Genesis and diagenesis of microporous micrites

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Thesis

Reference

Genesis and diagenesis of microporous micrites

VOLERY, Chadia

Abstract

Microporous limestones made of rhombic to sub-rhombic low-Mg calcite crystals generally smaller than 4 µm (micrites) account for many carbonate reservoirs, especially in the Middle East. However, despite their substantial economic interest, the genesis of microporous limestones is poorly understood. The main factors involved in the development of the intercrystalline microporosity, as well as the timing of this development, remain a matter of debate. In order to determine the factors and the conditions of formation of microporous limestones, five different studies were conducted: 1) bibliographic inventory of the microporous carbonate formations in the Middle East and comparison of their stratigraphic occurrence with calcite/aragonite seas periods and the relative position of sea-level, 2) study of the lacustrine microporous and tight micrites of the Madrid Basin (Late Miocene, Spain), 3) investigation of the marine microporous and tight limestone alternations in the Urgonian Formation (late Hauterivian to early Aptian, France), 4) description of core sections composed of microporous and tight limestones from the A [...]

VOLERY, Chadia. Genesis and diagenesis of microporous micrites. Thèse de doctorat : Univ. Genève, 2010, no. Sc. 4205

URN : urn:nbn:ch:unige-121222

DOI : 10.13097/archive-ouverte/unige:12122

Available at:

http://archive-ouverte.unige.ch/unige:12122

Disclaimer: layout of this document may differ from the published version.

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UNIVERSITÉ DE GENÈVE FACULTÉ DES SCIENCES Département de Géologie et Paléontologie Professeur E. DAVAUD

TOTAL EXPLORATION-PRODUCTION Département de Sédimentologie Docteur B. CALINE

Genesis and Diagenesis of Microporous Micrites

THÈSE

présentée à la Faculté des Sciences de l’Université de Genève

pour obtenir le grade de Docteur ès Sciences, mention Sciences de la Terre par

Chadia VOLERY

de

Aumont (Fribourg)

Thèse N° 4205

GENÈVE

Atelier d'impression ReproMail

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Volery, C.: Genesis and diagenesis of microporous micrites.

Terre & Environnement, vol. 96, 112 pp. (2010)

ISBN 2-940153-95-7

Section des sciences de la Terre et de l'environnement, Université de Genève, 13 rue des Maraîchers, CH-1205 Genève, Suisse Téléphone ++41-22-379.66.28 - Fax ++41-22-379.32.11

http://www.unige.ch/sciences/terre/

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Abstract

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Abstract

Microporous limestones made of rhombic to sub-rhombic low-Mg calcite crystals generally smaller than 4 μm (micrites) account for many carbonate reservoirs, especially in the Middle East. However, despite their substantial economic interest, the genesis of microporous limestones is poorly understood. The main factors involved in the development of the intercrystalline microporosity, as well as the timing of this development, remain a matter of debate.

In order to determine the factors and the conditions of formation of microporous limestones, fi ve different studies were conducted:

1) bibliographic inventory of the microporous carbonate formations in the Middle East and comparison of their stratigraphic occurrence with calcite/aragonite seas periods and the relative position of sea-level, 2) study of the lacustrine microporous and tight micrites of the Madrid Basin (Late Miocene, Spain), 3) investigation of the marine microporous and tight limestone alternations in the Urgonian Forma- tion (late Hauterivian to early Aptian, France), 4) description of core sections composed of microporous and tight limestones from the A reservoir of the Mishrif Formation (Cenoma- nian to early Turonian, Mesopotamian Basin) and an outcrop laterally equivalent to these cores in the Natih Formation (Cenomanian to early Turonian, Oman), and 5) analysis of the Mg distribution inside micrite crystals by using scanning transmission electron microscopy (STEM) combined with X-ray energy dispersive spectroscopy (X-ray EDS).

This multi-approach permitted to observe important similarities in the different study objects, to highlight the main factors responsible for the development of the intercrystalline microporosity and to propose a diagenetic model explaining the formation of these microporous limestones.

The mineralogical composition of the precursor mud must be dominated by low-Mg calcite crystals. Aragonite and high-Mg calcite muds constitute unstable sediments that transform during diagenesis into low-Mg calcite limestones. On the contrary, muds made up of low- Mg calcite crystals are able to resist a moderate diagenesis and can thus partly preserve their primary microfabric and intercrystalline microporosity.

The formation of microporous limestones implies an early cementation of the precursor carbonate mud mainly made up of low-Mg calcite crystals rapidly after sedimentation. In the ionically active zone of a meteoric phreatic lens, the dissolution of the most unstable crystals (aragonites and high-Mg calcites coming from the disintegration of organism tests and the smallest low-Mg calcites) leads to the precipitation of calcite overgrowths around the most stable micrite crystals (the largest low- Mg calcites). The process was named “hybrid Ostwald ripening”. This early and moderate cementation rigidifi es the original microporous framework before burial, while partly conserv- ing its microfabric with intercrystalline microporosity, and allows the precursor carbonate mud to resist compaction.

In conclusion, two main factors are essential to create microporous limestones: 1) a precursor mud mainly composed of low-Mg calcite crystals, and 2) an early cementation of the precursor mud before burial to allow the sedi- ment to resist compaction and to partly con- serve its original microfabric with intercrystalline microporosity.

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Résumé

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Résumé

Les calcaires microporeux composés de cristaux de calcite faiblement magnésienne (LMC) de taille généralement inférieure à 4 μm (micrites) constituent de nombreux réservoirs carbonatés, spécialement au Moyen-Orient.

Cependant, malgré un enjeu économique important, la formation des calcaires microporeux est peu comprise. Les principaux facteurs impliqués dans le développement de la microporosité intercristalline, de même que le timing de ce développement, demeurent un sujet de controverse.

Afi n de déterminer les facteurs et les conditions de formation des calcaires microporeux, cinq différentes études ont été menées:

1) inventaire bibliographique des formations microporeuses carbonatées du Moyen-Orient et comparaison de leur occurrence stratigraphique avec les périodes de mers à calcite/aragonite et la position relative du niveau marin, 2) étude des micrites lacustres microporeuses et imper- méables du Bassin de Madrid (Miocène sup., Espagne), 3) investigation des alternances formées par des calcaires marins microporeux et imperméables dans la Formation de l’Urgonien (Hauterivien sup. à Aptien inf., France), 4) description de sections de carottes composées de calcaires microporeux et imper- méables situées dans le réservoir A de la For- mation du Mishrif (Cénomanien à Turonian inf., Bassin de Mésopotamie) et d’un affl eu- rement latéralement équivalent à ces carottes dans la Formation du Natih (Cénomanien à Turonian inf., Oman), et 5) analyse de la distribution du magnésium à l’intérieur de cristaux de micrite en utilisant un microscope électro- nique à balayage et transmission couplé à un spectromètre à énergie dispersive.

