Inorganic GeochemistryApplications toPetroleum Geology

Dominic Emery & Andrew RobinsonBP Exploration, 4/5 Long Walk, Stockley Park

Uxbridge, Middlesex U8111 BP, UK

WITH CONTRIBUTIONS FROM

Andrew Aplin Newcastle University

& Craig Smalley BP Group Engineering & Research

OXFORD

BLACKWELL SCIENTIFIC PUBLICATIONSLONDON EDINBURGH BOSTON

MELBOURNE PARIS BERLIN VIENNA

Inorganic GeochemistryApplications toPetroleum Geology

Inorganic GeochemistryApplications toPetroleum Geology

Dominic Emery & Andrew RobinsonBP Exploration, 4/5 Long Walk, Stockley Park

Uxbridge, Middlesex U8111 BP, UK

WITH CONTRIBUTIONS FROM

Andrew Aplin Newcastle University

& Craig Smalley BP Group Engineering & Research

OXFORD

BLACKWELL SCIENTIFIC PUBLICATIONSLONDON EDINBURGH BOSTON

MELBOURNE PARIS BERLIN VIENNA

This book is dedicated to our families:Helen, Edward, David, Flo, Sandra, Bob, Avril and Dan

© 1993 byBlackwell Scientific PublicationsEditorial Offices:Osney Mead, Oxford OX2 OEL25 John Street, London WC1N 2BL23 Ainslie Place, Edinburgh EH3 6AJ238 Main Street, Cambridge

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A catalogue record for this titleis available from the British Library

ISBN 0-632-03433-5

Library of CongressCataloging-in-Publication Data

Emery, Dominic.Inorganic geochemistry:

applications to petroleum geology/Dominic Emery & Andrew Robinson,with contributions from Andrew Aplinand Craig Smalley.

p. em.Includes bibliographical references

and index.ISBN 0-632-03433-51. Geochemistry. 2. Petroleum- Geology.

I. Robinson, Andrew. II. Title.QE515.E44 1993553.2'8 - dc20

Contents

Preface ixAcknowledgements IX

1 Introduction

1.1 Background1.2 How is inorganic geochemistry applied to

petroleum geology?1.3 What is in this book1.4 Overview1.5 What is not in this book

2 Textural and MineralogicalAnalysis 7

2.1 Introduction2.2 Transmitted light microscopy

2.2.1 Introduction2.2.2 Sample preparation2.2.3 Mineral identification and

differentiation of detrital grains fromdiagenetic cements

2.2.4 Mineralogical quantification2.2.5 Mineral paragenesis2.2.6 Porosity description

2.3 Cathodoluminescence microscopy2.3.1 Introduction2.3.2 Analytical techniques2.3.3 Sample preparation2.3.4 Applications of CL

2.4 Ultraviolet fluorescence microscopy2.4.1 Introduction2.4.2 Applications of UVF

2.5 Scanning electron microscopy2.5.1 Introduction2.5.2 Sample preparation2.5.3 Applications of emission mode SEM2.5.4 Applications of backscatter mode SEM

2.6 Transmission electron microscopy2.6.1 Introduction2.6.2 Sample preparation

2.6.3 Applications of TEM2.7 X-ray diffraction

2.7.1 Introduction2.7.2 Sample preparation2.7.3 Applications of XRD

2.8 Thermogravimetry-evolved water analysis2.8.1 Introduction2.8.2 Analytical techniques2.8.3 Applications of TG-EWA

2.9 Pore image analysis2.9.1 Introduction2.9.2 Analytical techniques2.9.3 Applications of PIA

3 Fluid Inclusions 41

3.1 Introduction3.2 Relationship to host mineral3.3 Microthermometry I - principles

3.3.1 Introduction3.3.2 Melting temperatures of solid phases3.3.3 Homogenization temperatures3.3.4 Data collection - precision and

accuracy3.4 Microthermometry II - interpretation

3.4.1 Introduction3.4.2 Stretching and leakage - a terminal

problem?3.4.3 Pressure corrections: can we and should

we?3.4.4 Example 1: calci te filled fractures, Li ttle

Knife Field, North Dakota3.4.5 Example 2: mineral cementation,

offshore Angola3.5 Non-destructive analysis of individual inelusions

3.5.1 Introduction3.5.2 Laser Raman spectroscopy3.5.3 Fourier transform infrared spectroscopy3.5.4 Ultraviolet fluorescence

3.6 Bulk analysis of petroleum inclusions3.6.1 Introduction

v

vi Contents

4

4.14.2

4.3

4.4

4.5

5

5.15.25.3

3.6.2 Isolation of a fluid sample3.6.3 Gas chromatography3.6.4 Gas chromatography - mass

spectrometry

Stable Isotopes 73

IntroductionPrinciples4.2.1 Terminology4.2.2 Isotope fractionation4.2.3 Isotope geothermometry4.2.4 Analytical methods4.2.5 Data interpretation: general problem"Oxygen and hydrogen4.3.1 Water4.3.2 Silicates4.3.3 Example 1: quartz cement in a

Pennsylvanian sandstone, West TuscolaField, north-central Texas

4.3.4 Example 2: illite cement in fluvialsandstone, Brent Group, NorthernNorth Sea

4.3.5 Carbonates4.3.6 SulphatesCarbon4.4.1 Principles4.4.2 Example 3: calcite cement in a Miocene

carbonate reservoir, Liuhua Field,Pearl River Mouth Basin, offshoreChina

Sulphur4.5.1 Principles4.5.2 Example 4: thermochemical sulphate

reduction in a carbonate reservoir, deepFoothills region, Alberta, Canada

Radiogenic Isotopes 101

IntroductionRadiogenic isotope systemsK-Ar dating5.3.1 Principles5.3.2 Analytical methods: precision and

accuracy5.3.3 Assumptions5.3.4 Example 1: illite cement in aeolian

sandstone, Rotliegend Group,

5.4

5.5

5.65.7

6

6.16.2

6.3

Southern North Sea5.3.5 Example 2: illite cement in fluvial

sandstone, Brent Group, NorthernNorth Sea

5.3.6 Example 3: K-feldspar cement, offshoreAngola

40Ar- 39Ar dating5.4.1 Principles5.4.2 Example 4: chlorite cement, Triassic,

Central North Sea5.4.3 Example 5: illite cement in aeolian

sandstone, Rotliegend Group,Southern North Sea

5.4.4 Example 6: K-feldspar overgrowthsThe Rb-Sr system5.5.1 Principles5.5.2 Analytical methods5.5.3 Rb-Sr dating of clay minerals5.5.4 Example 7: illite cement in aeolian

sandstone, Rotliegend Group,Southern North Sea

5.5.5 Sr isotope stratigraphy5.5.6 Example 8: dating Tertiary sediments,

V~ring Plateau, offshore Norway5.5.7 Tracing the origin ofSr in subsurface

fluidsThe Sm-Nd systemU- Th- Pb dating of carbonates

Porosity and PermeabilityPrediction 129

IntroductionReservoir quality prediction in frontiersexploration: Flemish Pass Basin, offshoreNewfoundland6.2.1 Introduction6.2.2 Geological background6.2.3 Approach6.2.4 Establishing a relationship between

permeability and depth6.2.5 Prediction of uncemented reservoir6.2.6 ConclusionsNet to gross prediction: Norphlet Formation,Gulf of Mexico6.3.1 Introduction6.3.2 Geological background6.3.3 Approach

