relationships between stylolites and cementation in sandstone reservoirs: examples from the north...

94 (2007) 17–35www.elsevier.com/locate/sedgeo

Sedimentary Geology 1

Relationships between stylolites and cementation in sandstonereservoirs: Examples from the North Sea, U.K. and East Greenland

Martin Baron ⁎, John Parnell

Department of Geology and Petroleum Geology, Meston Building, University of Aberdeen, King's College, Aberdeen, AB24 3UE, UK

Received 14 November 2005; received in revised form 26 April 2006; accepted 28 April 2006

Abstract

The reservoir potential of hydrocarbon sandstone reservoirs may be significantly reduced by compartmentation as a result of thedevelopment of stylolites. A petrographic and fluid inclusion microthermometric study was performed on sandstones containingabundant stylolites from the Buchan, Galley and Scott Fields in the Outer Moray Firth, offshore Scotland, and from a palaeo-oilbearing sequence in East Greenland. The main objective of this study was to further constrain the temperatures and burial depths atwhich stylolitization occurs in sandstone reservoirs. The sandstones containing abundant stylolites are also characterized by theirhighly cemented nature. Numerous occurrences of quartz overgrowths clearly truncated by sutured stylolites are evident in all ofthe samples. Fluid inclusion microthermometry reveals that quartz cementation, which is interpreted to be coeval withstylolitization, occurred at minimum temperatures of between 86 and 136 °C. Basin modelling of the Scott and Galley Fieldsindicates that quartz cementation and stylolite development formed at depths greater than 2.5 km which were attained during rapidTertiary burial. The occurrence of hydrocarbon fluid inclusions within healed microfractures orientated at high angles to thestylolites suggests that these microfractures provided pathways for hydrocarbon migration in the highly cemented, lowpermeability zones associated with highly stylolitized sandstones.© 2006 Elsevier B.V. All rights reserved.

Keywords: Stylolites; Diagenesis; Fluid inclusions; Basin modelling

1. Introduction

Pressure solution is the process by which material isremoved by solution or diffusion from a discretesurface, the sides of which remain in close contact(Groshong, 1988). Pressure solution in sandstones iscommonly divided into two types: intergranular pres-sure solution and stylolitization. Intergranular pressure

⁎ Corresponding author. Present address: Badley Ashton andAssociates, Winceby House, Winceby, Horncastle, Lincolnshire,LN9 6PB, UK.

E-mail address: [email protected] (M. Baron).

0037-0738/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.sedgeo.2006.04.007

solution (sometimes referred to as grain-to-grain stylo-litization) takes the form of adjacent detrital grainshaving interpenetrating sutured or smooth contacts(Trurnit, 1968). In contrast, stylolites are intergranularserrated surfaces that are lined by insoluble constituentsof the enclosing rock (Tada and Siever, 1989). Both theprocesses are the same; the only difference is one ofscale.

Stylolites are common in carbonates and sandstones,but are also known to occur in a variety of other rocktypes, including cherts (Cox and Whitford-Stark, 1987),pegmatites and other igneous rocks (Burg and Ponce deLeon, 1985). They can be divided in to those which are

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18 M. Baron, J. Parnell / Sedimentary Geology 194 (2007) 17–35

orientated approximately parallel to bedding planes andthose which are not. Stylolites that are bedding parallelhave formed in response to compression as a result ofprogressive burial, whereas stylolites that are generallynot orientated parallel to bedding are believed to haveformed as a result of tectonic compression (Railbeck andAndrew, 1995). This is because stylolites are believed toform perpendicular to the direction of maximumprincipal stress (Onasch, 1994).

A number of factors are thought to influence styloliteformation in sandstones, including temperature, pres-sure, mineralogy of the sandstone host and mineralogyof the stylolite. Although most sandstones that haveexperienced temperatures of more than 250 °C havedeveloped some evidence of pressure solution (Gratz,1991; Wu and Groshong, 1991; Dewer and Hajash,1995) the minimum burial depth required for stylolitesto develop in sandstones remains uncertain. Theimportance of pressure in stylolite development remainsa matter of much debate. Some worker believe thatdissolution is dependent on the stress at grain contacts(e.g. Dewers and Ortoleva, 1990; Mullis, 1991; Sheldonet al., 2003), whereas others believe that pressure onlyhas a limited affect on stylolite development insandstones (see Oelkers et al., 2000). Numerous studieshave also shown that the presence of mica and/or illite

Fig. 1. Major structural elements in the Outer Moray Firth of the Nor

clays promotes pressure solution in quartz (Pittman etal., 1992; Bjørkum, 1996; Oelkers et al., 1996). Thesestudies have illustrated that pressure solution in somequartzose sandstones only occurred at interfaces be-tween quartz and mica/illite clays, with no pressuresolution occurring at quartz/quartz interfaces. The causeof this has been attributed to both enhanced diffusion atmica/clay surfaces (Weyl, 1959) and to an increased pHassociated with mica/clay decomposition (Bjørlykke,1979). Bjørkum (1996) further suggested that stress-induced intergranular pressure solution in quartzosesandstones is either not as common as believed, or maynot have existed at all in conditions encountered duringdiagenesis. It is also believed that the mineralogy of theactual stylolite seam may also influence the rate ofquartz dissolution and stylolite development. Walder-haug et al. (2006) suggested that stylolites dominated bykaolin show less pressure solution than those dominatedby illite and mica because kaolin possesses a lowersurface charge which leads to a reduced thermodynamicdrive for quartz dissolution.

Pressure solution, including stylolitization, is thoughtby many workers to be a major source of silica forsandstone cementation in sandstones. Pressure solutionwill have a detrimental effect on reservoir quality insandstones by removing intergranular porosity due to

th Sea with the location of the Galley, Scott and Buchan Fields.

19M. Baron, J. Parnell / Sedimentary Geology 194 (2007) 17–35

greater packing and increased quartz cementation,which is the main cause of porosity loss in sandstonesburied in excess of 2–3 km (Giles et al., 2000). Thisstudy documents stylolites from four sandstonesequences that contain, or have contained in the past,significant hydrocarbon accumulations. It investigatesthe ambient temperatures, compositions and relativetiming of the fluids present in the sandstones duringdiagenesis and stylolitization. The main objective of thisstudy is to further constrain the temperatures and burialdepths at which stylolitization occurs in sandstonereservoirs. The locations presented are the Galley, Scottand Buchan Fields in the Outer Moray Firth region ofthe North Sea, offshore U.K. (Fig. 1). Observations froma stylolite-bearing sequence from the Lower–MiddleJurassic of East Greenland (Fig. 2) are also presented.This sequence is the best known onshore analogue to theJurassic rocks which host more than 80% of thediscovered oil and gas resources in the North Atlantic

Fig. 2. Simplified geological map of East Greenland with sampledlocality. Modified from Stemmerik et al. (1997).

region. The occurrence of extensive pore-filling bitumenattests the presence of past oil accumulations (Price andWhitham, 1997).