Cette approche diversifi ée a permis d’observer d’importantes similitudes parmi les objets d’étude, de mettre en lumière les principaux facteurs à l’origine du développement de la microporosité intercristalline et d’élaborer un

modèle diagénétique expliquant la formation de ces calcaires microporeux.

La composition minéralogique de la boue précurseur doit être dominée par des cristaux de LMC. Les boues constituées d’aragonite et de calcite hautement magnésienne (HMC) sont des sédiments instables qui se transfor- ment durant la diagenèse en calcaires pauvres en magnésium. En revanche, les boues com- posées de cristaux de LMC sont capables de résister à une diagenèse modérée et peuvent partiellement conserver leur microtexture et leur microporosité intercristalline primaires.

La formation des calcaires microporeux implique une cimentation précoce d’une boue carbonatée principalement composée de cristaux de LMC rapidement après la sédimenta- tion. Dans la zone ioniquement active d’une nappe météorique phréatique, la dissolution des cristaux les plus instables (aragonites et HMC résultant de la désintégration de tests d’organismes et les plus petits cristaux de LMC) provoquent la précipitation de surcrois- sances calcitiques autour des cristaux micri- tiques les plus stables (les plus grands cristaux de LMC). Le processus a été nommé “hybrid Ostwald ripening”. Cette cimentation précoce et modérée rigidifi e la charpente microporeuse originelle avant l’enfouissement, tout en préservant la microtexture et la microporosité intercristalline, et permet au sédiment carbon- até de résister à la compaction.

En conclusion, deux principaux facteurs sont essentiels pour créer des calcaires microporeux: 1) une boue carbonatée précurseur principalement composée de cristaux de LMC, et 2) une cimentation précoce de la boue pré- curseur avant enfouissement, qui permet au sédiment de résister à la compaction et de conserver partiellement sa microtexture ainsi que sa microporosité intercristalline originelles.

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Remerciements

J’aimerais dédier ces quelques lignes à ceux qui ont participé à la réussite de ma thèse et de mon parcours géologique et personnel.

Tout d’abord, je tiens à remercier Eric pour son soutien au long de ces quatre années de travail.

Grand géologue et adroit pédagogue, il a été pour moi un excellent directeur. Il m’a aidée à franchir les pics et les creux de ma recherche de thèse. Jamais envahissant, toujours disponible, il a su me responsabiliser et me donner confi ance en mon travail. J’espère avoir acquis au cours de ces années un peu de ses qualités scientifi ques et de sa fi nesse intellectuelle.

J’aimerais également remercier Bruno de m’avoir donné l’opportunité de travailler avec Total.

Une telle collaboration est une expérience précieuse pour une jeune chercheuse fraîchement sortie des études. J’ai ainsi eu l’occasion de me confronter et de me former au monde de la géologie appliquée durant mes nombreux séjours au centre de Total à Pau. Et je garderai toujours des sou- venirs savoureux des excellents repas au Berry!

Durant ces séjours à Total, j’ai rencontré des personnes de grande valeur. Frédéric en fait partie.

Je le remercie pour ses conseils toujours avisés sur ma recherche et pour son calme rassurant au milieu de quelques tempétueuses réunions.

Je tiens à remercier Gregor de s’être déplacé de Miami pour assister à ma défense de thèse. Les deux semaines de terrain que j’ai eu la chance de faire avec lui dans les Caraïbes m’ont énormé- ment appris sur les carbonates et resteront inoubliables. Je le remercie également de m’avoir offert la possibilité de faire un post-doc à RSMAS. J’espère que nous trouverons un moyen de collaborer à l’avenir pour résoudre quelques énigmes sédimentologiques.

Au centre de Total à Pau, j’aimerais remercier particulièrement Serge et l’équipe du laboratoire L1 pour leurs très nombreuses et précises analyses pétrophysiques, minéralogiques, géochimiques et pour leur bonne humeur. Je tiens aussi à remercier Patrick et son équipe d’avoir rendu mon travail en carothèque possible et sympathique.

Plusieurs géologues ont collaboré à différents sujets de ma thèse. Merci à Anneleen Foubert de m’avoir coachée et montré la voie dans la recherche scientifi que et la publication de papiers. Le week-end pascal passé avec elle dans le Bassin de Madrid, entre neige et ciel bleu polaire, restera un fameux souvenir. Merci à Christophe Durlet d’avoir partagé ses connaissances en stratigraphie des ciments et de m’avoir accueillie à Dijon. Merci à Jean Charollais et Bernard Clavel pour leur aide précieuse sur la datation des orbitolines.

J’aimerais remercier Rossana pour les heures d’analyse au MEB. Son enthousiasme et son énergie ont toujours su réchauffer la salle climatisée et sans fenêtre du MEB.

Merci à Daniel d’avoir répondu à mes questions sur les sédiments lacustres. A François et Pierrot d’avoir confectionné de si magnifi ques lames minces. A Jacques d’avoir imprimé de grands et jolis posters. A Jacqueline d’avoir désamorcé tous les problèmes administratifs.

Remerciements

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J’aimerais également remercier Mario avec qui j’ai eu la chance de faire mon diplôme et de vivre quelques magnifi ques semaines de terrain en Corse. Il m’a initiée à la recherche, au souci du géologue. Discuter avec lui a toujours été source d’enrichissement et d’élargissement des horizons.

Merci de m’avoir lancée avec confi ance dans le bain de la géologie et de m’avoir rendue amou- reuse des roches, des mouvements et des plis, des couleurs.

Merci à tous mes collègues de thèse, Mélanie, Chloé, Katrina, Lina, Cristina, Claude-Alain, Greg, Jérôme, Sylvain, Matar, Olivier, Aurèle, Mortaza, Neda, Fabienne, Ralph, Fiore, embarqués sur le même navire, ils sont pour beaucoup devenus des amis.

Merci aussi à tous les étudiants que j’ai eu le plaisir d’avoir en cours. Les questions ont souvent été source de doute et d’approfondissement de mes connaissances en géologie. J’espère avoir pu les aider à chercher.

Sans la possibilité de s’aérer la tête, de sortir de sa recherche, une thèse ne serait pas possible!

Je remercie tous ceux qui m’ont soutenue au cours de ces quatre années par un verre, un pique- nique, une randonnée, une partie de tennis ou un beau voyage: Vanessa, Aurélie, Andréa, Ariane, Mathilde J., Sabrina, Diana, Amélie, Sophie, Berivan, Christine, Fabienne, Mathilde B., Géraldine, Pernelle, Emma, Julia.

