Facies Analysis and Depositional Environments of the Nahr Umr Formation in Rumaila Oil Field, Southern Iraq

Abstract


Introduction
Nahr Umr Formation (Albian) is one of the important formations in central and southern Iraq, as this formation represents the sandy deposition of the Albine period in the country.It is also considered one of the important reservoirs due to its petrophysical characteristics and oil assemblies.
The Nahr Umr Formation has been described by the researcher Glynn Jones, in 1948(Bellen et al., 1959) in the well (Nu-2) of Nahr Umr, which is the typical location for this formation and was adopted by (Owen and Nasr, 1958).This consists of sandstone with high porosity and permeability, and contains a few shale overlaps that permeate the sandstone in the lower part of it, while it consists of shale and limestone rocks with the presence of organic materials from the electrification in the upper part.The upper contact surface of this formation is conformity and graded with the Mauddud Formation.The limestone at the base of the Mauddud Formation may rise the top of the black shale of the Nahr Umr Formation, and this formation is unconformable bounded from below by the Shuaiba Formation, which corresponds when the black shale of the Nahr Umr Formation sits on the limestone to Shuaiba Formation, but this formation may be unconformable in other areas of central and southern Iraq, where the crumbly sandstone to form the Nahr Umr sits on the dolomitic limestone of the Shuaiba formation (Alsultan and Manhi, 2021), as in the Dujail region ( Macleod, 1961and Safer, 1961in Buday and, Jassim 1987), It is believed (Owen and Nasr,1958) that the upper and lower contact surfaces are conformity in the southern part of the Salman zone and Mesopotamia the thickness of the Formation reaches more than (360 m), the thickness of the formation reaches higher than either in southern Baghdad and northwestern Iraq (400 m) and the greatest thickness of the formation is (160 m) in Iraq and Kuwait (Jassim and Goff, 2006).(Al-Naqib, 1959) This formation rewards the upper part of the Sarmord Formation in northern Iraq (Owen and Nasr, 1958) As for southern Iraq, the formation of Burqan is rewarded in Kuwait, while rewarding the Khafji and Safaniya training in northern Saudi Arabia (Power, 1968) and the Al-Kazemi Formation in eastern Iran (James and Wynd, 1965).
The study area is located in Rumaila oil fields, Southern Iraq.The studied region is located in the Mesopotamian zone's southernmost unit within the Zubair subzone (Fig 1).The location of the studied borehole; depth and thickness of the Nahr Umr Formation are listed in Table 1.
The aim of the present study is to interpret the depositional environment based on detailed petrographic study and facies analysis.Well logs such as gamma ray (GR) and density of relating the log response to facies were conducted as a main requirement of stratigraphic analysis to depict the Nahr Umr Formation sequence development.

Methodology
Preparing thin section and wireline logs of wells Ru-403 and R-555 (North Rumaila oilfield) and Ru-463 was provided by the Basrah Oil Company (South Rumaila oilfield).120 core samples were used to create thin sections in the University of Babylon's Department of Applied Geology laboratories.Over 86 thin sections from accessible cores and cutting samples were petrographically for examined this purpose, its lithology are either sandstone or shale.This is in addition to the description and interpretation of core is one of the first steps in facies analysis of clastic reservoirs.Approximately 195.17 m of core available for the selected wells have been described.The description included the nature of the formation rocks, whether they were sandy, shale, or silt rocks, and their facies characteristics such as colour, hardness, and granular size, as well as the sedimentary structures and inclusions (plant and biological residues and charred materials) and oil evidence, and using them in diagnosing facies and then determining the sedimentary environment.This is in addition to lithological information collected from available log sets of spontaneous potential (sp), acoustic, and gamma-ray data (GR).
The primary and minor lithofacies were investigated, as well as diagenetic variables, porosity kinds, origins, and evolution.To supplement facies identification, well logs such as gamma ray (GR), spontaneous potential (SP), and sonic log were combined with lithofacies.Using available logs (gamma ray and density), as well as stratigraphic correlation, the lithological units, reservoir units, and sedimentary facies were anticipated and correlated.
• Making thin sections from cores and cuttings samples collected from investigated wells.
• Examine thin sections to determine the petrography and lithofacies of the wells under investigation.
• For each thin segment, 400 grains were counted using the clastic point counting technique.
• Examine the whole collection of well logs for lithofacies distribution changes.

