Facies Architectuer and Stratigraphic Evolution of Campanian Succession in Balad, East Baghdad and Kifl Oil Fields

Abstract


Introduction
The Late Campanian -Maastrichtian sequence is an important succession in the stratigraphic evolution of North Arbian Plate (Iraq).This sequence started with a widespread sea level rise and almost covered the whole country which had been took place after the termination of this cycle (Budy, 1980).
The sediments of the Hartha Formation was deposited during the Late Campanian Stage (Bellen et al., 1959).Due to the importance of this sequence in terms of stratification and economics in the oil industry, it has been focused on in this study.
The Campanian succession three oil fields in central Iraq within the Mesopotaminan Zone which are East Baghdad, Balad and Kifl oil fields were selected to the current study.
East Baghdad Oil Field represents a group of oil fields, which lies in the middle of Iraq located east of Baghdad, far about 20 Km from the center of Baghdad to the east, the whole south area of East Baghdad project is 1201 Km 2 contains three parts (S1, S2 and S3) ( OEC, 2006).The East Baghdad Oil Field covers in length about 120 km and about 25 km in width, with general direction NW-SE (Fig. 1).Balad Oil Field is one of Iraqi oil fields where 9 wells were drilled, which located in the central Iraq along the Tigris River within Salah Al-din Povince about 70Km to the north of Baghdad (Fig. 1).Kifl Oil Field is sited in south of Baghdad to the southwestern Hilla City about 35km, and lies within the Mesopotamian Zone near the east boundary with Stable Shelf.This area located between Najaf and Karbala governorates to the west side of the Euphrates River, and to the northwest Razaza Lake. Al

Field Work
Sampling and description of selected boreholes from three oil fields, picked about 190 rock samples from the cuttings and core (Table 1).

Laboratory Work
• Preparing a thin section for the cuttings and core samples.
• Petrographic description of the prepared thin sections by transmitted light microscope.
• Interpreting available well logs and their responses to identify microfacies and stratigraphic changes.
Table 1.Succession interval and thickness with number of sample and well log types.

Geological and Tectonic Settings
This interval of time was considered by many authors to be of great importance for the paleogeographic evolution of the Middle East, with special interest in Iraq.Assigned the beginning of this widespread transgressive cycle to the termination of the Subhercynian movements (Late Cretaceous) in the inner parts of the Suture Zone, and with the general shifting of the sedimentary basins towards the Neo-Tethys (Buday, 1980).In the Late Campanian to Early Maastrichtian time Neo-Tethyan ophiolite was thrusted further into the northern part of Arabian Plate, uplifted above the sea level, and highly rates of them were eroded.The products of erosion were deposited as flysch sediments with thickness more than 2000 m in a narrow foredeep basin along the northern and eastern parts of the Arabian Plate extending from southeast of Turkey through northeast of Iraq and southwest of Iran and Oman.The extensional basins was strongly subsided during the Late Campanian to Early Maastrichtian.These subsidence areas are including the Euphrates, the Azraq -Sirhan Grabens and the extending to the E-W direction at Anah and Tayarat grabens.These basins had originally generated during the rifting stage in early Late Cretaceous Period (Jassim and Buday, 2006).The Campanian to Maastrichtian section comprises the Hartha Formation and the Shiranish Formation in southern and central Iraq (Fig. 2).This succession was started with a major sea level rise (transgression), and ended with the tectonic uplifting stage and sea level fall (regression) (Sharland et al., 2001).Many studies had been done to classify the lithologic units which deposited during this period depending on microfacies and biostratigraphy as well as it's paleogeography position.The more importance of these studies was Budy's Devision (1980), which merged these units according to resemblance lithofacies and paleogeography position and subdivided these rock units into groups depending on it's depositional conditions and environments as following: Hartha Facies: which deposited in fore reef environment and may be classified into: Shiranish Facies: which deposited in off-shore environment and may be classified into: • Shiranish Formation.The Hartha succession was diagnosied at the first time in Zubair Oil Field by Rabanit (1952), in Bellen et al.(1959).This succession is included of 200-250m in thickness of detritus bioclastics with glauconitic limestone and interbeding with green or gray shale.In some area the limestone was dolomitized (Bellen et al., 1959).The succession was deposited in the greater parts within a marginal marine, fore reef, neritic marine and shoal environments, while the back reef facies are identfed in local area.The lower boundary of the succession with Saa'di Formation is unconformable surface and often is marked by the precence of conglomerate beds (Buday, 1980).In other areas the limestones are appeared strongly dolomitized with chalky limestone beds occur frequently.The oolitic beds were diagnosed in some oil wells with appearence nodules, lenses and thin beds of anhydrite often occur in the succession along the southwestern part of the Mesopotamian Zone.Argillaceous limestones appeares more frequently in the eastern parts of this zone where the succession passes into the Shiranish succession (Fig. 3) (Aqrawi et al., 2010).To the northern part of Iraq, the Hartha succession is including parts of the Pilsner Formation.Bellen et al. (1959) mentioned that the Pilsner facies comprising of organic detrital limestones, argillaceous limestone with fossil debris, and medium grained of dolomite.Evapotritic and limestone beds bearing oolitic are identified in the Hartha succession between the Ramadi and Makhual areas.In northwestern part of Iraq, the oolitic limestones was appeared (Ditmar, 1971).This facies refers to deposition in lagoon environment within the middle part of the Hartha carbonate platform to the west side of the Mesopotamian Zone (Jassim and Buday, 2006).