2. Methodology

The samples chosen for this study include sandstonescontaining well developed stylolites that are approxi-mately parallel to bedding planes. The stylolites haveamplitudes ranging from 0.1 to 1.0 cm and thickness ofbetween 0.2 and 4 mm. The stylolites from EastGreenland, the Scott Field and Galley Field havesharp-peak and sutured types, whereas those from theBuchan Field possess more primitive wave-like andsutured types according to classification of Park andSchot (1968). Most of them reach the entire length of thecore indicating that they have minimum lengths of10 cm. The stylolites are generally composed of illiteoften with minor muscovite, iron oxides and pyrite.

Fluid inclusion microthermometric analysis wasperformed on selected quartz, dolomite and calciteminerals that were prepared as 100 μm thick doublypolished wafers using standard techniques (Shepherd etal., 1985). The petrography of fluid inclusion assem-blages was first examined at low magnifications using aNIKON Eclipse E600 microscope equipped with bothtransmitted white and ultraviolet light sources. Inclu-sions were divided into primary, pseudosecondary andsecondary types based on the criteria of Roedder (1984).Fills were estimated at room temperature using thestandard charts of Roedder (1984). Ultraviolet light(UV), with an excitation wavelength of 365 nm, wasprovided by a high-pressure mercury lamp with a420 nm barrier epi-fluoresence filter that allows only thelong wavelength ultraviolet light to reach the sample.Images were captured using Leica DC 200 computersoftware and a digital camera. Microthermometricanalysis was carried out using a calibrated LINKAMTH-600 stage and measured with the aid of a videoscreen coupled to a Nikon OPTIPHOT 2-POL micro-scope. Homogenization temperature (Th) measurementswere determined using a heating rate of 10 °C/min−1.First ice melting (Te), final ice melting (Tm) and salthydrate melting (TmHH) temperature measurementswere determined using a heating rate of 1 °C/min−1.Salinities were estimated from Tm measurements usingthe methods of Bodnar (1993).

Fluid inclusions in all the samples occur withinprimary and secondary settings in quartz, calcite anddolomite cements as well as detrital quartz grains. Theprimary inclusions in quartz, calcite and dolomitecements possess a variety of morphologies including


negative crystal shape, sub-negative crystal shape andirregular morphologies. They range in size from <2 to8 μm and at room temperature they are two-phase(liquid- and vapour-filled) with fills of approximately0.94 to 0.97. Secondary fluid inclusions are moreelongate in morphology and range in size from <2 to18 μm. At room temperature they are also two-phase(liquid- and vapour-filled) with approximate fills of 0.80to 0.95.

When viewed under ultra-violet light the samplesfrom the North Sea, all contain fluorescing hydrocarbonfluid inclusions. The samples from the Scott and GalleyFields contain hydrocarbon fluid inclusions in primaryand secondary settings, whereas the hydrocarbon fluidinclusions in the samples from the Buchan Field onlyoccur in secondary settings. The hydrocarbon fluidinclusions from the Galley Field possess two-phase(liquid and vapour) with a liquid phase that mainlyfluoresces green in colour, whereas those in the samplesfrom the Buchan and Scott Fields typically possess anon-fluorescing vapour phase and a yellow-fluorescingliquid phase. These fluorescence colours suggests thatthe hydrocarbons present in the fluid inclusions havemoderate API gravities (Bodnar, 1990).

Many workers have argued that fluid inclusions inquartz regularly thermally re-equilibriate due tointernal overpressuring of individual inclusions as aresult of heating during burial. High internal overpres-sure in individual fluid inclusions results in stretchingthrough permanent plastic deformation. If this occurs,the fluid inclusions will increase in volume anddecrease in density, which will result in measured Thvalues being higher than the original entrapmenttemperature. In order to try and eliminate Th valuesfrom fluid inclusions that have thermally re-equili-briated the methods of Goldstein and Reynolds (1994)were applied. This method involves dividing groups offluid inclusions into fluid inclusions assemblageswhich are defined as the most finely discriminated,petrographically associated groups of fluid inclusions.In these samples, a fluid inclusion assemblage is agroup of fluid inclusions that are contained within asingle cement growth zone (quartz, calcite or dolomite)or a single healed microfracture. 90% of fluidinclusions within a single assemblage should yield Th

values that are within 10–15 °C of each other. Onlyfluid inclusions values from fluid inclusion assem-blages that obey the criteria of Goldstein and Reynolds(1994) are reported.

SEM-CL images were collected on a Phillips XL-30CP SEM. The carbon coated doubly polished waferswere carbon coated and mounted on glass slides using a

double-sided adhesive. Images were captured using anaccelerating voltage of 15 kV, a beam current of 1 nAand a working distance of 1 mm from the cathodolu-minescence detector to the sample.

3. Galley Field, Outer Moray Firth, U.K.

3.1. Geological setting

The Galley Field is situated in the Witch Groundgraben, in the Outer Moray Firth, offshore Scotland(Fig. 1). The Outer Moray Firth is part of the North Searift system which is a failed rift of Jurassic–Triassic ageformed on a basement transected by Palaeozoic lines ofweakness developed principally along NE–SW Cale-donian trends and NW–SE trans-European Fault zonetrends (Erratt et al., 1999). The reservoir is composed ofthick packages of Upper Jurassic sand-dominatedgravity flow deposits (Boote and Gustav, 1987). Thesedeposits are enclosed within the Upper Jurassic toearliest Cretaceous Kimmeridge Clay Formation andwere deposited in a deep marine environment (Parting-ton et al., 1993). During deposition of this sequence,rapid subsidence in the Witch Ground graben created adeep depression in response to incipient rifting. As thegraben shoulders rose, unconsolidated sediments wererecycled and deposited as gravity flow sands insubsiding lows (Boote and Gustav, 1987). It is uncertainwhether the Galley field is sealed by a stratigraphic or afault seal trap (Moseley, 1999).

3.2. Observations

The Upper Jurassic sandstone reservoir of theGalley Field contains numerous stylolites towards thebase of the sequence. The sandstones are well sortedquartz arenites, containing over 95% of unstraineddetrital quartz. Grains sizes are medium to coarse(Wentworth scale), with most of the grains in the 400to 800 μm size range. Extensive cementation close tothe stylolites has occluded the intergranular porosity.From visual estimations cement volumes are greatest inareas around the stylolites. The dominant cement isquartz with subordinate pyrite, clays, calcite and iron-titanium oxides. Quartz cements occur primarily asfine-grained (50–500 μm in size), syntaxial over-growths, which are in optical continuity with thedetrital grains (Fig. 3). A minor amount of detritalfeldspar grains have been partially dissolved prior toquartz cementation. Pyrite form euhedral, fine-grainedcrystals (20–50 μm) enclosed within quartz over-growths, indicating that pyrite predated quartz

Fig. 3. (A) Thin section image from the Galley Field of a quartzovergrowth (O) coating detrital quartz grains (D) which are truncatedby a stylolite (S) viewed under plane polarised light. Also shown areprimary aqueous fluid inclusions hosted in a quartz overgrowth(arrowed), (B) image of a stylolite (arrowed) from the Galley Fieldand (C) image of quartz cement hosted fluorescing hydrocarbon fluidinclusions (arrowed) at the contact between detrital quartz grains (D)and quartz overgrowths (O) from the Galley Field. Viewed under UVlight.


precipitation. Minor authigenic clays (mainly kaoliniteand illite) are either partly enclosed by quartz cementsor occur as partial coatings to quartz overgrowths. The

authigenic clays are in turn partly enclosed by late-stage calcite cements indicating that calcite precipita-tion was the last diagenetic event.

