Clay mineral diagenesis and quartz cementation in mudstones: The effects of smectite to illite reaction on rock properties

Marine and Petroleum Geology 26 (2009) 887–898

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Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo

Clay mineral diagenesis and quartz cementation in mudstones: The effects of smectite to illite reaction on rock properties Christer Peltonen*, Øyvind Marcussen, Knut Bjørlykke, Jens Jahren Department of Geosciences, University of Oslo, P.O. Box 1047, Blindern, NO0316 Oslo, Norway

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 October 2007 Received in revised form 31 December 2007 Accepted 14 January 2008 Available online 5 June 2008

The Late Cretaceous to Early Tertiary sequence of the Vøring and Møre Basins from the Norwegian Sea has been examined with respect to mineralogy based on 319 cutting samples from five wells. A clear relationship between mineralogy and well log data is demonstrated. A significant change with respect to velocity, porosity and density occurs within the depth interval corresponding to 80–90  C. At shallow depths/temperatures (<2.0 km/70  C), compaction is mainly mechanical and the physical properties are similar to what has been measured by experimental compaction of mudstones. At greater depths, however, the log derived velocities and densities are higher than those produced by experimental compaction indicating significant chemical compaction. XRD analyses show a progressive alteration of smectite to illite (S–I) within this depth/temperature interval which results in the release of significant amounts of silica into solution. Detrital silt and fine-grained quartz showed no secondary quartz overgrowths. These grains are isolated within a clay matrix and surrounded by clay minerals, thus limiting the available surface area and pore space for quartz overgrowths. Chemical analyses (XRF) indicate that silica is conserved within this depth interval, and the amount released from S–I alteration was locally precipitated. Field emission gun-scanning electron microscopy (FEG-SEM) and cathode luminescence (CL) identified authigenic micro-crystalline quartz cement within the clay matrix at temperatures above w85  C. This is accompanied by an increase in velocity and density indicating that the S–I reaction and the precipitation of authigenic quartz caused a significant change in the rock stiffness. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: Compaction Mudstone Smectite Quartz cement Vøring Basin Clay diagenesis

1. Introduction At shallow depths (<2.0 km) and relatively low temperatures (<60–80  C), compaction of mudstones in subsiding sedimentary basins is mainly a result of mechanical processes which result in a reduction of porosity due to reorientation and breakage of grains as a function of grain strength and effective stress (Bjørlykke, 1998). Chemical compaction is the result of mineral dissolution and precipitation reactions, which are a function of mineral stability (thermodynamics) and kinetics. The main controlling factors for chemical compaction and cementation are primary mineral composition, pore-fluid composition and time–temperature history (Bjørlykke, 1998), and it is therefore difficult to duplicate these processes experimentally. One of the most common and important reactions in mudstones is smectite to illite (S–I), which results in the release of significant amounts of silica (Weaver, 1959; Towe, 1962; Boles and Franks, 1979; Srodon, 1999), but the question of what happens to this silica still remains unanswered. Quartz

* Corresponding author. E-mail address: [email protected] (C. Peltonen). 0264-8172/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2008.01.021

cementation in sandstones has been thoroughly studied in the past few decades, due to its obvious importance for reservoir quality, and semi-quantitative models have been developed (Walderhaug, 1996). At low temperatures (<80  C) quartz growth is very slow and micro-quartz can form as coatings on detrital quartz grains in sandstones if a silica source like amorphous silica from organic remains is present (Jahren, 1993; Aase et al., 1996; Jahren and Ramm, 2000). Quartz cementation in mudstones is, however, difficult to observe petrographically, but it may be a critical factor determining the physical properties. The purpose of the present study was to analyze the mineralogy, chemistry and petrography of a thick mudstone sequence in order to establish a relationship between sediment composition, diagenesis and physical properties as recorded by well logs. This may also provide information regarding elastic properties of mudstones and aid in the interpretation of seismic data. 2. Geological background The Vøring and Møre Basins are located on the Norwegian continental margin between 62 and 68 N in the Norwegian Sea (Fig. 1). They are northeast to southwest trending basins, which are

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thickest in the southeast and thin to the northwest and is important when evaluating present day geothermal gradients (Table 1) and time/temperature history. 3. Data and analytical methods

Fig. 1. Base map of the Møre and Vøring Basins – Norwegian Sea, with locations of the five studied wells. Modified from Brekke (2000).