Petrography
The mineral composition of sandstone rocks can be controlled by many factors, including the mineral composition of the source rock, the distance of transporting debris before it reaches the final deposition sites, the climate in the source area, and the diagenetic effect after the completion of deposition (Suttner and Dutta, 1986).The sandstones of Nahr Umr Formation are classified as quartz arenite.Which formed after a long-distance transport from the source area?It includes a variety of rocks (igneous, sedimentary, and metamorphic).It formed 95% has a texture showing well sorted, rounded quartz-arenite sandstone unit to < 25% in the same formation shale-dominated unit (according to Folk classification).Sedimentary rocks (sandstone) have been described to determine processes, depositional environments, and textures, as well as for determining porosity and permeability, which is an important tool for the study of rich reservoirs with hydrocarbons.
• Quartz is the main mineral of sandstone in the study area, as it constitutes a very high percentage of about 95% in the formation of Nahr Umr.The main reason for this ratio is that quartz is caused by weathering tropical, long-distance transportation, and recycling.(Dickenson, 1988).According to (Wentworth, 1932), quartz grains vary in size from medium to very fine, and there are two types of quartz: -Most of the quartz grains are monocrystalline (plt.1a),As the monocrystalline quartz may be derived from a plutonic source or other types of sources (Blatt, 1967;Folk, 1968).The size of the granules ranges from fine to medium, and the roundness of the granules ranges from semi-rectangular to round, and some are very round (plt.1j,k).Polycrystalline quartz (plt.1b) is a group of quartz grains aggregated in different directions of light (Tucker, 1985), and(Young, 1976) indicated that polycrystalline quartz can develop into monocrystalline quartz under the influence of increasing pressure and temperature.The reason is due to the absence of polycrystalline quartz it is unstable when transporting long distances or its absence in the source rocks.(Young ,1976) mention that polycrystalline quartz could be developed into monocrystalline quartz under the influence of increasing pressure and temperature.• Feldspar was found in less than (5%) among the mineral components of the studied models of the Nahr Umr Formation, and the reason for this is that it is less stable and sensitive to chemical reactions during weathering and long-distance transportation (Pettijohn et al., 1973).Feldspar is derived from coarse-grained subterranean rocks with less extension than the volcanic source (Pettijohn, 1975).In general, it includes orthoclase, plagioclase, and microcline (plt.1c).• Rock fragment are pieces of rock that contain different mineral components, the percentage of which is little or no non-existent during long-distance transportation (Pettijohn et al., 1973).The rock pieces are more important in sediments because they give specific information about the nature of the source rocks (Blatt et al., 1982).Two important points must be distinguished in the first rock pieces; the sand content of the rock pieces is related to the particle size of the source rock.
Second: The abundance of rock pieces depends on the size of the grains (Selley, 2000) (plt.1 d,e).

