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Diagenesis of upper cretaceous onshore and offshore chalk from the North Sea area

Hjuler, Morten Leth

Publication date:2007

Document VersionPublisher's PDF, also known as Version of record

Link back to DTU Orbit

Citation (APA):Hjuler, M. L. (2007). Diagenesis of upper cretaceous onshore and offshore chalk from the North Sea area. DTUEnvironment.

Institute of Environment & Resources

Diagenesis of Upper Cretaceous

onshore and offshore chalk

from the North Sea area

Morten Leth Hjuler

i

Diagenesis of Upper Cretaceous onshore and offshore chalk from the North Sea area

Morten Leth Hjuler

Ph.D. Thesis

August 2007

Institute of Environment & Resources

Technical University of Denmark

Diagenesis of Upper Cretaceous onshore and offshore chalk from the North Sea area

Cover: Torben Dolin & Julie Camilla Middleton Printed by: Vester Kopi, DTU Institute of Environment & Resources ISBN 978-87-91855-43-6

The thesis will be available as a pdf-file for downloading from the institute homepage on: www.er.dtu.dk

Institute of Environment & Resources LibraryBygningstorvet, Building 115, Technical University of Denmark DK-2800 Kgs. Lyngby

Phone:Direct: (+45) 45 25 16 10 (+45) 45 25 16 00 Fax: (+45) 45 93 28 50 E-mail: library@er.dtu.dk

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Preface One fact seems invariably connected to Ph.D. studies; they never follow the course of action originally intended! The present thesis is no different. When I back in august 2002 commenced the Ph.D. study at Høgskolen i Stavanger (HiS) as a geologist, the intention of my supervisors, Professor Rasmus Risnes and Professor Vidar Hansen, was to bridge one of the gaps existing between geology and engineering sciences. The vast engineering experience collected over the years at HiS with respect to mechanical properties of Upper Cretaceous chalk needed a geological explanation, especially after it became clear that mechanical strength depends on type of pore fluid as exemplified by the water weakening effect. The main purpose of the thesis was to study particle contacts by means of transmission electron microscopy (TEM) and scanning electron microscopy (SEM) and thereby unravel some of the secrets behind chalk cohesion.

Three months of the autumn of 2003 was spent at Department of Earth Sciences at the University of Aarhus performing SEM studies of chalk. Three months of the following spring was spent at Materials Research Department at Risø National Laboratory, mainly with TEM studies of chalk, but also with testing the usefulness of other microscope based examination techniques including low vacuum SEM (LV-SEM) and atomic force microscopy (AFM).

With the untimely and sad death of Rasmus Risnes in December 2004, a strong driving force behind the Ph.D. project was lost together with Rasmus’ essential insight into the problems, the project was intended to solve. In order to increase the geological aspects of the project, contact was established to associate professor Ida Fabricius at the Technical University of Denmark (DTU) in 2005 and a series of investigations planned and initiated in the autumn that year. During the same period, the first manuscript was prepared together with researchers from UiS and submitted.

In November 2005, it became clear that most scientific results obtained with TEM was based on sources of error, leading to the conclusion that completing the Ph.D. study was unrealistic. Consequently, my engagement at HiS, now the University of Stavanger (UiS), was terminated. However, Ida Fabricius accepted the task of being my supervisor, and based on a grant from BP Norway I was employed as research assistant at Institute of Environment and Resources at DTU in the spring 2006. The main theme of the thesis was modified to encompass diagenetic variations of chalk from the North Sea area based on petrographical and petrophysical investigations.

During the last year, the study has changed from describing entirely geological aspects to include estimates of how geological characteristics influence on engineering properties. Ultimately, the final content of this Ph.D. thesis then has come to more or less reflect the original intentions outlined by Rasmus Risnes and Vidar Hansen.

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An overview of the scientific work is presented in a summary covering the combined aspects of one published and two submitted manuscripts together with three published abstracts and one technical report. These are listed below and included in the thesis after the summery. Three additional enclosures provide petrographical descriptions of each locality, discussion of interpretational problems related to SEM and TEM, and the procedure used in calculating quantitative mineralogy.

The papers, conference abstracts, report and enclosures are not included in this www-version but may be obtained from the Library at the Institute of Environment & Resources, Bygningstorvet, Building 115, Technical University of Denmark, DK-2800 Kgs. Lyngby (library@er.dtu.dk)

INCLUDED MANUSCRIPTS:

Hjuler, M.L. & Fabricius, I.L., submitted. Variations in diagenetic history of Upper Cretaceous onshore and offshore chalk in the North Sea area. Journal of Petroleum Science and Engineering.

Strand, S., Hjuler, M.L., Torsvik, R., Pedersen, J.I., Madland, M.V. & Austad, T., 2007.Wettability of chalk: Impact of silica, clay content, and mechanical properties. PetroleumGeoscience, 13: 69-80.

Hjuler, M.L. & Hansen, V., submitted. Chalk seen through the eyes of scanning and transmission electron microscopes – interpretational challenges. Sedimentary Geology.

INCLUDED ABSTRACTS

Hjuler, M.L. & Fabricius, I., 2007. Diagenetic Variations between Upper Cretaceous Outcrop and Deeply Buried Reservoir Chalks of the North Sea Area. AAPG Annual Convention, Long Beach, California, abstracts volume, p. 63.

Hjuler, M.L. & Fabricius, I., 2007. Diagenetic Variations between Upper Cretaceous Outcrop and Deeply Buried Reservoir Chalks of the North Sea Area. Extended abstract. AAPG Annual Convention, Long Beach, California, abstracts cd-rom, 1-6.

Fabricius, I. & Hjuler, M.L., 2007. Chalk Reservoir Properties controlled by Composition, Age Temperature, Burial Stress and Pore Pressure. AAPG Annual Convention, Long Beach, California, abstracts volume, p. 44.

INCLUDED REPORT

Hjuler, M.L., 2006. Silica and clay mineralogy and distribution in reservoir chalk (Valhall) and outcrop chalks (Stevns, Aalborg and Hallembaye). BP technical report, 33 pp.

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AbstractUpper Cretaceous chalk of the North Sea area forms a saucer-shaped body with the hydrocarbon-bearing central part being buried beneath 2-3 km of tertiary sediments, whereas the rim is cropping out in surrounding countries. Burial history controls diagenetic alterations and diagenetic alterations determine the properties of chalk. Predicting properties of reservoir chalk by testing and modelling of easily obtainable outcrop chalk may thus lead to erroneous characterizations. Understanding diagenetic mechanisms and their consequences are thus a key issue when reservoir chalk characterization is based on outcrop chalk. The purpose of this thesis is to identify diagenetic differences between various Upper Cretaceous outcrop and reservoir chalks based on petrophysical and petrographical evidence. Main emphasis has been on high magnification characteristics obtained by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in order to learn more about recrystallization, cementation and particle contacts.

Outcrop chalk East and South of the North Sea shows clear signs of recrystallization, however shallow maximum burial depth have in general prevented cementation. West of the North Sea, deeper maximum burial of English outcrop chalk has caused porosity-reducing cementation. Regional overpressure in deeply buried offshore chalk has delayed mechanical compaction thereby preserving high outcrop chalk-like porosities. Calcite redistribution and reshaping of calcite particles are, however, more pronounced than observed in English and other outcrop chalk. Non-carbonate mineralogy changes with burial depth as illustrated by transformation of opal-CT into submicron-size quartz aggregates.

Geomechanically, English chalk resembles reservoir chalk from the Dan, South Arne and Ekofisk fields, and Valhall field chalk resembles chalk from Rørdal and Stevns. With respect to flooding tests, specific surface area of chalk from England, Précy sur Oise, Hallembaye, Lägerdorf and Stevns are comparable to that of reservoir chalk. These outcrop chalks may constitute suitable proxies for reservoir chalk with respect to flooding properties, provided that porosity and permeability of outcrop and reservoir chalk are equally comparable.