bounded to the west by a volcanic margin escarpment and to the east by passive margin platforms and terraces. The tectonic history of the Norwegian Sea is discussed in detail by previous authors (Blystad et al., 1995; Bjørnseth et al., 1997; Lundin and Dore, 1997; Brekke et al., 1999; Dore et al., 1999; Brekke, 2000; Skogseid et al., 2000; Ren et al., 2003; Lien, 2005). The Vøring and Møre Basins are characterized by a very thick Cretaceous sequence, which is composed primarily of mudstones and reaches depths of up to 9000 m (Færseth and Lien, 2002). Deep marine heterolithic sandstones also comprise a volumetric minor component of the Late Cretaceous package and are the primary hydrocarbon in the Vøring Basin (Brekke et al., 1999). In the latest Maastrichtian to Early Paleocene time, regional uplift produced an unconformity across the Vøring Basin. This period of uplift was followed by continental break-up and the eruption of thick, extrusive basaltic lavas in the Late Paleocene to Early Eocene (Brekke et al., 1999). The opening of the NE Atlantic was associated with subaerial volcanism that produced ash and tuffs, which eventually covered the entire basin. The Vøring Basin was also tectonically active in the Late Eocene to Middle Miocene. Tectonic compression created arches and domes that controlled sediment transport paths and depocenters, and the most distal facies were eventually filled with mud and siliceous ooze during the Late Miocene (Brekke et al., 1999). During the Late Pliocene to Pleistocene, mainland Norway was uplifted causing westward tilt and erosion, which resulted in the deposition of a thick prograding wedge across the Møre and Vøring Basins. The burial/uplift and temperature histories of the Norwegian Sea basins are quite complex and vary spatially throughout the basins. Heating events are postulated in association with regional uplift in the latest Maastrichtian to Early Paleocene as well as in the Miocene (Brekke et al., 1999). Magmatic underplating and intrusion are interpreted by some authors to have had both local and regional effects on heat flow and geothermal gradients (Fjeldskaar et al., 2003; Svensen et al., 2004). The sediments of the Møre and Vøring Basins are, however, at maximum burial depth, due to the thick Plio-Pleistocene sediment package. This prograding wedge is

A total of five deep-water wells from the Møre and Vøring Basins were chosen for this project (Table 1 and Fig. 1). Borehole petrophysical logs were acquired from the Norwegian data repository for petroleum data (NPD DISKOS – PetroBank). In order to reduce the amount of data, and to extract representative depth trends, average values of the physical properties were calculated. Data points (log measurement spacing w15 cm) were averaged in 5 m intervals and plotted. This is approximately the same resolution as borehole cutting samples and provides a reasonable basis for comparison between the different data. Porosity was calculated from wire-line bulk density values (RHOB) using a matrix density of 2.65 g/cm3 and water density of 1.02 g/cm3. The sample material consisted of both washed and unwashed cuttings obtained from core storage facilities of the respective operators or from the NPD. Well logs and final well reports were used to develop a sampling strategy for each well, and cutting samples were taken at approximately 30–50 m spacing, which best corresponded to changes in the petrophysical logs or lithological/ stratigraphical interval. All wells are vertical with only minor deviation and all depths are given as measured depth, in meters from sea floor unless otherwise specified. 3.1. X-ray diffraction (XRD) – bulk and clay mineralogy A total of 319 cutting samples were analyzed for whole-rock (bulk) and clay fraction (<2 mm) mineralogy using XRD. Preparation, analysis and interpretation procedures are modified from Moore and Reynolds (1997) and Hillier (2003). Semi-quantification or quantitative representation (QR) is based on calculation of the integrated peak area of respective mineral phases, multiplied by inhouse calibrated and/or published weight factors (Pearson and Small, 1988; Ramm, 1991). Clay fraction analyses were completed with multiple treatments; air-dried, ethylene glycol, heated-1 (300  C), and heated-2 (550  C). Slow scans were completed for accurate feldspar determination (bulk; 26–28.5 2q; 0.012q/4 s), in addition to kaolinite/chlorite determination (clay AD; 24–26 2q; 0.012q/4 s). In this study, smectite includes random-ordered mixed-layer illite/smectite (R0: 10–50% illite), whereas ordered illite/smectite (R1 > 50% illite) is represented as I/S. The quantity of smectite or I/S was determined by the integrated area of the expanded 17 Å peak with ethylene glycol treatment, whereas the type of ordering (R0, R1 or R3) was determined by the location of the smectite 003/illite 002 peak (Moore and Reynolds, 1997). All expanding clays were quantified as smectite or illite/smectite. The integrated area of the 7 Å (chlorite 002/kaolinite 001) peak was used to quantify both

Table 1 Summary of wells included in this study. Well

6305/1-1

6405/7-1 6505/10-1

Ormen Lange Ellida Water depth (m) 840 Kelley bushing (m) 24 WD þ KB (m) 864 TD MD mRKB 4560 141/3.84 BHT/DT ( C)

6706/11-1 6704/12-1

Helland-Hansen Vema Dome

1206 684 36 26 1242 710 4300 5028 129/4.25 151/3.49

1238 26 1264 4317 115/4.3

Gjallar Ridge 1352 25 1377 4103 142/5.25

Depth and temperature data are from the Norwegian Petroleum Directorate website.