Contents vii

6.3.4 Quantitative mineralogy: controls on 7.2.3 Approachreservoir quality 7.2.4 Fluid inclusions: petrography,

6.3.5 Conditions of mineral cement growth microthermometry and GCMS analysis6.3.6 Prediction of Tight Zone thickness 7.2.5 Conclusions6.3.7 Conclusions 7.3 Prediction of the occurrence of diagenetic

6.4 Influence of kaolinite on sandstone porosity: celestite cap rock: Central North SeaBrent Province, Northern North Sea 7.3.1 Introduction and approach6.4.1 Introduction 7.3.2 Conditions and cause of celestite6.4.2 Geological background precipitation6.4.3 Approach 7.3.3 Simulation of celestite precipitation6.4.4 Petrography and isotopic composition 7.3.4 Conclusions

of kaolinites 7.4 Regional mapping of migration pathways:6.4.5 Conclusions Weald Basin, onshore UK

6.5 Appraisal from a discovery well: Magnus Field, 7.4.1 Introduction and approachNorthern North Sea 7.4.2 Geological background6.5.1 Introduction 7.4.3 Fluid inclusions in ferroan calcite6.5.2 Geological background cement6.5.3 Approach 7.4.4 Conclusions6.5.4 Controls on porosity and relationship of 7.5 Filling history of a reservoir: Waalwijk,

cementation to oil filling onshore Netherlands6.5.5 Conclusions 7.5.1 Introduction

6.6 History of fracturing in a Chalk reservoir: 7.5.2 Geological background

Machar Field, Central North Sea 7.5.3 Approach6.6.1 Introduction 7.5.4 Petrography, K-Ar illite ages and6.6.2 Geological background dolomite stable isotope ratios6.6.3 Approach 7.5.5 Conclusions6.6.4 Geochemistry of fracture fills I fluid

inclusions8 Correlation 187

6.6.5 Geochemistry of fracture fills IIstable and radiogenic isotopes 8.1 Introduction

6.6.6 Conclusions 8.2 Stratigraphic correlation6.7 Controls on permeability and the origin of high- 8.3 Lithological and reservoir property correlation

permeability streaks: Forties Field, Central 8.4 Stratigraphic correlation in exploration:North Sea Tertiary of offshore Norway6.7.1 Introduction 8.4.1 Introduction6.7.2 Geological background 8.4.2 Geological background6.7.3 Approach 8.4.3 Approach6.7.4 Image analysis of Forties Formation 8.4.4 Strontium isotope ages

sandstones 8.4.5 Conclusions6.7.5 Conclusions 8.5 Stratigraphic correlation in exploration:

Plio-Pleistocene of the Gulf of Mexico

7 Fluid Migration 1718.5.1 Introduction8.5.2 Geological background

7.1 Introduction 8.5.3 Approach7.2 History of petroleum migration from outcrop 8.5.4 Oxygen isotope stratigraphy

samples: Aquitaine Basin, France 8.5.5 Conclusions7.2.1 Introduction ·8.6 Reservoir connectivity: Ekofisk Field,7.2.2 Geological background Cretaceous of offshore Norway

viii Contents

8.6.1 Introduction8.6.2 Geological background8.6.3 Approach8.6.4 Isotopic analyses of chalk and residual

salts8.6.5 Conclusions

8.7 Reservoir correlation: Gullfaks Field,Triassic-Jurassic of offshore Norway8.7.1 Introduction8.7.2 Geological background8.7.3 Approach8.7.4 Sm- Nd isotopic correlation8.7.5 Conclusions

9 Petroleum Recovery 213

9.1 Introduction9.2 Secondary recovery9.3 Enhanced oil recovery9.4 Production of corrosive fluids9.5 Secondary recovery: Forties Field, offshore UK

9.5.1 Introduction9.5.2 Geological background9.5.3 Approach9.5.4 Chemical and isotopic analyses of

produced fluids9.5.5 Conclusions

9.6 Secondary recovery and gas souring: WytchFarm Oilfield, Dorset, UK

9.6.1 Introduction9.6.2 Geological background9.6.3 Approach9.6.4 Chemical and isotopic analyses of

produced fluids9.6.5 Conclusions

9.7 Enhanced oil recovery - steam injection: ColdLake Area Oil Sands, Alberta, Canada9.7.1 Introduction9.7.2 Geological background9.7.3 Approach9.7.4 Petrographic and isotopic investigations

of steam-induced reactions9.7.5 Conclusions

9.8 Enhanced oil recovery - fireflooding:Lloydminster Area Oil Sands, Saskatchewan,Canada9.8.1 Introduction9.8.2 Geological background9.8.3 Approach9.8.4 Petrographic investigation of

mineralogical changes9.8.5 Conclusions

References 231

Index 245

Preface

During the spring of 1990 we were approached by acolleague, a geologist with a problem. He had read apaper in the latest issue of a major journal thatpresented fluid inclusion data from an area in whichhe was exploring. The paper had concluded that aTertiary 'thermal event' had been responsible forgenerating petroleum; if this were the case, ourfriend assured us, it would have far reaching implica­tions for the prospectivity of the basin. Could weread the paper and give an opinion as to the validityof the conclusion? We read the paper. Our view wasthat the data were probably fine but that they hadbeen poorly and over-optimistically interpreted sothat the evidence for a thermal event was very shakyindeed. Our friend was duly grateful.