The ability of SEM and TEM to resolve details on the nanometer scale exposes morphological and structural alterations caused by sample preparation and interaction between sample and electron beam. SEM microscopists should look out for artificial surface textures produced by conductive coatings, carbon spreading from carbon adhesive discs, acceleration voltage control on surface detail, morphology changes due to electron bombardment, organic material mimicking clay flakes, authigenic non-carbonate minerals settling on the calcite surface from pore fluids, and precipitation of suspended ions due removal of pore fluids by heating. TEM microscopists should look out for mineralogical changes due to beam damage, graphite contamination mimicking

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siliceous coatings, apparent cohesion of calcite particles due to mutual overlap, and cementation-like features due to residual adhesive from the ion milling process or carbon from a conducting carbon coating.

v

Dansk résumé Øvrekretassisk kridt i Nordsø-området danner et tallerkenformet legeme, hvor den oliebærende centrale del er begravet under 2-3 km tertiære sedimenter, i modsætning til randområdernes kridtforekomster som er blottede i omliggende lande. Kridtets begravelseshistorie styrer de diagenetiske ændringer, og de diagenetiske ændringer styrer kridtets egenskaber. Derfor er forudsigelser af reservoirkridts egenskaber på basis af tests eller modelleringer udført på blotningskridt ofte behæftet med fejl. Det er derfor af stor vigtighed at forstå diagenetiske mekanismer og heraf afledte konsekvenser for materialeegenskaberne, hvis karakterisering af reservoirkridt skal baseres på blotningskridt. Denne afhandling har til formål at afdække diagenetiske forskelle mellem øvrekretassiske reservoirers og blotningers kridtforekomster baseret på petrofysiske og petrografiske data. Hovedvægten er lagt på højopløsningsoptagelser af detaljer relateret til rekrystallisation, cementering og partikelkontakter. Den nødvendige forstørrelseskapacitet blev opnået med skanningelektronmikroskopet (SEM) og transmissionselektronmikroskopet (TEM).

Blotningskridt øst og syd for Nordsøen viser tydelige tegn på rekrystallisation, mens den begrænsede begravelsesdybde i vid udstrækning har forhindret cementering. Vest for Nordsøen har dybere begravelse af det engelske kridt været tilstrækkelig til at aktivere porøsitetsreducerende cementering. Det regionale overtryk i det dybt begravede reservoirkridt har udskudt den mekaniske kompaktion, hvorved høje porøsiteter sammenlignelige med blotningskridts er bevaret. Dog har opløsning og genudfældning af calcit fundet sted i større udstrækning end i kridt fra England og andre blotningslokaliteter. Den mineralogiske sammensætning af ikke-karbonater ændres med begravelsesdybden som illustreret af opal-CT’s transformation til kvartsaggregater, hvor de enkelte krystaller er mindre end 1 μm.

Ud fra et geomekanisk synspunkt findes flere lighedspunkter mellem engelsk blotningskridt og reservoirkridt fra felterne Dan, Syd Arne og Ekofisk. Reservoirkridt fra Valhall-feltet er sammenligneligt med kridt fra blotningerne ved Rørdal og Stevns. Med hensyn til floodingeksperimenter har kridt fra England, Précy sur Oise, Hallembaye, Lägerdorf og Stevns et specifikt overfladeareal, som er sammenligneligt med overfladearealet for reservoirkridt; blotningskridt fra disse lokaliteter udgør fornuftige reservoirkridtsubstitutter forudsat at porøsitet og permeabilitet også er sammenlignelig.

SEM’ets og TEM’ets evne til at opløse detaljer på nanoskala afslører morfologiske og strukturelle ændringer forårsaget af prøvepræparering og vekselvirkningen mellem prøve og elektronstråle. SEM-mikroskopører skal være opmærksomme på sekundær overfladeornamentering dannet af ledende overfladefilm, kul som spreder sig fra en dobbeltklæbende kulfilm, accelerationsspændingens kontrol over overfladedetaljerig-dommen, morfologiske ændringer fremprovokeret af elektronstrålen, organisk materiale

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som minder om lerflager, autigene ikke-karbonater udfældet på calcitoverfladen fra porevandet og udfældning af opløste ioner som følge af fordampning via opvarmning. TEM-mikroskopører skal være opmærksomme på mineralogiske ændringer forårsaget af stråleskader, grafitkontaminering mindende om silikafilm, tilsyneladende sammenhæftede calcitpartikler og cementlignende strukturer dannet enten af resterne af klæbemidlet fra ionætsningsprocessen eller af kul fra en ledende kulfilm.

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Acknowledgements Several persons have made dedicated and kind contributions to this Ph.D. project and to me personally. I would sincerely like to express my gratitude to all these persons.

Associate professor Ida Lykke Fabricius has shown unwavering faith in me as person and scientist since accepting the task of becoming my supervisor. The scientific outcome and thus approval of this Ph.D. project was at all times uncertain, but Ida has put considerable effort and patience in making the present thesis possible. I most sincerely appreciate her efforts.

At DTU I especially want to thank Ph.D. student Casper Olsen, associate professor Niek Molenaar, head of lab section Sinh Ha Nguyen and technician Hector Diaz for all the good times together and for beneficial discussions. And, of course, the friendly staff and students at E&R are acknowledged for making my stay a pleasant one.

I sincerely appreciate the obligingness and patience exhibited by the DTU administration; without their efforts, finishing the present Ph.D. thesis had not been possible.

Professor Rasmus Risnes was the one who introduced me to chalk seen from an engineer’s point of view. His social and scientific engagements with students were notorious and I enjoyed the privilege of being part of his world for a while. With his untimely death the University of Stavanger and the chalk society lost an exceptional open-minded scientist, and I lost a true mentor and a good friend.

From the University of Stavanger, I am most grateful to associate professor Merete Vadla Madland for the incredible amounts of friendly help she has granted me scientifically and personally through the years. Professor Vidar Hansen is greatly acknowledged for always being available for discussions and for being a sturdy support during microscopy. I am most thankful to Professor Tor Austad for his friendly guiding support. Sharing office with fellow Ph.D. student Atle Kverneland provided me with beneficial technical discussions and a true friend. The staff and students at faculty of Science and Technology are truly appreciated for giving me three good years in Stavanger.

I warmly thank associate professor Erik Thomsen, computer expert Søren Bo Andersen and the helpful staff at Department of Earth Sciences at the University of Aarhus for making my three months there a pleasant and rewarding time.

Equally warmly, I wish to thank senior research scientist Jørgen Bilde-Sørensen together with staff and students at Materials Research Department at Risø National

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Laboratory for three splendid months, which I enjoyed scientifically and not least socially.

I also would like to express my gratitude to BP Norway AS with partners Hess, Norske Shell AS and Total E&P Norge AS for providing economical support to this study.

During the last year my superheated brain has been relaxed each weekend by performing carpentry at Castle Grumstrup in the daytime and being waited on like a king in the evening. This weekly climax and refueling sharpened my focus during the week and provided the hardly needed energy to finish the thesis. Thank you, Ulla, Søren & Jan.

Finally, my dear parents, brother and sister have been an immense support during these five years. When my spirits failed they were always there to throw a lifeline. Without this solid base, I would not have made it! Thank you!

