Iraqi

Abstract


Introduction
The Wasia group represents a widespread Cenomanian-Early Turonian carbonate succession in the Arabian Gulf and surrounding areas, including the Mishrif Formation.It is a prolific reservoir of hydrocarbons and hosts many of large oilfields (Aqrawi et al., 1998).The Mishrif Formation has a wide distribution of marine carbonate deposits of great thickness in central and southern parts of Iraq.
It represents a unique architecture of different secondary environments within the carbonate platform (Al-Musawi et al., 2019).The Mishrif Formation in the Mesopotamian Zone forms one of the world's most prolific hydrocarbon reservoirs in this area, such as the oilfield that occur in the Zubair and Tigris Subzones (Idan, 2017;Idan and Faisal, 2019).The Mishrif Formation represents the gradual deposition of shallow marine sediments, southern and central of Iraq (Mahdi et al., 2013).As in Al-Najaf Basin, the formation occurred in a broad shallow-water carbonate platform associated with an intra-shelf depression that is closed to the related Middle Eastern Cretaceous carbonate reservoirs (Murris, 1980 andIdan et al., 2019).The shelf margins were the best locations for the prolific growth of rudist biostromes during the Cenomanian-Early Turonian.In general, these facies form significant petroleum reservoirs in other Cretaceous Arabian Peninsula basins (Harris et al., Burchette and Britton, 1985;Videtich et al., 1988;Burchette, 1993;Aqrawi et al., 1998;and Idan et al., 2015).
The Mishrif Reservoir is composed of relatively shallow environment of marine argillaceous limestone with rudist, coral, algal, and lagoon facies.In the rudist interval that accumulated along the margins of shelf, the main hydrocarbon accumulations are occurred (Aqrawi et al., 2010 andMahdi et al., 2013).In addition to the shoal facies of inner-shelf environments, some fields have producing reservoirs from successions of open marine shallow environments facies.
In the exploration of the Arabian reservoirs, the significance of rudist accumulations is indicated as a significant contributor in the reservoir facies of the Cretaceous strata.The Rudist shoal-reefs are widespread in the Lower-Middle Cretaceous carbonates that span wide areas of the Middle East (Bathurst, 1975).The rudist-bearing have high value of porosities (more than 30%) and high value of permeability, which are greater than 1000 mD, while the thickness of net pay is greater than 150 feet or 50 m.Such the perfect reservoir characteristics, which are characterized by very wide geographic extension, resulted in that the Mishrif is classified as the top one reservoir in Iraq.So, the total proved reserves in the Mishrif reservoir are more than one third of the Iraqi total reserves (Al-Sakini, 1992).
The formations that equivalent to the Mishrif Formation are the upper part of the Magwa Formation in Kuwait (Owen and Nasr, 1958).While the Sarvak Formation in Iran (Jassim and Goff, 2006), the lower unit of the Judea in central and northeastern Syria and to the Mardin Formation in southeastern Turkey.This study focused on the Mishrif Formation in the studied oilfield, located 35km southwest of the Basra city that is one of the biggest field's city in Iraq (Idan et al., 2019 andAl-Khafaji et al., 2021).The gamma-ray log was used for estimation the shale volume in the Mishrif, as a result, shale volume relative to the bulk volume was approximately about 19%, and an increasing volume was clear in the upper reservoir, reaching the highest value in the MA and MB11 units (Aziz et al., 2022).
According to litho-geochemistry profile, porosity and shale content which is constructed by Al-Mimar et al. (2019), the Mishrif Reservoir is divided into eleven subzones which includes reservoir and non-reservoir units.The main objectives of the study are studying the diagenetic processes development in the Mishrif Formation and the effects of these processes on the petrophysical properties in term of total porosity and clay content depending on the thin sections and well logs interpretations.The well logs include porosity logs (sonic, density and neutron) as well as the gamma ray log.

Location of the Study Area
The study area is located in the Basrah, southern Iraq, 20 km south west of the Basrah city as illustrated in Fig. 1.

