ORIGINAL PAPER

Depositional environments and porosity distributionin regressive limestone reservoirs of the Mishrif Formation,Southern Iraq

Moutaz Al-Dabbas & Jassim Al-Jassim &

Saad Al-Jumaily

Received: 27 February 2009 /Accepted: 30 April 2009 /Published online: 9 June 2009# Saudi Society for Geosciences 2009

Abstract Eight subsurface sections and a large number ofthin sections of the Mishrif Limestone were studied tounravel the depositional facies and environments. Theallochems in the Mishrif Formation are dominated bybioclasts, whereas peloids, ooids, and intraclasts are lessabundant. The sedimentary microfacies of the MishrifFormation includes mudstone, wackestone, packstone,grainstone, floatstone, and rudstone, which have beendeposited in basinal, outer shelf, slop followed by shoalreef and lagoonal environments. The formation displaysvarious extents of dolomitization and is cemented by calciteand dolomite. The formation has gradational contact withthe underlying Rumaila Formation but is unconformablyoverlain by the Khasib Formation. The unconformity isrecognized because the skeletal grains are dominated byChaophyta (algae), which denotes the change of environ-ment from fully marine to lacustrine environment. Thus, thevertical bioclast analysis indicates that the Mishrif Forma-tion is characterized by two regressive cycles, whichcontrol the distribution of reservoir quality as well as thepatterns of calcite and dolomite cement distribution. MishrifFormation gradationally overlies Rumaila Formation. Thiswas indicated by the presence of the green parts ofChaophyta (algae) as main skeletal grains at the uppermostpart of well Zb-47, which refer to lacustrine or fresh waterenvironment. Petrographical study shows that the fossils,peloids, oolitis, and intraclasts represent the main allochem.Calcite and dolomite (as diagenetic products) are thepredominant mineral components of Mishrif Formation.Fossils were studied as an environmental age and facial

boundaries indicators, which are located in a chart usingpersonal computer programs depending on their distribu-tions on the first appearance of species. Fifteen principalsedimentary microfacies have been identified in the MishrifFormation, which includes lime mudstone, mudstone–wackestone, wackestone, wackestone–packstone, pack-stone, packstone–grainstone, grainstone–floatstone,packstone–floatstone, packstone–rudstone, and wacke-stone–floatstone. Markov chain analysis has been used tostudy the transitional pattern of different microfacies typesvertically in each well and laterally in all wells as acomposite section. The vertical analysis indicates that theMishrif Formation characterized by two regressive cycles,the main one started with basinal or outer shelf environ-ment, slop environment followed by shoal or reefalenvironment, and ended with a lagoonal environment. Thelateral analysis shows the same regressive cycle, and byusing the lithofacies association concepts, we built thedepositional model of the Mishrif Formation environment.

Keywords Mishrif Formation . Porosity . Facies analysis .

Depositional environment

Introduction

The Mishrif Formation (Cenomanian–Early Turonian age)is one of the important carbonate reservoirs in middle,southern Iraq and throughout the Middle East (Videtich etal. 1988; Alsharhan and Nairn 1993; Alsharhan 1995;Aqrawi et al. 1998). In southern Iraq, the formationprovides the reservoir in oilfields such as Tuba, Zubair,West Qurnah, and Nasryia (Fig. 1). The formation containsup to 40% of Cretaceous oil reserves in Iraq and about 30%of total Iraqi oil reserves (Aqrawi et al. 1998).The Mid-

Arab J Geosci (2010) 3:67–78DOI 10.1007/s12517-009-0057-x

M. Al-Dabbas (*) : J. Al-Jassim : S. Al-JumailyCollege of Science, University of Baghdad,P.O. Box 47138, Baghdad, Iraqe-mail: prof_aldabbas@yahoo.com

Cretaceous (Early Albian–Early Turonian) sequence in theMesopotamian basin of southern Iraq consists of twosedimentary cycles (Buday 1980). The Mishrif and under-lying Rumaila formations were originally described insouthern Iraq in well Zubair no. 3 at Zubair oilfield. Thecontact between the Mishrif and Rumaila Formations is

gradational, as is the contact between the Mishrif and KifilFormations. However, in many oilfields (such as WestQurnah), the Mishrif Formation is unconformably overlainby the Khasib Formation, where the Kifil formation isabsent (Aqrawi 1995).The thickness of the Formationvaries according to the location within the Mesopotamian

