.

The growth of euhedral, largely undeformed calcite crystals indicates that the fault fractures formed

open cavities during cementation; calcite cemented a cavity and did not grow displacively. Where the

faults cut lithologies of different competence (such as carbonate cemented horizons) dilational jogs

and pull-aparts are developed. These irregularities in the fault surface result in large cavities that are

filled with several generations of calcite cement. Occasionally, walls to dilational jogs are brecciated,

with fault plane rugosity being smoothed, and fragments of early cement generations being included

within the breccia.

Figure 10. Fractures along Fault 3 at Clavell’s Hard, showing open fractures partially filled withlarge, euhedral calcite crystals that nucleate on fracture wall and grow towards fault plane centres.Note abrupt fracture wall boundary.

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Although individual veins show little evidence of deformation subsequent to calcite

precipitation, multiple generations of calcite veining are clearly present (Figure 11). Later calcite

veins do not occur within or re-open earlier calcite veins, but develop adjacent to earlier formed

veins, indicating that the process of faulting and calcite precipitation results in strain hardening, i.e.

that the earlier formed cement veins are more competent than the host lithology. Where the fault

surface has rugosity, many veins of calcite (10’s to 100’s) are developed.

Figure 11. Cemented fault plane in massive clay-rich mudstones at Clavell’s Hard. Note thenarrow width of the damaged zone and the multiple generations of calcite veins indicatingdisplacement took place before calcite cementation, and that strain hardening has taken place.

The sequence of cementation is from the rock-wall to the middle of the fracture, where in

some cases a cavity exists, showing euhedral crystals. Well cemented dilational jogs appear in the

vertical exposure (Figure 12) and scarce cataclastic events (brecciation) remain transverse to the

hanging-wall in the northern direction.

22

Figure 12. Detail of a dilational jog in a well cemented cementstone band enclosed between massive(above) and laminated (below) mudstones. Note the pull-apart extensional nature of the jog, nowpartially filled with multiple generations of calcite veining. Note the displacement across the faultplane slip surface of approx. 35 cm and the increase width of the damage zone in the cemented layer.Calcite veining is present along the fault plane surface. Pen knife 7cm for scale.

23

Sampling

After mapping, faults and veins were photographed and representative samples were systematically

collected along different points of the fault system encompassing all the variability observed at the

outcrop (See figure 13 as example for location), including fault surfaces, small veins, dilational jogs,

breccias, and any cross-cutting relationships. Special care was taken to check that veins of all sizes

and faults of variable displacements were appropriately sampled.

Figure 13. Location of samples collected from conjugated fault (Faults number 3 and 4).

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Conclusions of the fault system

The N-S faults described in the Kimmeridge Clay Formation are all normal, conjugate faults with a

mean strike of 10° and a mean dip of 60°, and are spatially restricted to an area south of the

Purbeck-Isle of Wight inversion monocline (Arkell, 1947; Hunsdale and Sanderson, 1998), with

similar fault orientations and characteristics being recognised in the adjacent offshore Kimmeridgian

sediments (Donovan and Stride, 1961).

There are 3 possible tectonic settings in which these faults could have formed:

1. During overpressure generation whilst the Kimmeridge Clay Formation was undergoing burial in

the late Jurassic early Cretaceous, comparable to hydrofracturing (Cartwright, 1995);

2. During compressional tectonics associated with the formation of the Kimmeridge Bay anticline in

the hangingwall of the Purbeck Monocline (Arkell, 1936; Phillips, 1964; Bevan, 1985) comparable

to structures described for the Liassic sequence on the margins of the Bristol Channel (McGrath and

Davison, 1995); and

3. During overpressure related to the inversion and removal of the Cretaceous-Tertiary cover

sequence, similar to inversion hydrofracturing reported by Sibson (1995).

The focussing of the damage zone deformation within the hangingwall indicates that as the

faults propagated, extensional tensile stresses were asymmetrically distributed at the front of

upward-propagating fault tips, and that the Present Day coastal section represents a high level

tectonic window on the fault systems. The extensional nature of the faulting, absence of reactivation

structures and upward-propagating nature of the faults is consistent with a explosive hydrofracture

origin.

The scale of the faulting, despite its high level nature, and the presence of conjugate fault

networks, suggests that these faults do not penetrate for significant depths beneath the Present Day

exposure. Given the thickness of the Kimmeridge Clay Formation at this locality, these faults and

25

associated calcite-filled veins have therefore most probably trapped fluids that were expelled

upwards from within the Kimmeridge Clay Formation and are thus ideal for investigating the nature

and composition of fluids generated in this mudstone sequence during early burial.

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