Geologic and Stratigraphic Setting
The study area is flat and contains buried longitudinal structures differing size below the Quaternary cover and separated by broad synclines.The fold structures mainly with north -south trend, its effect appear on the pre-Tertiary period layers, which are caused due to the north -south and northeastsouthwest faults of the crystalline basement.These structures are probably induced by diapiric salt caused by the infracambrian Hormoz salt series which is believed to underline parts of southern Iraq.The association of negative gravity residuals with the main structures such as Zubair and Nahr Umr confirm these salts (Ditmar, 1971).The type section of the Mishrif Formation was described as heterogeneous carbonate succession of argillaceous limestone.This description was given by Rabanit (1952), who first described the formation in the Zubair oilfield, southern Iraq within Zubair-3 well (Buday, 1980).The Mishrif Formation consists of two main sedimentary stages that are rapidly capped by the unconformity between the former and the Khasib Formation (Jassim and Goff, 2006;Al-Ameri et al., 2013;and Faisal et al., 2016), while the lower contact is gradual with the Rumaila Formation (Worthington and Cosentino, 2005).The Mishrif Formation in X oilfield was divided into three primary units by using the Geolog software version 2020.The main reservoir units are mA, mB1 and mB2 which are characterized by good porosity and permeability which then subdivided into secondary units based on their petrophysical characterizations (Al-Eissa and Al-Shahwan, 2021).The stratigraphic column of the study area is illustrated in Fig. 2.

Laboratory Work
The Diagenetic processes were carried out along the selected wells X-46, X-41 and X-42 in the X oilfield.Petrographically, more than 100 thin sections were prepared to evaluate the effects of the diagenesis processes on the petrophysical properties of the reservoir.

Software Programs
The Petrel software (version 2009) is used to interoperate the porosity logs (Neutron, density and sonic) and gamma ray log to determine the petrophysical properties of the Mishrif reservoir.

Diagenetic Processes
Diagenetic processes refer to all the changes that affected the deposits by different factors which include chemical, biological and physical factors without reaching the stage of metamorphism (Flugel, 1982).These processes considered as very important processes due to their effect on the primary textures of the carbonate beds.In addition of its effect on the reservoir properties by adding or destroying porosity and permeability.The studying of the diagenetic processes of the formation involving definition the major diagenetic processes and description the diagenetic features and textures and their relation to the diagenetic environments.The primary diagenetic processes that affecting the Mishrif in the study area include micritization, neomorphism, cementation, dissolution, dolomitization and pressure solution or stylolite.

Micritization
It is the most abundant process affecting the biological, as well as rock fragments within the studied microfacies.Micritization is an initial diagenetic process due to the fungal activity, which micrite envelopes the skeletal grains shortly after deposition (Plate 1-A).This process formed a crust enveloped the bioclastic fragments like the echinoderms, rudist, and other skeletal grains (Plate 1-B).In some intervals the structures of these grains are mainly destroyed and could not be recognized.The micritization is a destructive process on the porosity and permeability of the Mishrif rudist as seen in the thin section of well X-42 at the depth of 2270 m (Plate 1-A).
Plate 1. Thin section of micritization processes A) surrounding grains; B) valve of Ostracoda at the depth of 2270m in well X-42

Neomorphism
The neomorphism process is dominated in the middle and upper parts of the Mishrif Formation that effects on the skeletal grains making them difficult or impossible to identify while in the rudist shells the neomorphism transformed the aragonite to calcite particles (Plate 2-A).In addition, Neomorphism affected the micrite particles altering it to microsparites, pseudosparite or sparite particles (Plate 2-B).This process is related to meteoric condition (Longman, 1980) and its occurrence increase in humid conditions (Bathurst 1975).Neomorphism affected the bioclasts, as well as skeletal grains and micrite.
Neomorphism is not highly affected the quality of the reservoir intervals but little effect is recognized at the depth of 2435 m within the rudist intervals in the well X-46 (Plate 2b).
Plate 2. Thin section photograph of Neomorphism A-large benthic foraminifera; B-microspar to pseudospar at the depth of 2435, Well X-46

Cementation
Cementation is the most important process that affected the carbonate reservoirs.Cementation has resulted the deformation of primary, as wells as secondary porosity that take place in various intervals and microfacies.This process is filling intergranular and intragranular voids and fractures.It is thought that are of relatively late origin.The rudist intervals of Mishrif Formation is highly affected by Cementation, which is represented the highly destructive process on the reservoir intervals quality especially at the intervals of 2375m, different types of cement are identified like druzy mosaic cement, syntaxial rim cement and blocky cement as illustrated in plate 3-A, B, C, and D respectively (After Abbas and Mahdi, 2020).