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chlorite and kaolinite, and the ratio of the two was determined by the peak heights of kaolinite 002 (3.58 Å) and chlorite 004 (3.52 Å) from the clay slow scan. Illite was determined by the integrated area of the 10 Å peak, which can represent illite, mica, muscovite and glauconite. In order to represent clay mineral amounts in bulk QR, the clay fraction QR results for each mineral were multiplied by the total clay percentage determined from bulk QR (4.5 Å). 3.2. X-ray fluorescence (XRF) – major element analyses Approximately 1.0 g of crushed cutting sample material was placed in a ceramic container and weighed with an accuracy of four decimals before it was heated for 1 h at 1000  C to obtain loss on ignition. Exactly 0.45 g heated material was mixed with Spectroflux (Li2B4O7), in a 1:9 relationship. The mixtures are melted and cast as glass beads at 1350  C in an Automatic Bead Machine (Philips Perl’x 3). The glass beads were analyzed on a Philips PW 2400 Spectrometer. XRF methodology is more thoroughly described by Ahmedali (1989). 3.3. Electron microscopy Scanning electron microscopy (SEM – JEOL JSM 6460LV) and backscatter electron imaging (BEI) with energy dispersive X-ray spectroscopy (EDS) was used for element mapping and qualitative mineral identification. For better resolution, a field emission gun-scanning electron microscope (FEG-SEM – FEI Quanta 200) was used. FEG-SEM provides high spatial resolution (w2 nm at 15 kV) and the high beam brightness optimizes EDS analysis. A monochromatic, wavelength dispersive cathode luminescence system (Gatan MonoCL) was used to distinguish detrital quartz from authigenic quartz based on Go¨tze et al. (2001). 4. Results Mineralogical results within a litho-stratigraphic framework are discussed in detail in Peltonen et al. (2008). The Late Cretaceous to Early Tertiary sequence of the Møre and Vøring Basins consists of thick mudstones with only occasional thin layers of silty sandstone and carbonate. The main variable, with respect to mineralogy, is the smectite content. Results from mineralogical analyses show a decrease in smectite content with increasing burial depth and a corresponding increase in illite, chlorite and mixed-layer (ML) clay minerals (Figs. 2 and 3). The transition from random-ordered mixed-layer I/S (R0; >50% smectite) to ordered mixed-layer I/S (R1;<50% smectite) occurs in all of the studied wells between approximately 60 and 90  C, based on present day geothermal gradients (Fig. 2) The clay fraction (<2 mm) of the shallow interval (1.0–2.0 km) in the southern wells (6305/1-1, 6405/7-1 and 6505/ 10-1) is dominated by smectite (up to 80%), whereas the deeper sediments are composed of relatively consistent amounts of illite (w35% average), chlorite (w35% average) and kaolinite (w25% average). The northern wells (6706/11-1 and 6704/12-1) have less overall smectite at shallow depth and the deeper sediments are dominated by kaolinite (6704/12-1). At depths/temperatures greater than 130  C in wells 6305/1-1, 6505/10-1 and 6704/12-1, mixed-layered minerals (ML I/S) are present (<5%), while K-feldspar is absent. The kaolinite content is also relatively high at these depths indicating that it is stable at temperatures greater than 130  C in the absence of K-feldspar. Bulk chemical analysis was performed on well 6505/10-1 for the depth interval between 2015 m and 2620 m, which corresponds to temperatures of 70–90  C and the transition from random I/S (R0) to ordered I/S (R1). Chemical analyses (XRF) indicate only slight variation in the bulk chemical composition within this depth/temperature interval

