Depositional Environment and Microfacies Analysis of Yamama Formation in North Rumaila Oil Field, South Iraq

Yamama Formation is an important sequence in southern Iraq. Petrographic analysis was used to determine and analyze the microfacies and pore types. The diagenetic processes and the impacts on the petrophysical properties of the rocks were also identified. The petrographic identification was based on data of 250 thin sections of cutting and core samples from four wells that were supplied by the Iraqi Oil Exploration Company (O.E.C). The present study focuses on the depositional environment and the microfacies analysis of Yamama Formation. The results revealed several types of microfacies, including peloidal wackestone-packstone, algal wackestone-packstone, bioclastic wackestone-packstone, foraminiferal wackestone, bioclastic wackestone, and mudstone. The latter was divided into two submicrofacies, namely argillaceous lime mudstone and sparse fossiliferous lime mudstone. The study showed that Yamama Formation was deposited through various settings within the carbonate platform; these comprise inner ramp, middle ramp, and outer ramp settings. The inner and middle ramp settings, characterized at the top and middle Formation, have high effective porosity and low clay volume, due to dominant packstone.


Introduction
The Cretaceous rocks represent good oil potential rocks within the stratigraphic column in southern Iraq. The main oil reservoirs are of the Cretaceous age, in addition to some important reservoirs from the Tertiary and upper Jurassic ages at most of the hydrocarbon within this point of view were confined to Suitable deposits for reserve especially in the lower and middle Cretaceous Formations in the south of Iraq [1,2,3]. This study targets Yamama Formation due to its good oil potential which attracted the interest of many researchers [4].
The Yamama paleogeography is represented by the Late Tithonian-Valanginian time at the Southern Neo-Tethys opened with the separation of the Bisitoun (Avroman) microplate from Arabia. The Upper Berriasian to Lower Valanginian palaeogeography (Yamama Formation) is summarized in (Figure 1) [5]. The Mesopotamian Zone witnessed repeated open marine incursions, leading to the deposition of alternating shallow water carbonates and outer shelf marls. Sedimentation began with the deposition of the transgressive Sulaiy and Yamama formations and ended with deposition of the Ratawi Formation during the high stand stage [5]. In southern Iraq, The Yamama Formation comprises outer shelf argillaceous limestones, as well as oolitic, pelloidal, pelletal, and pseudo-oolitic shoal limestones.
Oolitic reservoir units are present in several NW-SE trending depocentres [5]. The formation is of the Berriasian-Valanginian age. From its stratigraphic position, it can be implied that the age range of the Upper Berriasian to Lower Valanginian is likely 140 to 136 Ma. The Yamama Formation was deposited in alternating oolitic shoal and deep inner shelf environments, probably controlled by subtle structural highs within a carbonate ramp [5].  [5] Mohammed and Nasser Iraqi Journal of Science, 2022, Vol. 63, No. 1, pp: 202-213 204 This study is intended to complement previous studies of the relationship between sedimentary environments and diagenetic processes and their effects on the petrophysical properties in north Rumaila oil field. The Yamama Formation occurs in the upper Berriasian-Aptian rock package. This package comprises the environments from shallow tidal flat to the deeper basin environments represented by the Shuiaba, Zubair, Ratawi, Yamama, Sarmord, and Lower Balambo formations .Yamama Formation is composed generally of limestone, with some thin dolomitic intervals, as well as narrow shale lamina [5].
Yamama Formation overlies the underneath Sulaiy Formation in a conformable manner. The latter is made of Jurassic-Lower Cretaceous carbonates composed of mud-dominated argillaceous limestone and contains the small-size benthic foraminifera. Yamama Formation changes upward gradually into Ratawi Formation (Figures 2 and 3). Ratawi Formation is a heterogeneous package of dirty limestone, shale, siltstone, and sandstone. The Ratawi Formation can be considered as a seal interval of the Yamama reservoir rocks [5].
The studied oilfield is the North Rumailawhich is located about 70 km west of Basra, forming a convex asymmetrical sub-surface, with a slight slope and axis towards the north and south .The field is about 41 km long and 15 km wide and has a structural closure of approximately 110 meters ( Figure 4). This field represents the northern extension of North Rumaila field. The formation is widely distributed in Iraq and represents reservoirs in the southern parts.
The Yamama Formation, in southern Iraq, comprises an outer shelf argillaceous limestones, in addition to oolitic, pelloidal, pelletal and pseudo-oolitic shoal limestones [6]. Sadooni [6] suggested that, in SE Iraq, the formation comprises three depositional cycles. Cycle tops contain oolitic grainstone inner-ramp facies which pass down into finer-grained peloidal facies and middle-ramp bioclastic/coral/stromotoporoid pack-wackestoncs. Outer-ramp cycle bases comprise thick grey shales with stringers of chalky micrite ( Figure 5). The main goal of this study is to find a relationship between the reservoir properties and sedimentary environments in carbonate rocks, concerning the effects of the diagenesis processes.