Compaction
The compaction due to the high thickness of sedimentary strata (more than 1km) destroys the porosity (Flugel, 2010).Compaction appeared significantly in grain-supported microfacies that made them oriented in the internal structure (Plate 5-A).Compaction on the other hand, can also roles as an important agent in increasing porosity and as a result permeability by creating fractures and/or microfractures in the formation.The porosity of the rudist intervals is appeared affected by the compaction which reducing the pore space by changing the grains geometry.
Plate 5. Thin section photograph of A) the orientation property in the internal structure sutured; B) Stylolite seam with low peak amplitude at the depth of 2465, well X-41

Pressure dissolution (Stylolization)
The stylolite is a thin interval of discontinuity within the bed developing from chemical compaction or pressure dissolution due to the overburden pressure and/or tectonism.In the thin sections, they have represented by sutures of Zigzag; in general, they are consisting of columnar to conical projections with intervening depression.These structures vary in size from contacts of sutured microscopic (microstylolites) to stylolite that have lengths of several meters.Stylolization within the Mishrif succession was almost characterizing the upper part and represented by high amplitude stylolite while the middle and the lower part of the succession that were dominated by peaks of low amplitude and irregular stylolite in addition to hummocky stylolite (Plate 5-B) particularly in rudist bearing intervals.Stylolite enhances the reservoir intervals, which act as conduits to the moving fluids.

Dissolution
The dissolution is the most important process that effect the Mishrif Formation because it is responsible of forming the vuggy, channel and moldic porosities and enhancing the pre-existing interparticle and intraparticle primary porosity making the Mishrif is one of the most giant reservoirs in the southern of Iraq.The dissolution affecting the Mishrif succession in most intensive within the shoal grainstone and rudstone facies in addition to the upper part of the Mishrif succession below the upper unconformity surface (Choquette and Pray, 1970;Lucia, 1995).Dissolution generates the all types of the secondary porosity in addition to dissolve the cemented intervals (Plate 6-A).This process enhances the porosity by not only creating secondary porosity, but also enlarging and touching the separated voids by making conduits among the structure of the formation (Plate 6-B).

Porosity logs
The porosity logs include the logs, which studied along the X-46, X 42, and X 41 wells, which are collaborated with the diageneses results to determine the petrophysical properties.The porosity logs include Neutron, Density, and Sonic logs in the X-46 well the porosity results which indicated by the sonic logs show a wide variation, the primary porosity is decreased in the well X-46 at the depth of 2300-2400 due to the cementation and compaction while the secondary porosity, which indicated by the neutron, density and sonic logs is enhanced due to the dissolution and fractures.A little enhanced is observed at the depth of 2400-2450 due to the effect of diageneses process like solution and fracturing.In well X-41, at the depth of 2400-2475m there is an increasing in the secondary porosity due to the increasing of grains to clay ratio as well as the effect of diagenesis processes such as dolomitization and solution in addition to the nature of the pore-throat associated with the rudist facies.while in the X-42 at the depth of 2215-2300m, the primary porosity is very low due to cementation, compaction and micritization affects as well as the medium content of shale.In general, the porosity is enhanced in the intervals of rudist zone (The lowermost units).The results of porosity are presented in Figs. 5, 6 and 7.

Gamma ray log
The formation radioactivity is used to determine lithology.Natural radioactive elements include the elements of uranium (U), thorium (Th), potassium (K) and radium (Ra), which are associated with the clay minerals that are undesirable in the oil reservoirs.The standard formula for calculating shale volume from a corrected borehole gamma ray log is depended on (Hassan et al., 2021).
Shale volume (VSH), in the oil reservoir is computed depending on gamma ray log.The volume of shale is best estimated by gamma ray logging measurement that respond primarily to shale.In the studied well the shale content is recognized in different intervals, in well X-46 medium shale content is recognized at the depth of 2300-2335m indicated marker shale bed which lowering the effective porosity of the reservoir.In the well of X-42 a low content of shale is recognized, the Mishrif Formation in this well seems clear except at the depth of 2175-2200 (MA unit, seal unit) but the porosity is low due to the compaction and cementation.In the well of X-41, the low content of shale volume is determined in the interval of 2400-2475 with high porosity due to the nature of reefal zone and the diagenesis effect of dissolution and dolomitization and high shale volume is determined at the depth of 2300, 2375, 2400 m (Figs 5, 6 and 7).

