Facies Analysis and Paleoenvironmental Reconstruction of Ghar Formation in Al-Muthanna Governorate, Southern Iraq

Abstract


Introduction
The Mesopotamian Basin of southeast Iraq hosts important hydrocarbon reservoirs within the Oligocene-Miocene succession (Asad and Hamd-Allah, 2022;Abdul Majeed, 2023).However, fewer studies are done on the Oligocene-Miocene formations compared to the carbonate reservoirs of Cretaceous formations (Handhal et al., 2018 and2020;Al-Humaidan et al., 2022).The Ghar Formation of the Miocene age (Bellen et al., 1959) was mainly deposited in the Mesopotamian Basin and exposed in the Early Miocene (Goff et al., 1995).The Asmari Formation consists of a thick succession of platform carbonates, including the Ahwaz sandstone member correlated with the Ghar Formation (James and Wynd, 1965) (Fig. 1).
The type section of the Ghar Formation is located in Zubair-3 Well, southern Iraq.It consists of 422 feet thick interbedded units of sandstone, sandy limestone, streaks of gypsum, and shale (Owen and Nasr, 1958).According to Alsharhan and Nairn (1995), the siliciclastic influx from the western Arabian Plate was responsible for the deposition of Oligocene-Miocene siliciclastic formations.The high rate of sediment supply affected the carbonate platform through an easterly directed wadi system.This was observed in the Oligocene-Miocene formations of southern Iraq (Al-Mishkilet et al., 2023).
Ghar Formation was deposited in marginal marine (partly fluvio-deltaic) environments (Jassim and Goff, 2006).The siliciclastic sediments of the Ghar Formation sourced from the uplifted areas of the Arabia plate were a result of the Red Sea rifting during the Late Oligocene-Early Miocene (Al-Juboury et al., 2010;Khalaf et al., 2019).Al-Juboury et al. (2010) studied the Ghar Formation in southern Iraq, near Kuwait, and it was concluded that the formation consists of alluvial to shallow-marine and paralic facies.Khalaf et al. (2019) suggest that thin beds of carbonates within sandstone succession proposed a short-term marine effect interrupted by a fluvial environment.The deposition was episodic and remarkably affected by the tectonic movements related to particularly Red Sea rifting.Therefore, it is postulated that the Ghar Formation was deposited in a fluvial braided system along the paleo-shore line.The main aim of the current study is to understand the depositional paleoenvironments of the Ghar Formation by using a combination of field, petrographic identification, and microfacies analysis.Jassim and Goff, 2006;Du et al., 2022)

Geological Setting and Stratigraphy
The Ghar Formation outcrops only in the inner platform, within the Western and Southern Deserts (Sissakian and Fouad, 2012;Sissakian et al., 2016).The formation interferes with the Euphrates Formation south of Al-Najaf and Al-Nasiriyah areas (Sissakian et al., 2016).The research area is located within the Stable Shelf of the Arabian Plate (Jassim and Goff, 2006).In the present work, the Ghar Formation consists of sandy limestone, sandy dolomitic limestone, dolomitic limestone, calcareous sandstone, and sandstone.The upper boundary of the Ghar Formation with the Nfayil Formation is conformable, and this is identified by the presence of calcareous sandstone at the top of the upper unit of the Ghar Formation and the first appearance of green marl at the bottom of the lower Member of Nfayil Formation.Nfayil Formation is equivalent to the lower Fars Formation (Sissakian et al., 1997).The lower boundary with the underlain Dammam Formation is unconformable, and it is distinguished by the occurrence of dolomitic limestone at the top of the upper member of the Dammam Formation.

Location
The project area is located in the southern desert of Iraq, within Al-Muthana Governorate (Fig. 2).Four outcrops and two boreholes are selected with the coordinates shown in Table 1.

Materials and Methods
The lithofacies of the Ghar Formation are described at four outcrops and two boreholes (Fig. 3), and the results are presented as lithologic sections (Figs.9 to 14).A total of 40 samples of the lithofacies were selected.A petrographic study of thin rock sections was done using a polarizing microscope.The mineralogy of selected samples was identified by using chemical analysis and X-ray diffraction (XRD).
The XRD analysis was done in the Ministry of Sciences and Technology laboratory to identify the mineralogical composition of ten samples.Three samples were studied with a Scan Electron Microscope (SEM) in Al-Khora Company (Baghdad) to identify the surface topology of clay minerals.The carbonates rocks of the formations are classified based on Dunham (1962) classification depending on the depositional textures of the rocks, while the sandstone rocks are classified after Pettijohn et al. (1972) depending on texture that is whether the sandstones are composed of grains only, the arenites, or contain more than 15% clay matrix, forming the wackes (Fig. 8).The depositional environments of the carbonate facies are interpreted based on Flugel (2010) models.