SEM-CL reveals that there are two phases of quartzcement: a non-luminescing quartz and a mottledluminescing quartz. The non-luminescing quartz alwaysoccur as 20 to 100 μm wide crystals which coat thedetrital grains and are therefore interpreted as precipi-tating before the mottled luminescing quartz. Themottled luminescing quartz occurs towards the centreof the intergranular pores. The detrital grains containnumerous healed microfractures that are orientated athigh angles to the stylolite seams. These microfracturesare filled by quartz with a similar luminescing characterto the surrounding non-luminescing quartz cement. Thehealed fractures are continuous with the early-stage,non-luminescing quartz cements indicating that theywere formed simultaneously. Only very minor amountsof intergranular pressure solution are evident with manyof the detrital grains possessing point-to-point contacts.

The sandstones adjacent to the stylolites possess anumber of healed transgranular microfractures many ofwhich are orientated at high angles to the stylolitesurfaces. These healed microfractures are identified bythe presence of abundant, small (1 to 5 μm in diameter)fluid inclusions (Fig. 3). Cross-cutting relationshipsbetween stylolite seams and the healed microfracturestypically vary within individual samples, with thehealed microfractures either terminating at the stylolitesurface or occasionally cutting across it. Open micro-faults with very small amounts of shear displacementsare also present in association with some of thestylolites. These microfaults have typical shear dis-placements of 10 to 100 μm and only occur in theimmediate vicinity of the stylolite surfaces. Thesemicrofaults are also typically oriented at high angles tothe stylolite surfaces and are therefore likely to haveformed at a similar time to the healed microfractures.These relationships suggest that the healed microfrac-tures and open microfaults are broadly coeval with thestylolites.

Total Th values of primary aqueous inclusions hostedin quartz overgrowths range from 85.8 to 128.9 °C(Table 1 and Fig. 4). The inclusions hosted within theearly-stage non-luminescing quartz cement typicallypossess Th values in the 90 to 100 °C range, whereas theinclusions hosted within the late-stage mottled lumi-nescing quartz typically possess slightly higher Thvalues in the 100 to 120 °C range. A number of healedfractures, containing fluid inclusions, are orientated athigh angles to the stylolite seams. Four inclusionshosted within these healed microfractures have similar

Table 1Fluid inclusion microthermometric data: Th represents homogenisation temperatures

Location Composition Locationand host

Th range Th mean(no. measurements)

Salinity(no. measurements)

Galley Field(15/23-4a: 4151.5 m)

Aqueous-salt Primary quartzcement

98.6 to 122.3 110.1 (10) 2.1 to 17.3 (5)

Hydrocarbon Primary quartzcement

87.6 to 98.5 92.9 (10) –

Aqueous-salt Secondary quartzcement

111.2 to 111.4 111.4 (2) –

Galley Field(15/23-4a: 4155.9 ft)


84.8 to 127.5 105.0 (17) 2.4 to 11.7 (10)


117.6 to 118.7 118.2 (2) –


83.7 to 94.6 87.2 (10) –

Hydrocarbon Primary calcitecement

85.2 to 86.6 86.0 (3) –

Galley Field(15/23-4a: 4159 m)


90.2 to 128.9 113.2 (13) 2.1 to 6.5 (5)

Hydrocarbon Secondary detritalquartz

70.6 to 84.8 78.2 (5) –

Scott Field(15/22-c2z: 3898.7 m)


88.4 to 118.6 104.3 (17) 0.4 to 19.3 (12)


128.7 to 132.2 130.2 (3) –


74.8 to 78.5 77.2 (3) –

Scott Field(15/21a-alz: 4095.0 m)


84.8 to 109.5 96.3 (16) 2.6 to 19.0 (9)


126.3 to 135.3 129.7 (6) 5.1 to 7.7 (2)


81.6 to 89.9 85.7 (2) –


74.8 to 106.0 94.0 (6) –

Scott Field(15/21a-alz: 4119.7 m)


89.9 to 119.4 101.8 (16) 4.5 to 8.0 (9)


121.4 to 128.3 125.4 (3) 4.5 to 5.3 (2)


66.3 to 88.3 81.3 (10) –


81.4 to 85.5 83.8 (3) –

Buchan Field(21/1-8: 2125.05m)

Aqueous-salt Primary dolomitecement

133.3 to 178.9 159.9 (8) 13.9 to 22.2 (8)

Aqueous-salt Primary calcitecement

81.5 to 90.8 86.5 (7) 20.9 to 22.0 (4)

Buchan Field(21/1-8: 3080.65 m)


90.5 to 131.0 118.5 (5) 0.5 to 1.4 (3)


131.2 131.2 (1) –

Buchan Field(21/1-8: 3117.69 m)


72.0 to 107.0 85.9 (8) 12.4 to 14.7 (7)


132.0 to 144.4 140.3 (5) 15.2 to 17.5 (4)


81.1 to 112.6 101.4 (8) 4.8 to 12.7 (7)

Buchan Field(21/1-8: 3137.30 m)


122.4 to 147.7 135.9 (4) 1.7 to 3.6 (4)


Table 1 (continued)

Location Composition Locationand host

Th range Th mean(no. measurements)

Salinity(no. measurements)

Buchan Field(21/1- 3137.30 m)

Hydrocarbon Secondary quartzcement

100.2 to 104.1 103.1 (5) –


136.8 to 151.6 145.1 (4) 7.0 to 8.4 (2)

Aqueous-salt Primary dolomitecement

150.8 to 160.7 159.6 (6) 8.4 to 11.7 (5)

Buchan Field(21/1-8: 3141.75 m)


85.0 to 129.2 103.9 (11) 1.1 to 4.1 (7)


84.1 to 86.9 85.5 (2) –

Buchan Field(21/1-8: 3162.20 m)


93.7 to 107.2 99.9 (6) 0.5 to 4.2 (5)


132.0 to 146.1 140.4 (5) 0.7 to 1.4 (3)


103.0 to 123.4 113.1 (9) 13.4 to 18.2 (7)

Bjorndal, SE GreenlandOxfordian OlympenFormation


71.1 to 116.8 93.0 (31) 1.9 to 5.1 (9)


132.0 to 136.1 134.0 (4) 9.1 to 5.7 (2)

Bjorndal, SE GreenlandCallovian VardeklÆftFormation


69.5 to 121.6 94.1 (16) 4.18 to 9.60 (4)


115.2 to 129.8 122.5 (2) 1.6 (1)

Salinities calculated using the regression of Bodnar (1993).


