Clay minerals in deeply buried paleoregolith profiles ... 64/64_5_588.pdf · CLAY MINERALS IN DEEPLY BURIED PALEOREGOLITH PROFILES, NORWEGIAN NORTH SEA LARS RIBER 1,*, HENNINGDYPVIK

CLAY MINERALS IN DEEPLY BURIED PALEOREGOLITH PROFILES,

NORWEGIAN NORTH SEA

LARS RIBER1 ,* , HENNING DYPVIK

1 , RONALD SØRLIE2 , AND RAY E. FERRELL, JR.3

1 Department of Geosciences, University of Oslo, P.O. Box 1047, Blindern, NO-0316 Oslo, Norway2 Lundin Norway AS, Strandveien 4, NO-1366 Lysaker, Norway

3 Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803-4101, USA

Abstract—Recent discoveries of oil in deeply buried paleoregolith profiles on the Utsira High, NorwegianNorth Sea, was the first time basement rocks had been demonstrated to be petroleum reservoirs on theNorwegian continental shelf. The present study aimed to establish the processes responsible for theprimary weathering sequence, distinguish them from other phases of alteration, and create a model for thedevelopment of reservoir properties in crystalline basement rocks.Hand-specimen and laboratory tests revealed a link between reservoir properties in weathered granitic

rocks and alteration facies. Samples were obtained from two distinct paleoregolith profiles on the UtsiraHigh. The core samples were studied in detail by optical microscopy, X-ray powder diffraction, scanningelectron microscopy, and X-ray fluorescence. In the altered coherent rock facies, porosity and permeabilitywere mainly created by joints and fractures prior to subaerial exposure. In the altered compact rock andaltered incoherent rock facies, the development of reservoir properties was increasingly affected byphysicochemical interactions between the rock and percolating fluids during subaerial exposure and earlydiagenesis. In well 16/3-4, the altered coherent rock facies contained R0 illite-smectite (I-S), well orderedkaolinite, and a mixture of fine-grained mica and illite, produced in semi-open and closed microsystems. Inthe altered compact rock and altered incoherent rock facies, disordered kaolinite became more abundant atthe expense of R0 I-S, well ordered kaolinite, plagioclase, and biotite, suggesting alteration in semi-openmicrosystems. The collapse of the rock structure and clogging of mesofractures by clays contributed toreduced permeability in the clay-rich upper part of the altered incoherent rock. In contrast, well 16/1-15represented a more deeply truncated weathering profile compared to 16/3-4, characterized by open andinterconnected mesofractures and moderate formation of clay. R0 I-S was present and kaolinite was rarethroughout the profile, suggesting stagnant conditions. During burial, a porosity-reducing serpentine-chlorite Ib b = 90º polytype formed in the overlying sandstone and the regolith. Application of theseresults should improve the success of exploration and production efforts related to hydrocarbon reservoirsin the altered crystalline basement.

KeyWords—Alteration Facies, Basement Reservoirs, Kaolinite, Micromorphology, Norwegian NorthSea, Paleoregolith, Utsira High.

INTRODUCTION

The discovery of oil in altered and fractured granites

below a cover of shallow marine sandstones of Late

Jurassic�Early Cretaceous age on the Utsira High,

Norwegian North Sea (Figure 1), was the first time

crystalline basement rocks on the Norwegian continental

shelf were demonstrated to be hydrocarbon reservoirs

(Riber et al., 2015 presented a detailed geological

description of the Utsira High area). The search for

similar targets in the northern North Sea (Marello et al.,

2013; Osnes, 2013; Trice, 2013) has stimulated research

into the role of clay minerals in the generation of

microtextures affecting the porosity and permeability of

the regolith reservoirs. Understanding the origin of

basement reservoirs in the North Sea has created new

opportunities for applications of clay mineralogy to

hydrocarbon exploration in mature hydrocarbon

provinces.

Clay-mineral composition has been linked to weath-

ering and diagenetic processes in sedimentary sequences

(partially summarized by Galan and Ferrell, 2013).

Recently, kaolinite and other clay minerals in altered

crystalline rocks on the Utsira High, northern North Sea

area have been recognized in potential hydrocarbon

reservoirs, thus stimulating research on the role of

textures and secondary mineral content on the porosity

and permeability of crystalline basement rocks. In an

earlier investigation (Riber et al., 2015) two well

developed paleoregolith profiles were identified from

the Utsira High, a Silurian�Ordovician batholith

(Slagstad et al., 2011; Lundmark et al., 2013) currently

* E-mail address of corresponding author:

[email protected]

DOI: 10.1346/CCMN.2016.064036

Clays and Clay Minerals, Vol. 64, No. 5, 588–607, 2016.

This paper is published as part of a special section on the

subject of ‘Clays in the Critical Zone,’ arising out of

presentations made during the 2015 Clay Minerals

Society-Euroclay Conference held in Edinburgh, UK.

located ~2 km below the sea floor. Changes in the

abundance and micromorphology of kaolinite, other

secondary minerals, and the primary minerals in the

granitic basement suggested an alteration sequence

related to the timing of development of porosity and

permeability. Questions related to the spatial distribution

and relative timing of the events remain to be answered.

The present study, based on detailed mineralogical and

petrographical analyses, extends the earlier investigation

(Riber et al., 2015) and focuses on mineral distribution

patterns and micromorphological features produced by

dissolution and precipitation processes in relationship to

specific, near-surface alteration facies. The results

provide new insight into the role that secondary minerals

in deeply buried regolith profiles play during the

formation of hydrocarbon reservoirs.

The coupled near-surface interactions of biochemical

and physical processes in the so-called ‘Critical Zone’

are responsible for the alteration of solid rock to regolith

and soil (Brantley et al., 2006, 2007; Buss et al., 2008;

Lin, 2010). Recognition of preserved palaeoregolith

profiles, however, can be complicated because of

multiple stages of partly overprinting weathering epi-

sodes (polygenetic) (Molina et al., 1991) or the

succeeding diagenetic or hydrothermal alteration during

burial (Nesbitt and Young, 1989; Rainbird et al., 1990;

Nesbitt, 1992; Ziegler and Longstaffe, 2000; Retallack,

2001; Driese et al., 2007; Srivastava and Sauer, 2014;

Liivamagi et al., 2015). The regolith may be divided into

three main alteration facies based on the mechanical

properties of the rock: the altered coherent rock, altered

compact rock, and altered incoherent rock facies. The

alteration facies display similar mechanical properties to

those of corresponding weathering facies: altered

coherent rock, saprock, and saprolite (Velde and

Meunier, 2008), but are more neutral terms with respect

to conditions of formation. The recognition of alteration

facies is important when evaluating the hydraulic

properties (porosity and permeability) of the rock

(Schoeneberger and Amoozegar, 1990; Negrel, 2006).

