Iraqi

Abstract


Introduction
The effectiveness for hydrocarbons, which determines a reservoir's productivity, depends on porosity, but deliverability depends on permeability. Recognizing reservoir quality, which is determined by its hydrocarbon storage capacity, is a major goal in the reservoir characterization process. Porosity and permeability are crucial factors in determining the quality of reservoirs as a consequence (ElSharawy and Nabawy, 2019 ). Data quality is identified, described, and rated using flow units. Lithological, petrographic, and petrophysical data are a few examples of flow units. The best way to uncover and understand connections between these forms of data is to study them all at once. (Ahr, 2008).A better knowledge of reservoir characterization is a precondition for efficient and better management of heterogeneous reservoirs, and appropriate reservoir characterization is a fundamental aspect and critical effective field development planning. (Shedid, 2018). The variety of pore networks, which are the result of facies variations and digenetic processes, makes identifying rock facies and flow units n carbonates challenging. (Riazi, 2017).In this article, the flow units and rock fabric number of the major reservoir were identified and characterized using two approaches. Among the techniques used were: The Lucia (1999) technique separates rock into three classifications based on rock fabric and grain size, whereas the flow zone indicator approach uses unique criteria to characterize each hydraulic flow unit that controls fluid flow. Three wells from the Buzurgan Field have been selected as carbonate reservoirs because they penetrated Mishrif Formation and were consistently dispersed to dentify reservoir flow units and rock facies n the carbonate reservoir.

The Study Area
The Buzurgan oil field is located in Missan governorate south eastern raq near the raq -ran borders approximately 300 Km to the south-east of Baghdad and 40 Km to the north-east of Amara city and within an area of 354.4 Km2 (Fig 1). Buzurgan field s composed of two domes run along a NW-SE direction. Three available wells have been studied (BU-1, BU-2, and BU-3) that t has been chosen n the field. The geographic coordinates, Top and bottom for studied wells of Buzurgan field are presented in the Table 1. The Buzurgan field s placed tectonically n the Mesopotamian Basin (Buday and Jassim, 1987). This setting directly influences depositional setting, structural style, and fracture intensity.

Geological and Stratigraphic Setting
In the study area and surrounding areas, the Mishrif (and comparable Upper Sarvak) reservoirs, and the sealing Upper Cretaceous shale, the Middle and Upper Cretaceous layers comprise one of the richest petroleum systems in the Middle East (including Iraq). The Mishrif was widely deposited with shallowmarine platform carbonates throughout the Cenomanian to Turonian periods of the early Late Cretaceous. Many previous studies showed that this formation is one of the main oil carbonate reservoirs in the Mesopotamian and Zagros Basins, and it contains about 30% of the ratified oil reserves in Iraq (Sadooni and Aqrawi, 2000); (AlBahadily and Nasser, 2017); (Al-Khafaji et al., 2021); (Al-Aradi et al., 2022).
The Mishrif Formation is a rudist and benthonic foraminifera-rich carbonate series (Aqrawi et al., 2010). The transition from the basinal Rumaila Formation to the Mishrif Formation's shallow open marine facies is the Mishrif Formation's bottom limit. An unconformity surface between the Middle and Late Cretaceous truncates the upper border with the Khasib Formation (Fig.2). The Mishrif is represented by the Gir-bir Formation in the north, the Balambo Formation in Iran, the top portion of the Sarvak Formation in Kuwait, and the Magwa Formation in Kuwait (Alsharhan and Nairn,1988). Local tectonic influence is reflected In variable depositional facies within the study area. The Mishrif formation (lasted about 5 million years) represents low gradient shelf that fringed the Arabian platform during the Cenomanian and early Turonian. The rocks are characterized by extensive rudist platforms, with a sedimentation pattern controlled mainly by eustatically driven sea level changes, which during the Cenomanian, became gradually controlled by tectonism (Buchem et al. 2002). 30 percent of Iraq's entire oil reserves are contained there, with a 26-28 API rating ( Aqrawi et al., 2010). Carbonates from basins in the Upper Jurassic and Lower Cretaceous are most likely where the hydrocarbons came from. (Aqrawi et al., 2010) . The AP9/AP8 megasequence border is formed by the top Mishrif truncation at 92 million years (Sharland et al., 2001).
The genetic sequence known as K140, which is in the third order, is represented by the Rumaila, Mishrif High Stand System Tract ( Aqrawi et al., 2010). The Mishrif is regarded as the culmination of a marine shelf series. Rudist reefs as well as other related buildups were evidence of the Mishrif formation's deposition following the transgression of the Ahmadi and Rumaila formation's marly limestones and shales (Aqrawi et al., 2010).