It would be tempting to write that at this point wedecided that we could make a lot of money bywriting a book that would explain fluid inclusionand other geochemical techniques to petroleumgeologists. In fact, we had been planning the projectfor a couple of months already but the incident didserve to confirm our view that there was room for abook that would enable non-specialists to make uptheir own minds about the large number of papersnow appearing in print every month which includesome facet of inorganic geochemistry as a majorconstituent. This book is the result. Its purpose isto bring together the most important inorganic geo­chemical methods in a single volume, to explaintheir potential and limitations in a form that isaccessible to the non-specialist, and to demonstratetheir application to a wide range of problems inpetroleum geology, from exploration, throughappraisal and development, to production. Thebook is therefore intended for geologists, geo­physicists and production engineers in oil companieswho wish to broaden their knowledge of the geo­chemical methods available for solving problemswith which they are routinely faced. We hope that it

will also be of interest to final year undergraduates,postgraduates with an interest in the inorganic geo­chemistry of sedimentary rocks and waters, and tothose attending petroleum geology and related MSccourses.

Acknowledgements

Andrew Aplin (Newcastle University) contributedthe majority of Chapter 4 and Craig Smalley (BPResearch) wrote the sections on Rb-Sr, Sm-Ndand U-Th-Pb in Chapter 5. The book is far betterfor these contributions. Numerous friends and col­leagues have also contributed to the book by provid­ing prize specimen photomicrographs, and byreviewing the text at the many and varied stages ofits development. Thanks in no particular orderto Chris Rundle (British Geological Survey), JimMarshall (Liverpool University), Ian Hutcheon(University of Calgary), Christine Knox, Jon Gluyas,Jonathan Henton, Ed Warren, Tim Primmer,Norman Oxtoby, Andy Brayshaw, Shona Grant,Andy Leonard, Mike Bowman, Joyce Neilson,Steve Rainey, Max Coleman and Keith Mills.

This book would have been particularly difficultto write had we not had the support of BP Explora­tion and BP Research which we thank for permissionto publish. Organizations are the sum of individualshowever and we would like to thank in particularIan Vann and Jon Bellamy for their generous sup­port. We are grateful to many other people through­out BP for allowing us to use case study materialfrom the fields or areas in which they work. Wewould also like to acknowledge the co-operation ofBP's partners in allowing us to publish informa­tion on many of the fields and licences mentionedthroughout the text.

Dominic Emery & Andrew RobinsonBP Exploration, London

ix

Chapter 1 Introduction

1.1 Background

1.2 How is inorganic geochemistry applied topetroleum geology?

1.3 What is In this book

1.4 Overview

1.5 What is not In this book

1.1 Background

Petroleum is not as easy to find as it used to be. Mostof the accessible sedimentary basins in the worldhave been explored and a large proportion of themore obvious petroleum targets have been drilled.The more risky and costly exploration becomes, themore important it is to develop new discoveriesas efficiently as possible and to extract a greaterproportion of the petroleum in place from existingfields. The last 15 years have seen this imperativegive birth to some important new topics within theearth sciences, most of which cross the boundariesbetween the traditional divisions of geology as taughtto undergraduate students. For example, sequencestratigraphy - now a principal tool of the explora­tion geologist - has emerged from classical strati­graphy and seismic interpretation; and reservoirdescription has been born of a long overdue rela­tionship between reservoir engineering, sophisti­cated geophysics, stratigraphy and sedimentology.

Inorganic geochemistry is one more relatively newweapon in the armoury of the petroleum geologist.In its broadest sense, the subject includes the studyof all of the chemical constituents of rocks andsubsurface fluids, excluding only organic com­ponents based on carbon. This book makes noattempt to cover this potentially vast field in its en­tirety. It may appear at first glance to contain rathera mixed bag of subject matter but the choice of whatto include has not been arbitrary. The themes ofthe book are the characterization of fluids in sedi­mentary basins, understanding their interaction witheach other and with rocks and the application of thisinformation to finding, developing and producing oiland gas. This might include dating quartz cementgrowth in a sandstone in an attempt to predictporosity distribution, or determining the extent ofseawater breakthrough into a reservoir during pro­duction. There is a considerable degree of over­lap with the field of sediment diagenesis but thisbook covers important topics with a geochemicalcomponent which would never find their way intoa diagenesis book, such as strontium isotope strati­graphy, correlation and production chemistry.

2 Chapter 1

The subject of inorganic geochemistry is not ofcourse in itself new, but its application to sedi­mentary basins has lagged behind igneous andmetamorphic geochemistry. There are a numberof reasons for this. One may be the relativelyunglamorous nature of a sandstone when set besidea peridotite. Perhaps more important is the fact thatsedimentary, particularly clastic rocks are inherentlydifficult to analyse in any meaningful way becausethey contain constituents with many different originswhich for most purposes must be analysed separ­ately. The study of sediment geochemistry hashowever been stimulated in recent years by theincreased availability of core from deeply buriedsediments taken by petroleum exploration com­panies and by their readiness to finance researchinto controls on porosity and permeability (we areourselves beneficiaries of this largess). The lastfew years have also seen the development of newmethods - the use of lasers, for example whichhave helped to overcome sampling problems.

1.2 How is inorganic geochemistry appliedto petroleum geology?

It is relatively easy to watch wave ripples form or tosnorkel over a growing reef and study the depositionof sediments. It is a lot harder to study how sedi­ments are modified during burial in a basin; pro­cesses cannot in the main be directly observedbecause they occur at substantial depths and inmany cases may well have ended long ago. Onlythe products of fluid-rock interaction. diageneticminerals and present day formation waters, are leftto act as records. Inorganic geochemistry provides ameans of interpreting these records. The main typesof information it provides are as follows.1 Timing. Relative and absolute ages of mineralgrowth and dissolution, and of the presence andmigration of fluids (both water and petroleum).2 Temperature. Temperatures at which mineralsgrew or dissolved and at which particular fluids werepresent in a rock's pores. Temperature and timingare correlated for a sediment in a subsiding sedi­mentary basin and can be related by modellingburial and thermal history.3 Chemical composition. The bulk and isotopechemistries of minerals and water contain informa­tion about the history of fluids, especially theirinteraction with rocks.

It is one matter to obtain this information fromminerals and fluids but another matter entirely to

interpret the processes involved. Given a sampleof quartz cemented sandstone, the geochemist willquite probably be able to find out what temperatureit grew at and say something about the origin of thewater involved, but will not be able to say why therock is cemented or what caused the quartz toprecipitate. This is the level of current understand­ing of most diagenetic phenomena: we can charac­terize them but not explain them. The trick to usinginorganic geochemistry to solve problems in the fieldof petroleum geology is to accept these limitationsand do the best possible with the information thatcan be obtained. In many cases, this is quite a lot.At the very least, geochemical methods provide ameans of integrating diagenetic phenomena intothe temporal framework that forms the basis ofbasin analysis. For example, the timing of mineralcementation and dissolution, and consequentchanges in porosity and permeability can be relatedto phases in the development of a basin and to oilmigration. The models that emerge for porosityand permeability prediction invariably involve alarge empirical component, but they represent animprovement over empiricism alone.