The present study focuses on the secondary minerals

that formed from near-surface alteration processes

during the initial subaerial exposure and subsequent

shallow burial of the Utsira High crystalline materials,

and relate the mineralogical and micromorphological

changes of the altered granites to specific alteration

facies observed in hand specimen. The results will

benefit hydrocarbon exploration efforts in the North Sea

and other regions of the world where crystalline bedrock

may be the host for hydrocarbon reservoirs.

MATERIALS AND METHODS

Samples

Two wells, 16/3-4 and 16/1-15, from the Utsira High

(Figure 1) were sampled at Weatherford Labs, Sandnes,

Norway, in 2012�2014. A visual estimate of the degree

and intensity of alteration, noted on a scale from A1 to

A5, was determined by methods recommended by the

International Society for Rock Mechanics (ISRM, 1978).

The cored intervals from wells 16/3-4 and 16/1-15 are

located 28 km apart in the Johan Sverdrup and Edvard

Grieg oil fields, respectively (Figure 1). Two cored intervals

in 16/3-4 were available (Figure 2). The upper interval

ranged from 1940.80 m down to the core gap at 1946.60 m

(measured depth from the kelly bushing, a drilling device).

The lower interval came from 1955�1960.60 m. Sixteen

samples were collected from the available 12 m of core

(Figure 2). Twenty-two samples were collected from 47 m

of basement core from well 16/1-15 (Figure 3). The upper

interval came from 1920�1934 m and the lower interval

from 1943�1973.10 m. All samples from both wells were

prepared for thin-section analysis and X-ray powder

Figure 1. Location of sampled wells on the Utsira High, northern

North Sea. The wells are located ~28 km apart, on the Avaldsnes

high (informal name) and the Haugaland high (informal name),

respectively.

Vol. 64, No. 5, 2016 Clay minerals in deeply buried paleoregolith profiles 589

diffraction (XRD) of the whole sample (bulk) and the

<2 mm size fraction.

Petrographical and mineralogical analyses

Polished thin sections were prepared at the University

of Oslo and by the Petrological Section Service (Petro-

Sec) after impregnation with blue epoxy. Petrographic and

mineralogical analyses were performed in the Department

of Geosciences, University of Oslo on a Nikon Labophot-

Pol microscope and SEM-EDS (JEOL JSM-6460LV, with

a LINK INCA Energy 300 (EDS)) with detectors for

secondary-electron images (SEI), backscattered electron

images (BSE), and cathodoluminescence (CL) from

Oxford Instruments (Oxfordshire, UK).

XRD analysis

The XRD analyses were carried out in the Department

of Geosciences, University of Oslo using a Bruker D8

ADVANCE (40 kV and 40 mA) diffractometer with

Lynxeye 1-dimensional position-sensitive detector (PSD)

(Bruker Corporation, Billerica, Massachusetts, USA),

using CuKa radiation. Micronized powder specimens

were prepared as randomly oriented whole-rock samples

by the front-loading procedure and analyzed by counting

for 0.3 s at steps of 0.01º2y from 2 to 65º2y. Phase

identification was performed with Diffrac.EVA (Bruker)

using the PDF4 minerals database. The entire XRD

pattern (Rietveld, 1969) was applied for quantification of

mineral abundances using Siroquant V4 (Sietronics Pty

Ltd, Canberra, Australia; Taylor, 1991). The results were

quantitative representations (QR) of mineral weight

percentages with approximate precision of 10�15% at

the 10 wt.% level, except for clay minerals where a larger

uncertainty was expected due to greater influence of

structural disorder and compositional variability in the

measured intensities.

Figure 2. Sketch from the regolith profile in well 16/3-4 showing the increase in the degree of alteration (A1�A5) and changes in

plagioclase and kaolinite abundances. Insets display the contrasting rock fabric in the altered incoherent rock compared to the

altered coherent rock.

590 Riber, Dypvik, Sørlie, and Ferrell, Jr. Clays and Clay Minerals

The <2 mm fraction was extracted by gravity settling

and used to prepare oriented aggregate mounts by the

Millipore filter transfer method (Moore and Reynolds,

1997) The XRD patterns were collected after four

treatments: air-drying, saturation with ethylene glycol

vapor for 24 h, heating at 350ºC for 1 h, and heating at

550ºC for 1 h. The XRD data were recorded by counting

for 0.3 s at steps of 0.01º2y from 2 to 35º2y. One-

dimensional oriented-aggregate XRD patterns of inter-

stratified clay minerals were simulated using NewMod II

(Reynolds and Reynolds, 2012).

Randomly oriented clay powders were prepared by

sprinkling dried <2 mm material on a zero-background

slide with a drop of acetone and then improving the

random orientation with the aid of a razor blade,

followed by another drop of acetone. The XRD data

from randomly ordered clay powders were recorded by

counting for 2 s at steps of 0.02º2y from 30 to 65º2y.

The degree of structural disorder in kaolinite was

approximated by measurements of the full width at half-

maximum (FWHM) and positions of the 001 and 002

reflections (Trunz, 1976; Singh and Gilkes, 1992; Amigo

et al., 1994; Battaglia et al., 2004).

Geochemical mass-balance calculation

From five key samples from each profile, detailed

geochemical data were obtained by X-ray fluorescence

(XRF) in the Department of Geosciences, University of

Oslo. The XRF analysis was carried out using a

Panalytical AxiosmAX Minerals spectrometer (Almelo,

The Netherlands) utilizing a 4 kW rhodium tube and the

Panalytical WROXI calibration for major and minor

elements.

An evaluation of the geochemical data was performed

by Dr Steven Driese, Baylor University, USA, and

followed a geochemical mass balance approach

Figure 3. Sketch from the regolith profile in well 16/1-15 showing the increase in the degree of alteration (A1�A5) and changes in

plagioclase and kaolinite abundances. Insets display the contrasting rock fabric in the altered incoherent rock compared to the

altered coherent rock. The vertical scale in well 16/1-15 has been compressed by a factor of 5 compared to well 16/3-4.


(Brimhall and Dietrich, 1987; Driese et al., 2000). The

enrichment and depletion trends of mobile elements

require the identification of an immobile index element.

In earlier studies of paleosols and paleoweathering

profiles Al was applied successfully in this regard, also

when diagenetic overprinting has occurred (Rye and

Holland, 2000; Driese et al., 2007). A critical aspect in

mass-balance reconstructions is the choice of parent

material (protolith). In order to minimize the effect of

local inhomogeneities, the protolith composition was

determined by the average of the two lowermost samples

displaying the least alteration.

Loss on ignition (LOI) represents the weight loss

after heating to 1050ºC and has been applied success-

fully as an indicator of the degree of weathering (e.g.