J’aimerais également remercier ma famille du Sud-Ouest. Elle m’a donné un foyer quand j’allais travailler chez Total à Pau, mais aussi et surtout beaucoup d’amour, de repas délicieux et de soirées folles. Merci à ma Tatie Noëlle, Chrystèle, Didier, Jérôme, Annick, Franck, Sylvie, et aux petits Béarnais, Lucile, Axel, Hugo, Tom, Manon.

Merci à ma famille d’ici, d’avoir tout vécu avec moi, le meilleur et le pire. Merci à ma Marraine, pour sa compréhension profonde. Merci à Lucia pour sa chaleur, son humanité exceptionnelle.

Merci à mon Parrain, d’avoir aimé les roches et mon travail, d’être là entre les nuages et les pierres, entre les lignes de cette thèse.

Merci à mon grand frère Igor qui depuis toujours m’a montré la voie. De m’avoir appris aussi bien à faire des ronds de fumée, à skier, qu’à mettre les mains dans le cambouis ou la farine.

J’ai découvert grâce à lui que bien des domaines techniques sont intéressants, compréhensibles;

il suffi t d’y plonger! Merci à Sophie, ma belle-sœur, pour sa confi ance. Et merci à mes deux crapouillons: Pauline et Nicolas!

Finalement, je remercie mes parents, pour leur patience, leurs erreurs et leurs réussites, l’initiation au voyage, la possibilité des études, et leur amour.

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Table of Contents

Abstract 1

Résumé 5

Introduction 21

Chapter 1: Shallow-marine microporous carbonate reservoir rocks in the Middle East:

Relationship with seawater Mg/Ca ratio and eustatic sea level 27 INFLUENCE OF SEAWATER CHEMISTRY ON THE PRECIPITATION OF CALCIUM CARBONATE 28

Calcite versus aragonite seas and the Mg/Ca ratio 28

Discussion of the different approaches 30

METHODS AND RESULTS 30

Number of microporous carbonate formations 30

Eustatic changes and occurrence of microporous carbonate formations during the Cretaceous and Cenozoic 31

DISCUSSION 34

The Late Carboniferous to Triassic versus the Cretaceous 34

The Cenozoic 35

Factors controlling the development of microporous limestones 36

CONCLUSIONS 38

Chapter 2: Lacustrine microporous micrites of the Madrid Basin (Late Miocene, Spain) as analogues for shallow-marine carbonates of the Mishrif reservoir Formation (Cenomanian

to early Turonian, Middle East) 39

GEOLOGICAL SETTING 40

METHODS 41

RESULTS 41

Petrography and petrophysical properties 42

Tight facies (T) 42

Microporous facies (M3, M2, M1) 42

Mineralogical composition 46

Stable isotopes 46

DISCUSSION 46

Similarities between lacustrine and marine micrites 46

Differences between the microporous and the tight facies 47

Diagenetic models 49

Early diagenesis: scenario 1 49

List of Figures

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Fig. 7 - The obvious resemblance in microfabrics between Late Miocene lacustrine microporous micrites of Colmenar de Oreja (Spain) and Cenomanian to Early Turonian shallow-marine microporous carbonates of

the Mishrif reservoir Formation (Qatar) 47

Fig. 8 - SEM pictures from the microporous facies of the lacustrine micrites of Colmenar de Oreja showing

a bi-modal grain size 48

Fig. 9 - Sketch illustrating the early diagenetic model of the scenario 2 49 Chapter 3: Microporous and tight limestones in the Urgonian Formation (late Hauterivian to early Aptian) of the French Jura Mountains: Focus on the factors controlling the formation of microporous facies

Fig. 1 - Stratigraphy of the Hauterivian to Turonian sequence of the southern part of the Jura Mountains and

localisation of the studied section 54

Fig. 2 - Localisation of the Pierre-Blanche section in the southern part of the French Jura Mountains 54 Fig. 3 - Synthetical log with petrographical descriptions, biostratigraphy, stable isotopes values, mineralogical composition, petrophysical properties, macro-cements proportions, and interpretations of the deposi-

tional environments and the sequence stratigraphy 56

Fig. 4 - Thin-section photomicrographs of samples from the Pierre-Blanche section 57 Fig. 5 - Sketch with photographs showing the spectacular subaerial erosion surface in the upper outcrop 58 Fig. 6 - Diagram presenting the relative chronology of the 18 diagenetic phases recorded in the Pierre-

Blanche section 59

Fig. 7 - Non-polarized light microscopy and cathodoluminescence photomicrographs illustrating the diage-

netic phases registered in the Pierre-Blanche section 61

Fig. 8 - Sketch illustrating the Loreau’s classifi cation 62

Fig. 9 - SEM photomicrographs of the micritic matrix 63

Fig. 10 - Backscattered-SEM photomicrographs of the micritic matrix 65 Fig. 11 - Graph of porosity versus permeability for the four different types of matrix microfabric 66 Fig. 12 - Graph of porosity versus permeability in relation to the different depositional textures 66 Fig. 13 - Sketch illustrating the differentiation between the microporous and tight facies 68 Chapter 4: Comparative study of Cenomanian to early Turonian microporous limestones in the Mishrif Formation (Wells X and Y, Mesopotamian Basin) and in the Natih Formation (Jabal Madmar, Oman)

Fig. 1 - Schema of the depositional environments of the Mishrif Formation (Mesopotamian Basin) 73 Fig. 2 - Localisation and picture of the Jabal Madmar outcrop in the Natih Formation (Oman) 74 Fig. 3 - Synthetical log of the Jabal Madmar outcrop presenting the petrotexture, the sedimentological content,

the depositional environment and the SEM matrix microfabric 79

Fig. 4 - Thin-sections and SEM photomicrographs of samples from the Jabal Madmar outcrop 80 Fig. 5 - Diagram of permeability versus porosity for the different petrotextures found in the A reservoir

of X and Y 81

Fig. 6 - Diagram of permeability versus porosity for the different facies and depositional environments

characterizing the A reservoir of X and Y 81

Fig. 7 - Diagram of permeability versus porosity for the different matrix microfabrics observed in the A res-

ervoir of X and Y 82

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List of Figures

Fig. 8 - SEM photomicrographs of serrate microfabrics of samples from X and Y 83 Fig. 9 - Thin sections photomicrographs showing molds of bioclasts in Y limestones 85 Chapter 5: TEM study of Mg distribution in micrite crystals from the Mishrif reservoir Formation (Middle East, Cenomanian to early Turonian)

Fig. 1 - SEM image of micrite crystals from the Mishrif Formation 96 Fig. 2 - Backscattered SEM image of micrite crystals presenting different orientations relative to each other