Fig. 2 A simplified stratigraph-ic correlation of Cenomanian–Early Turonian sequences

Fig. 1 Distribution of the studied wells of the Mishrif Formation

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basin. It reaches 350 or 400 m in SE Iraq near the Iranianborder and thins or wedges out in the west and SW (Al-Siddiqi 1978).The Mishrif Formation represents a hetero-geneous formation originally described as organic detritallimestones, with beds of algal, rudist, and coral reeflimestones, capped by limonitic fresh water limestones(Bellen et al. 1959). Chatton and Hart (1960, 1961)included all the organic detrital neritic limestone units ofCenomanian–Early Turonian age, such as the M'sad, GirBir, and Mergi Formations in the Mishrif Formation. Theyplaced the freshwater limestone in the newly introducedKifil Formation. Ditmar and the Iraqi–Soviet Team (1971)continued to recognize the Gir Bir and M'sad as separatedformations. The Gir Bir and Mergi Formations are hereincluded in the Mishrif, but the M'sad is retained as anindependent unit because of its distinctive facies (Fig. 2).Unlike the Mishrif Formation, the M'sad Formation wasdeposited in a coastal, locally supratidal environment.

The Mishrif Formation in its typical area is composed ofgray-white, dense, algal limestones with gastropods andshell fragments at the top and of brown, detrital, porous,partly very shelly and foraminiferal limestones and rudistdebris at the bottom. The formation is variably dolomitizedW and NW of the type section (Fig. 3). The MishrifFormation is thickest in the Rumaila and Zubair fields(270 m), in the Nahr Umr andMajnoon fields along the Iraq–Iran border (435 m) and in the Abu Amud field between Kutand Amara (380 m). Other isolated occurrences are situatednear Kifil (255 m) and Samarra (250 m). In the western

desert, some rudist limestones of the M'sad Formationoutcrop about 100 km ESE of Rutba (50 m). The formationwas deposited as rudist shoals and patch reefs overgrowingsubtle structural highs developing in an otherwise relativelydeeper shelf on which open marine sediments of theRumaila Formation were deposited. The lower contact ofthe Mishrif Formation is usually conformable. The under-lying unit is usually the Rumaila Formation in the southand the M'sad Formation in the west. In the northernmostarea where the Mergi and Gir Bir facies occur, the lowerboundary is unconformable, and those units overlie theAlbian Mauddud (Upper Qamchuqa) Formation. The upperboundary is unconformable in the Awasil–Samarra area incentral Iraq and in the foothill and high folded zones (GirBir and Mergi occurrences). In parts of south Iraq, wherethe Kifil Formation is present, the upper contact isconformable, while in parts where the Kifil is absent, thetop of the Mishrif Formation is marked by an unconformity.

The aim of this research is to study the microfacies andthe depositional environment of eight boreholes west ofTigris and west of Shatt al-Arab River by studying thepetrographical and stratigraphic characteristics of the rocksamples and to prepare the depositional model of the MishrifFormation with their microfacies sequence relationship byapplying Markov chain analysis to analyze the microfacies(1) vertically to identify the cyclic nature of these differentmicrofacies and the main and secondary cycles and (2)horizontally to indicate the microfacies extinction and naturefor the Mishrif depositional model construction.

Fig. 3 Litho-facies of MishrifFormation in type section

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Materials and methods

To determine and construct the depositional environment ofthe Mishrif Formation, eight subsurface sections have beenselected from the wells of four oil fields located in southIraq. They are, Wq-215, Wq-57, Wq-21, West Qurnah, Zb-47, Zb-43, Zubair, Tu-5, Tu-4, Tuba, and Ns-2 Nasiriya.For this purpose, more than 1,800 thin sections (mostlycore) were described and interpreted, together with severalhundred thin sections previously prepared by the Iraq OilExploration Co. (OEC), were studied petrographically byapplying the modified Dunham 1962 classification byEmbry and Klovan (1972). In addition, to determine themineral composition, we used X-ray powder diffractiontechnique to analyze a number of samples.