Regassa et al., 2014).

Wireline logs

A suite of conventional wireline logs from wells

16/3-4 and 16/1-15 was provided by Lundin Norway AS.

Mineralogical changes and porosity development may be

identified by variations in bulk density (DEN) and neutron

porosity (NEU) (Tiab and Donaldson, 2011) also in

intervals that were not cored. Log analyses were carried

out using Interactive Petrophysics v.4.2 (by Senergy).

RESULTS

Alteration facies

Based on the upward reduction of the rock’s

mechanical strength, represented by the degree of

alteration scale (A1�A5), the two profiles were each

divided into three alteration facies (Figures 2, 3). In

16/3-4 the interval from the base of the core up to the

gap at level 1955 m was classified as A1�A2 and

comprised the altered coherent facies (Figure 2). The

altered compact rock facies represented the interval from

1946.5 to 1944.25 m, as the rock displayed reduced

mechanical strength (A3�A4) (Figure 2). The transition

to the altered incoherent rock facies was observed at

1944.25 m where the granite appeared friable (A4�A5)and secondary clay minerals were abundant (Figure 2).

In the lower core segment f rom 16/1-15

(1943�1973.10 m), the facies was classified as altered

coherent rock where alteration was mainly observed

along fractures (A1�A3) (Figure 3). No major changes

in rock strength were observed until above the core

break (1934 m) where the rock displayed reduced

mechanical strength (A2�A3), typical of the altered

compact rock facies (Figure 3). From ~1927.50 m the

rock appeared friable, indicative of the altered incoher-

ent rock facies (A3�A5) (Figure 3).

Whole-rock mineral content

The host protolith granites from wells 16/3-4 and

16/1-15 were classified as medium-grained monzo-

granite (Riber et al., 2015). Plagioclase, quartz, and

K-feldspar, in decreasing order, were the major primary

minerals (Table 1) in the least altered samples (proto-

lith) from both granites. In well 16/3-4, a two-mica

composition with biotite and muscovite made up a

combined average contribution of <5 wt.% (Table 1). In

well 16/1-15, however, the protolith contained >10 wt.%

biotite, with only traces of muscovite (Table 1).

Quartz and K-feldspar showed the least visible signs

of dissolution. The general resistance of quartz through

the studied profiles made it the best candidate to use as a

reference base to track the relative degrees of dissolution

and precipitation. K-feldspar grains (microcline)

displayed minor surface dissolution in both profiles

and showed no decreasing amounts upward relative to

quartz. In the profile from 16/3-4, a decrease in the

abundance of plagioclase compared to quartz was

observed in the altered compact rock and altered

incoherent rock facies (Figure 2, Table 1). In the most

altered samples, plagioclase was depleted by >50%

relative to quartz in the protolith (Table 1). In 16/1-15,

no significant decrease in plagioclase was noted in the

altered incoherent rock compared to the protolith

(Figure 3, Table 1).

Clay minerals

The clay-mineral assemblage of the altered coherent

rock facies in 16/3-4 was dominated by a phase that

expanded to ~16.5 A after EG treatment (Figure 4a), and

collapsed to 10 A after heating at 550ºC for 1 h

(Figure 4a). NewMod II simulations of the EG-treated

patterns recreated the shift to larger d spacings and

provided the best fit of 002 and 003 peak positions with

a mixed-layered R0 I-S with 75% expandable layers in

16/3-4 (Figure 5a). R0 I-S with 85% expandable layers

was detected in 16/1-15 (Figures 4c, 5b). The 060

reflection at ~1.49 A on randomly oriented powder

mounts was consistent with a dioctahedral smectite

component such as montmorillonite, but may also be due

to the presence of kaolinite, quartz, and fine-grained

micas (Moore and Reynolds, 1997) (Figure 4a,c).

Reflections at ~1.54 A indicate the presence of tri-

octahedral chlorite (Moore and Reynolds, 1997)

(Figure 4a), but may also have been affected by small

amounts of fine-grained quartz (4.24 A) in the sample

(Figure 4a). Fine-grained mica and possibly illite (I+M)

were identified by a broad, slightly asymmetrical peak at

~10 A, probably a physical mixture of fine-grained

muscovite (sericite), altered biotite, and possibly illite

(Figure 4a). The fit was excellent between the observed

and simulated XRD patterns for a representative sample

of altered coherent rock (Figure 5a) and altered

incoherent rock (Figure 5b).

In 16/3-4, R0 I-S was only present in the altered

coherent rock facies (Figures 2, 4a), and kaolinite

became gradually more abundant in the altered compact

rock and altered incoherent rock facies. Kaolinite was

recognized by peaks at ~7.15�7.16 A and 3.57�3.58 A


(Figure 4a,b) and their destruction after heating at 550ºC

for 1 h (Moore and Reynolds, 1997). In 16/3-4 the

kaolinite reflections showed an increase in d spacings

and FWHM from <7.15 A and 0.13 A, respectively, in

the altered coherent rock facies to >7.16 A and 0.24 A,

respectively, in the compact altered rock and altered

incoherent facies (Figure 6). Kaolinite from the over-

lying sandstone exhibited sharper peaks (0.10 FWHM)

and smaller d spacings (<7.1475 A) than the kaolinite in

the regolith (Figure 6).

In addition to I-S, an interstratified Fe-rich serpen-

tine-chlorite phase was identified in the clay fraction

from 16/1-15. The 14 A peak was partly overshadowed

by the dominant I-S in the AD and EG-treated samples,

but became visible after collapse of the I-S component to

10 A after heating at 350ºC. Furthermore, the 14 A peak

increased in intensity after heating at 550ºC

(Figure 4c,d). The 7 A and 3.53 A peaks were partially

destroyed after heating at 350ºC and almost completely

destroyed after heating at 550ºC (Figure 4c,d). In all

samples, the 002 reflections were considerably more

intense than the 001 suggesting an Fe-rich composition

(Brindley and Brown, 1980) (Figure 5b). The serpentine-

chlorite displayed narrow even-numbered reflections

(002 and 004) and broad odd-numbered reflections

(003 and 005), which were recreated in NewMod II by

applying a trioctahedral Fe-rich chlorite interstratified

with ~25% of a 7 A mineral (serpentine) (Figure 5b).

Random powder XRD patterns were consistent with the

Ib b = 90º polytype (Moore and Reynolds, 1997), and

the observed pattern was comparable to calculated

positions and intensities by Hillier (1994) (Figure 7).

R0 I-S and serpentine-chlorite were present in all three

alteration facies with the latter becoming more abundant

in the uppermost altered incoherent rock samples

(Figure 4d). Serpentine-chlorite was also identified in

Table 1. Results from quantitative Rietveld XRD analysis by Siroquant. Values are in wt.% of the specified minerals.