Mineralogy
The result of the XRD analysis shows that the major minerals in Ghar formation are quartz, calcite, and dolomite, whereas feldspar and gypsum are the minor minerals.Clay minerals such as Palygorskite and Montmorillonite were also identified.The high content of clay minerals is recorded in the lagoonal facies of the Oligocene-Miocene formations (Turki and Awadh, 2022).
The chemical results show that the average of SiO2 is 31.8%,and its range is between 0.79% and 91.19%.The highest percentage of silica was found in sandstone rock in core BH3.The average of Al2O3 is 1.09%, ranging from 0.13 to 5.84%.The average of Fe2O3 is around 0.46% and ranges from 0.07% to 1.9%.Sulfate oxide shows a high concentration of 7.39% in sandstone rock and a low concentration of 0.03% in carbonate rock, with an average of about 0.86%.Besides that, CaO, MgO, and the loss on ignition (LOI) illustrate significant distinctions between their values in carbonate rocks and sandstone rocks.The average CaO is 30.63%, and it ranges between 2.78% and 45.56%.The average of MgO is about 5.29%, ranging from 0.25% to 15.28%.The average LOI. is 29.50%, ranging between 3.18% and 45.29%.Most of the LOI is CO2 gas that comes from burning carbonate in rocks.Changes in their chemical concentrations obviously reflected the variation of rocks in this formation.For example, silica, iron, and aluminum elements could indicate continental deposits, called terrestrial detritus elements that enrich clastic rocks or sediments (Morelli et al., 2012).Therefore, this formation is more likely to be deposited in a mixing environment, possibly of deltaic origin, ranging from delta top to inner shelf conditions.

Facies Associations of the Ghar Formation
The rocks of the Ghar Formation are the only non-pure marine sediments within the Early Miocene sequence.It is subdivided into three facies associations: siliciclastic, mixed, and carbonate facies associations.In its major part, the formation represents a transitional environment from continental to marine conditions, possibly of deltaic origin.

Siliciclastic facies associations
• Distributary channels facies associations: It includes sandstone, calcareous sandstone, and laminated sandstone.1. Sandstone (Quartz arenite) facies The rocks belonging to this facies are grayish white and medium tough.This facies contains quartz (90%) and feldspar (trace), including orthoclase cemented with gypsum (10%) (Fig. 5A&B).The absence of marine fossils is remarkable in this facies.The quartz grains are mostly monocrystalline with unit extinction, and few of them display undulose extinction (Fig. 5D), rarely polycrystalline (Fig. 5C), mostly subrounded to rounded, subhedral to anhedral in shape and ranging in size from very fine to coarse (0.1-0.8) mm.Polycrystalline granules in sandstone show sutured internal boundaries between crystals reflecting a metamorphic origin (Blatt, 1967).This facies contains subhedral and subrounded fine to medium feldspar such as orthoclase (Fig. 5 E&F).The low percentage of unstable grains (feldspars) and the dominance of monocrystalline quartz indicate long-term intense chemical weathering with a warm humid climate that affected the source area (Amireh, 1991).

Calcareous sandstone (Quartz arenite) facies
The rocks are whitish-gray and medium tough.In thin section, this facies consists of quartz (75 -85%), feldspar (1% -2%), rock fragments (2% -3%) cemented with calcite (12% -20%) (Fig. 6A &B).The quartz grains are mostly monocrystalline and rarely polycrystalline, subhedral to anhedral, have subrounded outlines, and range in size from medium to coarse (0.25-0.52) mm.The polycrystalline grains are mostly of sedimentary origin (Fig. 6C).Rock fragments are subrounded to rounded, ranging in size from fine to coarse, and are composed of carbonate and clay.Previous studies documented such facies (calcareous sandstones) to have been deposited in braided fluvial channels in the distal parts of the braid plain (Du et al., 2022).

Laminated Sandstone (Subarkose) facies
The rocks of this facies are white and medium tough.Microscopically, this lithofacies contains sand grains, including quartz grains (70%-80%), feldspar (10%-12%), and rock fragments (3%-4%) (Fig. 7).Montmorillonite is also present in trace mount admixed with the groundmass.The quartz grains are anhedral to subhedral and slightly euhedral in shape with rounded outlines.They are mostly monocrystalline and range in size from very fine to fine sand (0.08 -0.15) mm.Feldspar grains are present in trace amounts consisting of orthoclase.The rock fragments include clay.Their size ranges from very fine to medium sand (0.07 -0.26) mm.The thin-bedded planar sandstone indicates deposition within intermittently flooded broad, shallow channels (Fedo and Cooper, 1990).