97 Fig. 3 - STEM and TEM images and electron diffraction pattern of micrite crystals 97

Fig. 4 - EDS map of Mg superimposed on STEM image 98

Fig. 5 - EDS maps showing homogeneous distributions of Mg, geometric Mg impoverishments near crystal edges, and Mg-enriched areas relatively close to the centre of the crystal 99

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List of Tables and Plates

Relationship with seawater Mg/Ca ratio and eustatic sea level

Table 1 - Cretaceous microporous carbonate formations in the Middle East 34 Table 2 - Cenozoic microporous carbonate formations in the Middle East 34 Chapter 2: Lacustrine microporous micrites of the Madrid Basin (Late Miocene, Spain) as analogues for shallow-marine carbonates of the Mishrif reservoir Formation (Cenomanian to early Turonian, Middle East)

Table 1 - Values of porosity, permeability and pore threshold radius from capillary pressure for samples 1 to

12 and 9top 42

Table 2 - Bulk compositions, CaO, MgO and SrO contents, and stable isotopic ratios of samples S1 to S5,

1 to 12 and 9top 46

Chapter 3: Microporous and tight limestones in the Urgonian Formation (late Hauterivian to early Aptian) of the French Jura Mountains: Focus on the factors controlling the formation of microporous facies

Table 1 - Main characteristics of the 18 diagenetic phases found in the Pierre-Blanche section 60 Table 2 - Porosity and permeability values for the three types of limestones 64 Table 3 - Calcite, clays, magnesium, strontium contents and stable isotopes values 64 Chapter 4: Comparative study of Cenomanian to early Turonian microporous limestones in the Mishrif Formation (Wells X and Y, Mesopotamian Basin) and in the Natih Formation (Jabal Madmar, Oman)

Table 1 - Porosity and permeability values for X, Y and both wells according to the facies and depositional

environment and to the matrix microfabric 75

Table 2 - Mineralogical composition of 18 samples from X and 18 samples from Y 76 Table 3 - Interval transit time and neutron porosity mean values from well logging data for the A interval of

two wells in X, seven wells in Y, and one well in Z 78

Plate A - Synthetical log of the A interval in X presenting the well logging data, the macroscopic, thin section and SEM descriptions, the petrophysical properties, the mineralogical composition and the interpretations of

the depositional environments and the sequence stratigraphy 87

Plate B - Synthetical log of the A interval in Y presenting the well logging data, the macroscopic, thin section and SEM descriptions, the petrophysical properties, the mineralogical composition and the interpretations of

the depositional environments and the sequence stratigraphy 88

Plate C - Correlation between Wells X and Y 89

List of Tables and Plates

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Plate 1 - F1 facies: bioturbated muddy limestone with subaerial features 90 Plate 2 - F2 facies: bioturbated muddy limestone with large foraminifera 91

Plate 3 - F3 facies: well-sorted bioclastic limestone 92

Plate 4 - F4 facies: bioturbated bioclastic limestone 93

Plate 5 - F5 facies: bioturbated bioclastic limestone with large foraminifera 94

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Introduction

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Shallow-marine limestones characterised by a microporous matrix account for many carbonate reservoirs, especially in the Middle East (Alsharhan and Nairn, 2003; Wilson, 1975). This matrix is made of sub-rhombic low-Mg calcite crystals with sizes generally smaller than 4 μm (micrites) (Folk, 1959) and has an intercrystalline microporosity (Fig. 1). Larger crystals can be up to 8 μm in size (microspars) (Folk, 1959).

The porosity and permeability values of these reservoirs respectively rate about 20 % and 100 mD. Pore throat radii are generally smaller than 1 μm.

Despite their economic importance, the formation of shallow-marine microporous carbonate reservoir rocks remains poorly understood. Several hypotheses explaining the origin of these limestones have been advanced in the light of specifi c case studies (e.g. Ahr, 1989; Budd, 1989; Kaldi, 1989; Moshier, 1989;

Perkins, 1989; Saller and Moore, 1989). The development of the micrite crystals and the associated intercrystalline microporosity is often explained in terms of late diagenesis, but divergences exist on the nature (marine or meteoric) of the involved fl uids. In recent years, only a few studies have focused on microporous limestones (Cantrell and Hagerty, 1999;

Lambert et al., 2006; Richard et al., 2007).

However, at present no conclusive model sat- isfactorily explains their formation and some

fundamental questions remain unresolved or are controversial. What is the mineralogy of the precursor carbonate muds changing with diagenesis into microporous limestones? Are the petrophysical properties related to the sedimentological facies and/or the mineralogical composition? Does the intercrystalline microporosity of the microcrystalline framework develop during early or late diagenesis?

The purpose of this work is to determine the conditions and the factors responsible for the development of microporous limestones. To do so, a multi-approach based on fi ve different studies (three petrographical, one bibliographic and one analytical) was chosen. Each of these investigations constitutes a chapter of the thesis:

Chapter 1: bibliographic investigation of the occurrence of shallow-marine microporous carbonate formations in the Middle East;

Chapter 2: petrographical and petrophysical description of lacustrine microporous micrites from the Madrid Basin (Late Miocene, Spain);

Chapter 3: petrographical and petrophysical description of marine microporous limestones from the Urgonian Formation (late Hauterivian to early Aptian, France);

1

10μm 4μm

Fig. 1 - SEM photomicrographs illustrating the matrix of a typical shallow-marine microporous carbonate reservoir rock composed of sub-rhombic low-Mg calcite crystals. Samples are from the Mishrif Formation (Cenomanian to early Turonian.

Middle East).

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Introduction

Mg/Ca ratio) and the Cenozoic (transition from calcite to aragonite seas; progressive increase of the Mg/Ca ratio). If microporous limestones are related to the evolution of precursor muds mainly composed of low-Mg calcite crystals, such microporous limestones shall occur during periods of calcite seas. Moreover, to observe if the development of microporous limestones is linked to specifi c eustatic conditions, the stratigraphic position of the microporous carbonate formations was compared with the relative position of sea-level.

The aims of this fi rst part are: 1) to review estimates of the Mg/Ca ratio and the nature of carbonate precipitates in seawater during the Phanerozoic, 2) to compare the occurrence of microporous carbonate formations in the Middle East during the Late Carboniferous to Triassic, the Cretaceous and the Cenozoic, and 3) to determine the main factors controlling the development of microporous limestones.