Petrography of the Mishrif Formation

More than 1,800 thin sections were studied to understandthe nature of the microfacies and to identify the micro-petrographical facies, their main constituents, and thediagenetic processes such as dolomitization, micritization,neomorphism, compaction, cementation, and dissolution(Folk 1965, 1980). The results revealed that the carbonaterocks of the Mishrif Formation in southern Iraq are consistmainly of micrite and, to a lesser extent, various skeletalgrains such as foraminifera (of different sizes), echinoder-mal plates, ostracods, and the shells of molluscs (mainlyrudists). Non-skeletal components are restricted mainly topeloids and to less-abundant intraclasts and ooids.

Skeletal grains

Planktonic foraminifera are less common representing thetransitional zone between Rumaila and Mishrif Formations,especially with the existence of the Calcispheres. Hedber-gella washitensis, Heterohelix globules, and various Oli-gosteginids are the dominant planktonic forms and are verycommon in the lower parts of the Mishrif Formation of thebasinal facies, which rests gradationally on the underlyingRumaila formation (Aqrawi et al. 1998). These planktonicfaunal assemblages are usually interpreted to indicate a low-energy, open-marine, outer-shelf depositional environmentbelow the wave base (Flugel 1982). Benthic foraminifera ofvarious sizes are the most common skeletal grains in theMishrif Formation, particularly increasing in slope, lagoonal,and intertidal environments, with less abundance in shoal,fore, and back reef environments, although some of themwere destroyed due to dolomitization and recrystalization,making them difficult to identify. Alveolinids, such asPraealveolina, Dicyclina, and Ovalveolina, are dominant,whereas Nezzazta and Textularia are less common. Otherbioclasts included molluscs shell fragment, rudist, thin rudist

biostroms with chondrodonta sometimes, and echinodermsthat represent the open sea life, indicated by either echinoidsor crinoids and exists as plates and spines; algae are lessthan molluscs and echinoderms and are represented bydasyclaacean and permoculculus, encrusting algae, and redalgae. Charophyte was located in upper Zb-47, indicatingnon-marine, lacustrine environment. Ostracods, coral, poriferaspicules, cup coral, septet fragments, sponge spicules, andbrachiopods existed as well. Rudists are the most importantmollusc fragments and occur either in a distinct rudstonefacies or a rudistid packstone/grainstone or rarely (with othergrains) in bioclastic wackestones and packstones.

Rudists are good environmental indicators and areinterpreted to indicate patch- and fore-reef slope settings(Wilson 1975; Flugel 1982; Tucker 1985).

Non-skeletal grains

In the Mishrif Formation, peloids are the main non-skeletalgrains, especially in packstone and grainstone microfacies,and range in size from silt to sand grade. Some peloids areprobably micritized ooids. Peloids occur in packstones andwackestones and are characteristic of shoal and subtidalenvironments, respectively (Tucker 1985). Intraclasts areinterpreted to be reworked grains within the subtidal andintertidal environment arising from current agitation and areless common than peloids and are present in low percentagesin peloidal and bioclastic wackestones and packstones.

Micrite

Micrites represented by microcrystalline, neomorphosed tomicrospare, with agglutinating like peloids with clottedtexture are the most common in most of the studiedboreholes, where the Mishrif Formation carbonates aremainly composed of mud-supported fabrics such as limemudstones and wackestones.

Depositional microfacies

The Mishrif Formation carbonates were classified followingFolk's (1962) and Dunham's (1962), modified by Embryand Klovan (1972) and revised by Wright (1992), classifi-cation into mud- or grain-supported textural types. Eachtype consists of principal microfacies, as shown in Table 1.

Lime mudstone microfacies This microfacies occurs atvarious levels throughout the studied sections but wascommon in the lower parts. Micrite is the main component,but planktonic foraminifera (mostly Oligosteginids orCalcispheres, Hetrohelix, etc.) also occur in variousproportions usually less than 50%.This microfacies domi-nates the underlying Rumaila formation (Aqrawi et al. 1998).

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Lime mudstone–wackestone microfacies This microfaciesoccurs at all the studied boreholes throughout the studiedsections except Tu-5. It represents a thin microfacies.Pelagic lime mudstone–wackestones are usually interpretedas outer shelf or basinal deposits (Wilson 1975).