Morten Leth Hjuler, 2007

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Table of contents

Preface ........................................................................................................... iAbstract ........................................................................................................iiiDansk resumé ............................................................................................... vAcknowledgements ..................................................................................... vii

1. Introduction .............................................................................................. 1 1.1. Problem statement ..................................................................................................... 1 1.2. Scope of thesis............................................................................................................. 2

2. Relation between diagenetic variations of Upper Cretaceous onshore and offshore chalk and engineering properties of chalk ....................... 3

2.1. Background ................................................................................................................ 32.2. Choice of methods ...................................................................................................... 4

2.2.1. Petrophysical data ........................................................................................................ 5 2.2.2. Petrographical data....................................................................................................... 6 2.2.3. Mineralogy................................................................................................................... 7

2.3. Main results ................................................................................................................ 7 2.3.1. Recrystallization and cementation features of outcrop and reservoir chalk ................. 7 2.3.2. Opal-CT quartz conversion................................................................................... 10 2.3.3. Clay minerals ............................................................................................................. 11 2.3.4. Distribution of silica and clay minerals in three types of outcrop chalk .................... 11 2.3.5. Meteoric diagenesis ................................................................................................... 162.3.6. Outcrop chalk as substitute in mechanical tests ......................................................... 17 2.3.7. Outcrop chalk as substitute in flooding tests.............................................................. 18

3. Conclusions ............................................................................................ 21

4. Manuscript abstracts .............................................................................. 23 4.1. Engineering properties of chalk related to diagenetic variations of Upper

Cretaceous onshore and offshore chalk in the North Sea area............................ 23 4.2. Wettability of chalk: Impact of silica, clay content, and mechanical properties 24 4.3. Chalk seen through the eyes of scanning and transmission electron microscopes

– interpretational challenges................................................................................... 25

5. References............................................................................................... 27

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MANUSCRIPTS

Hjuler, M.L. & Fabricius, I.L., submitted. Engineering properties of chalk related to diagenetic variations of Upper Cretaceous onshore and offshore chalk in the North Sea area. Journal of Petroleum Science and Engineering.

Strand, S., Hjuler, M.L., Torsvik, R., Pedersen, J.I., Madland, M.V. & Austad, T., 2007. Wettability of chalk: Impact of silica, clay content, and mechanical properties. Petroleum Geoscience, 13: 69-80.

Hjuler, M.L. & Hansen, V., submitted. Chalk seen through the eyes of scanning and transmission electron microscopes – interpretational challenges. Sedimentary Geology.

ABSTRACTS

Hjuler, M.L. & Fabricius, I., 2007. Diagenetic Variations between Upper Cretaceous Outcrop and Deeply Buried Reservoir Chalks of the North Sea Area. AAPG Annual Convention, Long Beach, California, abstracts volume, p. 63.

Hjuler, M.L. & Fabricius, I., 2007. Diagenetic Variations between Upper Cretaceous Outcrop and Deeply Buried Reservoir Chalks of the North Sea Area. Extended abstract. AAPG Annual Convention, Long Beach, California, abstracts cd-rom, 1-6.

Fabricius, I. & Hjuler, M.L., 2007. Chalk Reservoir Properties controlled by Composition, Age Temperature, Burial Stress and Pore Pressure. AAPG Annual Convention, Long Beach, California, abstracts volume, p. 44.

REPORT

Hjuler, M.L., 2006. Silica and clay mineralogy and distribution in reservoir chalk (Valhall) and outcrop chalks (Stevns, Aalborg and Hallembaye). BP technical report, 33 pp.

ENCLOSURES

Petrography, petrophysics and mineralogy of investigated outcrop and reservoir chalk

Interpretational challenges related to scanning and transmission electron microscopy of chalk

The calculation procedure used to estimate quartz, smectite and mica content from chemical data. (In Danish)

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1. Introduction

1.1. Statement of problem In the central North Sea chalk buried to depths of 2-3 km plays a major role as a hydrocarbon reservoir rock. Decades of research into chalk properties have improved our ability to characterize chalk as a material, and understand chalk reservoir behavior. Still, predicting chalk response during hydrocarbon production is a difficult task, not least due to the regional overpressure of the pore phase, the effects of which have proven hard to control as illustrated by the subsidence of the Ekofisk field.

The overpressured pore fluids assist in supporting the sediment overburden thus relaxing the effective stress acting at particle contacts and consequently counteracting mechanical compaction. At the Ekofisk field the extraction of hydrocarbons relaxed the pore overpressure and increased the effective stress acting at particle contacts causing these to break. As a consequence mechanical compaction was reinitiated which proved beneficial in squeezing out more hydrocarbons, but as a negative side effect the seafloor and producing facilities above started to subside. To solve this problem and improve oil recovery the overpressure was reestablished by injecting water into the reservoir; however this seemingly had little effect on the rate of compaction (Hermansen et al.,2000). The unknown mechanism causing continued subsidence was termed the “water weakening effect”.

In order to explain the altered reservoir properties at Ekofisk and to provide a more thorough understanding of diagenetic influence on the chalk structure, extensive research into geomechanical, chemical and petrophysical properties has been conducted which to a large extent has focused on identification of cohesion mechanisms between chalk particles and describing particle contacts (e.g. Risnes & Flaageng, 1999; Hellmann et al., 2002a, b; Madland et al., 2002; Røgen & Fabricius, 2002; Fabricius, 2003; Heggheim et al., 2005; Risnes et al., 2005; Fabricius & Borre, 2007; Strand et al.,2007).

Another important and related aspect of chalk research is concerned with chalk material used for geomechanical and flooding experiments. As deeply buried reservoir chalk is inaccessible in larger amounts for testing, easily obtainable outcrop chalk is used instead. However, variations in onshore and offshore chalk properties will expectedly occur due to different diagenetic history. Understanding diagenetic mechanisms and their consequences are thus a key issue when reservoir chalk characterization is based on testing of outcrop chalk.

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1.2. Scope of thesis In order to gather insight into the problems stated above, the thesis has been focusing on describing and comparing diagenetic variations of a number of reservoir and outcrop chalks based on petrophysical, petrographical and mineralogical data. In this context it has been considered important to provide a detailed visual understanding of the chalk structure at the particle level during different stages of diagenesis. Imaging was accomplished by high magnification scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Manuscript 1 presents diagenetic characterizations of 20 outcrop chalk samples covering 6 counties located around the present-day North Sea, and 9 chalk samples from deeply buried reservoirs in the central North Sea. The observed variations in diagenetic history of outcrop and reservoir chalk are related to burial depth and the overpressured pore phases in reservoir chalk. Based on diagenetic similarities and dissimilarities it is assessed whether or not sampled outcrop chalk qualifies as proxy for reservoir chalk when performing geomechanical and flooding tests.

Manuscript 2 focuses on and morphology, occurrence and distribution of silica and clay content in chalk. Three types of outcrop chalk are investigated in SEM and TEM with respect to occurrence, morphology and distribution of silica and clay minerals. It is evaluated how silica and clay may influence on chalk properties.

High magnification SEM and TEM observations of chalk frequently contain erroneous information originating from sample preparation, sample contamination, or from interaction between chalk and the electron beam. Manuscript 3 presents the interpretational challenges associated with these sources of error encountered in chalk during SEM and TEM study. With respect to SEM the visual effects of conductive coatings, sample substratum, sample alteration by electrons and contamination is described. With respect to TEM, contamination and calcite structure alteration by electrons are discussed.

The relations existing between the manuscripts are as follows: the first manuscript provides insight into different diagenetic environments and diagenetic alterations of outcrop and buried reservoir chalk and discusses which outcrop chalks qualify as substitutes for reservoir chalks. The second manuscript narrows this research area further and concentrates on improving our knowledge of the impact of silica and clay content on chalk properties; again, the aim is to provide better control when choosing testing material. Finally, the extensive data collected for the first two manuscripts has demonstrated how easily SEM and TEM observations of chalk may be misinterpreted. As SEM and TEM constitute the main scientific tools in this thesis and directly visualize the effects of diagenesis, it is considered of importance to communicate these negative experiences to other chalk researchers.

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2. Diagenetic variations of Upper Cretaceous onshore and offshore chalk related to engineering properties of chalk

2.1. Background The chalk deposition of north-western Europe was controlled by the Upper Cretaceous sea-level highstand (Fig. 2.1). Subsidence of the central North Sea took place during the Cenozoic whereas rim area inversions in Late Cenozoic caused removal of Cenozoic overburden as well as chalk layers (Hillis, 1995; Japsen, 1998). As a consequence the North Sea chalk constitutes a coherent, saucer-shaped body with the rim cropping out in several countries surrounding the North Sea Basin while the central part is buried beneath more than three km of Cenozoic sediments (Japsen, 1998).