Well Depth(m)

Quartz(wt.%)

Plagioclase(wt.%)

K-feldspar(wt.%)

Illite+mica(wt.%)

Kaolinite(wt.%)

Biotite(wt.%)

Chlorite(wt.%)

Calcite(wt.%)

Illite-smectite(wt.%)

16/1-15 1920.50 23.1 40.5 20.7 4.0 – 9.2 2.4 – –16/1-15 1920.82 26.2 32.2 24.9 6.4 – 7.3 2.8 – 0.316/1-15 1920.83 27.3 40.1 19.6 4.5 – 5.7 1.7 – 1.116/1-15 1922.50 22.7 40.2 22.3 5.0 – 6.9 1.6 – 1.316/1-15 1924.90 26.1 40.2 19.2 5.9 – 6.5 2.1 – –16/1-15 1925.77 29.8 25.3 18.9 2.3 – 22.8 0.9 – –16/1-15 1926.55 31.0 36.8 17.6 3.2 – 9.2 2.2 – –16/1-15 1927.50 27.3 41.6 16.4 3.7 – 9.7 1.2 – –16/1-15 1927.88 21.1 36.5 20.7 4.2 – 15.3 2.1 – –16/1-15 1927.90 31.9 35.2 18.5 3.7 – 9.3 1.4 – –16/1-15 1928.58 26.2 30.8 22.5 3.7 – 15.4 1.5 – –16/1-15 1930.85 29.2 35.2 17.7 2.9 – 13.1 1.8 – –16/1-15 1947.50 32.0 30.7 17.1 3.4 – 13.6 1.3 – 1.816/1-15 1949.20 29.0 37.2 19.1 4.0 – 8.7 1.6 – 0.416/1-15 1949.50 28.3 42.0 16.4 3.6 – 6.8 1.7 – 1.116/1-15 1956.92 27.8 34.6 16.1 3.2 – 15.5 2.1 – 0.716/1-15 1966.25 24.6 36.8 20.1 4.4 – 11.9 1.9 – 0.216/1-15 1966.80 28.6 36.4 16.4 3.2 – 13.1 2.3 – –16/1-15 1971.35 27.9 36.7 16.9 3.5 – 11.7 2.3 – 1.116/1-15 1972.50 32.2 26.3 20.2 5.1 – 9.9 2.3 – 4.116/1-15 1973.16 30.8 28 18.9 4.5 – 12.7 2.3 – 2.816/3-4 1940.90 46.1 14.8 19.6 7.0 9.1 2.6 0.7 0.1 –16/3–4 1941.40 39.2 20.3 21.2 6.5 9.5 2.6 0.7 – –16/3–4 1941.80 44.1 20.6 17.2 9.8 7.4 0.6 0.3 0.1 –16/3–4 1942.77 36.1 32.4 17.9 7.1 4.8 1.7 – – –16/3–4 1943.50 39.3 27.4 15.8 7.9 6.2 3.0 0.2 0.2 –16/3–4 1944.05 34.3 30.3 17.4 5.7 8.5 2.8 0.8 0.2 –16/3–4 1944.40 38.1 30.8 14.8 8.9 4.7 2.4 – 0.2 –16/3–4 1944.80 34.5 32.7 18.6 6.6 5.3 1.8 0.1 0.3 –16/3–4 1945.80 31.7 38.8 11.0 12.6 3.1 2.3 – 0.4 –16/3–4 1946.50 36.9 33.0 16.9 7.0 3.7 2.4 – – –16/3–4 1956.40 32.9 37.5 16.5 7.1 1.2 4.5 – 0.2 –16/3–4 1957.90 31.9 35.7 22.9 6.6 1.1 1.6 – 0.1 –16/3–4 1958.60 37.1 29.2 24.3 6.2 1.2 1.9 – – –16/3–4 1959.30 32.8 35.6 20.1 8.7 1.2 1.6 – – –16/3–4 1959.50 34.3 35.4 15.3 9.1 1.3 4.3 – 0.3 –16/3–4 1960.40 33.7 38.5 14.4 9.7 1.2 2.3 – 0.2 –


the sandstone sample from 1919.75 m, just above the

basement-sediment contact at 1920 m. Traces of kaolin-

ite were only observed in two samples in the altered

incoherent rock facies (Figure 3).

In addition to clay minerals, most samples contained

minor amounts of fine-grained quartz (4.24 A),

K-feldspar (3.24 A), and plagioclase (3.19 A) (Figure 4).

Petrography and microfabric

16/3-4. Intracrystalline porosity created by incipient

dissolution of plagioclase (Figure 8a) was the typical

microfabric of the altered coherent rock facies.

Plagioclase exhibited polysynthetic twinning and was

heavily affected by sericitization. Plagioclase commonly

displayed concentric zonation accentuated by preferred

dissolution in the central parts of the grain. Cavities

formed from dissolution of plagioclase were commonly

occupied by clay minerals. Clays, displaying ‘cornflake’

morphology and probably corresponding to the R0 I-S

identified from XRD, were identified within intracrystal-

line pores and dead-end fractures, while vermicular

kaolinite booklets were observed in open microfractures

Figure 4. XRD patterns of the <2 mm fraction in air-dried (AD), ethylene-glycolated (EG), heated at 350º for 1 h (350º), and heated at

550º for 1 h (550º) states from: (a) altered coherent rock facies (1959.30 m) in 16/3-4. Inset show 060 reflections from random

powder patterns; (b) altered incoherent rock facies (1941.40 m) in 16/3-4; (c) altered coherent rock facies (1972.50 m) in 16/1-15.

Inset shows 060 reflections from random powder patterns; (d) altered incoherent rock facies (1927.50 m) in 16/1-15.


(Figure 9a). Fresh biotite displayed straight cleavage and

homogeneous composition (Figure 8a). Chloritization of

biotite was common in all samples from both 16/3-4 and

16/1-15, and the chlorite pseudomorphs were recognized

by increased Fe/(Fe+Mg) ratios and K+ and Ti4+

depletion compared to the parent biotite. Altered biotite

hosted a variety of secondary mineral inclusions such as

K-feldspar, apatite, and TiO2 (Figure 9b).

In the altered compact and altered incoherent rock

facies the intensified dissolution of plagioclase and the

development of mesofractures created voids (Figure 8b).