Discussion
The Mishrif Formation consists of major carbonate reservoirs in southeast Iraq, with 32 structures (Awadeesian et al., 2019, Awadeesian et al., 2018).Rumaila, as well as the basal part of Mishrif Formation were deposited in a transgressive phase in a relatively deep-water environment that belongs to the Early Cenomanian age.While the lower regression phase deposited in the eastern parts of the Mesopotamian Zone.Microfacies analyses and the depositional environments were utilized to produce the diagenetic model of the Mishrif Formation.These analyses indicate Mishrif succession consists of two main regression phases.The first phase began with deposition of the outer shelf and/or deep basin environments.These deep environments graduated to the relatively shallower slope facies associations and finally, the intermittent rudist patch reefs alternated with the high-energy shoal environments in multi-story successions (Mahdi and Aqrawi, 2014;Al-Musawi et al., 2020 andIdan et al., 2020).This multi-story terminated with inner shelf, low-energy, mud-dominated lagoon environment facies.These mud-supported facies acted as a cap rocks to the underneath reservoir intervals.
As illustrated in Fig. 4, the limestone of the Mishrif Formation was influenced by the early and late stages of diagenesis.Marine phreatic and late diagenetic processes are principally altered mudstones and wackestone.The lack of pyrite crystals as well as calcite cements that fill the skeletal particles and the notable planktonic foraminifer's activity are the main features of digenesis in these microfacies.Clear rimmed and cloudy centered dolomite crystals and stylolite are the main activity of the late, deep burial diagenetic stages.These fabrics and structures are representing the mud-supported facies of the lower parts of Mishrif Formation and the gradually related underlying Rumaila Formation (Aqrawi et al., 1998).
Meteoric phreatic and mixing zones are the main environments that indicted from the features of the Mishrif microfacies analysis, which this alteration occurred underneath the shallow depositional surfaces.The dolomite rhombs of the mud-supported facies are referring to the mixing zone environments that located in the intertidal flat.While the dolomite microcrystals may indicate the brackish-water diagenetic environments (Badiozamani et al., 1977;Aqrawi, 1995).Local meteoric phreatic diagenesis is indicted from the over-growth of equant calcite and syntaxial cements on the echinoderm's fragments (Longman, 1980).In these diagenetic environments, the development of vuggy, channels, and mold porosity are occurred, precisely in the rudist-bearing and rudstone facies.In the current study porosity is enhanced in the reefal zones due to the nature of pore throat and the effect of some diagenesis processes.

Fig. 1 .
Fig. 1.A) Regional location of the study area in Iraq; B) enlarged of the study area, after (Al-Zubaidi and Al-Neeamy, 2020)

Plate 3 .
Thin section photograph of A) Druzy mosaic cement; B) Syntaxial rim cement; C) Blocky cement, and D) Syntaxial rim cement enveloping rudist and bioclasts at the depth of 2375, well X-46 5.1.4.Dolomitization Two types of dolomitization were recognized within the Mishrif succession formed by different mechanisms, late local source dolomite as indicated by Plate 4-A.In addition to early mixing dolomite that particularly with the formation of secondary intercrystalline micro-voids within mud-supported microfacies, dolomitization has enhanced the porosity and reservoir quality of some intervals of the rudist bearing intervals at the depth of 2450 m (Plate 4-B) (After Abbas and Mahdi, 2020).

Plate 4 .
Thin section photograph showing A) micrite with intensive dolomitization; B) early mixing dolomite at the depth of 2450, well X-41

Plate 6 .
Thin section photograph, show dissolution and the resulted pores at the depth of 2445, well X-46 5.1.8.Paragenesis As indicated by Fig. 3, micritization and neomorphism represented the earlier stages of diagenesis in the Mishrif Formation, as well as, the dissolution that extended toward the intermediate stages, especially to that, which related to the upper exposure surface.Cementation effected the Mishrif succession directly after deposition in the early stages, extended intermediate stages, which represented by the early syntaxial rim cement and the intermediate-late druzy cement and granular cement.The major effect of the massive mixing dolomite represented an early to intermediate stages followed by the latter effect of the local source dolomitization occurred laterally after burial and consisted dolomite cement and cloudy center-clear rim dolomite.Different types of pressure solution (Stylolization) are representing the deep burial diagenetic stages.