Mixed facies associations
mixed siliciclastic-carbonate facies contain very fine to medium-grained quartz sand, minor feldspar grains, and admixtures of dolomitized carbonate material.These mixed sediments were formed where carbonate platforms and basins are close to source areas of terrigenous supply (Tucker, 2003).

• Shoreface facies associations
This association includes: 1. Sandy dolomitic shelly wackestone These facies contain bioclasts (10%), quartz grains (30%), and minor amounts of feldspar (orthoclase) embedded in a groundmass consisting of micrite, which affected by dolomitization (Fig. 15A).The quartz grains are anhedral to subhedral and slightly euhedral in shape with subangular to subrounded outlines, mostly monocrystalline, ranging in size from very fine to medium sand (0.06-0.25) mm.The observed shell fragments are gastropods and pelecypods.The main diagenetic processes affected by these facies are dolomitization, dissolution, and porosity development (biomolds and intercrystalline).Then, cementation by blocky secondary calcite (Fig. 15B).The very fine to medium quartz sand grains and the maturity of minerals indicate that the siliciclastic sediments were transported by wind during sea level fall (Fischer and Sarnthein, 1988).
Palygorskite (4%) is also present as fibers covering the dolomite grains and spanning the intergranular pore spaces (Fig. 16A & B).

This association includes: • Sabkha (Supratidal) facies association
This association consists of only one facies.

Sandy microcrystalline dolomite
The rock of this facies is pale yellow and medium to tough.Microscopically, the groundmass of this facies consists of micrite that is entirely replaced by microcrystalline dolomite.These facies contain quartz grains (25%-30%) (Fig. 17A).They are mostly monocrystalline with unit extinction, and few of them display undulose extinction (Fig. 17C), rarely polycrystalline (Fig. 17B), mostly subrounded and ranging in size from medium to coarse (0.45 -0.6) mm.These facies are related to standard microfacies SMF23 of evaporitic coasts facies zone (FZ9) as categorized by Flugel (2010).

• Lagoon facies association:
This association includes lime mudstone and wackestone facies.

Lime mudstone facies
It mainly consists of micrite as groundmass (Fig. 18A).The rocks of this facies are pale gray and very tough.Quartz grains (1%-3%) are present in this facies.They are anhedral subhedral in shapes, subrounded to rounded outlines indicating eolian transportation in quiet marine environments (Plumley et al., 1962), and ranging in size from silt to very fine sand (0.05-0.07) mm.The main diagenetic processes that affected these facies are selective early dolomitization, dissolution, and porosity developments such as intercrystalline.These facies are related to standard microfacies SMF23 of lagoonal facies zone (FZ8) as categorized by Flugel (2010).

Wackestone facies (Dolomitic bioclastic wackestone)
This facies consists of micrite as groundmass with fossils >10%.The rocks of this facies are pinkish white and medium tough to tough.This facies is characterized by the occurrence of bioclasts (10% ─ 15%) embedded in micrite.The bioclasts include miliolids, Peneroplis sp., and Textularia sp. (Fig. 18B).Early dolomitization, micritization, and dissolution result in the formation of various types of pores, such as vugs, intercrystalline and intraparticle.This facies indicates restricted circulation on marine platforms (cut off ponds or lagoons) (Al-Hashimi and Amer, 1985).These microfacies are distinguished by the occurrence of algae with fenestral porosity within micritic groundmass (Fig. 19A).These microfacies contain quartz grains (15%).The quartz grains are mostly monocrystalline and rarely polycrystalline, anhedral to subhedral in shape with subrounded to rounded outlines and ranging in size from fine to coarse (0.15-0.51) mm.Polycrystalline quartz can be formed from monocrystalline quartz during metamorphism (Young, 1976).Palygorskite (3%) is also present as dots and admixed with the groundmass.The main diagenetic processes affected by these facies are dolomitization and dissolution, resulting in the formation of pores (fenestral and intercrystalline) (Fig. 19B) and then cementation of most pores by secondary calcite (granular, bladed, isopachous rim calcite coating quartz grains and drusy) (Fig. 19 C, D, E and F).The association of Quartz grains with lime mud and skeletal grains indicates the reworking of aeolian quartz sands in a low-energy subtidal environment (Fischer and Sarnthein 1988).Previous studies documented the absence of anhydrite nodules and occurrence of bioclasts, as well as sparse fenestrae in such facies and close association with other sabkha-type and peritidal facies, suggests a semi-restricted intertidal environment (Weber et al., 1995).

