Depositional Environment and Stratigraphic Evolution of Hartha Formation in Balad and East Baghdad Oil Fields, Central Iraq

Five subsurface sections and a large number of thin sections of the Hartha Formation (age Late Campanian – Early Maastrichtian) were studied to unravel the depositional facies and environments. The Hartha Formation is important as an oil reservoir in Iraq. Petrographic and microfacies analysis of selected wells from Balad and East Baghdad oil fields in Central Iraq, enable the recognition of three main Sedimentary paleoenvironments. These are restricted marine, the shallow open marine environment within the inner ramp, deep outer ramp. The studied Formation represents by two asymmetrical cycles bounded below by sequence boundary (SB1) the contact between Hartha and Saadi Formations. The deep outer ramp facies of the transgressive systems tract continue with the lower part of the Hartha Formation. Two depositional sequences consist of transgressive system tract and high stand system tract, separated by maximum flooding surface. These happened and represented the transgressive systems tract of the cycle that continues until the Shiranish Formation, which overlies the Hartha Formation.


Introduction
The Hartha Formation deposited in Late Campanian -Early Maastrichtian is an important carbonate formation in Iraq due to its petrographic and petrophysical characteristics that make it an oil reservoir in some regions. The Hartha Formation was defined by Rabanit in 1952 from well Zubair-3 in the Mesopotamian Zone in southern Iraq [1].
In central Iraq, the Hartha Formation is divided into upper and lower Members [2]. Sadooni identified five lithofacies based on petrographic and petrophysical characteristics. These include local rudist biostromal carbonates; echinoderm-rich packstones and grainstones with shallow-water larger foraminiferal shoal facies; and peloidal facies separated by deeperwater marly and fine-grained muddy carbonates. The formation is interpreted as an open platform deposition in central Iraq [3].
The lower contact of the Formation is usually an unconformity with Saddi Formation, often marked by a basal conglomerate. The upper boundary in the south of Iraq is conformable, and the Formation is often overlain by pelagic sediments of the Shiranish Formation [4].
The Hartha Formation was deposited in the forereef to shoal environment. Locally lagoonal backreef facies occur around the margins of the Stable Shelf. Reef facies may be present around Khlesia High, as indicated by the reef limestone debris in the detrital limestone beds in the formation in some wells [4].
Al-Zaidy, et al., 2013, [5] studied sequence stratigraphy and reservoir characterization of The Hartha the formation in Ahdab oilfield. Recognized six main facies associations in the studied succession; these are basinal, open shelf margin, fore slope, rudist biostorm, shoal and restricted platform (lagoon), and nine petrophysical horizons.
The tectonic and stratigraphic settings of the Late Campanian-Maastrichtian cycle begins with a widespread transgression and almost cover the whole of Iraq which occurred after the termination [6]. The sediments of the Late Campanian-Maastrichtian cycle were studied due to their importance in the oil industry in both the northern and southern parts of Iraq. Major onlap occurs in Late Campanian -Maastrichtian, the whole of Iraq was submerged by the Maastrichtian (Figure 1). It is characterized by strong westerly onlap on to a regional unconformity [3]. The Hartha Formation is the main lapping unit within the Late Campanian-Maastrichtian cycle. Over most of the Khleisia High and Stable Shelf (Figure 2), it is overlain by the Shiranish Formation [2]. These units pass towards the north and east, and locally within deep half-graben, into Shiranish Formation [3]. The aim of the present study is microfacies analysis to define the depositional environments of the Hartha Formation and stratigraphic sequence development in selected wells from Balad and East Baghdad oil fields.
The studied area is including two oil fields located in central Iraq East Baghdad and the Balad oil field (Figure 3).

Methodology
Five subsurface sections were selected from the wells of two oil fields: Balad and East Baghdad in the middle of Iraq, including East Baghdad: EB-53, EB-56, and EB-102; Balad: Ba-2 and Ba-3 to analyze the microfacies and to determine the depositional environments of the Hartha Formation and stratigraphic sequence ( Figure 3). For this purpose, more than 430 thin sections previously prepared by the north and center Oil Companies, were described, interpreted and were studied petrographically by applying the modified Dunham, 1962 [7] classification by Embry and Klovan, 1971 [8]. Analysis of gamma-ray, density, neutron, sonic, and resistivity logs was used to study the volume of shale, porosity types, and fluid saturation, which was applied. The thickness of Hartha Formation in the studied wells are range from 222.5 to 248 m for the East Baghdad oil field and from 314 to 593 m for the Balad oil field (Table 1).