Material and Methods
The Mishrif reservoir in the Buzurgan oil field was studied for this work, and the data gathered and research techniques were divided into three major groups: • The availability of data related to petrophysical properties and reservoir flow zone indicators, which make up a large portion of the overall data. This set of information was gathered from three wells (BU-1, BU-2, and BU-3),which represent wireline logs of the three well sections which include, Compensated Neutron tool (CNT), Formation Density compensated (FDC), Sonic log (borehole compensated BHC type), and Gamma Ray (GR).These logs were used to calculate the effective porosity. • More than 450m core plugs having detailed laboratory measurements (permeability) carried out by Missan Oil Company that were used in the petrophysical methods. • Thin sections examinations of available core samples for well BU-2 in order to study the types of microfacies in Mishrif Formation and then compare it's with the Rock-fabric numbers (RFN) for more accuracy.

Rock-fabric Numbers (RFN) Lucia Method
In terms of particle size, sorting, interparticle porosity, and distinct vuggy porosity Lucia (1999) developed the concept of the rock fabric method. Pore size and pore-size distribution, which regulate permeability and water saturation, can be conveyed by these phrases. The rock fabric, which is the consequence of geologic processes, is connected to the pore-size distribution. By categorizing pore space into grain-dominated and separate and touching vugs, as well as interparticle porosity and vuggy porosity (separate and contacting vugs), rock fabrics are associated to petrophysical characteristics (presence of open or occluded intergrain porosity and a grain-supported texture).
Since separate-vugs increase total porosity but only slightly improve permeability, they should be subtracted from total porosity in the case of separate-vugs (Lucia, 1999). The permeability in touchingvug pore systems rises 5 to 10 times beyond that anticipated from matrix permeability, despite the fact that this form of vuggy cannot be connected to porosity. Rock fabrics or petrophysical classes, according to Lucia (1999), are insufficient to accurately define the permeability in touching-vug pore systems. Lucia (1995) refers to the continuum of petrophysical classes as the "rock-fabric numbers" (RFN) The resulting global transform s given below: (1) Where K s the permeability, RFN s the rock fabric number with range from 0.5 until 4, and Фip s the fractional nterparticle porosity.According to RFN values, there are three classes of rock facies • Class 1 grainstone, thick crystalline dolostone, and grainstone that have been dolomitized.

Flow Units (Flow Zone Indicator Method)
A flow unit is a portion of sediment that has similar petrophysical characteristics to the sections above and below it, such as porosity, permeability, water saturation, pore-throat radius, storage, and flow capacity. Flow units are frequently grouped to define containers, and flow units were used to determine reservoir flow units by grouping rock types with similar flow capacities (Porras and Campos 2001). A flow unit is an identical reservoir processes rate that retains the geologic framework and rock type characteristics while being stratigraphically continuous. Rock types are reservoir units that, at a given height above free water, have a certain Porosity-Permeability relationship and a characteristic Water saturation (Gunter et al. 1997).
Rocks that make up carbonate reserves are often diverse. To properly understand reservoir dynamics, we must separate these reservoirs into zones, layers, and isolated units with lesser levels of heterogeneity. (Mohebian et al. 2017).
In this study, the flow zone indicator approach was used to split the Mishrif reservoir into a number of flow units. It is used to establish the geological qualities that are based on variations in pore-throat sizes that impact permeability in order to determine the geological characteristics that affect the flow of fluids. (Amaefule et al. 1993). In order to distinguish between different pore geometries (hydraulic units), FZI a special parameter, takes into account the geological characteristics of texture and minerology: ………………….. (2) ………….. (3) …………………… (4) Where FZI is a function of the reservoir quality index and void ratio (m), RQI is the reservoir quality index (m), ∅Z is a normalized porosity (pore volume-to-grain volume ratio) (fraction), and e is the effective porosity (fraction). Using Eqs. 2, 3, and 4, the functions for RQI against ∅z plot for each reservoir unit and have been created for all the wells. When the line and the data have the same slope, identical FZI values will fall on the line and similar flow units will be seen in the data (pore throat).

Microfacies Analysis using Microscopic Examination
For environmental analysis and correlation, the microfacies are totally sedimentological and paleontological standards which can be studied and characterized in thin sections (Flügel, 1982). The description of the core and thin sections that were available for the BU-1 well in the research region allowed for the identification of the significant facies of the Mishrif Formation in this study. The limestone microfacies of the Mishrif Formation were categorized according to Dunham's classification (1962).According to the types of microfacies which were identified n this study, the interpretations of the sedimentary environment are compared with standard microfacies of Wilson (1975) and facies zones for Flügel (1982 and 2004). Microscopic examination of thin sections results in to recognition of four major microfacies which are present in the Table 2, these are:

Lime mudstone microfacies
According to Dunham (1962), lime mudstone is a kind of limestone made primarily of microcrystalline calcite, also known as micrite. More than 90% of this microfacies is composed of micrite, and it also includes Allochems in ratios between 2% and 10%, Bioclasts, and lithoclasts, which are deposited in low-energy environments (Bathurst, 1975). Depending on its composition, this microfacies is deposited in deep marine skeletal lime mudstones with fine particle size (silt). Planktonic foraminifera, such as Hedbergella, Hetrohelix, and Oligostiginids, make up the majority of the skeleton grains. Most of the bioclasts are nice and unremarkable. There are also a lot of sponge spicules and less tiny echinoids. This facies is located in the bottom and topmost portions of the Mishrif Formation and is situated on the Rumaila Formation.