Lime wackestone microfacies It represents one of the mostcommon microfacies in the Mishrif Formation with 5 mthickness and reaches 18 m in Zu-43 and may locally bedominated by a specific type of bioclast at various levelswithin the succession. Bioclasts (such as Praealveolinids,

Table 1 Primary and secondary microfacies for Mishrif Formation in the studied boreholes

Main microfacies types Code Sub-microfacies Boreholes

Lime mudstone M Planktonic foram lime mudstone mBioclastic lime mudstone

Miliolids lime mudstone

Lime mudstone–wackestone M-W Microbioclastic lime mudstone–wackestone Wq-215,

Foraminiferal (planktons and benthos) Wq-57,

Lime mudstone–wackestone Wq-1,Zb-47,Zb-43,Tu-4, Ns-2Foraminiferal peloidal lime mudstone–wackestone

Lime wackestone W Orbitolina lime wackestone AllPeloidal bioclastic wackestone

Algal bioclastic lime wackestone

Miliolids bioclastic lime wackestone

Microbioclastic lime wackestone

Foraminiferal (planktonic) lime wackestone

Lime wackestone–packstone W-P Foraminiferal (pelagic) lime wackestone–packstone AllBioclastic peloidal lime wackestone–packstone

Bioclastic lime wackestone–packstone

Foraminiferal (benthonic) lime wackestone–packstone

Lime packstone P Bioclastic lime packstone AllPeloidal bioclastic lime packstone

Foraminiferal lime packstone

Fossiliferous lime packstone

Rudistid bioclastic lime packstone

Lime packstone–grainstone P-G Bioclastic foraminiferal peloidal Wq-215,

Lime packstone–grainstone Wq-57, Tu-5

Foraminiferal lime packstone–grainstone

Peloidal bioclastic lime packstone–grainstone

Lime grainstone G Peloidal bioclastic grainstone Wq-215,

Bioclastic grainstone Wq-21,

Rudist grainstone Zb-47,Tu-4, Tu-5,Peloidal algae pisolite, charophytal lime grainstone

Bioclastic benthonic (foram)lime grainstone Ns-2

Lime grainstone–rudstone G-R Bioclastic lime grainstone–rudstone Wq-215, Wq-57, Tu-4

Lime rudstone or rudist biostrome R Bioclastic rudstone Wq-215,

Rudistid rudstone Wq-21,

Bioclastic rudstone–floatstone Tu-5,Ns-2

Lime floatstone F Bioclastic rudistid floatstone Wq-57,

Intraclast bioclastic benthonic(foram) floatstone Zb-47

Lime boundstone B – Zb-47

Lime grainstone–floatstone G-F Bioclastic lime grainstone–floatstone Zb-47Bioclastic peloidal lime grainstone–floatstone

Lime packstone–floatstone P-F – Tu-4

Lime packstone–rudstone P-R – Tu-4

Lime wackestone–floatstone W-F – Tu-4

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algae, and echinoderms) comprise between 10% and <50%of the lithology, and limited pelagic foraminifera also occur.The microfacies is a characteristic of shallow, open-marineenvironments (Flugel 1982).

Lime wackestone–packstone microfacies It represents thethird common microfacies in the Mishrif Formation. Benthicforaminifera (such as Miliolids, Textularia, and Nezzata),sponge spicules, green algae, small mollusc fragments, andechinoderms occur in this microfacies in proportions up toabout 50%. The microfacies is typical of lagoons (Flugel1982) or restricted subtidal zones (Reulet 1982) with warmshallow waters and moderate circulation (Tucker 1985).

Lime packstone microfacies It represents one of the com-mon microfacies in the Mishrif Formation. This microfaciesis principally composed of peloids of various sizes, many ofwhich have an uncertain internal structure. In addition,benthic foraminifera, rudist fragments, and ostracods alsooccur. The microfacies is common in the upper parts of theMishrif Formation successions and is interpreted to indicateshoals and subtidal zones with moderate agitation.

Lime packstone–grainstone microfacies It represents theless common microfacies in the Mishrif Formation. It islocated in Tu-5, Wq-215, and Wq-57. This microfacies ischaracterized by a high content of rudist fragments, whichare associated with other bioclasts such as algal debris,benthic foraminifera, and peloids (in smaller proportions).It is one of the two principal reservoir facies of the MishrifFormation. Rudist grainstone is interpreted to be a reefbank or shoal deposit and rudist packstones to be a back-reef deposit. Lime mud may be present in small proportionsin intergranular pore spaces, indicating highly agitateddepositional conditions.