Microfacies Analysis
More than 190 thin sections which include cores and cuttings for the Hartha Formations from 3 boreholes (EB-55, Ba-2 and Kf-1) were used to explain the different depositional environments in the studied formation.The microfacies analysis is a good parameter to describe the sedimentary framework related to the basin development in the studied oil fields.The depositional environments can be explained in terms of physical, paleontologic, chemical composition, or geomorphic variables (Reineck and Singh, 1973).Seven major microfacies are diagnosed in the Hartha succession within East Baghdad, Balad and Kifl Oil Fields (Figs. 4,5 and 6); their characteristic skeletal and non skeletal grain types and the texture enabled to recognition of the paleoenvironments according to Dunham (1960), Wilson (1975) and Flügel (2010).

Microfacies B: Orbitoidal and Miliolids Wackestone
This microfacies is composed mainly of Orbitoides and miliolids, with bioclasts of mollusks, echinoderms, pellets and calcareous algaes.This represents the deposition in semi-restricted marine environment (Plt.1.2).In general, the rate of imperforate foraminfera indicates that depositions took place in a restricted environment.Furthermore, whole imperforate foraminifera often dominance near the shore environment in water depth less than 50m and can live in environment with extreme temperature and salinity (Murray, 1973;Flügel, 2004).Miliolid in some cases having been tolerant of hyposalinity or hypersalinity (Jones, 2014).The microfacies is comparable with RMF16 of Flügel (2010), where it was appeared in Kifl Oil Field within interval 925-965m only .

Microfacies E: Peloidal and Bioclastic Packstone and Grainstone
This microfacies is characterized by well rounded grains, these are inferred from their biform grain sized and ovoid shaped, with the dominance of relatively coarse to fine sized (sand sized) with moderate sorting peloids (Plt.1.4).The microfacies is related to RMF 26 which deposited in shoal environment, where it was observed in EB-55 (1705-1717m) only (Fig. 5).

Microfacies F: Gypsum and Anhydrite Mudstone
This microfacies is composed of gypsum and anhydrite with dolomitized mudstone which deposited in a lagoon and peridital environment (RMF 25).The depositional environment become more shallow (lagoonal) which the gypsum and anhydrite were deposited during the development of Hartha basin (Plt.1.6).This microfacies observed in kifl Oil Field within intervals 1060-1120m and 965-1000m (Fig. 6).

Microfacies G: Planktonic Foraminiferal Wackstone to Packstone.
This microfacies contains planktonic foraminifera such as Globogerinelloids sp. and Golobotruncana sp. with the presence of other small benthic foraminifera.This microfacies is representing the deposition in environments ranging from an open shelf to a deep open marine within outer ramp (Plt.1.7).