→ chapter 1 2) As aragonite and high-Mg calcite muds are probably not the precursor sediments of microporous limestones, recent carbonate muds deposited in shallow-marine tropical platforms cannot be used as valuable analogues to study the formation of these limestones. However, freshwater lakes may represent interesting environmental counterparts, as low-Mg calcite often precipitates from lake water (Dean and Fouch, 1983; Eugster and Kelts, 1983; Kelts and Hsü, 1978).

Late Miocene lacustrine microporous micrites of the Madrid Basin are characterized by similar mineralogy and microfabric to the Cenomanian to early Turonian shallow- marine limestones of the Mishrif reservoir For- mation in the Middle East, but have a much less complicated diagenetic history. In the Mesopotamian Basin, for example, the Mishrif Formation underwent hydrocarbon loading and was buried under more than 3700 m of Upper Cretaceous and Cenozoic sediments.

On the contrary, the Late Miocene micrites of the Madrid Basin were covered with less than 100 m of Cenozoic sediments and have never Chapter 4: petrographical and petrophysical

description of core sections from Wells X and Y in the A reservoir of the Mishrif Formation (Cenomanian to early Turonian, Mesopota- mian Basin) and a laterally equivalent outcrop in the Natih Formation (Cenomanian to early Turonian, Oman);

Chapter 5: analysis of the Mg distribution inside micrite crystals by using scanning transmission electron microscopy (STEM) combined with X-ray energy dispersive spectroscopy (X-ray EDS).

The fi ve parts are introduced separately below:

1) Recent carbonate muds deposited in tropi- cal platforms are mainly composed of aragonite and high-Mg calcite crystals. During meteoric (Lasemi and Sandberg, 1984) or burial diagenesis (Melim et al., 2002; Munnecke et al., 1997), aragonite and high-Mg calcite muds are generally transformed into limestones composed of low-Mg microspars. The microfabric of these limestones is very different from the microfabric encountered in microporous limestones in the Middle East. Crystals are generally larger than 4 μm, have an anhedral shape and often engulf relic aragonite needles. Moreover, although the porosity in fresh aragonite and high-Mg calcite muds may reach 70 %, it only attains about 5 % in the resulting consolidated limestones (Lasemi and Sandberg, 1984). This value is much lower than the porosity found in typical shallow-marine microporous carbonate reservoirs (~20 %). Thus, aragonite and high- Mg calcite muds are probably not the precursor sediments of microporous limestones.

In order to confi rm or undermine this hypoth- esis, an inventory of shallow-marine microporous carbonate formations in the Middle East depending on calcite (Mg/Ca ratio < 2) and aragonite seas (Mg/Ca ratio > 2) periods (Dickson, 2002; Hardie, 1996; Lowenstein et al., 2001; Sandberg, 1983; Siemann, 2003) was made. Three periods characterized by different seawater composition were chosen: the Late Carboniferous to Triassic (aragonite seas; high Mg/Ca ratio), the Cretaceous (calcite seas; low

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experienced oil saturation. Moreover, the Late Miocene section investigated in the Colmenar de Oreja quarry of the Madrid Basin presents both microporous and tight micrites.

The description of this lacustrine analogue and the characterization of the microporous and tight facies may permit to better understand the diagenetic processes involved in the formation and preservation of microporous limestones. Thus, this study illustrates: 1) the similarities between the late Miocene lacustrine microporous micrites of the Madrid Basin and the Cenomanian to early Turonian marine microporous limestones of the Mishrif Forma- tion in the Middle East, 2) the petrographical and petrophysical differences between the microporous and tight micrites of the Madrid Basin, and 3) the role of early diagenetic processes in the development of the lacustrine microporous and tight facies.

→ chapter 2 3) In the southern Jura Mountains in France, the Urgonian Formation (late Hauterivian to early Barremian) constitutes voluminous limestone deposits characterized by sedimentation in calcite seas (Dickson, 2002; Hardie, 1996;

Lowenstein et al., 2001; Sandberg, 1983). The Urgonian limestones are generally composed of hard, tight rocks and often form imposing cliffs. However, an outcrop with more than 150 meters of lateral extension was recently exposed in the Pierre-Blanche locality and reveals microporous limestones alternating with tight layers.

Because of their sedimentation conditions and geological age, the Urgonian platform deposits constitute interesting counterparts to Creta- ceous shallow-marine microporous carbonate reservoir formations of the Middle East. Thus, the examination of the conditions responsible for the creation of these unusual Urgonian rocks may improve the knowledge about the formation of marine microporous limestones and the modelling of shallow-marine microporous carbonate reservoirs made of microporous drains surrounded by tight intervals.

This part of the work proposes: 1) the petrographical and petrophysical description of the microporous and tight limestones composing the Pierre-Blanche section, and 2) a diagenetic model explaining the development of marine microporous limestones.

→ chapter 3 4) Making a profi t with hydrocarbon reservoirs composed of microporous limestones is more diffi cult than with conventional macro- porous reservoirs. Besides the complexity of oil extraction because of the strong capillary forces due to narrow pore throat size, the microporous carbonate reservoirs often possess a heteroge- neous distribution of the petrophysical properties that make their modelling problematic. In order to improve the modelling, it is necessary to understand the processes involved in the development or destruction of intercrystalline microporosity and to fi nd the factors (e.g.: petrographical, mineralogical, diagenetic) controlling the petrophysical properties.

The Mishrif Formation (Cenomanian to early Turonian, Middle East) represents a typical shallow-marine microporous carbonate reservoir (Aqrawi et al., 1998; Burchette, 1993;

Reulet, 1977). Several hydrocarbon fi elds are contained in this formation, notably the X, Y and Z fi elds in the Mesopotamian Basin.

Wells X and Y are about 35 kilometers apart.

However, well logging data and petrophysical laboratory measurements showed that X and Y present very contrasting petrophysical properties. For a better understanding and modelling of these reservoirs, the reasons for these differences have to be investigated.

In Oman, the Natih Formation (Cenomanian to early Turonian) constitutes a lateral equivalent to the Mishrif Formation in the Mesopo- tamian Basin. Microporous limestones surrounded by tight layers outcrop in the Jabal Madmar in the Adams Foothills (Homewood et al., 2008). This particular section may bring additional information about the conditions and factors leading to the differentiation of microporous and tight limestones.

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Introduction

The aims of this study are: 1) to petrographically and petrophysically describe cores from the A reservoir in Wells X and Y, 2) to highlight the similarities and differences between the two wells, 3) to petrographically describe the Jabal Madmar outcrop and to point out similarities with the Mishrif studied cores, and 4) to propose a model explaining the reasons for the contrasting petrophysical properties between Wells X and Y.

→ chapter 4 5) Cathodoluminescence microscopy is very useful to decrypt diagenetic processes involved in the formation of limestones. Unfortunately, such technology cannot provide information about the mineralogical composition evolution of micrite crystals, because of their smallness.