Th values (111.2–118.7 °C) to the primary inclusions inthe quartz overgrowths. Tm values of primary aqueousinclusions hosted in quartz overgrowths range from−13.5 to −1.2 °C, indicating that salinities are variable(2.07 to 17.34 wt.% NaCl eq.). The primary inclusionshosted in the early-stage non-luminescing quartzcements are less saline than the primary inclusionshosted in the late-stage mottled luminescing quartzcement. The Th values of primary and secondaryhydrocarbon fluid inclusions are broadly similar, withvalues ranging from 70.6 to 98.5 °C. Upon heating allthe inclusions homogenise via a bubble surfacetransition (i.e. L+V→L). The Th values of themajority of the hydrocarbon fluid inclusions arelower than the associated aqueous inclusions indicatingthe presence of an undersaturated petroleum. Manyworkers have argued that if hydrocarbon and aqueousinclusions were entrapped cogenetically they can beconsidered to have been trapped under equilibriumconditions and the aqueous inclusions are thereforelikely to be saturated with respect to the more water-soluble hydrocarbons (e.g. methane) at the time oftrapping (Nedkvitne et al., 1993; Munz et al., 1998).The Th values of aqueous-filled fluid inclusions thatare methane saturated will reflect the true temperature

of entrapment, with no pressure correction required(Hanor, 1980). The Th values of aqueous inclusionsthat do not occur in association with hydrocarboninclusions represent minimum temperatures of entrap-ment, whereas the Th values of aqueous inclusions thatdisplay good evidence for cogenetic trapping withhydrocarbon inclusions represent true temperatures ofentrapment.

4. Scott Field, Outer Moray Firth, U.K.


The Scott Field is located around 12 km west of theGalley Field in the Outer Moray Firth (Fig. 1). The fieldcame on-stream in 1993 and contains an estimatedreserve of 85 million m3 of oil (Brennand et al., 1998).The trap is series of complex fault blocks, which arearranged radially around a central crestal area. The fieldconsists of a series of fault compartments with differentoil–water contacts in each reservoir (Moseley, 1999).Oil is mainly reservoired in the Upper Jurassic PiperFormation, which is a fluvial to wave-influencedshallow-marine deltaic to shelfal sand-dominated suc-cession (Harker et al., 1993). These sandstones are

Fig. 5. (A) Image of a stylolite (arrowed) from the Scott Field and (B)thin section image from the Scott Field of a quartz overgrowth (O)coating detrital quartz grains (D) which are truncated by a stylolite (S)viewed under plane polarised light.

Fig. 4. Histograms of homogenisation temperatures of fluid inclusionsfrom the Galley Field.


massive and structureless, with most of the originalfabrics destroyed by extensive bioturbation. They arehigh quality sandstones with porosities in excess of 25%and permeabilities ranging from 1D to 2D in the crest ofthe field (Harker and Rieuf, 1996). Reservoir qualitydecreases progressively with depth due to increasedquartz cementation and compaction (Guscott et al.,2003). The Piper Formation is situated below theKimmeridge Clay Formation, which acts as both asource and a seal for this play.

4.2. Observations

The samples in this study are moderately well sorted,fine- to medium-grained (grain sizes 0.4 to 1.5 mm indiameter), subarkosic sandstones. The dominant detritalmineralogy is monocrystalline quartz, with subordinateK-feldspar, plagioclase feldspar, lithic fragments andpolycrystalline quartz grains. From visual estimations,cement volumes are greatest in areas around thestylolites. Cements are dominated by quartz withminor calcite, K-feldspar and clays also present. Fine-grained (0.02–0.8 mm in diameter) quartz and K-feldspar cements occur as syntaxial overgrowths. Quartz

cements also fill partially dissolved detrital feldspargrains indicating that there was a phase of dissolutionbefore quartz cementation. SEM-CL reveals that thequartz cements are composed of an early-stage non-luminescing and a late-stage mottled luminescingcement. Calcite cements are ferroan in compositionand occur as fine-grained (grain sizes of 0.1 to 0.2 mm)crystal masses that enclose quartz and K-feldsparovergrowths indicating that calcite post-dated quartzand K-feldspar cementation. Rare authigenic clays(kaolinite and illite) occur as uniform thin rims (2–8 μm) coating quartz, K-feldspar and calcite cementsimplying a late-stage authigenic origin. Minor pyrite,dolomite and barite cements are also present. Numerousoccurrences of quartz overgrowths truncated by thestylolites are evident (Fig. 5) in all of the samples. Evenclose to the stylolites no detrital grains with undulose


extinction are visible, indicating that the grains are nosignificantly strained. Only very minor amounts ofintergranular pressure solution are evident.

The quantity of volume loss by dissolution can becalculated from stylolites by assuming that columnamplitude equals width of dissolved material, surfaceswere initially planar and the amplitude increased asdissolution progressed (Tada and Siever, 1989). Thismethod is recognised to only yield a minimum volumeloss because it is possible that a flat stylolite could beproduced if the components of pressure solution werethe same on both sides of the stylolite (Groshong,1988). The stylolite spacing ranged from 10 to 75 cm,which equates to a minimum volume loss of 1.8%from the depth interval where the sandstones weresampled.

Th values of primary aqueous inclusions hosted inquartz overgrowths range from 84.8 to 142.7 °C, with anaverage of 100.8 °C (Table 1 and Fig. 6). Th values ofprimary inclusions hosted in the early-stage non-luminescing quartz are similar to those hosted in thelate-stage mottled luminescing quartz. They possess

Fig. 6. Histograms of homogenisation temperatures of fluid inclusionsfrom the Scott Field.

variable Tm values ranging from −15.8 to −0.2 °C,which equate to salinities in the 0.4 to 19.3 wt.% NaCleq. range. The inclusions with the more salinecompositions and higher Th values are always hostedin the late-stage mottled luminescing quartz. Since theseinclusions occur in association with hydrocarboninclusions, the Th values can be considered as truetrapping temperatures. Th values of hydrocarbon inclu-sions hosted in primary settings in quartz cements andsecondary settings in detrital quartz grains and quartzcements range from 66.3 to 106.0 °C (average 84.7 °C),with homogenisation occurring via a bubble surfacetransition. Th values of the aqueous inclusions hostedwithin healed fractures in quartz overgrowths anddetrital grains oriented at high angles to the styloliteseams have Th values of between 121.4 and 135.3 °C(average 128.7 °C). Many of these inclusions also occurin the same trails as hydrocarbon inclusions, whichmeans that they can also be considered as true trappingtemperatures.

The formation waters of the Scott Field are ofuniformly high salinity, in the range of 120,000–130,000 mg l−1 (Dean, 1994). This equates to a salinityof approximately 12–13 wt.% NaCl eq. which is similarto the salinities of the higher temperature primaryaqueous inclusions hosted in the late-stage quartzcements. The lower temperature fluid inclusions hostedwithin the early-stage quartz cement possess more dilutecompositions indicating that the salinity of the porefluids increased greatly during quartz cementation.