Diagenetic Processes
Diagenetic processes are defined as a set of chemical and physical changes that occur in sediments since the time of their deposition and through the stages of lithification.And its appearance on the surface of the earth and before reaching the stage of transformation under normal pressure and temperature (Flugel, 1982), as it refers first to the interactions that occur between one mineral and another within the sediment, or between one mineral and several minerals and suspended and intertwined liquids (Selly, 2000).Diagenetic processes affect the change of pore type and geometry of sandy rocks.And then it can control its porosity and final permeability, and thus it was divided into two types: the first: early diagenesis linked to the environment of sedimentation and sediment formation, the second: Late diagenesis and they are linked to regional fluid migration patterns (Boggs, 2009).The following are the most important transformational processes in the formation of the Nahr Umr.
Compaction: Under the influence of the sedimentary cover, there is a set of processes that occur that reduce the initial porosity of the sediments and reduce the size of the rocks through the exit of fluids that are between the grains (plt.1g).Between the grains, this is in the case of sandy rocks(plt.1i),and through this, the act of judgments was distinguished from the shattering, crushing, and expansion of the surface area of the grains.The content is reduced by increasing the clay and fine granules(plt.1h).(Searl, 1989) Structural components.Cementation: The cementing process reduces the porosity that was present at the time of sedimentation because cement is formed around the edges of the grains and its growth outward in the pores leads to partially or filling the pores, which leads to a decrease in the porosity (Nichols, 2009).In the rocks of Nahr Umr Formation, more than one type of cement was distinguished, and it was found that silica cement constitutes a proportion more than other types of cement, which is in the form of a superstructure for turf Quartz overgrowth (plt.1f).It is visually continuous with the quartz grain and appears partially or Complete, and in many cases, it can be distinguished by noticing a ring from iron oxides or clays, the original quartz grain is separated from the cement, and this is known as the dust line (Pettijhon et al., 1973).Calcite cement was also observed.In lower proportions than silica cement, which is either as microcrystalline site (Microcrystalline calcite).Or in the form of spots, and it is known as spray calcite cement.Also, some quartz grains appeared floating in the lime cement, resulting in the so-called fabric (Poikililotopic texture) (Boggs, 2009), It is known as inconsistent cement due to the difference in its chemical composition from the composition of the granules that make up the rock.It is believed that the most important sources of this type of cement are solutions saturated with calcium carbonate, resulting from dissolving calcareous particles and structures within the Formation itself, as well as dissolving some lime layers located below and above the Formation at the borders of contact with the lime Shuaiba and Mauddud lime Formation respectively, and then deposited in the form of cement in other locations, within the Formation of Nahr Umr.While iron oxide cement is distinguished in some models of formation rocks by its color reddish brown or black and is found with silica and lime cement and is in two forms, either filled the microfractures or between grains (intergranular).

Lithofacies of Nahr Umr Formation
The division of the core into lithofacies is one of the major findings of the description based on the thickness of the layers, the kind of rocky type, the color, the sedimentary structures, and the contents, which is one of the important phases in the facies analysis of the clastic reservoirs.The following is a list of the key strata in the Nahr Umr Formation sequence, described according to.

Shale lithofacies
These (F.A) are shale rocks with a high organic content and a black or dark lead hue (vegetation residues) (Fig. 2a).The main distinguishing feature of these facies is their high density, which is coupled with a large shale volume (high Gamma ray).The lower section Nahr Umr Formation is dominated by shale, and this facies reflects that part.

Lenticular bedded sandstone-mudstone lithofacies
This facies consists of alternating soft, ventricular sand rocks that are intertwined with shale rocks of gray color, non-foliation.This overlaps with shale rocks having a shale tint but no foliation (Fig. 2b).Due to energy fluctuations, this sandstone can change into sedimentary formations of the kind of wavy sand in one portion.This kind of rock was found in the top and lower parts of the Nahr Umr Formation, which is dominated by clay rocks.The volume of shale is rising (fennel-shaped gamma ray).

Falser Bedded Sandstone -Mudstone lithofacies
These facies (F.C) consist of simple and falser layers with a thickness of less than one meter and a limited distribution that may not be present in all, where it consists of falser bedding, fine-grained medium-sized sedimentary cycles of the examined wells.In a calm oceanic environment, sandstone is interspersed with shale-shaped structures.When the mud is trapped between the sand in the form of layers that are not connected and isolated, wave action will result, as it was formed as a result of alternating wave currents (Fig. 2c).(Falser bedding) (serrated shape gamma ray).
In stagnant water, where sand is deposited when the sedimentation energy is high and the clay remains suspended, then it is deposited on the peaks formed in the early stages of the period when the sedimentation energy is low.When the strong currents return, they will deposit sand again and remove the mud deposited on the tops of the ripples.Therefore, the mud present in the ripple depressions will remain trapped.Thus, these facies form in environments that have alternating energy (Alsultan, et al., 2021).