1.3 What is in this book

This book has two parts. Chapters 2-5 describe thegroups of techniques that we have found to be mostuseful in petroleum geology and Chapters 6-9describe case histories - mostly from our own workor that of our colleagues - grouped according to thenature of the problem that inorganic geochemistryhelped (or in some cases, failed) to solve.

The chapters on geochemical techniques empha­size applications to sedimentary rocks and the fluidsin sedimentary basins. Particular attention is paid toprecision and accuracy and to the questions of whatinformation can be plausibly obtained and underwhat circumstances: what do data mean and what dothey not mean? Particular difficulties and pitfalls areillustrated by the use of examples which are made upof complete sets of real data. The case historiesthat make up the second half of the book cover awide range of applications but inevitably reflectour interests and biases. They include clastic andcarbonate rocks from many parts of the world but anumber are from the North Sea (Fig. 1.1). Many of

Fig. 1.1 (Opposite page.) Location of North Sea oil andgas fields mentioned in this book.

6201 6202 6203 6204 6205

219

218

217

214 208 209 210 33 35

Magnus • \ • Snorre

NORWAY

14 15 18 19

NoeS

2020 21 22 8 9 10

Forties

27 28 29Machar 1 2 3

UKes

3035 36 37 38

42 43 44

GERMANY

ENGLAND

• OilfieldBELGIUM

""'-.

FRANCE t .1..-.1

o Gas field

4 Chapter 1

the case studies are from rift basins but there isusually no reason why analogous problems in othergeological settings should not be tackled in similarways. Coarse grained sediments figure more thanmudstones because they are of importance asreservoir rocks and because, in the main, theyare easier to study and more information can beobtained from them.

All of the case histories are taken from real lifeand we have not tinkered with the data or modifiedthe conclusions. Some were successful in solving theproblem they set out to solve, rather more solved itpartially, some solved a completely different prob­lem and others proved scientifically fascinating butof no use whatsoever. We feel that it is far moreinstructive, and interesting, to present relativefailures alongside relative successes. Negative resultsrarely get published but are often as instructiveas positive outcomes. In most cases, careful andcircumspect science can take a problem only so far.When that point is reached - and we try to make itclear when it is - we do not feel embarrassed tospeculate. Petroleum geology, particularly explora­tion, involves making the best of incomplete under­standing and is an essentially optimistic enterprise.

1.4 Overview

Chapter 2 (Textural and Mineralogical Analysis)describes five basic and three more advanced tech­niques for mineral identification and quantification.Inclusion of this topic in a book on geochemistrystretches the definition of the term, but analysis of asample's mineralogy is vital if any sense is to bemade of geochemical information obtained fromit. The basic techniques include thin-section petro­graphy, cathodoluminescence microscopy, fluores­cence microscopy, scanning electron microscopyand X-ray diffraction. Examples are given through­out the text of the application of the techniquesto mineral identification, differentiation of detritalgrains from diagenetic cements, mineral quantifica­tion and the construction of paragenetic histories forreservoir rocks. The three more advanced techniquescovered are transmission electron microscopy,thermogravimetric-evolved water analysis and poreimage analysis, which are playing an increasing rolein chemical analysis, clay mineral quantification andporosity analysis respectively. The chapter stressesthe application of techniques at the expense ofa detailed description of apparatus and sample pre­paration (for which see Tucker, 1988). Emphasis

is also placed on the value of combining the tech­niques with standard petrophysical and engineeringmethods for describing the reservoir quality of rocksamples.

Chapter 3 (Fluid Inclusions) explains how thestudy of these minute samples of fluid trapped dur­ing mineral growth or fracture healing can provideinformation about the temperatures of diageneticreactions, and about the composition of fluids pass­ing through sedimentary rocks and at what temper­atures they did so. The study of fluid inclusions inigneous rocks and particularly, metallic mineraldeposits, goes back more than a century but onlyover the last 10 years has much effort been put intothe study of inclusions in sediments. Much of theearlier work, and some current studies, suffer fromoverinterpretation and this chapter attempts toredress the balance by underlining some limitationsof the technique. Particular attention is paid tothe question of leakage because if fluid inclusions indiagenetic minerals routinely leak - and a case canbe made for thinking that they do - then they willbe of limited value for the study of sedimentaryrocks.

The applications of stable isotopes, the subject ofChapter 4, to petroleum geological problems rangefrom stratigraphic analysis to better understandingand predicting reservoir quality and reservoir fluidtype. Stable isotopes are used principally as naturaltracers for subsurface reactions and - in the caseof oxygen isotopes - can provide information aboutreaction temperature. The chapter first describesthe principles and nomenclature of stable isotopesystems (a necessary evil but surprisingly painless),the basic analytical techniques, and the uncertaintiesassociated with isotopic measurements. This isfollowed by a description of the most importantstable isotope systems in the context of the fluidsand minerals in which they are found: oxygen inwater, silicate, sulphate and carbonate minerals;hydrogen in water and clays; carbon in carbonateminerals, CO2 and CH4 ; and sulphur in sulphate andsulphide minerals and HzS.

Chapter 5 explains the use of radiogenic isotopesto unravel geological history. It begins with arefresher in the simple physics that forms the basisof radiometric dating, which serves to stress thecommon features of all of the dating methods. Fourisotope systems are described: K-Ar, Rb-Sr,Sm-Nd and U-Th-Pb. K-Ar dating is almostroutinely used to date K-bearing mineral cements,principally illite. The chapter explains why this must

be done with great care even when the samplematerial is particularly suitable. Ar-Ar dating aclever technique that also relies on the K-Ar decayseries - is also covered. The other isotope systemscan be used for dating sedimentary material onlyunder rather restricted circumstances. However,radiogenic isotopes also have value as naturaltracers. Strontium isotope ratios in particular can beused to trace the chemical evolution of naturalwaters. One consequence of this property is theiruse as a tool for dating marine carbonate and phos­phate (strontium isotope stratigraphy).