889

(Table 2) suggesting similar primary sediment type/source and a closed system with respect to silica diagenesis. High resolution scanning electron microscope (FEG-SEM) was used in an attempt to identify any changes in petrographic texture within the depth interval corresponding to the transition from random-ordered I/S (R0) to ordered I/S (R1). Authigenic microcrystalline quartz was identified within the fine-grained clay matrix of mudstone cuttings at depths greater than 2420 m (Fig. 4). These quartz grains appear as individual grains on the scale of 2–3 mm (Fig. 5A and 6B), but there is subtle evidence for some connectivity between these grains within the clay matrix (Fig. 5A). Monochromatic CL analysis produced a broad luminescence peak between 580 nm and 620 nm (Fig. 5A, B), which is consistent with luminescence found in low temperature authigenic quartz due to oxygen vacancies caused by impurities (Mu¨ller, 2000; Go¨tze et al., 2001). A comparison of monochromatic CL analysis for authigenic micro-quartz and detrital quartz can be seen in Fig. 5. Qualitative secondary electron image and CL analyses were also completed on a select number of detrital silt and fine-sand sized grains, floating within the clay dominated matrix, but no evidence of quartz overgrowth was identified. In addition to detrital silt-size quartz grains, abundant silt-sized quartz aggregates (5–20 mm) were identified and confirmed by CL analyses as authigenic. These aggregates are composed of small quartz grains (w1–3 mm), and may be crystallized amorphous Si remnants (Fig. 6A, C). A comparison of mineralogy, chemistry and petrophysical properties was completed for well 6505/10-1. A marked change in physical properties occurs contemporaneously with the S–I transition and the onset of micro-quartz cement. At depths greater than approximately 2.5 km (w85  C) significant changes in velocity (Vp and Vs), porosity, resistivity and acoustic impedance occur, while gamma-ray and bulk density (RHOB) show only gradual change (Fig. 7A–G). A cross-plot between velocity (Vp and Vs) and RHOB shows a sudden change in gradient (greater increase in velocity versus density) at temperatures greater than 80  C (Fig. 7H, I). XRF, mudlogs, sidewall core description and qualitative petrographic observations do not indicate any significant change in chemistry or lithology within this depth/temperature interval (1800–2600 m/ 60–90  C). 5. Discussion The decrease in smectite content and transition from random I/S (R0) to ordered I/S (R1) with increasing burial depth corresponds with a marked change in velocity (Vp and Vs) in the studied depth/ temperature interval of well 6505/10-1(Fig. 7). This is interpreted to be the result of temperature controlled chemical compaction, i.e. dissolution and precipitation of smectite to illite (S–I) and concomitant quartz precipitation. The dissolution of smectite and the precipitation of higher density illite may result in a decrease in the clay mass of up to 30% (Srodon, 1999) and releases significant amounts of silica (Weaver, 1959; Towe, 1962; Boles and Franks, 1979; Srodon, 1999). Authigenic micro-quartz, as observed in the present study, may have been formed from the surplus Si released from the S–I reaction, which precipitated as quartz cement within the micro-pores of the clay matrix. Incipient quartz precipitation resulted in only minor porosity reduction; however, a significant velocity increase is recorded and may be due to increased rock strength due to grain-framework stiffening. The stiffening effect measured by the velocity increase is more marked than the porosity reducing effect recorded by the density increase (Fig. 7H, I). While Fig. 7 shows the detailed log properties within a limited depth interval in well 6505/10-1, the complete log intervals are included in Fig. 8. The depth interval corresponding to thermally altered smectite have p-wave velocities approaching 3500 m/s, which are probably due to quartz cementation related to

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A

SOUTH

6305/1-1

6405/7-1

0 20 40 60 80 100

0 20 40 60 80 100

800

1100

1400

DEPTH (m)

70

1500

o

2000

90

2400

1900

o

3000

2500

3200

2700

3400

130

1600

1100

1800

1300

2200

1500

2400

1700

3600

1300

1700

PLAG

K-FSPR

XRD BULK QR%

2300 2500

XRD BULK QR%

CALCITE ZEOLITES PYRITE SMECTITE

1400

DEPTH (m)