Siltstone-shale lithofacies
Shale rocks containing silt and fine sand, as well as plant remnants, make up these facies (F.D).This structure originated in the top portion of the Nahr Umr Formation, which is dominated by shale, since the amount of shale here is smaller than it was in the shale lithofacies (Fig. 2d).The amount of shale here is lower than amount compared with the other shale lithofacies, resulting quick drops in values in the gamma ray log.

Cross-bedded sandstone lithofacies
These facies (F.E) consist of coarse sandstone medium in size, well-sorted, and their color varies between light and dark brown due to the increase in oil evidence in them.Where sedimentary structures were observed, two are trough cross-bedding and planar cross-bedding (Fig. 2e).Parallel to the level of shale were also observed, and small layers of shale and silt may be present within these facies, and sometimes the effect of the oxidation process on the rocks of this facies may be observed by noticing the red color on sandy rocks.The sandstone facies with flat cross-layering is one of the most common facies in the Nahr Umr Formation, particularly in the Middle section.It may not appear that there is a difference in granular size or that it is difficult to tell apart.

Parallel and Cross Lamination Sandstone lithofacies
It is facies (F.F) that consists of fine to medium foliated sand grains inclined at different angles from well-sorted sandstones.The intersecting foliar facies are mostly of the medium-sized type (Fig. 2f).The transverse foliar is in a low energy environment with a low current of flood plains, tidal flats (Qaradaghi et al. 2008), and calm water islands, banks, and dams.While the existence of the bioturbation as a mottled structure indicates an active deposition in river channels and prodelta (Qaradaghi et al. 2008).

Sedimentary Environment
The modern stratigraphic study aims to determine the sedimentary environments and try to develop a sedimentary model for the study area, through information obtained from sedimentary facies, electrofacies, and microfacies.
The sedimentary environment is defined as a geographically defined part of the earth surface, in which sediments accumulate and can be described from a geomorphological point of view, and it is characterized by complex physical, chemical, and biological conditions that distinguish it from its surroundings (Selley, 1978).
The facies are one of the smallest units of the environment, and the first to establish a relationship between the sedimentary facies and the sedimentary environment is (Walther, 1894), and this relationship was called Walther's Law.This states that the vertical succession of sedimentary facies is generated through the lateral succession of the environments.The vertical succession of facies defines the facies association, which is characterized by the sedimentary environment.It is considered a key to the environmental explanation and that this succession results from the change of the sedimentary environment itself, which in turn results from the change in sea level.
It is clear from the above the possibility of deducing sedimentary environments according to the availability and collection of sedimentary evidence, including the study of microfacies, sedimentary structures, sedimentary texture, and rock components available in the facies, and linking them with the sensory study (electroscopic) and thus it was possible to determine the sedimentary environment for the formation of Nahr Umr:

Fluvial Environment:
This environment has been characterized by fining upwards and the prevalence of high-energy sedimentary structures below these facies is represented by the cross-bedded and planer cross-bedded, respectively, which transform into the low energy structures, represented by the parallel and cross lamination at the top, which indicates the gradual low in the energy of sedimentation.
The study of modern rivers indicated that there are two types of sediments formed in the riverine environment, one of which represents coarse sediments at the bottom of the canal that moves as a bed load or as a suspended load.Suspension load in the stages of high or normal flow (Alsultan and Awad, 2021).while the second of them represents sediments above the banks represented by soft and transported sediments as a suspended load in the stages of floods, and thus the riverine environment can be divided into: • Channel Lag Deposits.This environment consists of cross-bedded sandstone lithofacies (F.E), including coarse grain sandstone, which is very rough sands resulting from very high energy flows transmitted as a bottom stratigraphic load.Erosion and transport occur in such an environment due to the high energy of the current, and when this energy of the current is reduced, this facies is deposited in the middle of the river channel, which is known as sediments.As a result of lateral growth (lateral accretion), which in turn leads to a decrease in the energy of the rivers responsible for the transport process, a decrease in the grain size occurs at the top, and its facies are precipitated by sandstone with intermittent (Parallel and Cross Lamination Sandstone lithofacies (F.F), respectively, forming sediments fining upwards.• Natural Levee Deposits The sediments of this environment are deposited adjacent to the river channels in the form of longitudinal bodies, extending over.The two banks of the river are characterized by a steep slope towards the channel and a little slope towards the flood plain, and it is more visible on the concave banks than on the convex banks.It consists of cross-bedded sandstone lithofacies (F.E), sandstone-mudstone lithofacies (F.C), and siltstone-shale lithofacies (F.D).The upper parts of these facies show a fine succession towards the top and that these facies (F.E) and (F.C) appear in succession, and finally, towards the top, the facies (F.D).As the horizontal surf is formed in conditions of high run-off at the beginning of the flood stage, as the quantities of water carrying suspended matter and the confined interior increase, so does the energy of this water.• Flood Plain Deposits.Floodplains represent wide and flat basins located along the two banks of the river, and sediments collect in them when water crosses into the floodplains.Container on charcoal, these facies are sometimes distinguished, especially the siltstone-shale lithofacies (F.D).

Deltaic Environment
This environment has been distinguished by the appearance of the coarsing sequences upward and the presence of organic materials, plant remains, and amber, which are found in the marshes and swamps environment within the deltaic environment, as well as the response of the spontaneous potential (SP) that shows the funnel shape, which indicates the cycles of coarsing upwards, and these deltas can be counted as the type dominated by the river, through the number of high sediments entering it and the large grain size of sandy grains and the prevalence of sedimentary structures of high energy represented by intermittent stratification and the emergence of shale rocks, and the self-voltage sensor tends to resemble the shape electro surgery of the deltaic environment controlled by the river according to Pirson, (1970).
It was feasible to identify it from the south and north Rumaila oil fields and its gradient upwards to a deltaic environment dominated by tides, as evidenced by the spread of sedimentation of channels and tidal flats in the well (Ru-403) and for the Nahr Umr (Ru-463).This deltaic habitat was developed at the boundary of the seam between the development of Nahr Umr and Mauddud, and it was formed during the drop in sea level, generating a gradual delta.
• Delta plain Deposits.The environment consists of cross-bedded sandstone lithofacies (F.E) and sandstone-mudstone lithofacies (F.C).The sands of this environment show a consistent gradation in the grain size (uniform in grain size).Carbonized flakes located within the weak levels of stratification are common, and spontaneous potential (SP) shows a cylindrical shape.These facies can be counted within the sub-aerial part of the deltaic deposits.The subaerial part of the delta represents the sediments of the secondary deltaic channels.• Distributary Mouth Bar Deposits.The sediments of this environment consist mainly of cross-bedded sandstone lithofacies (F.E).This shows coarse sequences towards the top due to the increase in sedimentation energy towards the top.These facies are deposited over the prodelta sediments.And such a sequence is explained by the fact that it was deposited in the environment of the developing Delta region (Prograding Delta Front).This may be observed through the behavior of the spontaneous potential, which shows the funnel shape, and these represent the underwater part of the deltaic deposits.(Subaqueous Part of Delta) It represents the environment of the delta front affected by rivers (River Dominated Delta Front).The effect of the riverine action is evident through the prevalence of discontinuous stratification structures and coarse granular size.• Distal Bar Deposits.This environment consists of false-bedded sandstone-mudstone lithofacies (F.C).And, with the presence of thin layers of siltstone-shale lithofacies (F.D) and shale lithofacies (F.A), the sands of these facies are of fine size.The facies of this environment show coarse sequences towards the top, and this is consistent with the response of the self-voltage sensor, which shows the funnel shape and that the thickness of this environment increases in the middle of the study area.It represents a transitional environment between the pre-delta environment from below and the environment of the river mouth barrier from the bottom.The upper part represents the sloping edge towards the sea, and it is characterized by an increase in the sedimentation speed and the grain size towards the top.Softwood grain is rich in organic matter (Canaanite, 2002).• Prodelta Deposits.This environment is represented by the shale lithofacies (F.A) and the siltstone-shale lithofacies (F.D) .respectively,thus reflecting the phenomenon of coarseness towards the top.This environment is characterized by its fine sediments, which result from the slow accumulation of sediments in a calm environment.We infer from the types of rocks and sedimentary structures that these facies belong to the underwater part of the deltaic sediments located within the environment of the front of the delta.• Tidal Channel & Tidal Flat.The appearance of facies in this environment confirms to us the transformation of the delta from a river-dominated delta.Several indications were observed in the pulp available in the wells of the study, especially in the well (R-555).It gradually fines in the granular size towards the top, thus reflecting the phenomenon of fining towards the top as well as the gradation in the sedimentary structures towards the top, from the sedimentary structures characterized by high energy (planar and trough stratification) at the bottom to the distinctive low energy sedimentary structures (flasher and lenticular stratification) at the top.The presence of the latter is one of the important indications of the sedimentation of mixed intertidal flats, with the presence of mudstone and sandstone and bioturbation in the upper part of these sequences.The following describes each of these sedimentary structures.In the lower part of this core, sandstone facies were observed.This type of application results from the effect of displacement of sand waves or large ripples marks, due to the currents of the flood and island cities.Then, in Zabeel, it is topped by the basin stratification of the same previous facies (Klein, 1977).This type of structure results from the displacement of the large ripples marks under the influence of tidal currents at the bottom of the tidal channels (subtidal sub environments).It is also called the Lower Tidal Flat (Klein, 1972Al-Karkaji, 1989) Then it grades upwards, revealing the facies of the Falser Bedded Sandstone-Mudstone lithofacies (F.C) and facies of the Lenticular bedded sandstone-mudstone (F.B) .Widespread in this part and finally the facies of shale lithofacies (F.A) all these environments and facies appear in the lower member of the mixed unite (Figs.3,4 and 5).