Skeletal relicts of K-feldspar were preserved as the

albitic sections of perthites were dissolved selectively

(Figure 9c). The resulting dissolution voids accommo-

dated the precipitation of fine-grained, massive, and

randomly oriented kaolinite (Figure 9c). From the upper

part of the altered incoherent rock facies (sample

1941.80 m) mesofractures and circular voids were

commonly occupied by amorphous, dark-brown material

resembling organic matter (Figure 8d). Iron oxides were

observed associated with the dark-brown material. The

presence of amorphous material and precipitation of

kaolinite in cavities and mesofractures possibly reduced

the connectivity between pores (Figure 8c). In the

sample from 1940.90 m, mesofractures were capped by

a coarser-grained, silty material. Pore-filling kaolinite

was present as hexagonal plates and as vermicules which

averaged 5 mm thick and displayed various stacking

faults along the c axis (Figure 9d). Quartz grains from

level 1941.40 m exhibited a repeating pattern of

protruding pyramidal shapes due to dissolution.

K-feldspar displayed etch-pitted surface features, but

pervasive dissolution had not occurred. Pseudomorphic

transformation of biotite to kaolinite increased upward

in the profile (Figure 9c). Minute pyrite inclusions were

observed within altered biotite in samples from

1944.40 m and above.

16/1-15. In the altered coherent rock facies, plagioclase

and biotite displayed modest alteration features and only

minor porosity modification (Figure 8e). The altered

incoherent rock facies (1924.90 m) showed open meso-

fractures and, in contrast to 16/3-4, limited dissolution of

plagioclase (Figure 8f). Mesofractures were intercon-

Figure 5. NewMod II-simulated XRD patterns compared to observed ethylene glycolated (EG) XRD pattterns from: (a) altered

coherent rock facies (1959.30 m) in 16/3-4; (b) altered incoherent rock facies (1927.50 m) from 16/1-15


nected and commonly radiated from expanded biotite

grains and cross-cut quartz and feldspar grains (Figure 8f).

Secondary mineralization was predominantly in intragra-

nular voids occupied by clays corresponding to the R0 I-S.

Altered biotite was recognized by the widening of wafers

and development of sinuous 001 planes (Figure 9e).

Furthermore, hand-picked biotite grains in the altered

incoherent rock facies experienced a slight shift to

~10.5 A in XRD that did not react to EG or heat

treatments. In contrast to 16/3-4, however, only a few

examples of pseudomorphic transformation of biotite to

kaolinite were observed in 16/1-15, exclusively in the

altered incoherent rock facies. Neoformation of serpen-

tine-chlorite with Fe/(Fe+Mg) ratios of ~0.75, measured

by SEM-EDS, reduced porosity and was commonly

observed in the vicinity of altered biotite (Figure 9f).

The serpentine-chlorite increased in abundance upward in

the profile and was also detected in the overlying

sandstone (displaying similar Fe/(Fe+Mg) ratios as in

the regolith) (sample from 1919.75 m). Two samples at

the top of the core (1925.77 and 1924.90 m) revealed Mg-

and Mn-siderite associated with the serpentine-chlorite in

thin sections

Results from wireline logs

In well 16/3-4, the density tool (DEN) recorded a

progressive decrease in the bulk density from the top of

the altered coherent rock facies at ~1952 m that

continued through the altered compact rock and altered

incoherent rock facies above the no-core interval. In the

upper part of the altered incoherent rock profile the trend

was less clear and density even appeared to increase in

some of the clay-rich samples (Figure 10a). In well

16/1-15, however, no apparent decrease in the bulk

density was observed (Figure 10b). In well 16/3-4 the

alteration facies could be distinguished by plotting

density vs. the neutron readings (NEU). More intense

alteration resulted in a decrease in DEN and increase in

NEU (Figure 10c). The increase in NEU and decrease in

DEN corresponded to an increase in the kaolinite/

(kaolinite + plagioclase) ratio (Figure 10c).

Figure 6. Cross-plot of FWHM vs. d value for the kaolinite 001

reflection from the <2 mm samples from well 16/3-4, represent-

ing the degree of crystallinity. Three groups can be distin-

guished: altered compact rock and altered incoherent rock

(1940.80�1946.60 m), altered coherent rock (1955�1960.60 m), and sandstones overlying the regolith profile

(1940.10�1940.50 m).

Figure 7. RandomXRD patterns of the <2 mm fraction from the altered incoherent rock facies (1920.82 m) in 16/1-15. The calculated

positions and intensities of chlorite Ib b = 90º polytype after Hillier (1994) are displayed for comparison.

Figure 8 (facing page). Typical microfabric of the alteration facies: (a) altered coherent rock facies from level 1959.50 m in 16/3-4;

(b) the altered incoherent rock facies in 16/3-4 represented by sample from 1941.80 m; (c) sericitized and dissolution pitted

plagioclase in sample 1941.80 m crossed by intragranular mesofractures that are partly clogged by kaolinitic clays; (d) dark

amorphous material in mesofractures in altered incoherent rock sample 1941.80 m in 16/3-4; (e) the altered coherent rock facies in

16/1-15 represented by a sample from 1972.50 m; (f) open mesofractures in altered incoherent rock facies in 16/-15 (sample

1924.90 m). Abbreviations: Qz = quartz; Pl = plagioclase; Kfs = K-feldspar; Bt = biotite; Kln = kaolinite; I-S = R0 illite-smectite;

Chl = chlorite; Ap = apatite; micropor. = microporosity (abbreviations from Whitney and Evans, 2010).



Figure 9. Backscattered SEM images displaying micromorphologic features of primary and secondary minerals observed in distinct

alteration facies: (a) the altered coherent facies (1957.90 m) in 16/3-4; (b) secondary K-feldspar and minor inclusions of apatite and

TiO2 in partly chloritized biotite in altered coherent rock sample 1949.50 m from 16/1-15; (c) selective dissolution of albitic

perthites in K-feldspar and example of kaolinitization of biotite from altered compact rock sample at 1944.80 m in 16/3-4. The bright

color indicates the Fe-rich biotite parent and the gray color, the kaolinitic pseduomorph; (d) disordered vermicular and hexagonal

platy morphology in kaolinite in the altered incoherent rock facies in 16/3-4 (1941.40 m); (e) altered biotite from saprolite in 16/1-15

(1924.90 m); (f) the altered incoherent rock facies in 16/1-15 (sample 1927.50 m). Abbreviations: I-S = R0 illite-smectite; Kln =

kaolinite; Srp-Chl = serpentine-chlorite; Ap = apatite; Kfs = K-feldspar; Qz = quartz; Bt = biotite (abbreviations from Whitney and

Evans, 2010).


Mass-balance calculations

The altered compact rock and altered incoherent rock

facies in 16/3-4 were characterized by net removal of

SiO2, Na2O, CaO, MgO, P2O5, and K2O, and a slight

increase in Fe2O3, compared to the protolith composition

(Figure 11, Table 2). In the uppermost and most altered

sample (1941.40 m), the loss of CaO (�72%), P2O5

(�78%), and Na2O (�50%) were more pronounced than

the decrease in SiO2 (�13%), K2O (�13%), and MgO

(�25%). Fe2O3, however, displayed an increase of ~5%

compared to the protolith composition (Figure 11,

Table 2). The leaching of soluble elements corresponded

to a progressive increase in LOI (Table 2).