Variations in burial diagenesis between buried reservoir chalk and outcrop chalk result in different rock properties, which is of importance when simulating reservoir conditions using outcrop chalk as models during either mechanical or flooding tests.

The burial diagenesis characteristics observable in chalk of the North Sea area are reflected petrographical, petrophysical and mineralogical characteristics (e.g. Scholle, 1974, 1977; Borre & Fabricius, 1998; Fabricius, 2003). However burial depth (i.e. temperature, stress and pressure) is just one diagenesis controlling parameter; others may be exposure to meteoric water (e.g. Molenaar & Zijlstra, 1997), clay and silica content (e.g. Røgen & Fabricius, 2002; Fabricius & Borre, 2007; Strand et al., 2007), hydrocarbon content (e.g. Hancock & Scholle, 1975), pore water chemistry (e.g. Fabricius & Borre, 2007), local tectonics (e.g. Hancock & Scholle, 1975; Mimran, 1975, 1977; Clayton, 1983; Hancock, 1990), size and shape of chalk particles (e.g. Neugebauer, 1974, 1975) and mode of deposition (e.g. Scholle et al., 1998; Damholt & Surlyk, 2004).

Understanding the interaction between different diagenesis influencing mechanisms is not straightforward. Vast amounts of studies have been concerned with various properties of chalk due to its huge economic importance as a hydrocarbon-bearing rock in the central North Sea. As an example the subsidence problems of the Ekofisk field spawned extensive research into mechanical properties of chalk (e.g. Risnes & Flaageng, 1999; Collin et al., 2002; Madland et al., 2002; Heggheim et al., 2005).

Based on petrographical and petrophysical characteristics of Upper Cretaceous chalk, this thesis aims to provide an overview of different types of diagenetic environments in the North Sea area. Outcropping onshore chalks from 6 countries and deeply buried offshore chalk from 4 oil fields in the central North Sea were sampled (Fig. 2.1).

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Fig. 2.1. Map of the North Sea area showing sampled localities and their present-day burial (modified after Ziegler, 1990).

Each investigated outcrop sample represent a set of diagenetic features which may or may not resemble diagenetic features of offshore chalk. Based on data obtained in this study, diagenetic variations between offshore and outcrop chalk is addressed and general conclusions made regarding outcrop chalk substituting for offshore chalk during mechanical and flooding tests.

2.2. Choice of methods A large amount of petrographical and petrophysical data was produced for this investigation. Each type of data (texture, porosity, silica content etc.) represents one aspect of the “chalk personality” and may hint at the depositional or diagenetic history of the chalk. Combining all data types from one locality and comparing this data set with data sets from other localities may provide detailed insight into the sequence of events which produced the different types of chalk. The significance of and mutual relations between different data types are explained below.

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2.2.1. Petrophysical data

Porosity: Porosity reflects burial depth, either as a measure of compaction (shallow burial) or porosity-reducing cementation (deep burial). Preserved high porosity in reservoir chalk indicates overpressuring of the pore phase.

Permeability: Permeability in chalk depends on porosity and specific surface area (Mortensen et al., 1998; Røgen & Fabricius, 2002). Reduced porosity results in reduced permeability. The higher the specific surface area, the higher the resistance to fluid flow, and the lower the permeability. Also texture plays a role. The larger pores in some pack- and grainstones increase permeability.

Carbonate content: Chalk mostly consists of calcite. The specific surface area of calcite is low compared to other non-carbonate constituents like silica and clay minerals. Reducing the carbonate content will expectedly increase the specific surface area and reduce permeability.

Particle density: Pure chalk has a particle density identical to or nearly identical to calcite (2.71 g/cm3). In less pure chalk (lower carbonate content) lower or higher particle density indicates a non-carbonate fraction containing e.g. silica (2.65 g/cm3) or apatite (3.2 g/cm3).

Specific surface area: Specific surface area of chalk is controlled by carbonate content and mineralogy of the non-carbonate fraction. According to Røgen & Fabricius (2002) the specific surface area of calcite is on average 2 m2/g, quartz about 5 m2/g, kaolinite about 15 m2/g and smectite about 60 m2/g. It follows that significant amounts of smectite may reduce permeability significantly.

Equivalent particle diameter: The equivalent particle diameter is another way of reflecting specific surface area and thus the consequences are the same.

18O: The 18O/16O ratio represents a measure of the water temperature at which calcite precipitated. The higher the temperature the lower the 18O value. In larger fossil shells composed of unaltered calcite, 18O values reflect the palaeotemperature. In chalk matrix, built from particles with sizes around 1 m, calcite dissolves from and reprecipitates at particle surfaces during diagenesis meaning that significant amounts of calcite has been redistributed; in this case,

18O values reflect the pore water temperature and thus constitute a relative measure of burial depth.

Loss on ignition: Loss on ignition estimates the percentage of organic material in the non-carbonate phase and is used to check for hydrocarbon remains in reservoir chalk. In outcrop chalk significant amounts of organic material indicate contamination by recent fungi, algae, bacteria etc.

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2.2.2. Petrographical data

Petrographical data, e.g. morphology and distribution of silica and clay minerals within the chalk matrix were obtained by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and backscatter electron microscopy (BSE).

Texture: Texture may influence on permeability and cohesion of chalk.

Recrystallization: Calcite surfaces exist in chemical equilibrium with pore water. The slow process of recrystallization involves the constant interchange of calcium and carbonate ions between calcite surfaces and pore water where ions will dissolve from less stable sites and precipitate at stable surfaces (Schlanger & Douglas, 1974). As a result some fossils or particles will grow (overgrowth) at the expense of others, surfaces will become smoothened, fossil detail will be lost, and specific surface area reduced (Wise, 1977; Borre & Fabricius, 1998).

Porosity-reducing cementation: Chemical compaction as a consequence of pressure dissolution between silicates and calcite particles results in formation of condensed dissolution seams of silicates (e.g. stylolites) (Fabricius, 2003). Dissolved calcite reprecipitates in the pore space as overgrowth causing a reduction in porosity, cementation of intrafossil cavities and reshaping of chalk particles.

Intrafossil porosity: Cementation of intrafossil porosity indicates active pressure dissolution (stylolitization) and thus significant burial depth.

Compactional crushing: Crushing of microfossils (e.g. foraminifers) requires the stress levels present at significant depth. Alternatively, stress induced by tectonics may cause crushing of microfossils.

Overgrowth: Overgrowth involves dissolution of calcite from less stable surfaces and reprecipitation on stable surfaces. Overgrowth is caused either by recrystallization or cementation. As burial depth increases overgrowth tends to become more pronounced.

Particle shape and particle reshaping: The shapes of calcite particles are in general reflecting the coccolith structures from which they originated. During recrystallization and cementation, calcite particles tend to attain more rhombohedral shapes. Calcite redistribution and particle reshaping increases with burial depth.

Particle interlocking and contact cement: Two calcite particles cannot coalesce to one crystal unless the crystallographic orientations of their lattices match at the contact point, which is probably rarely the case. During the dissolution-reprecipitation process, one particle will grow around the edge of the other at the

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contact point (Wise, 1977; Fabricius, 2003). When the particles are broken apart an angular depression (contact cement) then scars the surface or edge of the overgrowing particle (Fabricius, 2003). The depth of this scar may represent the amount of overgrowth.

Dissolution structures: At significant burial depth the effective stress at calcite-silicate contacts becomes sufficient to initiate pressure dissolution (Fabricius & Borre, 2007). During pressure dissolution calcite dissolves and reprecipitates locally or perhaps further away. At the points of dissolution calcite is removed and non-carbonates, mainly clay minerals, concentrate as flaser structures or stylolites.

2.2.3. Mineralogy

Mineralogy was determined by X-ray diffraction (XRD) of oriented samples combined with chemical analysis. An example of the calculation procedure used in the chemical analysis to determine mineral amounts is given in Enclosure 1. A graphical overview of mineralogical and petrophysical data is presented in fig. 2.2.