Chapter 6 (Porosity and Permeability Prediction)is the first of four chapters that group case historiesto illustrate a particular application of inorganicgeochemistry. It is also the bulkiest because it hasbeen up to now the most important application if thenumber of published studies is anything to go by.The introductory section explains how inorganicgeochemistry contributes to porosity and perme­ability prediction by reducing risk (neither canbe predicted using inorganic geochemistry alone).There follow six case histories which range fromexploration in frontier areas where the amount ofinformation available is very limited (Flemish Pass,Grand Banks) through progressively more matureexploration areas (offshore Louisiana and the BrentProvince) to porosity prediction in appraisal, devel­opment and production in three North Sea fields(Magnus, Machar and Forties).

Chapter 7 (Fluid Migration) considers inorganicgeochemical evidence for phases of water and petro­leum migration. The value of knowing about petro­leum migration need hardly be underlined butmigration of waters is also of interest as these affectreservoir quality by interacting with rocks and canalso alter oil through the processes of biodegrada­tion and water washing. The four case historiescover the use of fluid inclusions for sorting outmigration history in poorly and better known areas(Aquitaine and Weald Basins), the prediction ofthe occurrence of a rather odd diagenetic reservoirrock in the Central North Sea and the history offilling of a gas-condensate discovery (Waalwijk,Netherlands) .

The next chapter (Correlation) describes howstable and radiogenic isotope systems can be usedto correlate stratigraphic units, chiefly on a broadexploration scale, but also on the smaller scale ofindividual reservoir units within discrete oilfields.Four case studies are described. The first, from theNorwegian North Sea, shows how strontium isotope

Introduction 5

stratigraphy and radiometric dating can be usedto refine the stratigraphic correlation of Tertiaryclastic sediments, and assist in erecting a sequencestratigraphy. The second also uses isotope strati­graphy, but in this case oxygen isotopes are appliedto provide a very high resolution stratigraphy of afew tens of thousands of years for Plio-Pleistocenesandstone reservoir targets in the Gulf of Mexico.The third and fourth case studies are both from oil­fields in the Norwegian sector of the North Sea, theEkofisk Chalk Field, and the clastic Gullfaks Field.In Ekofisk, strontium isotopes are used to correlatereservoir zones using data obtained from the rockmatrix and from the formation waters. In Gullfaks,samarium and neodymium isotopic methods areused to predict the distribution of reservoir sandbodies by identifying changes in sand provenance.

Chapter 9 (Petroleum Recovery) outlines thebasic principles of secondary and enhanced oilrecovery, and the problems of corrosive fluid pro­duction (HzS and COz). The first two case studies,from the Forties Field, UK North Sea, and from theWytch Farm Field, onshore UK, demonstrate thevalue of oxygen and hydrogen isotopes as tracers forseawater breakthrough during oil production. Theapplication of sulphur isotopes for understandingsour gas production in Wytch Farm is also outlined.The last two case studies are from the Cretaceousheavy oil sands of Alberta and Saskatchewan inwestern Canada. These demonstrate the importanceof quantitative petrography linked to petrophysicsfor describing the effects of thermal recovery pro­cesses (steam injection and fireflooding) on shallowreservoir sandstones. The case study on steam injec­tion also shows how carbon isotopes can be used toidentify the source of COz produced during thermalrecovery.

1.5 What is not in this book

This is a book about the application of fluid-fluidand fluid-rock interaction to petroleum geology andas such contains only those methods which we havefound to be useful in this field. We avoid describingmethods of analysing whole rocks partly because wehave found bulk chemistry to be of rather limitedvalue in this respect and also because it is a subjectin its own right that has been better covered than wecould manage in other books. We have also avoidedchemical modelling of water-rock interaction andmodelling of fluid flow in sedimentary basins. Theseare used together by some (not by us) to predict

6 Chapter 1

porosity; we believe them to have explanatorypower and to be useful for understanding diageneticprocesses but feel that their predictive power islimited (see Section 6.1). Although geohistoryanalysis - especially burial and thermal modelling -

is repeatedly referred to in the text, it also con­stitutes a separate subject and we have not as aconsequence dealt with fission track analysis whichis principally a means of calibrating burial history.

Chapter 2 Textural and Mineralogical Analysis

2.1 Introduction

2.2 Transmitted light microscopy2.2.1 Introduction2.2.2 Sample preparation2.2.3 Mineral identification and

differentiation of detrital grains fromdiagenetic cements

2.2.4 Mineralogical quantification2.2.5 Mineral paragenesis2.2.6 Porosity description

2.3 Cathodoluminescence microscopy2.3.1 Introduction2.3.2 Analytical techniques2.3.3 Sample preparation2.3.4 Applications of CL

2.4 Ultraviolet fluorescence microscopy2.4.1 Introduction2.4.2 Applications of UVF

2.5 Scanning electron microscopy2.5.1 Introduction2.5.2 Sample preparation2.5.3 Applications of emission mode SEM2.5.4 Applications of backscatter mode SEM

2.6 Transmission electron microscopy2.6.1 Introduction2.6.2 Sample preparation2.6.3 Applications ofTEM

2.7 X-ray diffraction2.7.1 Introduction2.7.2 Sample preparation2.7.3 Applications of XRD

2.8 Thermogravimetry-evolved water analysis2.8.1 Introduction2.8.2 Analytical techniques2.8.3 Applications ofTG-EWA

2.9 Pore image analysis2.9.1 Introduction2.9.2 Analytical techniques2.9.3 Applications of PIA

2.1 Introduction

Mineralogical and textural analysis of samples is theessential first step in any inorganic geochemicalprogramme and provides the critical link betweenthe quality of a reservoir - chiefly its porosity andpermeability - and more advanced geochemicaltechniques such as fluid inclusion analysis (Fig. 2.1).Mineralogical and textural analysis can provideseveral types of information:1 mineral identification;2 differentiation of detrital from diagenetic phases;3 quantitative analysis of mineral abundance;4 mineral paragenesis (the sequence of mineralgrowth and dissolution);5 mineral chemistry;6 description and quantification of porosity; and7 identification of the main factors influencingporosity and permeability.The most appropriate technique for providing eachof these is shown in Table 2.1. Note that, to be mosteffective, certain techniques need to be applied inconjunction with others. Clay mineral X-ray diffrac­tion is most effective as a tool for quantifying claymineral content in rock samples when appliedwith thermogravimetric-evolved water analysis.Similarly, cathodoluminescence microscopy of lime­stone samples is most effective when applied withobservations made using transmitted light.

The objectives of this chapter are to introducemineralogical and textural analytical techniques, andtheir applications to understanding the post-deposi­tional evolution of siliciclastic and carbonate rocksin petroleum provinces. We will concentrate parti­cularly on the application of the variety of techniquesdescribed here, rather than on details of instrumen­tation and sample preparation. References coveringthese aspects in more depth are given in the text.