Materials and Methods
This study was based on subsurface data approved by the Iraqi Petroleum Exploration Company. Four wells were selected at the North Rumaila field structure, which are R-131, R-172, R-89, and R-158. The row data were collected from the final geological reports of the studied wells, which contain the plugs and thin sections that were identified.
Thin section analysis, by using polarized microscope, resulted in the petrographic description, identification and classification of microfacies, as well as identification of diagenetic processes to determine and reconstruction of the depositional environment of the Yamama Formation. The study involved more than 250 slides of cores and cuttings that were described and interpreted, together with several hundreds of thin sections previously prepared by the Iraq Oil ExplorationCompany and studied petrographically by applying the modified Dunham (1962) classification by Embry and Klovan (1972) [7,8 ].
Well logs analysis was used to predict the relationship between the porosity and permeability, along with the effects of the sedimentary environments and diagenesis in terms of decreasing or increasing the reservoir properties. Many of the characteristics of the limestone of Yamama Formation were recognized using cutting samples, which were utilized to choose samples for thin section preparation. The cutting investigation was accompanied by the use of geological well reports, including data on the depths to which oil showed up. Fossil dissolution pores were used to distinguish the limestone units (vugs and molds), cementation, nodular fabrics, pyritization, bioturbation, fractures and stylolite. In addition, microfacies analysis depended on the study of thin sections of cutting samples, from the Oil Exploration Company labs, for selected wells for Yamama Formation in North Rumaila. Well logs were digitized using NeuraLog software. Furthermore, available well log responses were related to facies changes for the studied succession intervals.

Peloidal wackestone-packstone Microfacies
In a low-energy sheltered area (plt. 1-I), these facies are often prevalent in the upper portion of the studied well The could correspond to the Wilson Standard Microfacies Types (SMF-17) and (FZ-8) of the Standard Facies Zones, according to the Flugel Classification (2010) [9], suggesting an internal ramp setting with minimal water circulation.

Algal Wackestone to Packstone Microfacies
The algal debris facies can be divided into two subfacies according to algae types, as described below.
A-The subfacies of dasycladacean wackestone These are primarily composed of green algae from the Dasycladacean group. Most of their skeletons were dissolved by spary calcite cement in order to be filled. In most cases, algae are mixed with shell fragments (mainly pelecypods).
The most favorable areas for green algae development are typically sheltered lagoons and protected reef flats [9] (Plt. 1-A, B).

B-Permocalculus.
This is the largest group of red algae preserved as fossils. These microfacies can represent open-circulation shallow marine waters (Plt. 2-M).

Bioclastic Wackestone Packstone Microfacies
Echinoderms and algae fragment are mixed in this microfacies as an indication of open marine environment without reef or that confined to the circulation of water (plt. 1-E, F). Wilson Standard Microfacies Types (SMF-10), (FZ-2) can correspond to it [10]. Because of the abundance of wackestone packstone, this suggests a shallow open sea climate.