Results and Discussion
The Hartha Formation is a conformable overlaying by the Shiranish Formation,while the lower contact of this formation is usually an unconformity by often marked a basal conglomerate with Saddi Formation.

Depositional Microfacies
The Hartha Formation carbonates were classified following Dunham's (1962) [7] and Embry and Klovan 1971 [8], into mud-or grain-supported textural types. Moreover, the microfacies descriptions have identified the environments according to Flügel, 2010 [9]. In this study, the Hartha Formation is divided into nine principal microfacies units through all the studied wells depending on the present fossils and lithological characters as a basis for division. The main factor responsible for facies developments and their distribution is relative sea-level frequent fluctuations. It appears that each of the selected wells characterized by definite microfacies units may be of similar or different constituents. Each microfacies unit is discussed in the following:

1-Lime Mudstone Microfacies
Micrite is the main component, which is slightly affected by recrystallization and dolomitization processes(Plt.3-A). Few shell fragments and echinoderm fragments are recognized, generally less than 10% (Plts.1-A and B). The noticed diagenetic processes are the recrystallization of micrite to micro-sparite due to the neomorphsim process, and dolomitization that affected this microfacies due to the existence of the anhedral -subhedral fine dolomite crystals which indicate an early dolomitization. In some parts, fractures are noticed that are caused by the compaction diagenetic process.
This microfacies reflects the deposition of low-energy under a deep quiet water environment. It can extend from basinal to mid-ramp compared with the RMF5 [9].
The presence of Orbitoids in a high number suggested the warm shallow marine (50-100m) or fore reef environment [11]. Therefore, it is concluded that this facies was deposited in the open marine environment in association with the inner ramp. This microfacies correspond to RMF 13 [9].
The abundance of Rotaliida in some cases may reflect the tolerance of hypo salinity or hyper salinity as well as normal marine salinity as a characteristic of the shallow marine environment [12]. This microfacies indicates the deposition in open and restricted marine (inner ramp) facies association compared with RMF 13 [9].
In general, the rate of imperforate foraminifera indicates a restricted depositional environment. Furthermore, whole imperforate foraminifera often dominates near-shore environments in water depths of less than 50m and can live in an environment with extreme temperature and salinity [13]. Miliolida in some cases has been tolerant of hypo salinity or hyper salinity [12]. It's comparable with RMF16 [9].
Many differentiated accumulations of benthonic foraminifera are also found in this microfacies. Dolomitization indicated late diagenesis (Plts. 3-D and 3-G). Moreover, the euhedral crystal of dolomites along the stylolite could be a good indication of late diagenesis, cementation, dissolution, and neomorphsim. Authigenic minerals are the main diagenetic processes affecting various particles as well as the micritic groundmass.
The nature of bioclastic in this microfacies suggests open marine in the inner ramp corresponds to RMF 14 [9].

6-Foraminifera -Bioclastic Wackestone -Packstone Microfacies
It represents one of the common microfacies in the Hartha Formation. It's composed of many types of bioclasts of various sizes such as Mollusca shell fragments, echinoderm plates, calcareous algae, coral, rudist association with differentiated assemblages of benthonic foraminifera such as Orbitoides, Rotaliida, loftusia and omplalocyclus and few peloids (Plts. 1-C and H). These microfacies have been affected by many diagenetic processes (neomorphism, cementation, authigenic mineral (pyrite crystals), chemical compaction (stylolitization), while the euhedral fabric indicates late dolomitization (Plts.3-F and 3-C) Dolomitization is record in both the matrix as a scattered crystals dolomite and allochems. Some genera of foraminifers were indicted to environments such as Nodosaria, which was founded only in normal marine salinity so it could be tolerant of shallow marine also Orbitoides lived in shallow marine. This microfacies belong to the open marine facies in the inner ramp and is correlated to RMF 14 [9].