5. Buchan Field, Outer Moray Firth, U.K.


The Buchan Field is also located in the Outer MorayFirth, offshore Scotland (Fig. 1). The reservoir of theBuchan Field is composed of Old Red Sandstone (ORS)siliclastic sediments which are unconformably cappedby Cretaceous calcareous mudstones with minorargillaceous carbonates creating an anticline type trap(Burnhill and Ramsay, 1981). The field, which wasonline between 1981 and 1995, contained an estimated13 million m2 of recoverable oil (Brennand et al., 1998).The ORS reservoir has a very low primary porosity, dueto extensive cementation and only produced from anopen fracture porosity (Hill and Smith, 1979). Thesequence is composed of Devonian to Visean conglom-erates, sandstones and siltstones, with calcrete horizonsoccurring towards the top of the sequence. Thesediments are often arranged in fining upward cyclesand have been interpreted by Richards (1985) to have


been deposited in a system of fluvial channels withassociated sheetflood sediments deposited in a hot aridclimate.

5.2. Observations

The samples in this study are pale orange coloured,moderately well sorted, medium- to very coarse-grained(grain sizes 0.5–1.5 mm in diameter), subarkosicsandstones. The dominant detrital mineralogy is mono-

Fig. 7. (A) Thin section image of a stylolite (arrowed) from the BuchanField, viewed under plane polarised light; (B) thin section image fromthe Buchan Field of a quartz overgrowth (O) coating a detrital quartzgrain (D) which is truncated by stylolite (S) viewed under planepolarised light; and (C) image of hydrocarbon fluid inclusions(arrowed) within healed microfractures in a detrital quartz grainfrom the Buchan Field. Viewed under UV light.

and polycrystalline quartz, with subordinate K-feldspar,plagioclase feldspar, chert, lithic fragments and micagrains. From visual estimations, cement volumes aregreatest in areas around the stylolites. Calcite, dolomite,quartz, K-feldspar, pyrite, iron oxides and clays are themain cement phases. Fine-grained (0.02–0.8 mm insize) quartz and K-feldspar cements occur as syntaxialovergrowths which are in optical continuity with theadjacent detrital quartz and K-feldspar grains (Fig. 7).SEM-CL reveals that only one phase of homogeneousquartz cement is present. Calcite and dolomite cementsare relatively fine-grained, with grain sizes mainly in0.10 to 0.20 mm range. There has also been minorreplacement of detrital grains prior to calcite cementa-tion, evidenced by grain boundary etching. Calcitecements are poikilotopic and inhibit overgrowthsforming on detrital quartz and K-feldspar grainsindicating that calcite cements pre-date both quartz andK-feldspar cements. Dolomite cements enclose quartzand K-feldspar overgrowths indicating that dolomitepost-dated calcite, quartz and K-feldspar cements. Pyritecements are often extensively or completely altered tooxide minerals, such as goethite. Fine-grained hematiteoccurs as thin 2 to 10 μm wide uneven rims coatingdetrital grains. Authigenic clays occur as uniform thinrims (2–8 μm thick) coating detrital grains and earlycements implying an authigenic origin.

Some of the detrital quartz grains display intergran-ular pressure solution evidenced by adjacent quartzgrains possessing concavo-convex and occasionallysutured contacts. Although the detrital feldspars grainsare highly fractured, no intergranular pressure solutionis evident in any them suggesting that stresses weredissipated through fracturing rather than pressuresolution.

Th values of primary aqueous inclusions hosted inquartz overgrowths range from 72.0 to 147.7 °C, with anaverage of 110.1 °C (Table 1 and Fig. 8). Theseinclusions possess variable Tm values ranging from−10.7 to −0.3 °C, indicating that salinities are in the 0.5to 14.7 wt.%NaCl eq. range. Th values of primary calcitecement-hosted aqueous inclusions range from 81.1 to131.2 °C. Variable Tm values indicate that theseinclusions possess salinities ranging from 4.8 to22.0 wt.% NaCl eq. The primary aqueous inclusionshosted in dolomite cements yield Th values of 133.3 to178.9 °C. These inclusions yield variable Tm valuesindicating that salinities were between 3.7 and 22.2 wt.%NaCl eq. A number of the larger, higher salinitydolomite-hosted inclusions yield Te values rangingfrom −54.5 to −50.1 °C (n=4), indicating the presenceof CaCl2 in addition to NaCl. These inclusions also yield

Fig. 8. Histograms of homogenisation temperatures of fluid inclusionsfrom the Buchan Field.


TmHH values in the range of −14.1 to −14.4 °C (n=3).According to the NaCl–CaCl2–H2O diagram of Oakes etal. (1990), Tm and TmHH values in this range correspondto molar Na/Ca ratios of approximately 3. Th values ofthe hydrocarbon fluid inclusions range from 84.1 and104.1 °C with homogenization always occurring viabubble surface transitions (i.e. L+V→L). The forma-tion waters of the Buchan Field are of high salinity, witha value of 190,000 mg l−1 (Smalley and Warren, 1994).This equates to approximately 19 wt.% NaCl eq. whichis a similar composition as the higher salinity values ofaqueous inclusions hosted in quartz, calcite and dolomitecements.

6. Traill ø, East Greenland


A Devonian to Cretaceous sedimentary successionunconformably overlies Archean metamorphic rocks inSouth East Greenland (Fig. 2). The Devonian toCretaceous succession occurs within westward-dipping,5 to 30 km wide, fault blocks which are bounded bynormal faults displaying multiple Devonian to Tertiaryactivation (Price andWhitham, 1997). These fault blocksformed during episodes of extension affecting the entirelength of East Greenland after the Caledonian orogeny.Until increased sea-floor spreading began during Palaeo-gene times, these rift basins occupied a position adjacentto the Vøring basin, which now lies offshore westernNorway (Talwani and Eldholm, 1977). In the Traill øregion of South East Greenland Devonian, Carbonifer-ous and Early Triassic, siliclastic and carbonate depositsare exposed. These are conformably overlain by Middleto Upper Jurassic sandstones and minor mudstones. ThisJurassic sequence has been used for many years as anonshore analogue for hydrocarbon reservoirs in thenorthern North Sea and offshore Norway. These strata,which record sedimentation during cycles of overalltransgression and regression, were deposited in delta,slope and base of slope environments (Surlyk, 1991).Unconformably overlying the Jurassic sediments areCretaceous mudstones with minor sandstones andconglomerates (Larsen et al., 1998). Numerous tholeiiticdykes and sills, dated at 55 Ma, and two syenite plutons,which have been dated at 35 Ma (Noble et al., 1988), arealso exposed on Traill ø.

6.2. Observations

The Jurassic sandstones at Bjørndal on Traill øcontain numerous stylolites (Fig. 9). The sandstone

Fig. 9. (A) SEM-CL image of rehealed microfractures (arrowed)orientated at high angles to a stylolite from East Greenland; (B) thinsection image from east Greenland of a quartz overgrowth (O) coatinga detrital quartz grain (D) which is truncated by a stylolite which iscomposed of illite (IS) and muscovite (MS) viewed under planepolarised light; and (C) SEM-CL adjacent detrital quartz grains,displaying intergranular pressure solution (arrowed), which arecemented by weakly zoned quartz.