2.2 Transmitted light microscopy

2.2.1 Introduction

Transmitted light microscopy is a basic tool of thetrade for the description of sedimentary rocks and

7

8 Chapter 2

Reservoirquality

e.g.

Porosity

Permeability

Water saturation

• Identificationof reservoirproblem;porosity lowerthan expectedbefore drilling.

Petrography

e.g.

Mineral identification

Differentiation ofdetrial/diageneticphases

Paragenetic history

Mineral quantification

• Identificationof cause;up to 20% quartzburial cement.

Furthergeochemicalanalysis

e.g.

Fluid inclusions

Stable isotopes

Radiogenic isotopes

• Identificationof approximatetiming of quartzby fluid inclusionanalysis; quartzpre-datespetroleum fill.

Possible solution: Identify future prospects inthis area where petroleum fillingpre-dates quartz cementation

Fig. 2.1 The link between reservoir quality, petrography and further geochemical analysis.

there are many texts which cover all aspects ofsedimentary petrology. Folk (1974), Pettijohn (1975)and Tucker (1981) are good general petrographictexts, whereas Moore (1989) and Pettijohn et al.(1973) concentrate on carbonate and siliciclasticpetrography respectively. The Atlas of SedimentaryRocks by Adams et al. (1984), and the AAPG colourguides to sandstones (Scholle, 1979) and carbonates(Scholle, 1978) contain superb colour thin-sectionphotomicrographs and are well worth referring to.

The objective of this section is to explain howtransmitted light petrography can be used to describethe diagenesis of sedimentary rocks (rather than thedepositional fabric). Petrographic description canprovide five main pieces of information:1 mineral identification;2 differentiation of detrital from diagenetic phases;3 quantitative analysis of mineral abundance;4 mineral paragenesis; and5 description of porosity.The problems associated with siliciclastic petro­graphy are quite different from those associatedwith carbonates. Siliciclastic rocks tend to be poly­mineralic, with several different diagenetic phasessuch as clay minerals, quartz and carbonates and it isusually easy to distinguish these from the detrital

components of the sediment. In contrast, carbonaterocks are composed of fewer minerals which tendto be relatively unstable during diagenesis so thatmuch of the primary depositional fabric may beobliterated. Recognition of subtle changes in car­bonate fabric and chemistry is therefore essential todifferentiate primary constituents from diageneticphases, and to allow different diagenetic carbonatesto be recognized.

2.2.2 Sample preparation

The thin-section is the basic sample requirement formicroscope petrography. It consists of a rock wafer30 11m thick, mounted on a glass slide. The thicknessof the rock wafer is standard to ensure uniformityof birefringence colours (which are determined inpart by sample thickness). Similarly, to ensure com­parability of samples, the rock wafer is usuallymounted on the slide in a medium of uniform refrac­tive index. Basic thin-section preparation is de­scribed in more detail by Miller (1988).

Before the section is cut, samples are commonlyimpregnated with a dyed resin. This fills and coloursporespace in the sample, allowing easier identifi­cation of porosity types in the sample, and also

Textural and Mineralogical Analysis 9

Table 2.1 Summary matrix of technique versus application.

Application

TechniqueMineralidentification

Differentiation ofdiagenetic1 fromdetrital phases

Mineralquantification

Mineral2

paragenesisMineralchemistry

High resolutionmineralogical andtextural analysis

Porositydescription

Transmitted light • •microscopy

CL Cold C •CL Hot S •CL SEM S •UVF

SEM SE • •SEM BSEM •TEM •XRD Whole • 11

rock

XRD Fine .12

fraction

TG-EWA

PIA

•

•

•

•

• •

•

•

•

Key:1 Includes sample screening2 Includes cement stratigraphy and cement fabric analysis3 With point-counting apparatus4 Qualitative only for stained carbonates5 Especially cement stratigraphy and cement fabric analysis in

carbonates6 Highly qualitative for Mn and Fe in carbonates7 Qualitative with energy dispersive X-ray analysis8 Software is available for BSEM quantification of very simple

mineral mixtures9 Semi-quantitative with energy dispersive X-ray analysis

10 BSEM-PIA provides quantitative porosity information only11 Very rapid whole rock quantification12 Especially for clay mineral identification

prevents poorly consolidated rocks falling apart.Thin-sections are usually stained. Two stains arecommonly used: a mixed stain for carbonates allow­ing the differentiation of ferroan and non-ferroancalcites and dolomites (Dickson, 1966); and a stainfor feldspars allowing the differentiation of potassiumfeldspars, plagioclase and quartz (Houghton, 1980).Table 2.2 summarizes the effects of the stainingprocedures on carbonate and silicate mineralogies.Note that it is essential to apply the feldspar stainbefore the carbonate stain, otherwise the latter willbe removed!

C, Chiefly carbonatesS, Chiefly siliciclasticsCL, cathodoluminescenceUVF, ultraviolet fluorescence microscopySEM, scanning electron microscopySE, emission modeBSEM, backscatter modeTEM, transmission electron microscopyXRD, X-ray diffractionTG-EWA, thermogravimetry-evolved water analysisPIA, pore image analysis

2.2.3 Mineral identification and differentiation ofdetrital grains from diagenetic cements

The main theme of this section is the differentiationof diagenetic phases from depositional grains, whichcan be a source of considerable ambiguity in thin­section description. The following provides a guideto the recognition of the most common diageneticphases in siliciclastic and carbonate rocks, and out­lines the pitfalls and problems of distinguishingdetrital grains and matrix from diagenetic phases. Adetailed description of sedimentary mineral iden-

10 Chapter 2

Table 2.2 Typical stain colours for carbonates andfeldspars.

* Carbonate stain colours using mixed stain of potassiumferricyanide and Alizarin red-S (Dickson, 1966).t Feldspar stain colours using stain of sodium cobaltinitrate.followed by potassium rhodizonate (Houghton. 1980).

tification is beyond the scope of this book. Thereader is referred to texts by Kerr (1959) and Deeret al. (1977). Table 2.3 summarizes the optical prop­erties of common minerals in sedimentary.rocks andtheir occurrence. If the nature of a mineral is indoubt from microscopy alone, microbeam tech­niques can be applied (see subsequent sections inthis chapter).