Lime grainstone microfacies It represents one of the mostcommon microfacies in the Mishrif Formation. Rudstone arealmost entirely composed of rudist fragment, most of whichare larger than sand grade, in addition to coral fragments of asimilar size. This microfacies is interpreted to be a fore-reefslope deposit (Wilson 1975). The rudstone and rudistidpackstone/grainstone are generally over- and underlain bysubtidal and outer-shelf facies, respectively, in the studiedboreholes. These two microfacies are characterized by highprimary and secondary porosities and permeabilities;together, they form the most important reservoir units inthe Mishrif Formation throughout the Mesopotamian basin.

Lime grainstone–rudstone microfacies It represents one ofthe intertunging microfacies between lime grainstonemicrofacies and the rudstone microfacies in the MishrifFormation. It is located in Tu-4, Wq-215, and Wq-57.

Rudstone microfacies It represents the non continuousmicrofacies with 0.5-m thickness in the Mishrif Formation.It is located in Tu-5, Wq-215, and Wq-21 and Ns-2.

Floatstone microfacies It shows the microstylolite inirregular anastomosing type that construct nodular struc-tures called stylonodular structures. It is located only inWq-57 and Zb-47.

Boundstone microfacies It represents the encrusting algaewith cellular structure that related to micrite which considerone of the important factors in fixation of the rudist in thereef formations. It is located only in Zb-47.

Lime grainstone–floatstone microfacies It represents poorlywashed, biosparite to unsorted biosparite/biomicrite. It islocated only in Zb- 47.

Markov chains analysis

Markov chains analysis has been applied to study thetransitional pattern of different microfacies types (Fig. 4),vertically in each well and laterally in all wells as a compositesection (Burchette and Birtton 1985; Miall 1973; Wells 1989).The tree diagram represents the best relationship between themain facies sequence. The best relationship was based onstatistical verification by using the chi-square test method,correlation coefficient, and correlation of determination toidentify the lithofacies association for each environment ofdeposition (Fig. 5). The tree diagram reflects the following:

1. The clear direction of the main regressive cycle, whichstarts by the lime mudstone (M), indicating the quietbasinal environment, and ends by the lime grainstone–rudstone (G-R), representing the shoal or reefalenvironments.

2. The repetition of cycle is reflected by the returningto the lime wackestone (W), to build the secondregression cycle that represents the lagoonal faciesenvironment.

3. The relationship is clear between the lime grainstone(G), lime grainstone–rudstone (G-R), and lime pack-stone–floatstone facies (P-F), with the building of theback reef or back shoal environments.

In conclusion, the vertical analysis indicate that the MishrifFormation characterized by two regressive cycles, the mainone starting with basinal or outer shelf environment, slopeenvironment followed by shoal or reefal environment andending with a lagoonal environment. The lateral analysisshows the same regressive cycles, and using the lithofaciesassociation concepts, we built the depositional model of theMishrif Formation environment (Figs. 5 and 6).

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Diagenetic processes

Themost important diagenetic processes inMishrif Formationare micritization, cementation, neomorphism, dolomitization,compaction, and dissolution. Carbonates of the Mishrif

Formation have been affected by both early and late-stagediagenesis.

Mudstones and wackestone were principally altered bymarine phreatic and later diagenetic processes. Diageneticfeatures in these microfacies include occasional pyrite

Fig. 5 Summary of the Markovchain analysis results for thesignificant relationships betweenthe microfacies in thedepositional environment of theMishrif Formation, southernIraq

Fig. 4 Description of theMishrif Formation based oncores and well logs at boreholeWq-215, west qurna oilfield(after Sherwani 1983; Aqrawiet al. 1998; Al-Jumaily 2001;Al-Khafaje 2006)

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crystal and borings in planktonic foraminifera, in additionto the rare occurrence of calcite cement filling skeletalgrains. Cloudy-centered, clear-rimmed dolomite rhombsand stylolites indicate later, burial-diagenetic processes.These diagenetic fabrics have been reported from wacke-stone and mudstones in the underlying Rumaila Formation(Aqrawi et al. 1998).