2.3. Main results

2.3.1. Recrystallization and cementation features of outcrop and reservoir chalk

Maximum burial of outcrop chalk on the eastern shore of the North Sea mostly stayed well below 1000 m as estimated by e.g. Japsen (1993, 1998, 2000) for Danish samples (500-750 m) and judged from the porosity which for most outcrop samples stays above 40%. Accordingly, pressures and temperatures remained low and incipient stylolites and porosity reducing cementation are only observed sporadically. Consequently high porosity is preserved and 18O values remain relatively high. Due to recrystallization, chalk particles have formed loose-firm contacts as witnessed by varying degrees of contact cementation. In general original particle shapes are relatively well preserved though overgrowth is always present and some reshaping into rhombohedra occurs (Fig. 2.3A).

The sampled English chalk is characterized by low porosity, rhombohedral particles, well-developed contact cements, intrafossil cements, occurrence of relatively well-developed stylolites and low 18O values; all indicative of significant maximum burial (Fig. 2.3B). As these alteration features are the most pronounced within investigated outcrop chalk, the English localities probably were subjected to the deepest burial.

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Fig. 2.2. Petrophysical and mineralogical results.

Although overpressured pore fluids in reservoir chalk may delay mechanical compaction and pressure dissolution, the effective stress at particle contacts generally has been sufficiently high to force particles and grains closer together and cause more significant calcite redistribution than observed in outcrops chalk. Thus porosity is generally slightly lower, reshaping of particles more pronounced and stylolites occur more frequently compared to outcrop chalk (Fig. 2.3C). Accordingly, 18O values around -3 and -4 per mil testify to calcite redistribution at depth.

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Fig. 2.3. Recrystallization and cementation. (A) High porosity (53 %) outcrop chalk from Stevns subjected to limited recrystallization and cementation. (B) Outcrop chalk from Whitecliff Bay showing signs of significant cementation resulting in moderate porosity (31 %). (C) Offshore chalk from the Dan field subjected to significant recrystallization and cementation causing porosity loss (30 %). (D) Offshore chalk from the Ekofisk field subjected to significant recrystallization but limited cementation thus preserving high porosity (45 %).

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In most offshore samples recrystallization and cementation have caused more euhedral particle shapes (mostly rhombohedra), more uniform particle sizes and often reduced porosity, even when high porosity is preserved (Fig. 2.3D). Especially in samples subjected to porosity reducing cementation (Fig. 2.3B) the tightly fused chalk particles clearly differ from the looser particle contacts in shallowly buried outcrop chalk. Contact cement is more developed in offshore samples compared to outcrop samples, even by identical high porosity. This indicates crystal equilibration via pore water and thus that the chalk is water wet. A higher degree of cementation and recrystallization is also reflected by more pronounced overgrowths in offshore chalk, significantly lower

18O values and the presence of well-developed stylolites. The specific surface area of the chalk samples is stable despite variations in carbonate content. This is probably an effect of equalization of particle sizes during recrystallization and cementation.

2.3.2. Opal-CT quartz conversion

The lack of opal-CT in buried chalk combined with higher temperatures and higher relative quartz content offshore than onshore indicate that any originally present opal-CT has transformed into quartz. In line with observations by Ehrmann (1986) and Maliva & Dickson (1992) this idea is supported by the presence of authigenic submicron-size quartz crystals arranged in clusters (Fig. 2.4A) with roughly the same size as opal-CT lepispheres distributed in the matrix (Fig. 2.4B).

Submicron-size quartz crystals are not restricted to offshore chalk, but were also observed in Queensgate and Schinkel chalk (Fig. 2.4C) both of which may have been buried sufficiently deeply to reach the temperature needed to convert opal-CT to quartz.

Fig. 2.4. Submicron-size quartz crystals. (A) Aggregate of submicron-size quartz crystals. Dan field. (B) Opal-CT lepisphere partly engulfed by overgrown calcite crystal. Saturn quarry, Lägerdorf. (C) Aggregate of submicron-size quartz crystals. Schinkel quarry, Lägerdorf.

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2.3.3. Clay minerals

Onshore, smectite is the dominant clay mineral whereas it was observed in only one offshore sample (Dan field). The opposite situation is true for kaolinite that was detected in one outcrop sample (Obourg) but is widespread offshore as authigenic crystals. Røgen et al. (1999) investigated mineralogy in several central North Sea fields and found smectite occurrences to be sparse and coincident with kaolinite absence.

A smectite to kaolinite transformation is considered unlikely based on observations by Fabricius (submitted) who found a depth-wise trend with smectite transforming into smectite-illite and subsequently into smectite-chlorite. Additionally some kaolinite may have a detrital origin (Fabricius et al., 2007). Authigenic kaolinite is probably precipitated from dissolved feldspar or clinoptilolite.

2.3.4. Distribution of silica and clay minerals in three types of outcrop chalk

Although the content of silica and clay in outcrop and reservoir chalk is generally small, it may affect chalk properties. The specific surface area of both silica and clay is significantly larger than that of calcite (Røgen & Fabricius, 2002), the main chalk-forming mineral, and thus these non-carbonate phases gain in relative importance. The Si-rich non-carbonate phases often constitute just a few percent of the chalk matrix, occasionally less than 1%. Among these phases, quartz and occasionally opal-CT predominates together with clay (Jeans, 1978; Bell et al., 1999).

Mineralogy based on chemical analysis is given in Table 2.1 for outcrop chalk from Hallembaye (Fig. 2.5), Stevns (Fig. 2.6) and Rørdal (Fig. 2.7). Rørdal chalk has the highest silica content including both quartz and opal-CT (Fig. 2.8) while clay minerals are present in minor amounts. Hallembaye and Stevns chalk do not contain opal-CT and in Stevns clay minerals occur more frequently than silica.

A closer inspection of the Rørdal chalk at elevated magnifications reveals the presence of significant amounts of thin clay flakes. Some of these flakes are stiff, generally 100 nm or less in thickness and may extend for more than 10 m; they are probably detrital mica (Fig. 2.9A). Other flakes are thinner (<50 nm), slightly curved and generally sized around 1 m or smaller. Due to lack of stacking they are interpreted as smectite (Fig.

Table 2.1. Mineralogy for selected localities.

Silica Clay minerals Locality Quartz Opal-CT Smectite Mica

Other

Hallembaye 50 10 10 30 Stevns 25 60 15 Rørdal 30 40 20 5 5

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Fig. 2.5. SEM picture of Hallembaye chalk.

Fig. 2.6. SEM picture of Stevns chalk.

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Fig. 2.7. SEM picture of Rørdal chalk showing local concentrations of opal-CT lepispheres.

Fig. 2.8. SEM picture showing the bladed structure of an opal-CT lepisphere. Rørdal chalk.

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Fig. 2.9. Clay in chalk. (A) SEM micrograph of large clay flake, possibly mica, in Rørdal chalk. (B) SEM micrograph of clay flake, possibly smectite, protruding into pore space in Stevns chalk. (C) SEM micrograph of extremely thin material, possible the clay mineral smectite, on top of a coccolith in Rørdal chalk. (D) TEM micrograph of clay flake, probably smectite.

2.9B). Some of these smectitic flakes were observed forming extremely thin flexible coats often with an irregular outline covering the surfaces of chalk particles (Fig. 2.9C and D). Due to their thinness, these coatings are hard to observe in SEM and consistent estimates of their size and distribution patterns are impossible to obtain.

In manuscript 2 the presence of Al, Mg and K obtained from EDS analysis is interpreted as indicating the clay minerals smectite and kaolinite. Based on analytical results from manuscript 1 (section 6.3.5) and based on observations by Røgen & Fabricius (2002) the presence of kaolinite is considered unlikely. Morphologically kaolinite is

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characteristic and relatively easy to recognize. Mineral members of the mica group or illite-smectite associations are considered more likely candidates.

Influx of terrestrial material from neighboring land areas is generally believed to be low; however, detrital quartz grains, feldspar and clay minerals may contribute to the amount of non-carbonate phases.