Foraminifera Wackestone Microfacies
These facies consist of rocks ranging from wackestone to packstone in texture and containing large benthic foraminifera, such as Miliolid and Trocholin (Plt.1-C, D). It may correspond to Wilson's (SMF-8) (FZ-2), which suggests a circulating shelf lagoon [10]. Environments of low energy below the wave base.

Mudstone Microfacies
The mudstone microfacies are divided into two main submicrofacies: Aargillaceous lime mudstone and Fossiliferous lime mudstone.

Bioclastic Wackstone Microfacies
This microfacies in features dissolved most of bioclasts and leaved effect stamps of bioclasts (Plate-2, N). it occurs as a shallow lagoon with open circulation at or just below the fair-weather wave base (FZ 7) and deep shelf (FZ 2); i.e. outer and mid ramp settings [9,10].
Hence, the Yamama Formation was deposited through various settings within the carbonate platform; these comprise the inner ramp, middle ramp and outer ramp setting.

SEDIMENTARY ENVIRONMENTS 2 Depositional Environments of the Yamama Formation
The carbonate ramp environments in Yamama Formation are classified into three categories: (1) inner ramp, (2) middle ramp, and (3) outer ramp [10] (Figure 4). The Mesopotamian Basin, which is built on a gently dipping slope of the Arabian Platform and considered a passive edge on a ramp setting, deposited the Early Cretaceous Yamama Formation [6]. In Yamama Formation, this diagram is arranged with shallowest water environments to the right side of the back ramp area, through deep basin environments to the left. In between, there is an intermediate area that indicates shallow ramp and deep ramp, with the confines between them being established approximately at the lower limit of the fair weather wave base (FWWB).
The absence of any reef margin development, the lack of primary slope break from the shoreline to deeper water, and the existence of high-energy grainstone facies were reported by an earlier work [11].These findings are consistent with prior investigations of facies relationships, which revealed facies organization along carbonate ramps and differences in hydrodynamic energy, depositional texture, sedimentary structures, and fossil content. The Yamama Formation's inner ramp develops the microfacies of lagoon, shoal, open marine, and restricted marine facies associations.
On a homoclinal ramp setting, lagoon ecosystems are also extensively formed. The degree of barrier restriction and permanence affects the organisms that live in lagoons, as well as the sediments that accumulate in this mostly quiet water environment [12]. Lagoonal, open marine, and restricted marine environments of Yamama Formation microfacies are rich in lime mud and dominated by benthic Foraminifers and green algae in the studied fields. Carbonate shoal distribution is one of the characteristics that describe inner ramp habitats with peloids, ooids, and intraclasts coated by micrite in Yamama Formation microfacies in the studied area. These microfacies are classified as shoal facies, meaning that they have a lot of energy [12]. Open marine environments with sponge spicule, echinoderm, bioclastic Foraminifera, algal debris, and peliod grains in the microfacies make up the inner ramp. Mudstone and occasional wackestone microfacies are present in the mid-ramp setting.   (Figures 1 and 7). Such results are in accordance with many researches [13 to 18]. Three major depositional cycles were determined. All the cycles exhibit normal sequential regression from the base upward. The upper part of the formation represents the fall of the sea level and marks the end of the regressive phase.

Diagenetic processes
Carbonates of the Yamama Formation have been affected by both early-and late-stage diagenesis. The most important diagenetic processes in the Yamama Formation are micritization, cementation, neomorphism, dolomitization, dissolution, and fractures (Plates 1 and 2). Mudstones and wackestone were principally altered by marine phreatic and later diagenetic processes. Cloudycentered, clear-rimmed dolomite rhombs and stylolites indicate later, burial diagenetic processes. Diagenetic features in the bioclastic wackestones, packstones, and grainstones indicate early and surface alteration in the meteoric phreatic and mixing zones. The development of moldic, vuggy, and channel porosity, particularly in packstones and grainstones, indicates a similar diagenetic environment. These processes are described briefly below.