Quartz cementQuartz cement is the most common diagenetic sili­cate mineral in sandstones (McBride, 1989). It occurschiefly in two forms: as microcrystalline cement andas syntaxial overgrowths on detrital quartz grains(Table 2.3). Microcrystalline quartz, also known asmicroquartz, chert or chalcedony, is relatively easyto differentiate from the depositional fabric of therock (Fig. 2.2a). Syntaxial quartz overgrowths arethe most common form of quartz cement but areoften difficult to distinguish from detrital grainsbecause of the optical continuity of quartz across thegrain-cement boundary. Quartz overgrowths canbe clearly differentiated using optical microscopyonly if the grain-cement boundary is visible. Figure2.2b shows a sandstone largely composed of quartzin which grain margins cannot be identified eventhough the euhedral crystal faces show that quartz

FeldsparsFeldspar cements are relatively common in arkosicsandstones but usually form only a minor or tracecomponent of the total rock volume (Waugh, 1978).Like quartz, feldspar cements chiefly occur as syn­taxial overgrowths on detrital grains and so presentthe same problems of identification. However, thereis a further complication in feldspar petrography:certain types of feldspar, whether grains or cement,cannot be distinguished from each other or, in somecases, from quartz unless the section is stained (seeSection 2.2.2).

Three types of feldspar are usually distinguishedin sedimentary petrography: plagioclase, orthoclaseand microcline (Table 2.3). Plagioclase is easilyidentified by common multiple or lamellar twinningwhich gives the mineral its characteristic stripedappearance under crossed polars (Fig. 2.2d; Deeret al., 1977). Microcline, the low-temperature formof potassium feldspar, is also easy to identify owingto its cross-hatched 'tartan' twinning visible undercrossed polars. Other feldspars are frequently un­twinned. A common feature of feldspars, which alsoassists in their identification, is their diageneticinstability relative to quartz (Burley et al., 1985).Feldspars dissolve more readily, leaving ragged,etched grain remnants in oversized secondary pores,or skeletal grains with microporosity (Schmidt& McDonald, 1979). As well as being leached,feldspars are commonly altered to fine-grained clayminerals.

cement is present. If quartz cementation had con­tinued to fill porespaces completely, only suturedcontacts between areas of quartz would be visible.One possible interpretation would be that thesutured contacts could represent compromise bound­aries between growing quartz crystals. Alternatively,we could infer that there is no quartz cement, andthat the sutured boundaries are pressure-solutioncontacts along which quartz cement has been dis­solved rather than precipitated (Houseknecht,1988). Distinction between these two quite differentinterpretations must involve recourse to furthertechniques such as cathodoluminescence microscopy(Section 2.3). Fortunately, many sandstones havea thin coating of depositional or diagenetic mater­ial, commonly haematite, between the grains andcement which allows grain outlines to be identified(Fig. 2.2c). Fluid inclusions are also often con­centrated along grain-cement boundaries (seeFig. 3.3f).

Pink, intensity is proportional toCa content

Yellow

No colour with rhodizonate stain

No colour

Ranging from pale to deepturquoise with increasing Fecontent

Very pale pink to red

Ranging from mauve to purpleto royal blue with increasing Fecontent

Stain colour

Pure Na-Albite t

Alkali feldspars t

Ferroan dolomite*

Plagioclase t

Dolomite*

Ferroan calcite*

Mineralogy

Non-ferroan calcite*

(a)

(c)

Textural and Mineralogical Analysis 11

(b)

(d)

Fig. 2.2 Silicate cements in sandstones. (a) Microcrystalline quartz cement between quartz and feldspar grains, Jurassicsandstone, Central North Sea. Plane polarized light. Courtesy A. Hogg. (b) Quartz cemented sandstone, Northern NorthSea. The boundaries between quartz grains and cements are invisible. Plane polarized light. Courtesy A. Hogg. (c) Quartzcemented sandstone, Rotliegend Group, Southern North Sea. Quartz grains and cements can be distinguished by thepresence of a dust rim. Plane polarized light. Courtesy A. Hogg. (d) Syntaxial overgrowth of plagioclase feldspar on adetrital grain. The grain-cement boundary is marked by a rim of haematite. Crossed polars. Width of photograph is4()()~m.

Clay mineralsClay minerals are best observed in the scanningelectron microscope because of their small size(usually a few to a few tens of micrometres). Never­theless, optical microscopy can still provide usefulgeneral information on certain clay mineral types.There are two main problems to be aware of in claymineral identification from thin-section petrography:the identification of clay mineral type and dif­ferentiation of clay mineral cements from matrixclay.

The common diagenetic clay minerals are sum­marized in Table 2.3. Kaolinite forms characteristicbooklets (Fig. 2.3a, b) or vermiform aggregateswith low birefringence (Fig. 2.3c). Chlorite, aferro magnesian clay mineral, is also relatively easyto identify on account of its green hue in transmittedlight and anomalous steely blue birefringence.It usually forms characteristic pseudohexagonalplatelets or a complex meshwork. Illite, a potassium­bearing clay, commonly forms plates parallel tograin surfaces, or micrometre-thick fibres extend-

Table 2.3 Common minerals in sedimentary rocks and their optical properties. Compiled from Kerr (1959) and Tucker (1988).

Group/mineral Crystal system Colour Cleavage Relief Birefringence Other features Cement Form and occurrence

Quartz Trigonal Colourless None Low + Grey Overgrowth cement As detrital grains and cement/replacivecommon phase

Cherts Colourless None Low + Grey Chalcedony and Chalcedony and microquartz aremicroquartz diagenetic unless as detrital grain

FeldsparsMicrocline Triclinic Colourless Present Low- Grey C,,,,,,,,,,,,, In;n, } Present as detrital minerals, but oftenOrthoclase Monoclinic Colourless Present Low- Grey Simple twins Overgrowth cements altered or leached. Generally minorPlagioclase Triclinic Colourless Present Low- Grey Multiple twins diagenetic phase, except in some

arkoses

MicasMuscovite Monoclinic Colourless Prominent, Mod+ Bright colours Parallel Common detrital mineral

planar extinctionBiotite Monoclinic Brown-green Prominent, Mod+ Bright colours, Parallel Common detrital mineral

planar masked by extinction,colour pleochroic

Clay minerals

} Common " oemen"Chlorite Monoclinic Green/blue green Planar Mod+ Grey/blue Present as detrital minerals, asKaolinite Triclinic Colourless Planar Low + Grey Fine grained alteration products of silicates and asIllite Monoclinic Colourless Planar Low + Grey-bright cementsSmectite Monoclinic Colourless Planar Low+ GreyGlauconite Monoclinic Green Planar Mod, Grey, masked Commonly Characteristic of low sedimentation

masked by colour replace pellets rates, may infill foram tests etc.by colour

Zeolites Most colourless Low, Commonly Common as cements in Associated with volcanogenicmost- grey volcanics sediments