6405/7-1

I/S

130

o

ILLITE

XRD BULK QR%

KAOLINITE CHLORITE

o

2000

900

1000

700

1100

1200

900

1400 1600 1800

1500

2000

1700

2200 90

o

1900

3200

2700

3600

2900

3800

1700

K/T 70

o

3200

2300

3400

2500

90

o

1900

2100

3000

o

1500

1900

2800

2500

130

1300

2600

3000

1100 1300

1700

2300

900

1100

2400

2800

0 20 40 60 80 100

1500

2200

2100

2600

0 20 40 60 80 100 500

800

NORTH 6704/12-1

6706/11-1

0 20 40 60 80 100 600

1300 70

6505/10-1

0 20 40 60 80 100 700

1200

3600

o

2100

2700

4000

XRD BULK QR%

0 20 40 60 80 100

1000

3400

90

4200 2900 3100 2700 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100

6305/1-1

2400

o

1900

SOUTH

1600 K/T 1800

70

1500

2500

3800

QUARTZ

800

K/T

2300

3400

2900

1100

2100

3200

0 20 40 60 80 100 900

1900

3000

o

XRD BULK QR%

B

900

2800

2300

2800

700

1200

2600

2100

2600

1000

2000

1700

2200

0 20 40 60 80 100 500

1400

1300

1600 K/T 1800

0 20 40 60 80 100 800

900

1200

NORTH 6704/12-1

6706/11-1

600

700

1000

3600

6505/10-1

2100 2300

2700

2500

130

o

4000 2900 3100 2700 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100

XRD CLAY QR %

XRD CLAY QR % SMECTITE

XRD CLAY QR % I/S

ILLITE

KAOLINITE

XRD CLAY QR %

XRD CLAY QR %

CHLORITE

Fig. 2. Mineralogical composition (bulk and clay fraction) and present day temperatures (red line) for the five studied wells transecting the Norwegian Sea: 6305/1-1, 6405/7-1, 6505/10-1, 6706/11-1, and 6704/12-1. The Cretaceous/Tertiary boundary is noted by the solid black line.

dissolution of smectite. The underlying sequence of the Kvitnos Formation – Lysing Member and the Lange Formation (>2800 m) consists of sandy intervals alternating with mudstones and marls and show significantly lower velocities. This may be due to lower contents of primary smectite and less quartz cementation.

The physical properties of shallow sediments from this study (<2000 m) are similar to what has been measured by experimental compaction in laboratory studies (Chuhan et al., 2002; Mondol et al., 2007): At greater depths, however, the velocities observed are generally higher than those produced by experimental compaction

C. Peltonen et al. / Marine and Petroleum Geology 26 (2009) 887–898

6505/10-1 250

Smectite

500

Illite + Chlorite + ML

750 1000 1250 1500

Depth (m)

1750 2000 2250 2500 2750 3000 3250 3500 3750 4000 4250 0

20

40

60

80

100

XRD % clay fraction Fig. 3. Smectite content versus illite þ chlorite þ mixed-layer clays for well 6505/10-1.

indicating significant chemical compaction (Fig. 9). Below burial depths corresponding to approximately 60–80  C, the effect of mechanical compaction is limited, and chemical compaction becomes the primary porosity occluding process (Bjørlykke et al., 1989; Bjørlykke and Hoeg, 1997). The onset of chemical compaction leads to a stronger grain framework which can withstand the forces of increasing effective stress, and from that point, temperature will act as the main controlling factor in porosity reduction (in siliciclastics). Mechanical and chemical compaction are thus very different with respect to the controlling factors and should therefore be treated separately during basin modeling. The transition from mechanical to chemical compaction in mudstones does not simply occur at a specific depth or temperature, but is rather a function of the stability of the primary minerals and burial history (Bjørlykke, 1998). Mineralogical and chemical compositions of mudstones can vary significantly throughout sedimentary basins (e.g. Peltonen et al., 2008) and thus the timing and degree of diagenesis will also vary.

891

Changes in primary sediment composition (provenance) and depositional setting (facies) will also influence changes in mineralogy/lithology with depth and thus changes in physical properties. The amount and type of smectite deposited in the Tertiary (particularly the Eocene and Oligocene), in association with extensive volcanic activity and the opening of the North Atlantic, cannot be directly compared to the smectite identified in the Late Cretaceous. The overall decrease in smectite content with depth is, at least in part, due to less volcanic ash deposited in the Late Cretaceous compared to the Early Tertiary. Such changes in sediment input source must be considered when discussing the diagenetic history of a thick stratigraphic sequence. Distinguishing the effects of diagenesis from variations in primary composition is challenging and a classical problem in this type of study. We have therefore chosen to focus on an interval with relatively constant sediment composition. Well logs (gamma-ray and resistivity), sidewall core descriptions, mudlogs, qualitative petrographic observations and XRF analyses do not indicate any significant primary lithology change within the depth interval of interest (1800–2600 m/60– 90  C). An increase in resistivity does occur; however, it does not correspond to any identified carbonate in XRF or XRD analyses, and may be a result of reduced porosity and dehydration of smectite, which reduces the salinity of the pore water. XRD results show a progressive alteration from random mixed-layer I/S (R0; >50% smectite) to ordered mixed-layer I/S (R1; <50% smectite), which is consistent with a temperature controlled reaction (Hoffman and Hower, 1979; Moore and Reynolds, 1997). The decrease in smectite and increase in illite/chlorite with depth are well documented in many sedimentary basins (Weaver, 1956, 1958; Perry and Hower, 1970; Hower et al., 1976), and are reported to occur at a temperature range between 60 and 100  C (Perry and Hower, 1970; Hoffman and Hower, 1979). The S–I reaction may be described as a dissolution–precipitation reaction (Boles and Franks, 1979; Nadeau et al., 1985; Inoue et al., 1987; Yau et al., 1987; Stixrude and Peacor, 2002) involving the addition of Al and K and the release of Si and H2O (Abercrombie et al., 1994). Due to the low solubility of Al it can be considered immobile, and the Al required for the S–I reaction is supplied by the destruction of additional smectite layers (Eberl and Hower, 1976; Boles and Franks, 1979). The S–I reaction is simplified in Eq. (1). Smectite D KD / Illite D SiO2 D H2 O