The net removal of mobile constituents was more

modest in 16/1-15 compared to 16/3-4. In the most

altered sample (1924.92 m), CaO (�24%), SiO2 (�7%),

MgO (�52%), Fe2O3 (�13%), P2O5 (�48%), and K2O

(�5%) were leached, but a slight increase in Na2O

(+15%) was noted (Figure 11, Table 2). The increase in

LOI was less prominent than in 16/3-4, corresponding to

lower net removal of soluble elements (Table 2).

Figure 10. (a) Bulk density log from 16/3-4, with approximate levels of boundaries between alteration facies; (b) bulk density log

from 16/1-15 with approximate levels of boundaries between alteration facies; (c) bulk density vs. neutron porosity cross-plot

comprising the regolith profile in 16/3-4. The light gray to black shading of squares indicates increasing kaolinite/(kaolinite +

plagioclase) ratios. The cross-plot was generated using the program Interactive Petrophysics (Senergy).


DISCUSSION

Physical fragmentation and chemical dissolution

responsible for the formation of reservoir properties in

granitic rocks on the Utsira High may be attributed to

alteration episodes occurring at or close to the surface.

The altered coherent rock facies contained evidence for

the initial alteration, and the changes became more

pronounced as alteration progressed near the top of the

paleoprofile in the altered compact rock and altered

incoherent rock facies. The main results from the

petrographical and mineralogical analyses are summar-

ized in Table 3.

Figure 11. Mass-balance calculations by S. Driese assuming immobile Al2O3. The protolith composition was calculated from the

average of the two lowermost and least altered samples in each profile.


The timing of exposure and onset of weathering of

the granitic basement on the Utsira High are not well

constrained. The youngest possible age of subaerial

exposure was determined by the ages of shallow marine

sandstones, Volgian in 16/3-4 and Valanginian in

16/1-15, resting directly on the altered basement

(www.npd.no; Riber et al., 2015). The transgression in

late Jurassic�early Cretaceous coincided with the

commencement of passive thermal subsidence of the

northern North Sea region (Nøttvedt et al., 2008).

Preliminary K-Ar dating of illitic clays from the

paleoregolith profiles in 16/3-4 and 16/1-15 suggested

a late Triassic age of formation (Fredin et al., 2014).

Uncertainties concerning the K-Ar dating exists because

Table 2. Major-element geochemistry (wt.%), determined by XRF, of a selection of elements.

————————— 16/3-4 ————————— ————————— 16/1-15 —————————

1957.90 m

(wt.%)

1959.30 m

(wt.%)

1944.80 m

(wt.%)

1944.05 m

(wt.%)

1941.40 m

(wt.%)

1972.50 m

(wt.%)

1966.25 m

(wt.%)

1949.5 m

(wt.%)

1927.5 m

(wt.%)

1924.92 m

(wt.%)

SiO2 72.7 72.7 72.6 72.5 72.6 69.4 68.4 68.4 67.6 68.2

Al2O3 15.3 15.4 15.9 16.9 17.6 15.2 15.2 16.0 16.7 16.1

Fe2O3 1.32 1.40 1.61 1.58 1.64 3.57 3.32 3.32 3.51 3.18

MgO 0.46 0.52 0.39 0.43 0.42 2.34 1.75 1.577 1.60 1.03

CaO 0.82 0.78 0.60 0.53 0.26 1.08 0.73 0.97 1.40 0.73

Na2O 3.77 3.73 3.38 3.36 2.15 2.77 3.89 4.24 4.22 4.07

K2O 4.43 4.22 3.90 3.66 4.32 4.67 5.13 4.12 4.04 4.92

P2O5 0.07 0.09 0.02 0.02 0.02 0.17 0.19 0.17 0.16 0.10

LOI 0.05 0.07 0.15 0.17 0.33 0.15 0.10 0.08 0.11 0.17

Table 3. Summary of the main results from petrographical and mineralogical analyses. Kln = kaolinite; Pl = plagioclase; Qz =quartz; Bt = biotite; I-S = R0 illite-smectite, Srp-Chl = mixed-layer serpentine-chlorite; por. = porosity; perm. = permeability

Alterationfacies

Interval(m)

Degree ofalteration(A1�A5)

Whole-rockmineralogy

Claymineralogy

Microfabric Reservoirproperties

16/3-4Alteredcoherent

1960.60�1952 A1�A2 Similar toprotolithcomposition

I-S > I+M >Kln

Incipient dissolutionof Pl and alterationof Bt

Por. and perm. con-trolled by macrofrac-tures. Intracrystallinemicroporosity

Alteredcompact

1952�1944.25 A3�A4 Formation ofKln at theexpense of Pland Bt

Kln > I+M Intensified Pldissolution andpseudomorphickaolinitization of Bt

Intercrystalline por-osity and meso-frac-tures after Pldissolution.

Alteredincoherent

1944.25�1940.80 A4�A5 Pl reduced by50% relative toqz. in protolith

Kln >> I+M Severe dissolutionof Pl and kaoliniti-zation of Bt Mas-sive Kln occupiedpore space.

Reduction of perme-ability after collapseof rock structure andclogging by clays.

16/1-15Alteredcoherent


Srp-Chl > I+M> I-S

Incipient dissolutionof Pl and alterationof Bt Invasive Srp-Chl

Por. and perm. con-trolled by macro-fractures

Alteredcompact

1934�1927.50 A2�A3 Similar toprotolithcomposition

Srp-Chl > I+M> I-S

Minor dissolutionof Pl and alterationof Bt Invasive Srp-Chl

Por. and perm. con-trolled by macro-fractures

Alteredincoherent


Srp-Chl >>I+M > I-S

Minor dissolutionof Pl and alterationof Bt InvasiveSrp-Chl moreabundant.