However, in order to understand the crystal- lization history of micrites, these data are pri- mordial.

Nowadays, no information exists on the distribution of minor elements inside a single micrite crystal. Magnesium (Mg) is the main element incorporated in calcite, but it often represents only a few mol % MgCO₃. Scanning electron microscopy (SEM) with X-ray energy dispersive spectroscopy (X-ray EDS) is generally not adapted to attain such high space resolution for such slight content variations.

However, ultra-thin sections analysed with X-ray EDS combined with scanning transmission electron microscopy (STEM) allow fi ne variations of minor elements inside small crystals to be highlighted.

Several (S)TEM studies have already explored the nanometric structures of synthetic calcite crystals (Chien et al., 2006; Paquette et al., 1996; 1999) and the early nucleation of calcium carbonate (Pouget et al., 2009), but no work was done on the distribution of minor elements inside a single micrite grain of ancient rock.

Thus, this study presents X-ray EDS combined with STEM maps showing the Mg distribution inside micrite crystals from the Mishrif Forma- tion (Cenomanian to early Turonian, Middle East).

→ chapter 5

Four of the fi ve parts constitute individual articles that are published or submitted in different scientifi c journals.

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Shallow-marine microporous carbonate reservoir rocks in the Middle East

ABSTRACT

The formation of shallow-marine microporous carbonate reservoir rocks remains poorly under- stood in spite of their economic importance, particularly in the Middle East. In this paper, we in- vestigate relationships between the stratigraphic occurrence of these carbonates in the Middle East and 1) the evolution of the Mg/Ca ratio in seawater; and 2) cyclic variations in relative sea-level.

An inventory of carbonate formations in the Middle East was compiled for three geological time intervals characterised by different seawater chemistries: the Late Carboniferous to Triassic (ara- gonite seas); the Cretaceous (calcite seas); and the Cenozoic (transition from calcite to aragonite seas). For each time interval, carbonate formations described as microporous have been listed.

During the Cretaceous calcite sea, eleven microporous carbonate formations were deposited in the Middle East. However, no microporous carbonates were formed during the Late Carboniferous to Triassic, a time of aragonite seas. During the Cenozoic, four of the fi ve microporous carbonate formations recorded were deposited before the transition from calcite to aragonite seas. Thus, these shallow-marine microporous carbonates appear to have developed from precursor muds which were mainly composed of low-Mg calcite crystals. Moreover, during the Cretaceous and the Cenozoic, microporous carbonate formations in the Middle East were generally associated with major transgressions and highstands of relative sea level.

The relatively high stability of low-Mg calcite muds may explain why shallow-marine micropo- rous carbonates formed during time intervals with calcite seas. In contrast to muds composed of aragonite or high-Mg calcite crystals, the original microfabric (including intercrystalline micropo- rosity) of low-Mg calcite muds can partly survive moderate diagenesis.

Chapter 1 Shallow-marine microporous carbonate reservoir rocks in the Middle East: Relationship with seawater Mg/Ca ratio and eustatic sea level

Article published in Journal of Petroleum Geology DOI: 10.1111/j.1747-5457.2009.00452.x

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INFLUENCE OF SEAWATER CHEMISTRY ON THE PRECIPITATION OF CALCIUM CARBONATE

The concentration of magnesium in seawater infl uences the precipitation of calcium carbonate (Folk, 1974), and numerous studies have shown that magnesium has an inhibitory effect on the growth of calcite (Berner, 1975; Davis et al., 2000; Reddy, 1986). High concentrations of magnesium facilitate the precipitation of aragonite and high-Mg calcite and prevent the formation of low-Mg calcite. Present-day seawater chemistry favours the precipitation of aragonite and high-Mg calcite on tropical platforms, but these conditions are not representa- tive for the whole of the Phanerozoic.

Calcite versus aragonite seas and the Mg/Ca ratio

Based on a study of the texture and composition of ancient ooids and cements, Sand- berg (1983) demonstrated that changes have

occurred in the primary mineralogy of marine non-skeletal carbonate grains during the Pha- nerozoic. Fluctuations in carbonate mineralogy point to an alternation between periods favouring the precipitation of calcite crystals (calcite seas) and periods characterized by the deposition of aragonitic sediments (aragonite seas) (Fig. 1). The correlation between these alternative carbonate mineralogies and climatic trends - calcite seas occur during greenhouse periods, aragonite seas occur during icehouse periods - led Sandberg (1983) to suggest that these changes can be explained by variations in the atmospheric pCO₂ content.

Wilkinson and Algeo (1989) determined variations in Mg/Ca ratios over the past 560 Ma by calculating the fl ux of magnesium and calcium between rivers, mid-ocean ridges and carbonate sediments (Fig. 1). Magnesium is delivered into oceanic basins by rivers and is removed either by hydrothermal alteration at mid-ocean ridges or as a result of carbonate

A

? C

A C

?

100 0

200 300

400 500

Time [Ma]

1 2 3 4 5

0

Mg/Caratioinseawater

Ng Penn

Miss D

S

C O P Tr J K Pg

Mg/Ca from echinoderms (Dickson, 2002, 2004) Mg/Ca from fluid inclusions (Lowenstein et al., 2001) Mg/Ca curve based on seafloor spreading rate (Hardie, 1996) Mg/Ca curve based on flux calculations (Wilkinson and Algeo, 1989) Calcite / Aragonite seas (Sandberg, 1983)

A C

Fig. 1 - Graph of time (Ma) versus Mg/Ca ratio in seawater during the Phanerozoic (data from Dickson, 2002; 2004;

Hardie, 1996; Lowenstein et al., 2001; Wilkinson and Algeo, 1989). See text for details. Along the bottom of the fi gure are time intervals with calcite and aragonite seas (C and A, respectively) determined by Sandberg (1983).

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sedimentation. Rivers and mid-ocean ridges both represent sources of calcium, but carbonate deposition is the only sink. In Wilkin- son and Algeo’s work, the behaviour of the Mg/Ca ratio is strongly controlled by variations in the fl ux of magnesium, and this was estimated from the dolomite volume.

Hardie (1996) showed that the mineralogical evolution of marine non-skeletal carbonates is concordant with that of potash evaporites.

Thus MgSO₄ evaporites occur in aragonite seas while KCl evaporites are formed in calcite seas.

Hardie (1996) suggested that seawater chemistry is primarily controlled by variations in hydrothermal fl uxes at mid-ocean ridge loca- tions, which in turn depend on the seafl oor spreading rate. By using estimated spreading rates and the relative abundance of granitic plutons, he calculated an Mg/Ca curve for the past 600 Ma (Fig. 1).