Fig. 10. Histograms of homogenisation temperatures of fluid inclusionsfrom East Greenland.


hosting the stylolites are moderately sorted quartzarenites containing approximately 95% monocrystallinedetrital quartz and minor metamorphic lithic fragments.They have fine to medium grain sizes (0.1 to 2mm), withmost of the grains in the 300 to 500 μm size range. Fromvisual estimations cement volumes are greatest in areasaround the stylolites. Extensive quartz cementation has

nearly completely occluded porosity. Quartz cementsoccur primarily as fine granular (0.05–0.30 mm in size),syntaxial quartz overgrowths, which are in opticalcontinuity with the adjacent detrital quartz grains.Additional cements include fibrous authigenic clays(mainly illite), which occur as uniform thin (2–8 μm)coatings to quartz overgrowths. Minor authigenic clayalso occurs as partial coatings of detrital quartz grains.This authigenic clay is partly enclosed by quartz cementsindicating that clay precipitation both pre-dated andpost-dated the main phase of quartz cementation. Solidbitumen occurs intermixed with authigenic late-stageclays coating the quartz overgrowths and is thereforeinterpreted as the final diagenetic event. Christiansen(1994) noted that the bitumen in this region had a veryhigh reflectance (Ro>4%), suggesting that it wasemplaced before Tertiary magmatism (35 to 55 Ma).

SEM-CL reveals that the quartz cements arecomposed of homogeneous mottled, sometimes zoned,luminescing quartz. The detrital grains close to thestylolite seams also often contain numerous non-luminescent healed fractures at high angles to thestylolite seams (Fig. 9). The healed transgranularmicrofractures, which contain numerous fluid inclu-sions, mainly cross-cut quartz overgrowths. Similar tothe other case studies the healed microfractures eitherterminate at the stylolite surface or occasionally con-tinue across it. Numerous occurrences of quartz over-growths that are clearly truncated along stylolitesurfaces are present in all of the samples (Fig. 9).


Even close to the stylolite surface none of the detritalquartz grains and quartz cements show any unduloseextinction, indicating that the detrital grains are notsignificantly strained. SEM-CL petrography reveals thatmany of the detrital quartz grains possess concavo-convex and sutured contacts (Fig. 9) indicating thatsignificant intergranular pressure solution occurred priorto quartz cementation. Th values of primary aqueousinclusions hosted in quartz overgrowths range from 69.5to 121.6 °C, with an average of 93.6 °C (Table 1 and Fig.10). Tm values in the range of −6.3 to −1.1 °C, indicatethat all of the inclusions are of low salinity (1.57 to9.60 wt.% NaCl eq.). Te measured from four separateprimary quartz cement hosted inclusions ranged from−42.2 to −36.9 °C, suggesting that the fluids containedMgCl2 in addition to NaCl.

7. Discussion

7.1. Relative timing of cementation and stylolitization

The precise source of silica for quartz overgrowths insedimentary basins remains uncertain. Many workersbelieve that silicate cements in sandstones originatefrom pressure solution (e.g. Oelkers et al., 1996;Bjørkum et al., 1998), whereas others argue thatpressure solution alone can not account for the largevolumes of quartz cements in some sandstones andsilica is more likely to have been transported byadvection into the host-rock via large-scale migrationof aqueous fluids released during the compaction ofdeeply buried sediments (Gluyas and Coleman, 1992;Ben Baccar and Fritz, 1993; Canals and Meunier, 1995).Although there is strong evidence that large-scale fluidmigration does occur in modern sedimentary basins (e.g.Land and Macpherson, 1992; Hanor, 1994; Eggenkampand Coleman, 1998), chemical mass balance calcula-tions and fluid flow modeling has indicated that thelarge scale advective migration of silica for quartzcementation requires far larger volumes of fluid flowthan could be attained in sedimentary basins (Bjørlykke,1993).

Although the precise source of silica involved in thecementation of these sandstones remains uncertain,silica released during pressure solution remains a goodpossible source. The zones containing stylolites arecharacterized by increased volumes of cement, inparticular quartz. It is more likely that stylolitizationincreased quartz cementation, rather than the stylolitesformed along well cemented zones, because the detritalquartz grains show extensive evidence of pressuresolution which would have provided a silica source for

the cements. Oelkers et al. (1996) also noted that, insome deeply buried stylolite-bearing North Sea sand-stones, porosity reduction due to increased cementationvaried from∼10% close to the stylolites to∼22% in theinterstylolite regions. This suggests that the increasedcementation in the low porosity zones was a result ofstylolitization.

Calculations by Bjørlykke (1994) showed that itrequires 109 volumes of water flow through a volume ofrock perpendicular to the isotherm to precipitate onevolume of quartz, making advective transport of silicaunlikely in most circumstances. In addition, the pre-sence of calcite and dolomite cements in these sandstonereservoirs also provides further evidence that silica wastransported via local-scale diffusion rather than large-scale advection, since precipitation of quartz requirescooling water and this would dissolve at least 30 timesmore carbonate (Bjørlykke and Egeberg, 1993). It istherefore not possible to precipitate quartz by advectiveflow without dissolving many times more calcite. Fromthese lines of evidence, it is likely that silica from thequartz cements in this study were derived from localpressure solution.

There are numerous quartz overgrowths that aretruncated by stylolites seams in all of the samples. Asstylolitization progressed, silica would have beenremoved by dissolution and the remaining relativelyinsoluble material was left behind forming the stylolite.Stylolites are thought to develop either along areascontaining a large number of intergranular pressuresolution contacts, which coalesce to produce stylolites(Raynaud and Carrio-Schaffhauser, 1992), along pre-existing joints, or along clay laminations (Spötl et al.,2000). As material is removed, the amplitudes of thestylolites increase, resulting in changes from gentlywave-like morphologies to complex sutured ones(Gratier and Guiguet, 1986). This means that thecomplex sutured stylolites which are described in thisstudy would have evolved from wave-like morpholo-gies. As progressive burial occurred and styloliteevolution progressed, more silica would be removedby pressure solution and deposited as quartz cements inthe inter-stylolite region. Quartz cements which formedduring the early stages of stylolite development wouldin turn be removed as stylolitization progressed. Thisimplies that although the quartz overgrowths aretruncated by the stylolites, the fact that quartz cementvolumes increase in areas containing stylolites indicatesthat pressure solution was coeval with quartz cementa-tion. It is therefore likely that the quartz overgrowthsthat are clearly truncated by sutured stylolites formedduring the early stages of stylolite development.