Diagenetic features in the bioclastic wackestones, pack-stones, grainstones, and rudstones indicate early and surfacealteration in the meteoric phreatic and mixing zones. Thedolomite rhombs in the wackestone and mudstones indicatemixing-zone diagenesis in intertidal flats, and the small sizeof the dolomite crystals (Badiozamani et al. 1977; Aqrawi1995) may support their brackish-water origin; syntaxialovergrowths on echinoderm plates and equant calcitecements are interpreted to be products of local meteoricphreatic diagenesis (Longman 1982).The development ofmoldic, vuggy and channel porosity, particularly in rudist-bearing packstones, grainstones, and rudstones, indicate asimilar diagenetic environment. Near-surface, meteoric-zonediagenesis probably affected structurally higher parts of thebasin, particularly in areas where rudist reefs developed.These processes are described briefly in turn below.

Micritization It is the most common process affecting theskeletal fragments in the packstones and wackestonesmicrofacies in shallow environment of the Mishrif Forma-tion. Micritization is an early diagenetic process and skeletal

grains were micritized shortly after deposition due to fungiaction and micritic envelops construction.

Cementation Cementation has led to occlusion of primaryporosity in Mishrif Formation carbonates such as packstoneand grainstone. There are many types of cements, such as (1)micrite cement, (2) overgrowth cement, (3) displacementcement, (4) granular cements, and (5) druzy mosaic cements,that are present in various microfacies filling inter- andintragranular pores and fractures. These cements are believedto be of later diagenetic origin, although the mosaic cementmay have formed during two or more phases.

Neomorphism There are two types in this formation: eithera change from aragonite to calcite or a change from calciteto another calcite, which is called recrystalization. Bothtypes were completed during dissolution and redepositionprocesses (Longman 1982). The neomorphism may changethe micrite partially or totally to sparite (4–10 μm) orpsuodosparite (10–50 μm). A process that could be usefulin preserving the original shape of the fossil shells ischanging the aragonite of their original shells into calcite,which is called calcitization (Tucker 1985).

Dolomitization Scattered, fine-grained dolomite rhombsoccur within the mud-supported microfacies and are oftenconcentrated along stylolite surfaces. The fine grain size ofthe rhombs and their occurrence within the mud-supported

Fig. 6 The environmental distribution of Mishrif Formation in borehole Wq-215. In addition, it indicates the significant relationships between themicrofacies

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facies indicates their early diagenetic origin, particularlythose present within intertidal mudstone. Larger dolomiticcrystals, of a clear-rimmed, cloudy-centered type, probablyformed during later diagenesis (Sibley 1982) but beforestylolitization. Dolomitization has slightly enhanced thereservoir quality of some of the microfacies, particularlywith the formation secondary intercrystalline microporeswithin muddy microfacies.

Compaction This process reflects the compaction due tothe heavy sedimentary cover (>1,000 m thickness), whichdecreases the porosity (Flugel, 1982). It was seen inpackstone and grainstone microfacies, which gave themthe orientation property in their internal structure. Thecompaction has indirect relationship with the originalprimary cementation (Flugel 1982).

Stylolitization Pressure solution has resulted in the forma-tion of dissolution surfaces, clay seams, and stylolites.

Stylolites in both mud- and grain-supported microfaciestook the form of horsetail and irregular anastomosing sets.Organic material and other relatively insoluble particles(dolomite rhombs, early calcite-cemented grains, and clayparticles) commonly occur on the stylolite surfaces,indicating a late-diagenetic origin.

Dissolution Dissolution is considered as one of the mostimportant diagenetic process in the Mishrif Formation. It isa more effective diagenetic process than cementation inmost of the reservoir facies. Dissolution in this environmentimproved the carbonates porosity, particularly that of thecoarse-grained, rudist-bearing facies.

This process acts to destroy the internal structures for theskeletal grains, leaving the micritic envelope to form themoldic porosity or vuggy porosity or enlarging the presentingvugs to form cavern porosity or channel porosity, andsometimes, there are open space structures due to the effectof the selective dissolution. The porosity was studied in the

Fig. 7 Representative types ofporosity in well WQ-21,westQurna oilfield

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Mishrif microfacies according to Choquette and Pray (1970)(Fig. 7).