Lowenstein et al. (2001) studied primary fl uid inclusions in marine halites. These inclu-

sions represent residual seawaters and were used to measure Mg/Ca ratios throughout the Phanerozoic (Fig. 1).

Dickson (2002; 2004) analysed echinoderms to reconstruct Mg/Ca ratios (Fig. 1) as well- preserved echinoderm tests retain their original chemistry. An empirical partition coeffi cient calculated for present day tropical echinoids was used to reconstruct the concentration of magnesium and calcium in the ancient seawater.

Siemann (2003) estimated variations in seawater chemistry over the past 550 Ma by ana- lysing the bromine content of primary marine halite (Fig. 2) as variations in the composition of seawater cause changes in the partition coeffi cient of bromine in halite. He used Mg/Ca ratios from Hardie (1996) to predict the bromine contents of halite for the Phanerozoic, and compared these values with bromine contents in halite derived from the literature.

100 0

200 300

400 500

Time [Ma]

0 Ng Pg K

J Tr

P D

S

C O

40 60 80 100 120

20

μgBr/ghalite

Br contents in marine halite from literature data.

Br contents in marine halite calculated according to Mg/Ca ratios in seawater from Hardie (1996).

Miss Penn

Fig. 2 - Graph of time (Ma) versus bromine content in marine halite calculated according to seawater Mg/Ca ratio (data in Hardie, 1996) (solid line). Bromine concentrations in marine halite from literature data (crosses) are in general consistent with the graph.

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Discussion of the different approaches

With the exception of the Wilkinson and Algeo (1989) curve (Fig. 1), the previously described methods used to estimate Mg/Ca ratios all give similar results. The method used by Wilkinson and Algeo (1989) to determine Mg/Ca ratios for the Phanerozoic can be criticized, because it assumes that the ratio is inversely proportional to the rate of removal of magnesium from seawater by marine dolo- mitization. However, the formation of dolomite is not the predominant factor controlling seawater chemistry (Hardie, 1996). Therefore, Wilkinson and Algeo’s calculations overesti- mate the marine dolomite sink for magnesium with respect to the hydrothermal source of calcium at mid-ocean ridges.

The work of Siemann (2003) validates the model of Hardie (1996) although the method- ology is different. The curve of bromine contents in marine halite calculated from Hardie’s (1996) Mg/Ca ratios clearly matches the data on bromine contents of halite from the literature (Fig. 2).

Because direct analyses of primary fl uid inclusions in marine halite (Lowenstein et al., 2001) and fossil echinoderm tests (Dickson, 2002; 2004) are based on measurements of original compositions and do not come from calculations, they are thought to be reliable.

The results are in general consistent with Har- die’s (1996) Mg/Ca curve (Fig. 1). Some differences between the Mg/Ca values of fossil echinoderm tests (Dickson, 2002; 2004) and the other methods can be observed from the Cambrian to the Devonian. These discrepan- cies are probably due to undetected diagenetic alterations of the fossil material, refl ecting its great age.

The experimental work of Füchtbauer and Hardie (1976) allows a comparison to be made between the Mg/Ca ratios (Dickson, 2002;

2004; Hardie, 1996; Lowenstein et al., 2001) and the model of Sandberg (1983). Füchtbauer and Hardie (1976) showed that for a solution at 25 °C and 1 atm pressure, low-Mg calcite precipitates when the Mg/Ca ratio is less than 2; however, aragonite + high-Mg calcite are produced when the Mg/Ca ratio is equal to or

greater than 2. Sandberg’s (1983) observations and estimates of the Mg/Ca ratio (Dickson, 2002; 2004; Hardie, 1996; Lowenstein et al., 2001) globally agree during the Phanerozoic (Fig. 1). Intervals with an Mg/Ca ratio below 2 match time periods during which there were calcite seas; whereas times when Mg/Ca ratios were greater than 2 can be correlated with periods during which there were aragonite seas.

METHODS AND RESULTS

Number of microporous carbonate formations An inventory of carbonate formations in the Middle East was compiled using data published by Alsharhan and Nairn (2003). Countries selected for the study were Bahrain, Iran, Iraq, Jordan, Kuwait, Oman, Qatar, Saudi Arabia, Syria, Turkey, United Arab Emirates and Yemen. Only carbonate formations with thick- nesses greater than 100 m, or which covered several countries in terms of lateral extent, were taken into account (a formation which extends over several countries was however counted as one). Intervals with only minor carbonate contents were not included. Carbon- ate formations described as being microporous (Alsharhan and Nairn, 2003) were individually enumerated (Tables 1 and 2).

Most of the carbonate intervals recorded are referred to by their formation name. Excep- tions occur, for example in Saudi Arabia, where they may be given the status of members. To simplify the discussion in this paper, however, only the formation names will be used.

According to the data in Alsharhan and Nairn (2003) and given the above criteria, a total of 148 carbonate formations can be listed for the study area. Of these, 69 carbonate formations are Cretaceous, of which eleven (16 %) are microporous (Table 1, Fig. 3).

Thirty-six carbonate formations were identifi ed for the period between the Late Carboniferous and the Triassic, but none of them was microporous (Fig. 3). Forty-three Cenozoic carbonate formations were identifi ed, of which four (9 %) are microporous (Table 2, Fig. 3).

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With the exception of the M’sad Forma- tion (Cenomanian, Iraq) and the Middle Atj Member (Campanian, Saudi Arabia), all the Cretaceous and Cenozoic microporous carbonate formations identifi ed constitute reservoir rocks. Porosities are about 20 % and permeabilities may reach a few hundred mD (Alshar- han and Nairn, 2003).

The most important microporous carbonate reservoir units of Cretaceous age (the Yamama, Kharaib, Shuaiba, Mauddud, Rumaila, Khati- yah, Mishrif and Simsima Formations) are mainly composed of bioclastic and/or peloidal limestones with signifi cant matrix microporosity. When unstable bioclasts are dissolved during subaerial exposure creating moulds and vugs, the porosity of grain-dominant facies may increase to 25 % with permeabilities of up to 1000 mD (Alsharhan and Nairn, 2003). This occurs for example in rudist-rich units such as the Shuaiba Formation (Aptian: Bahrain, Iraq, Qatar), the Mishrif Formation (Cenomanian to early Turonian: Iraq, Oman, Qatar) and the Simsima Formation (Maastrichtian: Oman) (Harris et al., 1984). The principal Cenozoic microporous carbonate reservoir units (the Jaddala and Dammam Formations) consist of bioclastic micrites (Alsharhan and Nairn, 2003). Intraparticle microporosity signifi cantly enhances the total microporosity when num- mulites are the dominant bioclasts present.