7.2. Pore fluid characteristics during diagenesis

When SEM-CL reveals that more than one episode ofquartz cement is present (e.g. in the Galley and ScottFields) all the quartz is truncated by the stylolites. Dueto the sparse distribution of quartz cement-hosted fluidinclusions, many of the Th values recorded were notfrom cements which were clearly truncated by stylolites.The microthermometric values of fluid inclusions fromquartz overgrowths that are not truncated by a stylolite,but are in close proximity to a stylolite, are within thesame range as fluid inclusions that are hosted in quartzovergrowths that are clearly truncated by the suturedstylolites. Since quartz cementation in all the samplesincreases with decreasing distance away from thestylolites, the Th values from all the quartz overgrowthsexamined are therefore believed to represent ambienttemperatures during stylolitization. The average Thvalues of primary aqueous inclusions hosted in quartzovergrowths range from 85.9 to 135.9 °C. The aqueousfluid inclusions hosted in quartz overgrowths from theScott and Galley Fields occur in association withhydrocarbon fluid inclusions which means that the Thvalues (averages: 96.3 to 110.1 °C) are more accuratemeasurements of the ambient temperatures duringquartz cementation. The average Th values of theprimary aqueous inclusions hosted in quartz over-growths from the Buchan Field is 110.1 °C and fromthe East Greenland sandstones is 93.6 °C.

It is not possible to distinguish if many of thedolomite and calcite cements post-date or pre-date thesutured stylolites. Since stylolitization is associated withsignificant compaction, it is unlikely that the poreswhich are now filled by late-stage calcite and dolomitecements would have remained open. Unless the cementsare extensively replacive, for which there is no evidence,it is most likely that these cements also pre-dated thesutured stylolites. The Th values of primary aqueousinclusions hosted in calcite and dolomite are notsignificantly different from the primary aqueous inclu-sions hosted in the earlier quartz cements (Table 1).

The sandstones from the Buchan Field containprimary fluid inclusions hosted in calcite and dolomitecements which yield average Th values that are between12 and 15 °C higher than the average Th values of theinclusions that are hosted in quartz cements. The largespread of Th values and lack of a unimodal distributionof the dolomite- and calcite-hosted inclusions suggestthat the inclusions with the higher Th values may havere-equilibrated to lower densities. Prezbindowski andLarese (1987) showed that even slight overheating ofinclusions hosted in calcite will result in stretching and

permanent volume change. This volume change resultsin an increase in the homogenisation temperature so thatthe Th values of re-equilibrated inclusions approach themaximum burial temperatures rather than the tempera-ture of original entrapment. The Th values of inclusionshosted in quartz cements are far more likely toaccurately record the temperature of entrapment. Bodnaret al. (1989) showed that quartz-hosted inclusions whichare less than 5 μm in diameter can withstand internaloverpressures (i.e. the difference between the internalpressure of the fluid in the inclusions and the confiningpressure acting upon the mineral host) of over 2 kbars(or overheating to >200 °C above the originalentrapment temperature). For lithostatic conditions,fluid pressures of 2 kbars correspond to burial depthsof approximately 8 to 9 km for sedimentary successions.It is clear that these rocks have not experienced suchextreme burial depths and the microthermometric datarecorded from quartz-hosted fluid inclusions accuratelyrecords the original temperature of entrapment.

All the samples possess open and healed transgranularmicrofractures orientated at high angles to the stylolitesurfaces. Cross-cutting relationships between the stylo-lites and healed microfractures typically vary withinindividual samples, with the healed microfractures eitherterminating at the stylolite or occasionally continuingacross it. This age relationship indicates that some of thehealed microfractures appear to post-date the formationof the stylolites and some pre-date it. From theirgeometries, it is possible that the stylolites and healedmicrofractures orientated at high angles to the stylolitesurfaces could have formed simultaneously. A maximumbedding plane-normal compression and a minimumbedding-parallel principal stress would form both thesetwo types of structures coevally. Additionally, some ofthe healed microfractures that terminate at high anglesagainst the stylolites could be analogous to coevalprocess-zone fractures in areas surrounding stylolites incarbonates which are believed to form coevally (seeRaynaud and Carrio-Schaffhauser, 1992). Watt (1983)also interpreted dilatant fractures associated with carbon-ate-hosted stylolites as forming coevally. We thereforeinterpret this range of age relationships between thehealed microfractures that terminate at high anglesagainst stylolites and the stylolites themselves as beingbroadly coeval. These healed fractures contain fluidinclusions which yield information on the temperatureand composition of the ambient fluid present during thehealing of the microfractures. Experimental work hasshown that open microfractures in quartz will havegeologically short lifetimes at elevated temperatures inthe presence of suitable aqueous solutions (Brantley et al.,


1990). Therefore, the individual fluid inclusions withinthe healed microfractures can be considered to provideinformation about the temperature and composition of thefluids present at the time of fracturing (Lespinasse, 1999).The average Th values of aqueous inclusions hostedwithin the post-quartz cement in healed transgranularmicrofractures oriented at high angles to the stylolitesurfaces are always higher than the average Th valuesfrom primary inclusions hosted in the quartz over-growths. This is consistent with their formation duringthe later stages of stylolite development. The average Thof the aqueous inclusions in these healed fractures rangefrom 111.4 to 145.1 °C. The Th measurements from theBuchan and Scott Fields are from healed microfracturescontaining hydrocarbon inclusions in addition to aqueousinclusions. These measurements (average 125.4, 129.7and 145.1 °C) therefore represent true trapping temper-ature as opposed to minimum trapping temperatures.

The effect of oil emplacement on quartz precipitationrates in sandstone reservoirs has been a matter of recentdebate. Some workers argue that oil has no effect onquartz precipitation rates, since quartz is water wet andthere is always water present in the system (Bjørkum etal., 1998; Walderhaug, 1994). In contrast, others haveargued that transport rates for aqueous species will bereduced and quartz precipitation rates will therefore beretarded or totally stopped (Worden et al., 1998; Marc-hand et al., 2001). The presence of oil-filled fluidinclusions hosted within primary settings in quartzcements in the samples from the Galley and Scott Fieldssuggests that quartz cementation continued after oilemplacement. However, it should be pointed out that thisonly indicates that some oil was present during quartzcementation and it is not conclusive evidence that quartzprecipitation rates were unaffected by oil emplacement.

7.3. Fluid pathways and timing of stylolitization

The porosity of carbonates in areas around stylolitesincreases even though they are areas of increasedcementation (Hofmann and Leythaeuser, 1995). Thisincrease in porosity is due to the development ofextensive fracture networks which develop to accom-modate the relative displacement between the styloliteand the surrounding rock (Raynaud and Carrio-Schaff-hauser, 1992). These fractures are thought to provideimportant conduits during the expulsion of hydrocar-bons from carbonate source rocks. Their presence in lowpermeability rocks is also thought to be essential inproviding conduits for fluids necessary for the devel-opment of stylolites (Hofmann and Leythaeuser, 1995).Similar fracture networks have also been observed to

develop in close proximity to stylolites in sandstones(Dunne and Caldanaro, 1997). The stylolite-bearingsandstones from the Galley, Scott and Buchan Fields allcontain hydrocarbon fluid inclusions within healedmicrofractures orientated at high angles to the styloliteseams. This indicates that these microfractures providedpathways for hydrocarbon migration during the forma-tion of the sutured stylolites. These microfractures couldalso be essential in providing conduits for the fluidsnecessary in the development of the stylolites as well asfluids exported during stylolitization.