7-Bioclastic Peloidal Packstone Microfacies
This microfacies is recorded at the East Baghdad oil field and very rare in the Balad oil field. The main constituents are peloids (Plts.2-G and H). Also contain different benthic foraminifers' like Miliolids and Rotalids with many types of bioclastic and shell fragments. Cryptocrystalline carbonate grains are generally smaller than other carbonate particles [13]. Cementation, neomorphsim, dissolution, and micritzation are the main diagenetic processes. The nature of bioclastic in this microfacies suggests very shallow marine water. The low diversity in this microfacies indicates restricted marine of the inner ramp [14]. Furthermore, the packstone texture indicates a relatively moderate energy system. The microfacies is related to RMF 13 [9].

8-Echinoderm Bioclastic Packstone Microfacies
This microfacies is principally composed of abundant echinoderm fragments. Few shell fragments, algae with benthonic foraminifera like Orbitoides, and few peloids are associated as well (Plts.2-A and B). Cementation, neomorphsim and dolomitization may indicate late diagenesis processes (Plts. 3-C and E). The authigenic minerals are affecting this Microfacies as well.
The presence of high levels of fragments indicates moderate energy. This microfacies ranges from mid ramp to inner ramp (open and restricted marine) and correspond to RMF7 [9].

9-Planktonic Microfacies
This microfacies consists of planktonic foraminifera as a main component in different percentages and texture such as Globotruncana sp. and Globogerinelloids sp., associated with small benthonic foraminifera and shell fragments (Plts.2-C, D, E, and F). The abundance of planktonic foraminifera suggests an open marine setting reaching slope of more than 200m and indicating basinal depositional environment [14]. The low energy hydrodynamic regime indicates deposition below the normal wave base [9 and 14]. This microfacies is reflecting a pelagic mudstone-wackestone correspond to RMF 5, [9]. This microfacies represents an environment ranging from a deep marine to basinal environment, and the Planktonic foraminifera are dominant in deep and quiet conditions of open sea environment [15].

Depositional Environments
Three major environments were indicated within the Hartha Formation in the studied wells. These include open and restricted marine belonging to the inner ramp and outer ramp environments. These environments were interpreted depending on the description of texture

Open marine Environment
This environment is recorded in all studied wells. It is characterized by microfacies which are deposited in open marine inner ramp Orbitoides wackestone, Bioclastic wackestone, Foraminifera bioclastic wackestonepackstone, and Echinoderm bioclastic packstonegrainstone. Many important fossils are included in this facies such as benthonic foraminifera, echinoderm fragments, coral, and shell fragment.

Outer ramp Environment
This environment was characterized by planktonic microfacies and lime mudstone microfacies. These microfacies contain mostly small bioclasts and shell fragments, planktonic foraminifera being less abundant.

Conclusions
The following conclusions are indicated from the relationships of the biota, texture, lithofacies, and vertical facies distribution, as well as the interpretation of the logs (Figure 9). 1-The absence of reef building facies, and widespread open marine deposits, are evidence that the Hartha Formation was deposited on a carbonate ramp. 2-The interpretation of microfacies allows for the recognition of nine facies associations that are assignable to ramp environments. 3-The depositional model and the facies model are presented from the inner to the outer ramp, corresponding with the distribution pattern of biota and foraminifera. 4-The facies model shows a depth gradient from: (a) Restricted marine inner ramp of low energy depositional environment, partially restricted hypersaline basin characterized by three microfacies association, which are Miliolids wackestone, Bioclastic Peloidal packstone, and Rotaliida wackestone microfecies. (b) Highly transparent and shallowest part of inner ramp characterized by the association of five microfacies. The paleo-environmental condition is open marine in inner ramp system are characterized by low to high energy richly fossiliferous bed. This carbonate system is represented by larger foraminifera, echinoderm, corals, calcareous algae, and shell fragments. (c) Outer ramp settings with two microfacies association, which are lime-mudstone and planktonic microfacies. Sediments of the outer ramp system are characterized by low energy, a specific type of foraminifera (planktonic), and little bioclastic debris that may reflect all organisms living below the photic zone.