Stratigraphic Development
The stratigraphic sequence is classified the sedimentary basin sediments into genetic packages which bounding by unconformity surfaces and their correlative conformity surfaces.The stratigraphic sequence term is used to provide a chronostratigraphic framework for mapping and correlating sedimentary facies and stratigraphic predictor (Emery and Myers, 1996).Sequence stratigraphic approaches were based on the sequence boundaries, system tracts, and the maximum flooding surfaces, keeping in mind, cycles during transgressive-regressive in vertical stacking patterns as a result of the seal level change vs. sediment supply.Wire line logs combining sonic and gamma ray logs were used for identifing these sequences.
The studied succession in Ba-2 represents one 3 rd order cycle with three 4 th order cycles (A1, A2 and A3).These cycles are asymmetrical and starts with cycle A where the open marine facies in Lower Hartha represents the transgressive systems tract (TST) underlying by a transgressive surface (TS) which coinside with the unconformable surface (SB1)with the underlying the Sadi Formation.The highstand systems tract of cycle A1 is reflected by the open marine facies of the Hartha Formation, A2 is reflected by the open marine and mid ramp facies represent thick TST and then appear the highstand systems tract (HST) which reflected by the open marine facies bounded by a TS reflected mid ramp and outer rampfacies in TST which represents A3 (Fig. 7).
In Eb-55 this succession represent three 4 th order (A1, A2 and A3).These cycles are asymmetrical and starts with cycle A where the mid ramp facies in Lower Hartha represents TST underlying by a TS which coinside with the unconformable surface (SB1) with the underlying Saa'di Formation.The highstand systems tract of cycle A1 is reflected by the open marine facies of the Hartha Formation and then recurrence the highstand systems tract where the restricted and open marine are appeare, A2 is reflected by the open marine and mid ramp facies represents thick TST and then appears transgressive systems tract which reflected by the open marine facies, and then appears the HST bounded by a TS reflected shoal and mid ramp in TST which represents A3 (Fig. 7).
The studied succession in Kf-1 represent two 4 th order cycles (A1 and A2).These cycles are asymmetrical and starts with cycle A where the mid ramp facies in Lower Hartha represents the TST bounded below by a TS which coinside with the unconformable surface (SB1) with the underlying Saa'di Formation.The highstand systems tract of cycle A1 is reflected by the open marine, restricted and lagoon facies of the Hartha Formation and then recurrence the highstand systems tract where the open marine, restricted and lagoon are appeared, A2 is reflected by the restricted, open marine, mid ramp, outer ramp and basinal facies represents thick TST which appears in lower part of the Shiranish Formation (Fig. 7).

Conclusions
Seven major microfacies were identified in the Hartha Formation in East Baghdad, Balad and Kifl oil fields; these microfacies are recognized according to their characteristic of skeletal and non-skeletal grain types and texture enabled to explain the paleoenvironments.These microfacies were deposited in lagoon, restricted, shallow open marine, shoal, mid and outer ramp.
The studied succession represents one 3 rd order cycle which divided into three 4 th order cycles (A1, A2 and A3).These cycles are asymmetrical and starts with cycle A where the open marine facies in the -Zaidy et al. (2013) studied stratigraphic sequence and petrophysical properties of The Hartha Formation in Ahdab Oil Field.They recognized mainly six associations facies in the studied sequence; they are restricted platform (lagoon), shoal rudistbiostorm, foreslope, open shelf margin and basinal, in addition to nine petrophysical horizons.Abd Aoun and Mahdi (2020) construct a two dimentional geologic model for Hartha succession in Majnoon Oil Field by using well logs data form eight wells.And during the same date studied Homadi and Al-Zaidy (2020) the carbonate microfacies and explained the development of Hartha basin in East Baghdad Oil Field.Albeyati et al. (2021) concluded that the Hartha Formation has the most promising hydrocarbon reserves in Ba-1 and Ba-4 wells based on integration of petrographic image analysis, well logs, and core analysis.The aims of the present study are identfing the microfacies of Hartha succession and stratigraphic sequence development in selected wells from Balad, East Baghdad and Kifl oil fields.

Fig. 1 .
Fig. 1.Base map showing location of the study area with the tectonic position according to Fouad (2012).

Fig. 7 .
Fig.7.Stratigraphic cross section showing the facies distribution and sequence cycles for studied oil fields.