Sandy intraclastic wackestone
The rocks of this facies are gray and white with yellowish iron oxide stains, medium tough, aphanitic, enclosing intraclasts.Microscopically, this facies consists of intraclasts (10% ─15%) and quartz grains (9% − 15%) embedded in a groundmass consisting of micrite (< 4µ) to microsparite (Fig. 21  A).Dolomite partially replaces the groundmass in borehole nine at 50-60 m depth.The intraclasts are anhedral in shape and consist of sandy lime mudstone.The quartz grains are anhedral to subhedral and euhedral in shape with subrounded to rounded outlines indicating eolian sediments, ranging in size from slightly silt to very fine sand (0.05-0.1) mm, primarily monocrystalline.Other constituents present in trace amounts are clay and orthoclase.Some of the quartz grains are corroded, and the corrosion area is filled with calcite.The main diagenetic changes that are affected by these facies are dolomitization of groundmass, recrystallization, dissolution, porosity development (intercrystalline and intraparticle), and cementation by granular calcite (Fig. 21 B).This facies is interpreted as an intertidal-subtidal environment depending on the lithology and type of adjacent facies associations (Grover and Read, 1978;Scholle et al., 1983).The intraclasts are re-transported and deposited in such facies due to desiccation and scouring by flowing water.

• Lacustrine facies association
This association includes bioclastic lime mudstone.

Bioclastic lime mudstone
This facies is characterized by freshwater bioclasts (5%), including charophyte, thin shell fragments of pelecypods, and ostracods with eolian rounded to subrounded quartz grains (Fig. 21 C).The quartz grains range in size from silt to very fine sand (0.05 -0.06) mm, anhedral and subhedral shape.The presence of such fossils reflected deposition in marine freshwater environments.

Diagenetic carbonate facies (Silicified limestone)
The rock of this facies is whitish gray and very tough (Fig. 22 A).In thin sections, the groundmass of this facies consists of micrite that is entirely replaced by micrograined quartz containing few detrital quartz grains of fine to coarse sized (0.1-0.55) mm.Subhedral to euhedral macrograined quartz grains are found as filling vugs as granular and drusy cement (Fig. 22 C).Clay (2%) is also observed in these facies as filling vugs and intercrystalline pores (Fig. 22B).This facies is formed by diagenetic replacement occurring in shallow marine carbonate rocks.Knauth (1979) considered that the diagenetic mixing zone might be particularly the suitable geochemical environment for chert replacement of carbonates.

Depositional Environment
The Ghar Formation, in its major part, represents a transitional environment from marine to continental conditions.
The carbonate rocks of the Ghar Formation reflected deposition in the evaporitic platforms (Sabkha), shallow restricted (lagoon), tidal, and lacustrine environments.Also, this formation is considered transitional between shallow, restricted marine environments and near-shore environments.Meanwhile, the clastic facies in the Ghar Formation indicate deposition in the delta front channel.The features of these depositional sites and their sediments are summarized below and illustrated in Fig. 23.

Conclusions
In the study area, the Ghar Formation (Upper Oligocene -Lower Miocene) is subdivided into three facies associations: siliciclastic, mixed, and carbonate facies associations.Six facies are identified in the mixed siliciclastic-carbonate deposit: the carbonate type, including sabkha, lagoon, intertidal-subtidal, and lacustrine; the mixed type, including shoreface and the siliciclastic type, including distributary channels.The formation, in its major part, represents a transitional environment from continental to marine conditions.The Ghar Formation consists of sandstone, sandy limestone, sandy dolomitic limestone, dolomitic limestone, and calcareous sandstone.The upper boundary of the Ghar Formation with the Nfayil Formation is conformable, and the lower boundary with the Dammam Formation is unconformable.The major minerals in the Ghar Formation are quartz, calcite, and dolomite, whereas feldspar and gypsum are the minor minerals.Clay minerals such as Palygorskite and Montmorillonite were also identified using XRD test and SEM.

Fig. 23 .
Fig.23.Suggested sedimentary model with the depositional environments of the Ghar Formation

Table 1 .
Location coordinates of the study area

Table 2 .
Samples and results of XRD Test

Table 3 .
Percentage of chemical analysis for Ghar Formation rocks