Dolomitization
The limestone beds of the Yamama Formation have been affected by dolomitization process at various degrees. The fine grain size of the rhombs indicates their early diagenetic origin, whereas larger dolomitic crystals of clear-rimmed, cloudy-centered types were formed during later diagenesis [12,13,14].
The fine grain size of the rhombs and their occurrence within the mud-supported facies indicate their early-diagenetic origin, particularly those present within intertidal mudstone. The coarser dolomite crystals are formed during a later dolomitization process but before stylolitization. Dolomitization has highly enhanced the reservoir quality of most the microfacies, particularly with the formation of secondary intercrystalline micropores within muddy microfacies (Pl.1 K , Pl.2 E).

Neomorphism
The Yamama Formation carbonates have been affected by neomorphism, which commonly affected the groundmass and particles ( Pl.2 A). Micrite is often recrystallized and aggrading to microspar or pseudospar. It also affects the molluscan shell fragments, foraminifera, and other skeletal grains, leading to the inversion of aragonitic skeletal fragments to calcite by the presence of solution. In shallow marine environments, the neomorphism process intensifies greatly by the solution under higher temperature and pressure conditions [9]. Cementation Cementation has been an effective diagenetic process in various microfacies, filling the intergranular and intragranular pores, cavities, fissure, and fractures and leading to the occlusion of primary porosity in the Yamama Formation carbonates. Many types of cements have been recognized: (1) micrite cement, (2) overgrowth cement, (3) displacement cement, (4) granular cements, (5) druzy mosaic cements, (6) syntaxial rim cement, and (7) blocky cement. These cements are believed to be of later diagenetic origin, although the mosaic cement and the syntaxial rim cement may have formed during two or more phases (Pl.1 J and Pl.2 J). Micritization It is the most common process affecting the skeletal fragments in the bioclastic packstones and bioclastic wackestones microfacies of the Yamama Formation. In a stagnant marine phreatic zone, the micritization process is affected greatly by blue-green algae or fungi [12]. Micritization is an early diagenetic process and micritized skeletal grains are very common in facies of the Yamama Formation (Pl.1 L).

Dissolution and Fracture
This process acts to destroy the internal structures for the skeletal grains, leaving the micritic envelope to form the moldic porosity or vuggy porosity (Pl.1 L, Pl.2 C,K,F,G,I and L). The results could distinguish the porosity and divided them into grain-dominated fabric and mud-dominated fabric. Also, interparticle porosity was distinguished and divided into grain-dominated fabric and mud-dominated fabric. In addition, interparticle porosity was distinguished and divided to grain-dominated fabric and mud-dominated fabric.1. 2.
The moldic porosity or vuggy porosity or enlarge the presenting vugs to form cavern porosity or channel porosity . Fracture porosity is found in mudstone and wackestone that are composed of soft mud, where the channel porosity is developed due to dissolution process.
In addition, Yamama Formation is effected by six diagentic processes, namely the dolomitization, cementation, neomorphsim, dissolution, fracture, and micritazation. These processes may increase porosity, as in the cases of dissolution and fractures, or lead to a decrease in porosity, as in dolomite and cementation. The reservoir properties of the formation decrease towards the bottom due to the increase in mudstone microfacies.
On the basis of microfacies analysis, the deposit environment of the Yamama Formation was divided into (1) inner ramp, (2) middle ramp, and (3) outer ramp. The inner ramp setting is characterized by Peloidal wackestone-packstone microfacies that have high effective porosity and low clay volume. The packstone inner ramp is dominant in the top and middle Formation at depths between 3719 and 3790 m, which is characterized by high reservoir quality. The middle ramp setting is characterized by Bioclastic wackestone microfacies and Bioclastic wackestone-packstone. The outer ramp setting is composed of Mudstone and Algal wackestone-packstone microfacies, characterized by a decrease in porosity and permeability due to the increase of clay volume. This results in a decrease in reservoir properties at the lower part of the Formation.