Microfacies analysis and petrographic study of the Mishrif Formation, in selected wells from southeastern of Iraq

Mishrif carbonates formation, is a major reservoir in southeast of Iraq and is one of the principle carbonate reservoir in central and southern Iraq, which is of Late Cenomanian to Early Turronian. A petrographic and depositional Environment study has been carried out by examining available thin sections in the Ministry of Oil for wells (No-1, No-2 (Noor field), Am-1(Amara field) and Hf-1 (Halfaya field)).


Introduction:
The purpose of this study is to identify the facies and depositional Environment of the Mishrif Formation, in selected wells from southeastern of Iraq. The Cenomanian Mishrif Formation is the main carbonate Cretaceous reservoir in southern Iraq, which belongs to the Cenomanian cycle, although this cycle probably ended during the early Turonian. In addition to the Mishrif, the cycle includes the Rutbah Sandstone Formation (mainly found in the Western Desert), the Ahmadi Shale Formation, the Rumaila and Kifl formations (both of southern Iraq), and the Gir Bir, Dokan and Balambo formations (of northern Iraq) [1].
The Mishrif formation is unconformably overlain by Khasib formation (late Cretaceous) at the top and gradationally underlain by basinal limestones of the Rumaila formation at the bottom. The Mishrif was deposited on an unstable platform where environments of deposition were generally shallow water limestones with rapid vertical changes.
The study area is located in the southeastern of Iraq in Missan province to the northeast of the Amara City, Missan Governorate, four wells (Noor-1, Noor-2, Amara-1 and Halfaya-1) have been studied, 2 wells from Noor field, 1 well from Amara field and 1 well from Halfaya field. Figure (1) shows the location of these fields.
This study aimed to study the Petrographic, facies and depositional environments of Mishrif Formation in southeastern of Iraq, Missan province.

1-Regional geological setting
The study area lies within the Mesopotamian basin as shown in Figure (2) according to the tectonic zones of Iraq [2]. The lower contact of the Mishrif with the Rumaila Formation is gradational; the formation is underlain by a deeper water carbonate shelf or ramp deposit, the Rumaila Formation, and overlain by the mid-Turonian unconformity. The top seal of the Mishrif reservoir comprises interbedded shale and deep-

2-Petrography
The petrographic analysis study for Mishrif formation have been carried out , which show that the Mishrif formation consists from skeletal grains (rudist, coral and calcareous algae, Foraminifera, echinoderms, ostracods, and molluscs), Rudist is common in Cretaceous platform, ramp and reef carbonates exhibiting a biofacies suspense to the windward/leeward position and shallowing-up wards cycles [7].
The Rudist is important in the building of the Mishrif formation and are found in two forms, small to large fragments (Plate 1-1, 1-2).
Corals are common in the Mishrif formation, were recognized within the rudist biostrome and as coral fragment associated with other organisms (Plate 1-3).Foraminifera are founded in two types, both benthonic and planktonic Foraminifera are found in the Recrystallization is primarily a change in size or shape of crystals (e.g. increase or decrease in the size of calcite crystals) where no change in mineral composition takes place [10].  No-2, 3881 m). 6-Recrystallization process, (No-1, 3413 m).

4-Microfacies Analysis
The available thin sections for selected wells in southeastern of Iraq have been examined, which shows that the Mishrif succession comprises six paleoenvironments which are:

4-1 Deep marine environment
These facies consist of mudstone to wackestone with small Bioclastic debris and consist mainly from planktonic Foraminifera such as Hedbergella, as well as Spicules and amounts of small echinoderms present (Plate 3-1).These facies exist mainly in the lower parts of the Mishrif Formation.

4-2 Shallow open marine environment
This environment is consist from benthonic and planktonic foraminifera in foraminiferal bioclastic Wackestone and Packstone, as well as, sponge spicules, molluscs, calcareous algae, coral, and echinoderms (Plate 3-2). This environment made up of wackestone with bioclastic debris, echinoderms, and the main diagenesis process exist are Stylolites, dissolution and Micritization.

E90
This Rudist Biostrome thickness observed that increase in the crestal part of the structure of Mishrif formation particularly in the Halfayah field (well Hf-1), and decreasing in the flanks along the axis parts, which indicate on the relationship of the growth of the Rudist Biostrome with the structure.

4-4 Shoal environment
The Shoal facies in Mishrif Formation consider important facies, consists from calcareous algae, Rudist, peloids, echinoderms, and benthonic Foraminifera (Plate 4-2). This environment is characterized by grain-supported and represent high energy level, have good reservoir potential and good porosity which enhanced by dissolution, no argillaceous and Stylolites.

4-6 Lagoonal environment
The lagoon environment exists in the top and middle part of the Mishrif Formation, consist mainly of benthonic foraminiferal wackestone and mudstone to wackestone. The benthonic Foraminifera are abundant and diverse including miliolids, Nezzazata, Alveolinids, and E93

5-Conclusion
This study is of a diagenetic and depositional Environment impact on carbonate rocks quality in the Mishrif formation has been carried out by examining available thin sections in the Ministry of Oil for wells (No-1, No-2 (Noor field), Am-1(Amara field) and Hf-1 (Halfaya field)), the following conclusions may be drawn: -