(1)

The amount of Si released in the S–I reaction will depend on the type of smectite present (e.g. trioctahedral or dioctahedral) and the source of potassium. Towe (1962) was one of the first to recognize that tetrahedral substitution of Al for Si in the S–I reaction would result in surplus silica, and that it could result in a significant amount of quartz cement. Depending on the composition of smectite, additional cations (Na, Ca, Mg and Fe) may also be released along with H2O and Si. In order for the S–I reaction to continue, however, the product cations (Si, Na, Ca, Mg and Fe) must

Table 2 Bulk chemical analysis (XRF) of well 6505/10-1. Helland-Hansen 6505/10-1 XRF results – major elements Depth (m)

SiO2 (%)

Al2O3 (%)

Fe2O3 (%)

MnO (%)

MgO (%)

CaO (%)

Na2O (%)

K2O (%)

TiO2 (%)

P2O5 (%)

L.O.I (%).

Sum (%)

Ba (ppm)

2015 2125 2220 2320 2420 2520a 2520b 2620

61.99 60.76 60.21 61.41 61.76 61.9 61.78 64.73

16.63 16.86 16.94 15.65 16.38 13.82 13.8 14.56

5.3 6.02 6.43 5.31 5.78 5.1 5.1 5.22

0.09 0.1 0.17 0.08 0.06 0.05 0.05 0.06

1.52 1.52 1.52 1.4 1.51 1.25 1.24 1.24

1.07 0.84 0.77 0.63 0.64 0.67 0.67 0.66

0.87 0.81 1.05 1.06 1.21 1.42 1.44 1.59

2.47 2.59 2.92 2.55 2.92 2.67 2.67 2.9

0.76 0.74 0.77 0.74 0.79 0.73 0.74 0.77

0.08 0.12 0.1 0.08 0.09 0.09 0.09 0.09

8.58 8.98 8.26 10.24 7.53 10.65 10.65 6.61

99.36 99.34 99.16 99.16 98.67 98.37 98.22 98.41

2631 2945 3328 9239 10,086 10,769 10,630 14,296

Average

61.82

15.58

5.53

0.08

1.4

0.74

1.18

2.71

0.76

0.09

8.94

98.84

7990.5

892

C. Peltonen et al. / Marine and Petroleum Geology 26 (2009) 887–898

Fig. 4. FEG-SEM backscatter electron imaging and EDS; (A) clay matrix; and (B) micro-crystalline quartz within the fine-grained clay matrix. Well 6505/10-1.

be removed from solution. This may be accomplished by the concomitant precipitation of quartz, chlorite, albite and carbonates (Hower et al., 1976; Boles and Franks, 1979; Abercrombie et al., 1994; Bjørlykke, 1998). If K-feldspar is the source for potassium and albitization does not take place, additional silica will be released

into solution. In the studied wells, however, albitization of K-feldspar is commonly observed by electron microscopy, and interpreted to be the source of K for the S–I reaction (Fig. 10). Carbonates do not show any significant increase with depth, but chlorite does show a slight increase (Fig. 2). The ratio of Si to other cations (Na,

Fig. 5. FEG-SEM backscatter electron imaging and monochromatic CL analyses; (A) and (B) authigenic micro-crystalline quartz; and (C) detrital fine-grained quartz. Well 6505/10-1.

C. Peltonen et al. / Marine and Petroleum Geology 26 (2009) 887–898

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Fig. 6. FEG-SEM backscatter electron imaging and EDS. (A) Authigenic silt-sized quartz aggregates–; (B) individual micro-quartz within clay matrix; and (C) patchy authigenic quartz cement surrounding chlorite. Well 6505/10-1.