RØRDAL CHALK

Opal-CT lepispheres (Fig. 2.7) visually dominate the Si content. Fully developed, the lepispheres normally reach diameters of 2–3 m, but few are allowed to finish their growth due to the confinements induced by the narrow pore space. Most opal-CT is present as imperfect lepispheres (Fig. 2.10A) or a number of interpenetrating blades (Fig. 2.10B), often including small chalk particles in their structures. Opal-CT may grow as irregular, bladed structures in narrow pore spaces, where it may reduce permeability locally by plugging pores (Fig. 2.10C).

An unpublished XPS analysis performed on Rørdal chalk shows that 50% of the free surface in chalk consists of Ca-compounds (calcite) and 50% consists of Si-compounds (silica and clay). These measurements reflect the high specific surface area of opal-CT blades and clay flakes compared to chalk particles. As growth of opal-CT blades mostly is directed into the pore space and not along the chalk particle surface, an apparent free chalk surface of 50% does not imply that the remaining 50% is covered with silica and clay. However, high magnification SEM observations indicate that clay coatings may cover a significant percentage of the chalk surface, but, as they tend to melt in with the chalk particle surface (Fig. 2.8C), the actual coverage is hard to estimate. Another possibility, discussed in manuscript 3 (section 5.1.7) is that the clay flakes grew suspended in the pore fluid and settled when the fluids were removed either during sample preparation or prior to SEM and TEM investigations.

HALLEMBAYE CHALK

In manuscript 2 opal-CT is observed to occur in small amounts and as poorly developed bladed structures. XRD and SEM analysis of Hallembaye chalk in manuscript 1 (section 2.3.3) did not show any signs of opal-CT presence though the Hallembaye chalk from manuscript 1 and 2 was sampled from the same quarry. This may be due to restriction of opal-CT to specific stratigraphic levels. The observable amount and distribution of larger clay flakes and clay coatings are similar to those of Rørdal chalk.

STEVNS CHALK

Opal-CT lepispheres are reported in manuscript 2, however in manuscript 1 (section 5.2), XRD and SEM analysis of Stevns chalk did not reveal opal-CT in the chalk matrix. As discussed in manuscript 1, section 6.3.1, the depositional environment near the coast at Stevns would provide a basis for a rich flora and fauna and thus favorable conditions for the decomposing actions of microorganisms at Stevns. The silica cycle may be

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Fig. 2.10. Growth of opal-CT. (A) SEM micrograph of imperfect lepisphere. (B) SEM micrograph of opal-CT blades growing within the central structure of coccolith. (C) SEM micrograph of intergrown opal-CT lepispheres producing bladed aggregates.

linked to biologic activity which may cause extraction of silica from the chalk matrix and concentration in flint bands. As a consequence clay minerals are relatively more abundant than silica in the chalk matrix. Due to low non-carbonate content, the observable amount and distribution of larger clay flakes and clay coatings are similar to those of Rørdal and Hallembaye chalk.

2.3.5. Meteoric diagenesis

The bioclastic grainstone from ENCI in Limburg combines high porosity with extensive recrystallization of grain surfaces which indicate meteoric diagenesis (Fig. 2.11). The extreme permeability (for a chalk) further facilitates meteoric diagenesis and is

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Fig. 2.11. Meteoric diagenesis. Significant recrystallization as seen from large calcite crystals. ENCI quarry.

explained texturally by the large interconnected pores in the grainstone. The high 18Ovalue reflects that recrystallization took place close to or at the surface. Despite extensive overgrowths the chalk is weakly consolidated which together with the high porosity indicates shallow burial.

Possibly chalk from St. Marguerite sur Mer, Précy sur Oise, Hallembaye and Lägerdorf was also subjected to some degree of meteoric diagenesis. Within the chalk matrix 5-50 μm large aggregates composed of rather tightly interlocking rhombohedral particles form characteristic cementation features. These aggregates are evenly distributed within the matrix and aggregate porosity is close to zero. In contrast, the surrounding matrix particles are far less reshaped indicating a maximum burial too shallow for pressure dissolution; at Lägerdorf, though, the presence of stylolites indicates some degree of pressure dissolution. The formation of cemented aggregates may be indicative of a near-surface setting where circulating fresh water had easy access to dissolve and reprecipitate calcite.

2.3.6. Outcrop chalk as substitute in mechanical tests

English outcrop chalk share similarities with investigated chalk samples from Dan field 1, South Arne field 1 and Ekofisk field 1 and 3. English chalk must have been subjected to significant maximum burial causing pressure dissolution which lowered porosity and accelerated calcite redistribution. In this aspect English chalk is similar to several of the investigated offshore chalks though this English chalk never was buried as deeply as the offshore chalk. Based on these observations English chalk assumingly possesses geomechanical properties close to those of chalk from Dan field 1, South Arne field 1 and Ekofisk field 1 and 3.

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The Valhall field 1 sample is characterized by extreme porosity (48%) together with limited particle reshaping and weakly developed contact cements between calcite particles, and no porosity-reducing cementation has taken place. These characteristics are shared with chalk from the Danish localities Stevns, Rørdal and Ellidshøj. With respect to geomechanical properties, the investigated chalk from Stevns, Rørdal and Ellidshøj is considered suitable as substitute for high porosity reservoir chalk where little calcite redistribution has taken place as observed in the Valhall field 1 sample.

Lägerdorf chalk may have attained burial depths slightly shallower than those of English chalk, which would assumingly increase calcite redistribution and make it similar to the Dan field 2, South Arne field 2 and Ekofisk field 2 samples. However, diagenesis is rather limited despite presence of stylolites and, additionally, indications of meteoric diagenesis were observed. Further investigations relating geomechanical properties to diagenetic characteristics has to be performed to decide whether or not Lägerdorf chalk may serve as proxy for reservoir chalk with diagenetic characteristics intermediate between English and Danish chalk.

With respect to geomechanical properties the weakly consolidated and highly permeable bioclastic ENCI grainstone must be considered unqualified as a substitute for any of the investigated reservoir chalks. Texturally, petrophysically and mineralogically the sampled phosphatic chalk from Beauval and Hardivillers deviates from investigated offshore and outcrop chalk. The geomechanical implication of the relatively low carbonate content combined with the completely apatite dominated non-carbonate fraction is uncertain. Thus the phosphatic chalk must be considered unsuitable as sub-stitute for offshore chalk. The organic contamination of unknown origin in investigated Kieler Bach samples make them unsuitable as substitutes for reservoir chalk.

Frequently occurring low porosity aggregates of calcite observed in chalk from St. Marguerite sur Mer, Précy sur Oise, Hallembaye and Obourg are interpreted as precipitations caused by meteoric diagenesis. As these heavily overgrown aggregates are situated in relatively unaltered mud matrix the calcite redistribution mechanism must be different from the more homogenous recrystallization and cementation observed in English and reservoir chalk. These outcrop chalks may qualify as substitutes for reservoir chalk.

2.3.7. Outcrop chalk as substitute in flooding tests

The porosity-permeability relationship is the main controlling factor when performing flooding tests of chalk. Fig. 2.12 shows that most outcrop and reservoir chalk samples generally follow a trend where permeability decreases with decreasing porosity.

Apart from porosity, permeability is also controlled by specific surface area; the higher the specific surface area, the higher the resistance to fluid flow. The curves in Fig. 2.12 represent porosity-permeability relationships calculated from the Kozeny equation at different constant specific surface area values.

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Fig. 2.12. Permeability versus porosity. Outcrop chalk and reservoir chalk follow the same trend despite variations in specific surface area.

Specific surface area is controlled by carbonate content and mineralogy of the non-carbonate fraction. The non-carbonate fraction of the Ellidshøj and Rørdal samples has high specific surface area due to opal-CT dominance, and accordingly, specific surface area of chalk in these samples is also high. Expectedly, permeability would be low, however, this is not the case; the specific surface area predicted from the Kozeny equation for the Ellidshøj and Rørdal chalk is close to 2 m-1 (Fig. 2.12) and thus considerably lower than the measured values of 6.0 and 4.6 m-1, respectively. Probably this deviation is explained by inhomogeneity of the chalk matrix due to the opal-CT distribution pattern. The Kozeny equation was developed to describe homogeneous material (Mortensen et al., 1998) and may not relate porosity, permeability and specific surface area properly in opal-CT dominated samples.