Eustatic changes and the occurrence of microporous carbonate formations during the Cretaceous and Cenozoic

The relationship between sea-level changes (Haq and Al-Qahtani, 2005) and the development of microporous carbonates during the Cretaceous (Fig. 4) and the Cenozoic (Fig. 5) in the Middle East was investigated. The results indicate that some stratigraphic intervals (Ber- riasian, middle to upper Hauterivian, Albian and Coniacian to Santonian in the Cretaceous;

middle Selandian to upper Ypresian and Lang- hian to middle Serravalian in the Cenozoic) do not include microporous carbonate formations even if shallow-marine carbonate deposition occurred (Ziegler, 2001). According to Haq and Al-Qahtani (2005), three periods of major sea-level lowstand occurred during the Creta- ceous. During these periods, microporous carbonates were in general not formed (with the exception of the Yamama Formation during the Valanginian in Iraq). By contrast, the development of microporous carbonate formations was in general associated with long-term rises and highstands in relative sea level (Fig. 4).

Similarly, during the Cenozoic, the three main microporous carbonate formations were deposited during periods of transgression and sea level highstand (Haq and Al-Qahtani, 2005) (Fig. 5).

36

58

39 11

4

L. Carbo. to Trias. Cretaceous Cenozoic

¬9%

total: 43 16%

total: 69

®

0%

total: 36

®

microporous carbonate formations carbonate formations (except microporous)

Fig. 3 - Bar chart showing the number of microporous carbonate formations during different time periods in the Middle East (data from Alsharhan and Nairn, 2003). During the Late Carboniferous to Triassic, 36 carbonate formations devel- oped, none of which is microporous. Of a total of 69 Cretaceous carbonate formations, eleven are microporous (i.e. 16 %).

Of 43 carbonate formations developed during the Cenozoic, four are microporous (i.e. 9 %).

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Onlap curve landward basinward

1.0 0.5 0.0

Relative sea-level changes

0 100 200

300 -100

Maastrichtian

Campanian

Santonian Coniacian Turonian

Cenomanian

Albian

Aptian

Barremian

Hauterivian

Valanginian

Berriasian

EARLYLATE

CRETACEOUS

70

75

80

85

90

95

100

105

110

115

120

125

130

135

140

145

Middle Atj (Saudi Arabia)

Judea (Syria)

Rumaila (Iraq, Bah.) Mauddud (Oman) Mishrif (Iraq, Om., Qat.)

Khatiyah (Qatar) M’sad (Iraq)

Shuaiba (Iraq, Qatar, Bahrain)

Kharaib (UAE, Qatar, Bahrain)

Yamama (Iraq)

Arabian platform regional sea-level changes Stage

Epoch

Period

Time[Ma]

Simsima (Oman)

Microporous carbonate formations in

Middle East

no microporous carbonate formations

Fig. 4 - Relative sea-level changes in the Middle East during the Cretaceous with the stratigraphic positions of micropo- rous carbonate formations (shaded areas) (modifi ed from Haq and Al-Qahtani, 2005; and Alsharhan and Nairn, 2003).

Hatched boxes correspond to sedimentary hiatuses. The absence of microporous carbonate formations in general coincides with major sea-level lowstands (other types of carbonates occur). The development of microporous carbonate formations is generally linked to major transgressions and highstands.

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no microporous carbonate formations

Onlap curve landward basinward

1.0 0.5 0.0

Relative sea-level changes

0 100 200

300 -100

Arabian platform regional sea-level changes Stage

Epoch

Period

Time[Ma]

Microporous carbonate formations in

Middle East

PALEOCENEMIOCENE

PALEOGENE

05

10

15

20

25

30

35

40

45

50

55

60

65

NEOGENE EOCENEOLIGOCENEPLIO.

Pleistocene-Holocene

Danian Selandian Thanetian Ypresian

Lutetian Bartonian Priabonian

Chattian

Rupelian Aquitanian Burdigalian Langhian Serravalian

Tortonian Messinian

Zanclean Placenzian^Gelasian

Dammam (Iraq, Kuwait) Jaddala (Syria) Fahud (Oman) Euphrates (Iraq)

Fig. 5 - Relative sea-level changes in the Middle East during the Cenozoic and stratigraphic positions of microporous car- bonate formations (shaded areas) (modifi ed from Haq and Al-Qahtani, 2005; and Alsharhan and Nairn, 2003). Hatched boxes correspond to sedimentary hiatuses. The three main microporous carbonate formations developed during the Eocene sea-level transgression and highstand.

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DISCUSSION

The Late Carboniferous to Triassic versus the Cretaceous

While eleven microporous carbonate formations occur in the Cretaceous of the Middle East, none were formed during the Late Car- boniferous to Triassic (Fig. 6). According to studies of seawater chemistry (Dickson, 2002;

2004; Hardie, 1996; Lowenstein et al., 2001;

Sandberg, 1983), the entire Late Carbonifer- ous to Triassic time interval was characterised by aragonitic seas. By contrast, the Cretaceous

was characterised by exclusively calcite seas.

Therefore seawater chemistry may explain the observed trend in the development of microporous carbonate formations.

However before any conclusions can be drawn, the question of paleogeographic variations during the different time periods must be addressed (Fig. 7). Did shallow-marine carbonate platforms have the same extent during the Late Carboniferous to Triassic as they did during the Cretaceous? During the Late Permian, a huge shallow-marine carbon-

Formation Country Stage

Euphrates Fm Iraq early Miocene

Dammam Fm Iraq, Kuwait middle to late Eocene

Jaddala Fm Syria middle to late Eocene

Fahud Fm Oman middle Eocene

The Cenozoic

Total : 4 microporous carbonate formations (9 % of Cenozoic carbonate formations) Table 1 - Cretaceous microporous carbonate formations in the Middle East (see text for defi nitions).

Table 2 - Cenozoic microporous carbonate formations in the Middle East (see text for defi nitions).

Formation Country Stage

Simsima Fm Oman Maastrichtian

Middle Atj Member Saudi Arabia Campanian

Judea Fm Syria Cenomanian to Turonian

Mishrif Fm Iraq, Oman, Qatar Cenomanian to early Turonian

Khatiyah Fm Qatar Cenomanian

M'sad Fm Iraq Cenomanian

Rumaila Fm Bahrain, Iraq Cenomanian

Mauddud Fm Oman late Albian to middle Cenomanian

Shuaiba Fm Bahrain, Iraq, Qatar Aptian

Kharaib Fm Bahrain, Qatar, UAE Barremian to early Aptian

Yamama Fm Qatar Valanginian

The Cretaceous

Total : 11 microporous carbonate formations (16 % of Cretaceous carbonate formations)