Interconnected meso-fractures increasedperm.


the present study could not positively identify illite as an

alteration product of subaerial weathering. Apatite

fission-track analysis from the area indicated that

basement rocks were at or close to the surface by the

late Triassic (Ksienzyk et al., 2013). Permo-Triassic

alluvial fan deposits, containing weathered granitic

clasts, have been described in the half graben close to

well 16/1-15 (Selvikvag, 2012; Asbjørnsen, 2015). In

sub-lithic to lithic Jurassic arenites from the Avaldsnes

high (Figure 1) an increase in granitic clasts and a

sudden diversity in Nd-isotope signatures occurred

(Sørlie et al., 2014). Similarly, granitic clasts with

weathering rinds were observed in immature shallow-

marine sandstones of late Jurassic age, resting on the

altered basement in well 16/3-4 and in nearby catch-

ments (Sørlie et al., 2014). The possible deep weathering

on the Utsira High during the Mesozoic was probably the

western continuation of the time-equivalent paleosurface

and associated regoliths in onshore Norway (Roaldset et

al., 1982, 1993; Olesen et al., 2006; Olesen et al., 2013)

and southern Sweden and Denmark (Lidmar-Bergstrom,

1982, 1993, 1995; Lidmar-Bergstrom et al., 1997;

Ahlberg et al., 2003). Recent H-O isotope studies from

kaolins in Mesozoic weathering sections from southern

Sweden and Bornholm suggested subaerial formation

under humid and tropical conditions (Gilg et al., 2013)

that may have been comparable to northern North Sea

conditions.

The onset of fracturing and chloritization of biotite

and sericitization of plagioclase can be attributed to late-

magmatic auto-metamorphism (Speer, 1984), and

reflected coupled processes occurring in the upper

portions of the batholiths prior to exposure. In the

process, K+ and Ca2+ exchanged between the two

minerals, resulting in the replacement of plagioclase

with fine-grained muscovite (sericite) and incorporation

of Ca2+ in the form of, for example, minor titanite

inclusions (Riber et al., 2015) within the altered biotite

(Eggleton and Banfield, 1985; Que and Allen, 1996).

The two paleoregolith profiles observed in wells 16/

3-4 and 16/1-15 (located ~28 km apart) demonstrated

contrasting initial stages of clay mineral development.

Alteration in the near-surface regime took place within

microsystems located along grain boundaries and

fractures (Korzhinskii, 1959). The microsystems con-

tained nucleation sites where the formation of secondary

products is greatly influenced by dissolution and

precipitation kinetics (Meunier et al., 2007). The early

stages of alteration in the altered coherent facies

(1960.60�1952 m) in 16/3-4 were characterized by the

coexistence of closed and semi-open microsystems,

promoting the precipitation of multiple types of clay

minerals (Meunier et al., 2007; Pacheco and Van der

Weijden, 2012). The interstitial porosity and dead-end

microfractures within plagioclase (Figure 9a) produced

near-equilibrium microsystems which acted essentially

as closed systems, which favored the formation of

discrete and mixed-layered smectitic clays (Nesbitt and

Young, 1989; Aoudjit et al., 1995; Taboada and Garcia,

1999; Wilson, 2004; Fisher and Ryan, 2006; Pacheco

and Van der Weijden, 2012). Kaolinite precipitated in

open microfractures, within plagioclase (Figure 9a),

which represented semi-open microsites intermediate

between far-from equilibrium and near-equilibrium

microsystems (Meunier et al., 2007). The altered

compact rock and altered incoherent rock facies in 16/

3-4 were dominated by open microsystems favoring the

formation of kaolinite (Nesbitt and Young, 1982; Nahon,

1991; Gardner, 1992; Velde and Meunier, 2008). The

geochemical mass-balance calculations from 16/3-4

displayed a progressive leaching of soluble elements

such as Na+, Ca2+, and Si4+ upward in the profile

(Figure 11), typical of depletion profiles (Brantley et al.,

2007) and facilitating the formation of kaolinite.

Kaolinite formed under intense weathering often

exhibits high-defect structures and reduced particle

size compared to standard reference kaolinites (DeLuca

a n d S l a u g h t e r , 1 9 8 5 ; H a r t e t a l . , 2 0 0 2 ;

Trakoonyingcharoen et al., 2006; Balan et al., 2007;

Hughes et al., 2009; Khawmee et al., 2013). The more

disordered kaolinite in the altered compact rock and

altered incoherent rock facies compared to the altered

coherent rock (Figure 6) reflected alteration of the more

ordered kaolinite crystals observed in the altered

coherent rock (Keller, 1977; Wilson, 1999; Drummond

et al., 2001; Balan et al., 2007; Khawmee et al., 2013).

In 16/1-15 the presence of R0 I-S in all alteration facies

and scarcity of kaolinite indicated that the drainage

conditions were restrictive and not favorable to the

formation of kaolinite. Compared to 16/3-4, the paleo-

regolith profile in 16/1-15 had possibly experienced

deeper truncation prior to burial. Paleoregolith profiles

are commonly truncated and are rarely preserved as

complete profiles in the sedimentary record (Thiry,

1999; Migon and Thomas, 2002). Thus, only the sand-

rich, poorly drained lower part of the saprolite (Acworth,

1987) was likely preserved in well 16/1-15.

Handpicked biotite grains from different alteration

facies in 16/3-4 did not reveal the presence of

hydrobiotite and/or vermiculite which are common

intermediate phases during biotite alteration (Rebertus

et al., 1986; Pozzuoli et al., 1992; Schroeder et al., 2000;

Van der Weijden and Pacheco, 2006; Apollaro et al.,

2013; Price and Velbel, 2014), and thus implying that

the direct transformation of biotite to kaolinite took

place (Ahn and Peacor, 1987; Kretzchmar et al. 1997;

Dong et al., 1998). The slight increase in d values in

altered biotite grains compared to fresh grains, from 16/

1-15, was similar to that of an oxidized biotite phase

reported from modern-day tropical weathering (Dong et

al., 1998; Buss et al. 2008; Bazilevskaya et al., 2013).

Volume expansion of altered biotite after initial oxida-

tion has, in certain cases, been interpreted to be the main

mechanism for the grusification (surface weathering) of


granitoids in the absence of extensive chemical weath-

ering (Isherwood and Street, 1976; Sørensen, 1988; Le

Pera and Sorriso-Valvo, 2000; Fletcher et al., 2006;

Scarciglia et al., 2007; Buss et al., 2008; Parizek and

Girty, 2014, Webb and Girty, 2016).

Upward intensification in the degree of alteration and

increased dissolution of primary minerals have been

demonstrated to correlate with increasing porosity

(Velde and Meunier, 2008; Jin et al. , 2011;

Bazilevskaya et al., 2015; Borelli et al., 2014). The

NEU-DEN crossplot (Figure 10c) suggests that the

alteration facies in 16/3-4 could be distinguished based

on petrophysical properties. The apparent increase in

density in certain samples in the clay-rich altered

incoherent rock was a result of reduction of the rock’s

mechanical strength, causing collapse of the rock

structure under its own weight, destruction of micro-

systems, and compaction (Meunier et al., 2007; Velde

and Meunier, 2008; Pacheco and Van der Weijden,

2012). Collapse of the clay-rich altered incoherent rock

created reduced permeability as a result of increased

length and tortuosity of fluid pathways, in addition to

clogging of pores by secondary clays (Figure 8c)

(Acworth, 1987; Driese et al., 2001; Meunier et al.,

2007). The observation of possible soil features such as

clay cutans, traces of rootlets and silty cappings

(Figure 8d), and traces of iron oxides in the clay-rich

uppermost samples may have been signs of pedogenesis

(Ransom et al., 1987; Faure, 1998; Retallack,1988,

2001; Kuhn et al., 2010). The alteration facies and major

textures were probably well developed before subsi-

dence occurred ending the weathering episode.