Although the Jurassic sandstones from East Green-land do not contain hydrocarbon fluid inclusions, thefact that bitumen occurs within pores indicates that oilcharging commenced after quartz cementation. Sincethe bitumen has been highly altered during Tertiarymagmatism, it must have been emplaced before the lastphase of magmatism, at 35 Ma. This also implies thatquartz cementation and stylolitization must have oc-curred before this time.

The timing and depth of mineral precipitation can beestimated by comparing Th values to burial history curves,if the palaeogeothermal gradient is known. Fig. 11 showsburial curves for two representative wells from the Galleyand Scott Fields generated using the Platte RiverBasinMod™ software. The present day geothermalgradient in the Outer Moray Firth of the North Sea is 34°C km−1 (Cornford, 1998). If we assume that thepalaeogeothermal gradient at the time of cementationwas similar to the present day, the depth and timing of thedifferent diagenetic phases can be estimated. Fig. 11shows that quartz cementation in the Scott and GalleyFields was associated with rapid burial during EarlyTertiary timeswhen depths exceeded 2.5 km. Oil chargingof these reservoirs must have already begun by this timebecause of the quartz overgrowths contain hydrocarbonfluid inclusions in primary settings. This is in agreementwith existing models for hydrocarbon charging in theOuter Moray Firth, which indicate that peak hydrocarboncharging occurred during Early Tertiary times inassociation with rapid burial of the Kimmeridge ClayFormation in the Witch Ground Graben (Bissada, 1983;Burley et al., 1989). This could explain why reservoirquality in the lower regions of the Scott Field is so low.The rehealed fractures orientated at high angles to thestylolite surfaces in the Scott Field formed at greaterdepths than the quartz overgrowths. Similar healedmicrofractures in the Galley Field formed at similartemperatures, and hence depths, as the later quartzcements. This is consistent with the healed microfracturesorientated at high angles to the stylolite surfaces beingbroadly coeval with the later stages of stylolitization.


Fig. 11 also attempts to detail the burial history of theJurassic sediments from East Greenland. The preciseEarly Tertiary burial is unknown and the burial depth of3.5 km prior to uplift at 20 Ma is only taken as a bestestimate (Hansen, 1992). Unfortunately the burial historyof the Buchan Field could not be constructed because ofthe lack of understanding of the Late Palaeozoic to EarlyMesozoic thermal history of the region.

7.4. Implications for stylolite formation

Although the processes that control quartz precipi-tation are not fully understood the main factors aretemperature, composition of the detrital grains, surfacearea of detrital quartz grains, solubility and compositionof pore fluids. Silica released by pressure solution atstylolite surfaces is transported into the interstyloliteregion by diffusion and precipitates as quartz on suitablesurfaces. The rate at which this occurs is dependent onthe rate of quartz dissolution, the rate of diffusion ofsilica and the rate of quartz precipitation (Bjørlykke,1999). Oelkers et al. (1996, 2000), Lander andWalderhaug (1999), Bjørkum et al. (1998) and Walder-haug et al. (2001) developed quantitative models toestimate the rate of dissolution and quartz cementationin sandstones. These models apply thermodynamic,kinetic and diffusion data combined with petrographicinformation to characterize the rate and extent ofchemical compaction and quartz redistribution insandstones. The validity of these models has recentlybeen questioned by Sheldon et al. (2003), because theyassume that pressure plays no role in quartz dissolution.Sheldon et al. (2003) argued that, since pressure solutionrelies on the transport of solutes by diffusion, whichrequires persistent gradients in chemical potential alonggrain contacts, stress gradients in sandstones aretherefore required to maintain continuous diffusion.

There is much evidence that the rate-limitingmechanism that controls the overall rate of quartzcementation at <3 km depths is the precipitation rate ofquartz (Walderhaug, 1996; Oelkers et al., 1996). Atdepths greater than 3 km, where a lot of the quartzcements in these sandstones were precipitated, the rate-limiting mechanism that controls the overall rate ofquartz cementation is diffusional transport (Renard etal., 2000). Fisher et al. (2000) also noted that quartz

Fig. 11. Composite burial history–temperature diagram for (A) theScott Field, (B) the Galley Field and (C) the Jurassic sandstones atBjørndal, East Greenland based on a constant 34 °C/km geothermalgradient. Also shown are the estimated timing of quartz cement and thehealed microfractures.


cementation in 28 studied oil and gas reservoirs in theNorth Sea were preferentially concentrated on newlyfractured, clean quartz grain surfaces generated duringcataclastic deformation. Therefore, by inference, the rateof stylolite formation must therefore also be controlledby the rate of quartz precipitation.

The results in this study draw similar conclusions tothese about the depth and ambient temperatures atwhich well developed stylolites form. Quartz cementa-tion in areas in the vicinity of the stylolites commencedat temperatures of ∼84 °C when depths exceeded∼2.5 km. Mass transfer calculations indicate that onlyminor chemical redistribution of quartz from stylolitesoccur at temperatures below 86 °C due to the highactivation energy required to precipitate quartz (Oelkerset al., 1996). The rate of quartz cementation once startedis an exponential function of temperature. Assumingaverage geothermal gradients, this temperature corre-sponds to burial depths of 2.9 km. Giles et al. (2000)and Bjørlykke et al. (1989) also noted that quartzcement and stylolite formation in many sedimentarybasins are apparent at depths from 2.5 to 3.5 km; withwell developed stylolites apparent at depths of between3.5 and 4.5 km.

8. Conclusions

Combined detailed petrographic and fluid inclusionmicrothermometric analysis have recorded the tempera-tures and compositions of the ambient pore fluidspresent during diagenesis and stylolitization of fourstylolite-bearing sandstone reservoirs. The main find-ings of this study are as follows:

1. The sandstones in the vicinity of the stylolites arecharacterized by high authigenic quartz volumes,suggesting that silica for quartz cementation waslocally sourced and transported.

2. Petrographic analysis documents quartz overgrowthsthat are truncated by sutured stylolites in all four casestudies. This relationship indicates that as stylolitiza-tion progressed early-formed quartz cements werepartially removed at the stylolite seam.

3. Fluid inclusion microthermometry reveals that thequartz cements and the stylolites formed at minimumtemperatures of between 86 and 136 °C.

4. Basin modelling of the Scott Field and the GalleyField indicates that the quartz cements and stylolitesformed at depths greater than 2.5 km during rapidEarly Tertiary burial.

5. The geometry of abundant healed microfracturesoccurring in association with the stylolites suggests

that they formed during stylolitization. The occur-rence of hydrocarbon fluid inclusions within thesehealed microfractures suggests that they providedpathways for hydrocarbon migration in the highlycemented, low permeability areas associated with thestylolites.

Acknowledgements

D. Strogen is thanked for providing the samples fromEast Greenland. K. Bjørlykke, R. Marfil and ananonymous reviewer are also thanked for their con-structive reviews.

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relationships between stylolites and cementation in sandstone reservoirs: examples from the north...

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