The results indicated that:

1. Interparticle porosity was distinguished in packstoneand grainstone microfacies.

2. Vuge porosity existed in mudstone and wackestonemicrofacies.

3. Moldic porosity was indicated in the grain- and mud-supported microfacies, where the algae disappears inthe primary microfacies and left their moldic shapes inthe packstone rich of algae. While in the secondary

microfacies, the dolomitic rhombs disappears in thedolomitic mudstone facies.

4. Growth framework represents the interparticles andintraparticles that resulted from growth builders likerudist and reefs or algae. It existed in the boundstone.

5. Fracture porosity in mudstone and wackestone, whichis composed of soft mud, developed the channelporosity due to dissolution process.

6. High porosity was indicated in the high-energy microf-acies as intraparticles especially in rudist-rich microfa-cies, in which porosity may reach more than 15–26%and permeability between 1 and 100 mD or more.

Fig. 8 Schematic block diagram representing the proposed depositional model for the microfacies of Mishrif Formation according to Markovchain analysis results for the cyclic sequence (note: the boreholes locations are not according to scale)

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The porosity and permeability were measured throughoutthe Mishrif Formation at the Nasiriya oilfield (Ns-3 well).Here, porosity was 2.1–28%, while permeability was <0.01–72 mD. High porosities (i.e., >14%) and permeabilities (i.e.,>98 mD and up to 1 Darcy) are restricted to the rudist-bearing facies, which therefore constitute the main reservoirzone analyses of the Mishrif Formation in southern Iraq.

7. Porosity decreases due to the cementation and compac-tion processes.

Diagenetic environments

The early Cenomanian transgression was responsible forthe deep-water conditions, in which Rumaila formation hasbeen deposited, as well as the lower part of the MishrifFormation in southern Iraq. Deposition of the lowerregressive sequence of the Mishrif Formation began in theeast of the Mesopotamian basin.

The depositional model of Mishrif Formation wasconstructed from the results of the microfacies studies,applying Markov chain for the facies sequence analyses ofeach borehole and for all the studied boreholes and therelationship of the lithofacies association with the environ-ment of deposition. The results of the vertical analysisindicate that the Mishrif Formation is characterized by tworegressive cycles, the main one started with basinal or outershelf environment, which is the extinction of the RumailaFormation depositional environment, then graduated to slopeenvironment followed by the restricted rudist reef thatexchange places with the shoal environment. The disconti-nuity of the thin rudist biostroms for each of the studiedboreholes is remarkably shown. The reefal environmentended with a lagoonal environment. The lagoonal environ-ment started to indicate the microfacies distribution, whichreflects the inner shelf conditions represented by thesubtidal to tidal environments to end the main depositionalcycle and cover the hydrocarbon bearing rocks. There wasrepetition of the main depositional cycle to the secondregressive depositional cycle, which indicates the highthickness in the studied boreholes and reflects two units ofthe rudist reef facies. The lateral analysis shows the sameregressive cycle, and using the lithofacies associationconcepts, the depositional model was build for the MishrifFormation environment from the studied boreholes (Fig. 8).

Conclusions

The Mishrif Formation (Cenomanian–Early Turonian) con-sists mainly of shallow marine environment carbonates, andmany microfacies have been identified: lime mudstone,

lime mudstone–wackestone, lime wackestone, lime wacke-stone–packstone, lime packstone, lime packstone–grainstone,lime grainstone, lime grainstone–rudstone, lime rudstone orrudist biostrome, lime floatstone, lime boundstone, limegrainstone–floatstone, lime packstone–floatstone, lime pack-stone–rudstone, and lime wackestone–floatstone. These car-bonates have been affected by a variety of diageneticprocesses. The rudist-bearing grainstones and rudstonesunites have porosities of >15% and permeabilities of between100 mD and 1 Darcy or more. The depositional environmentof Mishrif Formation in southern Iraq, represented by littoralor lacustrine, intertidal, and subtidal (lagoonal), as indicatedby the studied boreholes. Applying Markov chain analysisbased on statistical methods, to study the transitional patternand to identify the lithofacies association for each environ-ment of deposition, reflects that the Mishrif Formation ischaracterized by two regressive cycles, the main one startingwith basinal or outer shelf environment, slope environmentfollowed by shoal or reefal environment and ending with alagoonal environment. The lateral analysis shows the sameregressive cycles, and by using the lithofacies associationconcepts, we built the depositional model of the MishrifFormation environment.

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