In general, specific surface area of chalk from England, Précy sur Oise, Hallembaye, Lägerdorf and Stevns are comparable to that of reservoir chalk. These outcrop chalks may constitute suitable proxies for reservoir chalk with respect to flooding properties, provided that porosity and permeability of outcrop and reservoir chalk are equally comparable.

It is advisable to avoid using chalk containing larger quantities of opal-CT in flooding tests until more comprehensive data sets offer more details of opal-CT control on permeability. Also, the French phosphatic chalk should be avoided due to the high specific surface area caused by the significant influence of apatite. And obviously the extreme permeability of the ENCI sample disqualifies it as substitute for reservoir chalk.

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3. ConclusionsThe main topic of this thesis has been to improve our understanding of chalk diagenesis, in particular with emphasis on high magnification characteristics. Various petrographical, petrophysical and mineralogical investigation techniques have been applied in order to describe diagenetic variations and assess associated mechanical and flooding properties.

Main analytical instruments have been the scanning electron microscope (SEM) and the transmission electron microscope (TEM). As technical and interpretational experience increased during investigations with these instruments, it became clear that interaction between electron beam and chalk sample may generate several kinds of dubious signals prone to misinterpretation. It was decided to identify, explain and publish these results as an aid to other chalk researchers.

Outcrop chalk East and South of the North Sea shows clear signs of recrystallization, however maximum burial depth was in general too shallow to generate cementation. English outcrop chalk was subjected to deeper maximum burial and to porosity-reducing cementation. Deeply buried offshore chalk is affected by regional overpressure delaying mechanical compaction and to some degree preserving high porosities. However, higher temperatures at depth have increased calcite redistribution and caused more pronounced reshaping than observed in English and other outcrop chalk.

Non-carbonate mineralogy changes with burial depth. Outcrop samples frequently contain opal-CT and clinoptilolite and among clay minerals smectite dominates. In deeply buried offshore samples opal-CT and clinoptilolite are absent, smectite is mostly absent and kaolinite dominates. Assumingly, opal-CT is transformed into submicron-size quartz aggregates.

Distribution of silica (opal-CT) and clay minerals within the chalk matrix has been compared for three outcrop chalks with differing characteristics. Compared to Hallembaye and especially Stevns chalk, Rørdal chalk is richer in silica. In Rørdal chalk, silica produces opal-CT lepispheres in larger pores and irregular, bladed structures in narrow pores, where it may reduce permeability locally by plugging pores. Apparently, clay coatings may cover a significant percentage of the chalk surface in chalk from the three localities, but actual coverage is hard to estimate.

Geomechanically, English chalk represents a suitable proxy for reservoir chalk from the Dan, South Arne and Ekofisk fields. The less diagenetically affected Valhall field chalk may find a suitable geomechanical analogue in chalk from Rørdal or Stevns. Outcrop chalk subjected to meteoric diagenesis, phosphatogenesis and organic contamination chalk is considered less qualified as substitutes.

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In general, specific surface area of chalk from England, Précy sur Oise, Hallembaye, Lägerdorf and Stevns are comparable to that of reservoir chalk. These outcrop chalks may constitute suitable proxies for reservoir chalk with respect to flooding properties, provided that porosity and permeability of outcrop and reservoir chalk are equally comparable. The inhomogeneity of investigated chalk samples rich in opal-CT probably preserves permeability values higher than predicted from the high specific surface area (Kozeny equation). The effect of opal-CT on permeability needs to be analyzed on larger data sets before any conclusions can be drawn regarding the applicability of opal-CT rich chalk in flooding tests.

The ability of SEM and TEM to resolve details down to atomic or near-atomic scale also exposes morphological and structural alterations caused by sample preparation and interaction between sample and electron beam.

SEM microscopists should be attentive to following phenomena:

Artificial surface textures produced by conductive coatings. Carbon spreading from carbon adhesive discs. Acceleration voltage control on surface detail. Morphology changes due to electron bombardment. Organic material mimicking clay flakes. Authigenic non-carbonate minerals settling on the calcite surface from pore fluids.Precipitation of suspended ions due removal of pore fluids by heating.

TEM microscopists should be attentive to following phenomena:

Mineralogical changes due to beam damage. Graphite contamination mimicking siliceous coatings. Apparent cohesion of calcite particles due to mutual overlap. Cementation-like features due to residual adhesive from the ion milling process or due to conductive carbon film.

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4. Manuscript abstracts

4.1. Engineering properties of chalk related to diagenetic variations of Upper Cretaceous onshore and offshore chalk in the North Sea area

M.L. HJULER & I.L. FABRICIUS

The Upper Cretaceous North Sea area Chalk Group constitutes a saucer-shaped body with a deeply buried (2-3 km), hydrocarbon-bearing central part and the rim cropping out in the surrounding countries. Predicting properties of reservoir chalk may involve testing and modelling of easily obtainable outcrop chalk. However, variations in onshore and offshore chalk properties will expectedly occur due to different diagenetic history. Understanding diagenetic mechanisms and their consequences are thus a key issue when reservoir chalk characterization is based on outcrop chalk. The purpose of this study is to identify diagenetic differences between various Upper Cretaceous outcrop and reservoir chalks based on petrophysical and petrographical evidence.

Danish outcrop samples are moderately recrystallized with loose inter-particle connections and particle shapes remaining relatively close to original morphology. They resemble Valhall chalk with respect to porosity. In Valhall field high pore pressure and hydrocarbon presence have counteracted major recrystallization and cementation. In general calcite redistribution is accelerated with burial depth, so the moderate porosity of English chalk is presumably due to relatively deep burial as indicated by well-developed recrystallization and cementation features like rhomb-shaped particles, strengthened particle contacts, cementation of intrafossil porosity, low 18O values, reduction in specific surface area and occurrence of stylolites. These characteristics are shared with chalk of the Dan, South Arne and Ekofisk fields. Phosphatic French chalk and Dutch chalk subjected to meteoric diagenesis have properties deviating from reservoir chalk.

Because geomechanical properties presumably are controlled by porosity and degree of cementation, these results indicate that for geomechanical tests English outcrop chalk is best suited as proxies for reservoir chalk of the Dan, South Arne and Ekofisk fields, whereas Danish outcrop chalk best matches Valhall chalk.

Burial diagenesis influences non-carbonate mineralogy. Outcrop chalk is dominated by quartz, smectite and occasionally opal-CT and clinoptilolite. In offshore chalk opal-CT seems to have recrystallized into submicron-size quartz crystals, clinoptilolite has dissolved and in the studied samples kaolinite is the dominating clay mineral. As a result quartz dominance is more pronounced offshore. The specific surface area of chalk

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is controlled partly by calcite recrystallization partly by type and amount of non-carbonate minerals.

Outcrop chalk from England, Précy sur Oise, Hallembaye, Lägerdorf and Stevns have specific surface areas comparable to those of reservoir chalk, and may constitute acceptable substitutes in flooding tests; this substitution further implies comparable porosity-permeability relationship of outcrop and reservoir chalk. Opal-CT dominated samples present interpretational problems as permeability in these samples are hard to predict; the applicability of these samples in flooding tests is questionable.