Conclusions
The Nahr Umr Formation in the South and North Rumaila oilfields consists of sandstone, and contains a few shales overlaps that permeate the sandstone in the lower part of it, while it consists of shale and a few limestone rocks from the electrification in the upper part.The upper contact surface of this formation is conformable and graded with the Mauddud Formation.The limestone at the base of the Mauddud Formation may be on top of the black shale of the Nahr Umr Formation, and this formation is bounded from below by the Shuaiba Formation in an unconformable, which corresponds when the black shale of the Nahr Umr Formation sits on the limestone to form the Shuaiba Formation.
The Nahr Umr Formation in the Rumaila (South and North) oilfields of southern Iraq was formed in a fluvial environment and deltaic depositional conditions, and it contains six lithofacies: shale, lenticular bedded sandstone-mudstone, false bedded sandstone-mudstone, siltstone-shale, cross-bedded sandstone, and parallel and cross-lamination sandstone.
The Nahr Umr clasts were deposited in transitional environments, which represented oscillations in sea level rise and stillstands of the relative sea level controlled by tectonics as sediments of the basin transitioned from freshwater to marine conditions.
The finning upward mode was indicated when the deposition environment changed from delta front and delta plain to a highstand stage.The deposition of the upper part of the Nahr Umr Formation and the beginning of the Mauddud Formation mark the end of this stage as shallow carbonate marine.

Table 1 .
Thickness, coordinates and the depth of the Nahr Umr Formation at South Rumaila oilfield (Ru) and North Rumaila oilfield (R) In Southern Iraq.