CarbonatesAragonite Orthorhombic Colourless Rectilinear Mod-high High Commonly acicularCalcite Trigonal Colourless Rhombic Low-high Very high Carbonate cements Present as detrital grains, cements and

colours) Di"i~g"i'hed hy

display many cement replacive phases in carbonates and

Dolomite Trigonal Colourless Rhombic Low-high Very high morphologies (see siliciclasticscolours

stammgTable 2.4)

Siderite/ Trigonal Colourless Rhombic Low-high Very high(Table 2.2)

Common replacive phase in ironstones

ankerite colours

EvaporitesGypsum Monoclinic Colourless Planar Low GreyAnhydrite Orthorhombic Colourless Rectilinear Mod Bright colours Common burial cement Commonly crystalline, replacive after

evaporitesCelestite Orthorhombic Colourless Planar Low-mod Grey Burial cement Commonly partially replaciveBarite Orthorhombic Colourless Planar Low-mod Grey Burial cement Burial cement in sandstones, may be

associated with sulphidemineralization

Halite Cubic Colourless Rectilinear Low Isotropic Only present if Burial cementsectionprepared in oil

Iron minerals-f

Pyrite Cubic Opaque Distinguished in Yellow Common diagenetic mineral (1)

Magnetite Cubic Opaque reflected light Grey-black ><...c::Haematite Cubic Opaque, brown Red-grey Common diagenetic mineral in !.tinge siliciclastic red-bedsChamosite/ Monoclinic Green Mod Grey masked Occurs in ooids in ironstones and as t»

:::sberthierine by colour partial pore-fills Co

Collophane Non- Browns Mod Isotropic Replacive textures, commonly ini::i'

crystalline carbonates (1)""It»

Bitumens Non- Opaque Distinguished in Occurs in porespaces and in fluid 0"crystalline fluorescence inclusions CQ

(;"!.>:::sSo\)

.:<(I)

in"

c,.)

14 Chapter 2

(a) (b)

(c) (d)

Fig. 2.3 Silicate cements and dolomitized limestone. (a) Authigenic kaolinite filling porespace between quartz grains,Brent Group, Northern North Sea. Plane polarized light. Width of photograph is 500 ~m. (b) Field of view as above,crossed polars, showing individual kaolinite booklets. (c) Oil stained vermiform kaolinite (arrowed), Brent Group,Northern North Sea. Plane polarized light. Width of photograph is 700 ~m. (d) Euhedral rhombs of replacive, sucrosicdolomite, Jurassic Arab Formation, Saudi Arabia. Plane polarized light. Width of photograph is 500 ~m. CourtesyJ. Dravis.

ing away from the grain surface, and can also beidentified from its relatively high birefringence. Puresmectite is less common as a cement in sandstones,but interstratified illite/smectites may form fibrouscements. These can be distinguished from pure illitesusing X-ray diffraction (Section 2.7). Glauconite isrelatively easy to identify owing to its green hue inthin-section and its common occurrence as pelletsin marine sandstones. Differentiating detrital fromdiagenetic clay may be more difficult. Frequently,what is described as a clay matrix consists of a dense

brown mess which may contain any combination oforiginal detrital material, recrystallized detrital clay,or true diagenetic precipitate. Under such circum­stances, clay mineral identification is better leftto scanning electron microscopy (Section 2.5),transmission electron microscopy (Section 2.6) andX-ray diffraction.

CarbonatesIdentification of carbonate cements in siliciclasticrocks is relatively straightforward and may be as-

Table 2.4 Descriptive terms forcarbonate cement morphologies.Modified from Harwood (1988).

Cement terminology

Needle

Pendant ormicrostalactitic

Meniscus

Acicular

Peloidal

'Micritic' ormicrocrystalline

Columnar

Circumgranularisopachous acicular

Equant

Circumgranularequant

Overgrowth

Sparry

Poikilotopic

Baroque (or 'saddle')

Textural and Mineralogical Analysis 15

Description and characteristic environment ofprecipitation (where appropriate)

Thin « 10 J.1m) cements of single or en-echelon crystals

Cement forms in droplets beneath grains in vadosezone (zone where porosity is partially air- and partiallywater-filled)

Cement forms at or near grain-grain contacts, alsocharacterizes vadose zone

Thin, straight form (aspect ratios of 20-40, :=::::10 J.1mwidth). Characterizes marine phreatic (pores whollywater-filled) environment

Dark, microcrystalline coating to grains and pores.Characterizes marine phreatic environment

Microcrystalline cement which may coat grains and formbridges between grains. Characterizes marine phreaticenvironment

Broad cements (:=::::20 J.1m + ) commonly longer than broad

Equal thickness of acicular cements surrounding grains.Characterizes marine phreatic environments

Equidimensional crystals (commonly :=::::100 J.1m+).Characterizes freshwater or burial phreatic environments

Equidimensional cements surrounding grains.Characterizes meteoric phreatic environment

Cement is in optical continuity with substrate

Coarse (:=::::300 J.1m+), commonly equidimensional crystals

Coarse cement crystals enclosing grains and pre-existingcement phases

Coarse cement displaying undulose extinction.Characterizes burial environments

sisted by staining (Table 2.2; Dickson, 1966). Wheresamples are unstained, differentiation of carbonatecements from one another can be difficult andrelies chiefly on cement fabric criteria which aremuch more subjective (Table 2.4; Harwood, 1988).Replacive and cementing dolomite commonly formeuhedral rhombic crystals (Fig. 2.3d) or coarse crys­tals of baroque or saddle dolomite, showing unduloseextinction. Calcite may display a wide variety ofmorphologies, from acicular (or less correctly,fibrous) through to poikilotopic, where coarse crys­tals of uniform extinction enclose detrital grains andearlier cement generations.

In carbonate rocks, the major problems are indjstinguishing unaltered carbonate grains from thosewhich have been diagenetically modified, and in

differentiating genuine cements from neomorphiccarbonate. Again, staining the thin-section helps.Under normal marine conditions, all organismssecreting a calcite test will precipitate non-ferroancalcite with a low manganese content. Accordingly,the presence of any dolomite or ferroan calciteimmediately indicates that the skeletal particlehas undergone diagenetic alteration. However, thepresence of non-ferroan calcite in itself is insufficientevidence to guarantee that no diagenetic alterationhas taken place. Further screening for the presenceof manganese in the calcite by cathodoluminescence(Section 2.3) should be carried out, as well as detailedobservation of skeletal fabrics by scanning electronmicroscopy (Section 2.5).

Carbonate cements in carbonate rocks display