4.2. Wettability of chalk: Impact of silica, clay content, and mechanical properties

S. STRAND, M.L. HJULER, R. TORSVIK, J.I. PEDERSEN, M.V. MADLAND & T. AUSTAD

The success of improved oil recovery from naturally fractured chalk fields by injection of water is much depending on the wetting conditions of the reservoir rock, and also to some extent the compaction due to water weakening of the formation. Samples from outcrops are often used to mimic the reservoir properties in laboratory work. The present study illustrates that care must be taken when selecting outcrop material; especially the content and type of silica present will affect these important properties. Chalk samples from Rørdal, which contained significant amounts of silica and minor amounts of clay (6.3 wt% Si), have been studied by SEM, and the mineral properties of the silica were characterized. The surface chemistry of the porous medium is different from chalk containing smaller amounts of silica and clay (1.4-2.8 wt%). In the presence of a crude oil with high acid number and initial formation water, the water wet fraction of Rørdal chalk remained close to 1.0 after aging for 4 weeks at 90 °C in the crude oil. The Amott-Harvey wetting index showed, however, the wetting condition to be close to neutral, and only small amounts of water and oil imbibed spontaneously at the residual saturations. The difference in wetting conditions due to different content of silica and clay is also reflected in differences in mechanical properties. It appeared that the mechanical strength, as studied by a large number of Brazilian tests, became weaker as the water wetness decreased. The effect of wettability on the water weakening of chalk is discussed in terms of chalk dissolution and the chemistry associated to thin water films.

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4.3. Chalk seen through the eyes of scanning and transmission electron microscopes – interpretational challenges

MORTEN LETH HJULER & VIDAR HANSEN

The scanning electron microscope (SEM) and transmission electron microscope (TEM) are powerful instruments for characterizing chalk at high magnification. The SEM reflects external morphology and is useful for characterizing shape, size and spatial distribution of chalk particles. The TEM provides both morphological and structural information of probed particles. High magnification, observations may, however, also contain erroneous morphological and structural information originating from sample preparation or from interaction between chalk and the electron beam. This study presents some interpretational challenges associated with high magnification SEM and TEM investigations of artifacts produced by sample preparation and beam induced damage.

SEM: Conductive coatings applied to prevent sample charging may mask genuine surface details or generate artificial ornamentations; thus coating thickness should be reduced to a minimum. Chalk mounted on carbon adhesive discs may be covered with a thin film of carbon spreading from the carbon disc, not to be mistaken with any original organic coatings. By lowering the acceleration voltage, observation of surface detail is greatly improved and seemingly smooth surfaces may prove rich in details; considerable portions of these details may belong to conductive coatings. Thin particles like clay may bend or curl during interaction with the electron beam. Long organic threads frequently observed in chalk are of recent origin; their flattened thread terminations are easily confused with clay flakes. Authigenic non-carbonate minerals may have formed in the pore fluid and settled as the fluid was removed; these minerals do not occupy their original position and; some minerals, e.g. salt, probably precipitated from the fluid phase during sample preparation.

TEM: The high acceleration voltage used during TEM investigations may cause beam damage to calcite, as seen from diffraction patterns where a gradual decrease in crystallinity can be observed. Beam damage restricted to the calcite surface may produce an ultrathin layer of CaO on the calcite surface. Thin membranes scattered in suspension samples do not correspond to natural organic matter or clay; the membranes consist of graphite and are most likely contamination from the carbon film supporting the sample. Overlapping particles in suspension samples may give the impression of being cemented together. Residual adhesive from the ion milling process/conductive carbon films should not be confused with cementation features.

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5. References Bell, F.G., Culshaw, M.G. and Cripps, J.C., 1999. A review of selected engineering geological characteristics of English Chalk. Engineering Geology, 54, 237–269.

Borre, M., Fabricius (Lind), I.L., 1998. Chemical and mechanical processes during burial diagenesis of chalk: an interpretation based on specific surface data of deep-sea sediments. Sedimentology, 45(4), 755-769.

Bürki, P.M., Glasser, L.S.D. and Smith, D.N., 1982. Surface coatings on ancient coccoliths. Nature, 297, 145-147.

Cady, S.L., Wenk, H.-R. and Sintubin, M., 1998. Microfibrous quartz varieties: characterization by quantitative X-ray texture analysis and transmission electron microscopy. Contributions to Mineralogy and Petrology, 130, 320-335.

Calvert, S.E., 1974. Deposition and diagenesis of silica in marine sediments. In: K.J. Hsü and H.C. Jenkyns (Editors), Pelagic Sediments: on Land and under the Sea: Special Publication of the International Association of Sedimentologists 1. Blackwell Scientific Publications, pp. 273-299.

Clayton, C.R.I., 1983. The influence of diagenesis on some index properties of chalk in England. Géotechnique, 33(3), 225-241.

Collin, F., Cui, Y.J., Schroeder, C., Charlier, R., 2002. Mechanical behaviour of Lixhe chalk partly saturated by oil and water: experiment and modelling. InternationalJournal for Numerical and Analytical Methods in Geomechanics, 26, 897-924.

Damholt, T., Surlyk, F., 2004. Laminated–bioturbated cycles in Maastrichtian chalk of the North Sea: oxygenation fluctuations within the Milankovitch frequency band. Sedimentology, 51, 1323–1342.

Ehrmann, W.U., 1986. Zum Sedimenteintrag in das zentrale nordwesteuropäische Oberkreidemeer (Das Maastricht in Nordwestdeutschland, Teil 7). Geologisches Jahrbuch, A97, 1-139.

Fabricius, I.L., 2003. How burial diagenesis of chalk sediments controls sonic velocity and porosity. American Association of Petroleum Geologists Bulletin, 87(11), 1755-1778.

Fabricius, I.L., submitted. Chalk composition, diagenesis and physical properties. Bulletin of the Geological Society of Denmark.

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Fabricius, I.L., Borre, M., 2007. Stylolites, porosity, depositional texture, and silicates in chalk facies sediments. Ontong Java Plateau – Gorm and Tyra fields, North Sea. Sedimentology, 54, 183-205.

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Papers, conference abstracts, report and enclosures are not included in this

www-version but may be obtained from the Library at:

Institute of Environment & Resources Bygningstorvet, Building 115

Technical University of Denmark DK-2800 Kgs. Lyngby

(library@er.dtu.dk)

Manuscript 1

Engineering properties of chalk related to diagenetic variations of Upper Cretaceous onshore and offshore chalk in the North

Sea area

Morten L. Hjuler & Ida L. Fabricius

Submitted

Manuscript 2

Wettability of chalk: Impact of silica, clay content, and mechanical properties.

Skule Strand, Morten L. Hjuler, Randi Torsvik, J.I. Pedersen, Merete V. Madland & Tor Austad

Petroleum Geoscience, 13: 69-80, 2007.

Manuscript 3

Chalk seen through the eyes of scanning and transmission electron microscopes – interpretational challenges

Morten L. Hjuler & Vidar Hansen

Submitted

Conference abstract 1

Diagenetic Variations between Upper Cretaceous Outcrop and Deeply Buried Reservoir Chalks of the North Sea Area

Morten L. Hjuler & Ida L. Fabricius

AAPG Annual Convention, Long Beach, California, abstracts volume, p. 63, 2007.

Conference abstract 2

Diagenetic Variations between Upper Cretaceous Outcrop and Deeply Buried Reservoir Chalks of the North Sea Area

Morten L. Hjuler & Ida L. Fabricius

AAPG Annual Convention, Long Beach, California. Extended abstract, abstracts cd-rom, 1-6, 2007.

Conference abstract 3

Chalk Reservoir Properties controlled by Composition, Age Temperature, Burial Stress and Pore Pressure.

Ida L. Fabricius & Morten L. Hjuler

AAPG Annual Convention, Long Beach, California. Extended abstract, abstractsvolume, p. 44, 2007.

Report

Silica and clay mineralogy and distribution in reservoir chalk (Valhall) and outcrop chalks (Stevns, Aalborg and Liège)

Morten L. Hjuler

BP technical report, 33 pp, 2006.

Enclosure 1

Petrography, petrophysics and mineralogy of investigated outcrop and reservoir chalk

Enclosure 2

Interpretational challenges related to scanning and transmission electron microscopy of chalk

Enclosure 3

Beregning af kvarts-, smectit- og mica-indholdet ud fra kemisk analyse-data

(The calculation procedure used to estimate quartz, smectite and mica content from chemical data)

ISBN 978-87-91855-43-6

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