Ca, Mg and Fe) is greater in the reactant smectite than in any of the product precipitates (e.g. illite, chlorite and albite), thus illitization of smectite will result in a surplus of Si. The extremely low permeability in mudstone limits fluid-flow transport and diffusion, which favors local precipitation of quartz. Experimental investigations of laboratory induced S–I transformation have shown that it is in fact accompanied by the precipitation of quartz (Small, 1994) and results from other previous studies on mudstones have also indicated that some of the Si released from the S–I reaction is precipitated as authigenic quartz (Hower et al., 1976; Yeh and Savin, 1977). The K required for the formation of illite is likely supplied by dissolution of K-feldspar, which is supported by its decrease with depth and correlation to the I–S reaction in several studies (Hower et al., 1976; Boles and Franks, 1979; Bjørlykke and Aagaard, 1992). In the studied wells, however, K-feldspar amounts do not change significantly (Fig. 2). This is likely due to low overall

amounts (0–3%), which are difficult to accurately quantify using XRD. Alternatively, some of the smectite may have altered to chlorite (Reynolds, 1988), which does not require K and could also explain the lack of observed K-feldspar decrease in this study. Chemical analyses (XRF) show only minor variation in K, Al and Si amounts within this depth/temperature interval (Table 2). Assuming no significant changes in lithology within this interval, the surplus Si released during the S–I reaction must have been precipitated locally as a more stable silicate, i.e. quartz and the K required for the precipitation of illitewere supplied locally by the dissolution of K-feldspar. XRD quantification does not indicate any significant increase in quartz within this interval (variations in quartz quantity of <3% are difficult to resolve with the semiquantification techniques used in this study). However, the intensity of the quartz peaks (3.34 Å and 4.26 Å) from the <2 mm fraction shows a relative increase with depth (Fig. 11). Qualitative

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petrographic evidence supports this observation with micro-crystalline quartz increasing at depths greater than w80  C, indicating that at least some of the Si released from the S–I reaction was locally precipitated.

Some of the cuttings contained up to 20% silt and fine-grained sand which are predominantly isolated and floating within the clay matrix (Fig. 5C). CL analyses were performed on detrital silt-size quartz grains and showed no evidence of diagenetic quartz

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Fig. 10. SEM backscatter electron image and EDS analysis of albitized K-feldspar: (A) K-feldspar; and (B) Na-feldspar: Well 6505/10-1.

overgrowth. The fact that these quartz grains are completely surrounded by clays may limit the precipitation of secondary quartz on these surfaces; however, it may also enhance dissolution of quartz (Heald, 1955). This is consistent with clay coatings on detrital quartz grains which have been readily observed as a deterrent to secondary quartz precipitation in sandstones (Ehrenberg, 1993). Similar to sandstones (Aase et al., 1996; Jahren and Ramm, 2000), the combination of abundant unstable/metastable silica phase, lack of available substrate for secondary quartz precipitation and slow growth of macro-crystalline quartz at these temperatures may have lead to high quartz saturation. This created conditions which favored the precipitation of micro-crystalline quartz within the micro-porosity of the clay matrix. BEI images are interpreted to show a subtle network of quartz cement on the micron scale (Fig. 5A, B). Qualitative SEM analysis identified only trace amounts of this type of quartz cement at shallower depths (<80  C), but its occurrence increased at depths greater than 2420 m (w85  C). The increase in authigenic micro-quartz cement corresponds with a significant change in the velocity/density gradient for well 6505/ 10-1 (Fig. 7H, I). The precipitation of quartz within the micro-

Fig. 11. XRD diffractograms from <2 mm fraction (quartz peak at 3.34 Å): Depth interval 2125 m–2745 m, well 6505/10-1, showing increasing intensity (counts) with increasing depth.