Post-weathering � burial diagenesis

By early Cretaceous, rifting had ended in the northern

North Sea region and the Utsira High batholith

experienced passive thermal subsidence to present-day

depths of ~2 km (Nøttvedt et al., 2008; Riber et al.,

2015). Basement�sediment boundaries were reported

previously to be preferred pathways for post-weathering

fluid movements and commonly preserve the effects of

both paleoweathering and subsequent alteration events

(Sutton and Maynard, 1992, 1993, 1996; Ziegler and

Longstaffe, 2000). The limited tectonic activity and

volcanism in the area after burial of the regolith mantle

in early Cretaceous (Ziegler, 1992; Nøttvedt et al.,

2008), makes post-weathering hydrothermal alteration

(Ziegler and Longstaffe, 2000) of the paleoregolith less

likely, but diagenetic alteration probably took place in

both sections and altered the original weathering

profiles. In 16/3-4, the degree to which the kaolinite

observed in the regolith was influenced by early

diagenetic fluids is uncertain. Diagenetic kaolinite is

common in Jurassic sandstones from the North Sea

(Bjørlykke and Aagaard, 1992; Bjørlykke, 1998) and

was also identified in the intra-Draupne sandstone

resting on the regolith in 16/3-4 (Figure 6). The kaolinite

in the sandstone, however, displayed less disorder than

kaolinite in the altered incoherent rock (Figure 6), which

may indicate a different origin. Reduced permeability in

the upper altered incoherent rock interval in 16/3-4

probably prevented extensive diagenetic formation of

kaolinite after burial. The pyrite inclusions within

splayed biotite were indicative of reducing geochemical

conditions which stand in contrast to the formation of

kaolinite, and are generally attributed to diagenetic

alteration (Wright, 1986; Claeys and Mount, 1991).

The widespread porosity-reducing Fe-rich serpentine-

chlorite Ib b=90º polytype observed in the regolith and

overlying sandstone in well 16/1-15 (Figure 7) was

probably a diagenetic phase. Diagenetic serpentine-

chlorite with a comparable XRD signature suggesting

interstratification with 7 A layers has been reported from

Triassic�Cretaceous sandstones in the North Sea

(Humphreys et al., 1989; Hillier and Velde, 1992;

Hillier, 1994; Lindgreen et al,. 2002). The interstratifi-

cation with 7 A layers may imply that the serpentine-

chlorite originated from a 7 A precursor such as

berthierine (Hillier, 1994). Berthierine is reported to

form in marine and meteoric water mixing zones during

shallow burial in tropical regions, and develop through a

progressive loss of 7 A layers with burial (Jahren, 1991;

Hillier, 1994). Disappearance of kaolinite during burial

is reported to occur at temperatures between 150 and

200ºC (Boles and Franks, 1979), which are greater than

the bottom-hole temperatures recorded from 16/1-15

(94ºC) (www.npd.no). The scarcity of kaolinite in

16/1-15 (Figure 3) implies that a more deeply truncated

section of the original weathering profile was preserved

compared to 16/3-4. Siderite in the altered incoherent

rock facies in 16/1-15 is an indicator of stagnant, poorly

oxygenated water often formed during the early stages of

diagenesis (Retallack, 2001).

No evidence of K2O addition was indicated from the

geochemical mass-balance calculations (Figure 11) but

precipitation of authigenic K-feldspar and possible

illitization of smectite may represent local redistribution

of the available K+ in the system during burial.

CONCLUSIONS

On the Utsira High, Norwegian North Sea, reservoir

properties formed in deeply buried paleoregolith profiles

by near-surface physicochemical alteration of granitic

rocks. In well 16/3-4 porosity and permeability were

created mainly by intense dissolution of plagioclase,

generally increasing upward in the paleoregolith profile.

Clay minerals formed from plagioclase dissolution

progressed upward from dominant R0 I-S and kaolinite

in the altered coherent rock facies to kaolinite-domi-

nated in the altered compact rock and altered incoherent

rock facies. Increased porosity and clay formation were

reflected in the reduced bulk density in the altered

compact rock and altered incoherent rock facies as


measured by the density tool. In the altered incoherent

rock, collapse of the rock fabric and clogging of

fractures by clay are believed to have reduced perme-

ability significantly.

In 16/1-15, less intense chemical dissolution of

plagioclase was observed but the mechanical strength

of the rock was reduced upward in the profile. Highly

interconnected mesofractures were possibly created by

reaction-driven, volume-expanding processes in oxi-

dized biotite of the altered incoherent rock facies,

contributing strongly to the formation of porosity and

permeability. The limited formation of clay in 16/1-15

favored the regolith’s reservoir properties. The scarcity

of kaolinite in 16/1-15 compared to 16/3-4 may be a

result of deeper truncation of the profile, and thus

removal of the upper, well drained and kaolinite-

dominated part of the altered incoherent rock facies.

Reaction products occur in zones of increased overall

alteration but their lateral and vertical continuity can be

interrupted by juxtaposed microniches in which the

chemical and physical properties may be quite different.

The degree to which the preserved paleoregolith

profiles reflect the original paleoweathering conditions

remains to be discovered, but a strong post-burial

diagenetic overprinting probably occurred.

Deeply buried paleoregoliths on the Utsira High

represent the first hydrocarbon reservoirs in altered

granitic rocks in the North Sea, and require a new

approach in reservoir characterization. The degree of

dissolution of primary minerals and precipitation of

clays are of great importance in creating and destroying

porosity and permeability. The present study identifies

the possible sequence of processes responsible for the

creation of reservoir properties. The results presented

will help in evaluating similar hydrocarbon discoveries

and identifying new targets in crystalline basement

rocks.

ACKNOWLEDGMENTS

The authors thank Lundin Norway AS for the financialsupport for this project and for providing well data andaccess to cores. Thanks also to Steven Driese for providingmass-balance calculations. The SEM and XRD analyseswere supervised by Berit Løken Berg and Maarten Aerts atthe University of Oslo. The authors thank Paul A.Schroeder, Medard Thiry, and Kalle Kirsimae for valuablecomments and interesting discussions during the process ofcompiling this article and are grateful to A. Meunier andP.C. Ryan for helpful reviews.

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