porosity of the clay matrix may have contributed to a stiffening of the clay mineral framework thus resulting in an increase in velocity. The effect of micro-quartz cementation due to the release of Si from the S–I reaction is not the only factor influencing compaction in smectitic mudstones. An increase in clay particle size (Cambell et al., 1990), resulting from the S–I transition may increases the permeability, reduce overpressure and therefore increase the rate of compaction (Bjørlykke, 1998). In addition, the S–I reaction leads to dehydration and collapse of the smectite structure, which is shown to result in an increase in clay mineral alignment (Worden et al., 2005) that may reduce porosity and increase velocity. However, in this case, the increase in velocity is not associated with a proportional decrease in density, indicating no significant smectite collapse. It is unclear which of these mechanisms of the S–I reaction influence compaction most, and it may likely be a combined effect. Acoustic impedance (Fig. 7G) also shows a significant increase at depths greater than 2420 m (w85  C). P-impedance increases linearly down to 2420 m, representing gradual mechanical and chemical compaction. Below this depth, impedance values and gradient increase abruptly, possibly due to a change in the rocks’ physical properties caused by chemical diagenesis. A combination of fabric alignment and quartz cementation from the collapse of smectite and conversion to illite may have resulted in stronger grain framework and thus higher acoustic impedance values. It has been shown in granular material that even small amounts of cement at grain contacts will significantly increase rock strength and thus velocity (Bernabe´ et al., 1992), which may also be valid for mudstones. If the observed increase in acoustic impedance is in fact a result of diagenetic quartz cementation, it may also influence seismic properties. Since clay mineral diagenesis and quartz cementation are temperature controlled, and in this example are correlative to significant change in acoustic impedance, they may produce a horizontal, cross-cutting diagenetic reflector. Silicate reactions involving the dissolution and precipitation of opal A, opal CT and quartz may be the cause of anomalous horizontal reflectors (bottom simulating reflectors) in the Vøring Basin (Brekke et al., 1999) as well as the Northern North Sea (Rundberg, 1989) and the Barents Sea (Roaldset and He, 1995). Biogenic silica is a common component of marine mudstones and is also a potential source for quartz cement during burial diagenesis. However, the occurrence of isolated quartz crystals within the mudstone matrix (Figs. 5A and 6B) is not consistent with

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re-crystallization of biogenic silica tests. There is some evidence of re-crystallized biogenic silica fragments (Fig. 6A, B), but these are relatively large silt-size grains and formed earlier than the micro-quartz cement related to the S–I reaction. The CL techniques used in this study, however, cannot distinguish between authigenic quartz formed at 60 or 90  C. Additional work, possibly O-isotope analysis, may help to better constrain the formation temperature of the authigenic quartz cement. Petrographic evidence of quartz cement in mudstone is not well documented in the literature. Scheiber et al. (2000) used mono-CL to show the occurrence of quartz cement in mudstones, but this was restricted to larger grains consistent with those seen in Fig. 7A and C and interpreted to be sourced by or a replacement of siliceous planktonic organisms. Quartz cementation related to clay mineral diagenesis has been discussed to various extents by previous authors (Siever, 1957; Weaver, 1959; Towe, 1962; Yeh and Savin, 1977; Boles and Franks, 1979; Abercrombie et al., 1994), but clear petrographic documentation of authigenic quartz cement in mudstones is lacking. In the present study, however, we have observed microcrystalline quartz, which based on CL analysis is interpreted to be authigenic. These observations are based on a limited data set and additional work needs to be carried out in order to confirm and circumstantiate them. The relationship between compaction, clay mineral diagenesis and quartz cementation is very complex and difficult to constrain. Future work on larger data sets with better SEM/CL description should be carried out. 6. Conclusion 1. At depths/temperatures greater than w70  C, well log values, i.e. velocity, porosity and density, vary significantly from published experimental compaction results indicating significant chemical compaction. 2. Mineralogical results show progressive illitization of smectite with increasing depth which resulted in the release of significant amounts of Si into solution. Chemical analyses indicate that silica is conserved within this depth interval, and the Si released from the S–I reactions was locally precipitated. SEM and CL analyses support this interpretation with the identification of authigenic micro-quartz at corresponding depth interval. 3. The occurrence of authigenic quartz cement within the mudstone matrix corresponds to a marked change in velocity. The precipitation of micro-quartz from the S–I reaction may have produced a stiffer grain framework explaining the observed increase in velocity. 4. The amount of unstable/metastable silicate minerals or amorphous silica present within a mudstone will determine the potential for authigenic quartz cement and thus the degree of chemical compaction. Mudstones dominated by stable silicate minerals such as kaolinite will not undergo the same amount of chemical compaction as mudstones which are rich in smectite.

Acknowledgements We thank Sissel Jørgensen for assistance with the FEG-SEM located at the Centre for Materials Science and Nanotechnology, University of Oslo. We also acknowledge Turid Winje, Berit L. Berg and Mufak Naoroz at the Department of Geosciences, University of Oslo for help with CL, SEM, XRF and XRD preparation and analyses. Financial support from the Research Council of Norway through the PETROMAKS project and from the FORCE – NORWEGIAN SEA RESEARCH CONSORTIUM project sponsored by Statoil, Norsk Hydro AS, Gaz de France Norge and ConocoPhillips Scandinavia AS is acknowledged.

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