Wackestone microfacies
In this microfacies class, the proportion of skeletal and non-skeletal grains varies from ten to forty percent. Benthonic foraminifera like Dicyclina and Praealveolina, calcareous green algae, coral with echinoderms, and gastropods are among the significant fossils. Less frequent species included pelagic foraminifera, bryozoa, brachiopods, and sponge spicules. One of the frequent facies in the Mishrif Formation in the research region is the open marine microfacies. It is often found to the seaward of the rudist biostrome habitat. The predominant texture of the area is mudstone-wackestone.

Packstone microfacies
When compared to the microscopic matrix, this microfacies mostly consists of grains, which range in proportion from 40 to 90%. There are also small amounts of grains and other bioclasts, such as pellets and peloids. The packstone microfacies consists mainly of bioclasts of various sizes with echinoderm plates and mollusk bioclasts. This facies was deposited in Shoal environment that is principally consisting of medium to coarse-grained rudistid packstones. The shoal facies is formed in a high-energy environment

Grainstone microfacies
About 90% of this microfacies' fundamental structure is made up of grains, whether they are skeletal or not. The remaining 10% or so is made up of matrix with microsparite and pseudosparite textures. This facies is present in Mishrif formation of Buzurgan wells between 10 -15 m. The rudist biostrome microfacies was chiefly developed at Buzurgan wells. The microfacies consists of grainstone and interceded with packstone microfacies. The intergranular porosity have been present as well as vuggy and channels indicating varying intensity dissolution. The hydraulic flow unit ranges between 0.5-1.5 that s characterized by good porosity but very low permeability.

HFU-2
Class-3 RFN = 2.5-4 The hydraulic flow unit ranges between 1.5-2.5 that s characterized by good porosity but low to moderate permeability. The hydraulic flow unit ranges between 3.5-4.5 that s characterized by good porosity and high permeability. t s a classified a good reservoir zone

Calculation of Rock-Fabric Numbers (RFN)
Every carbonate reservoir's porosity and permeability data may be plotted to get the rock fabric number. As can be seen from the Lucia cross-plots for the three wells (Figs.3, 4and 5) the rock fabric number, which ranges from 0.5 to 4, is crucial for determining permeability. The distribution of rock fabric number along available porosity and permeability core shows the class 1 that range of rock fabric number from 0.5-1.5 (grainstone microfacies) as thin layer between other classes of Mishrif Formation. The class 2 between 1.5-2.5 that represents packstone microfacies was distributed where increasing porosity and permeability especially at the middle part of formation. The most common facies Wackestone to mudstone that represents class 3 of rock fabric number that ranges from 2.5-4. The Figs. 12,13,and 14 show vertical distribution for classes of rock fabric number n Mishrif Formation at available core measurements intervals. The results of these classes were compared with microscopic examination of microfacies from thin sections.

The Flow Units (Flow Zone Indicator)
iA cross plot of the logarithm of permeability versus. porosity data acquired from core and log studies is shown in Figs. 6.7 and 8. The enormous variation in pore throat diameters within each rock type shows large variations in particle size and sorting, which limit permeability. In the Mishrif Formation, four rock types and groups have been found, with the first representing poor reservoir quality (FZI-1) and the fourth showing a strong trend of permeability and porosity, indicating excellent reservoir quality (FZI-4 For various values of the Flow Zone Indicator, Figs. 9.10, and 11 show a cross plot of the logarithm of the reservoir quality index (RQI) vs the logarithm of the normalized porosity (FZI). All points on the similar FZI straight line are thought to have same pore throat characteristics (i.e., they represent the same hydraulic unit). Within the cored interval of the Mishrif Formation, these figures reveal the existence of four separate hydraulic units.

Conclusions
Lime mudstone, wackestone, packstone, and grainstone are the four principal microfacies identified by microscopic analysis of thin section studies. These rocks were deposited in basinal, lagoonal, open marine, and shoal settings. To demonstrate the classification of each rock type in the Mishrif Formation, a connection between rock types based on flow units and the sedimentary facies determined from core description and/or thin-section petrographic investigation is required. To sum up, figures 12, 13, and 14 show the changes in reservoir quality from BU-1 to BU-3. These graphs depict the vertical distribution of the hydraulic flow unit and RFN along the formation that was under study at the Buzurgan field. Even in diverse formations and depositional environments, the application of the petrophysical flow unit types technique on the empirical connections produces satisfactory results. Their reliability for various rock lithologic types is indicated by the calibration developed in this work. The petrophysical flow unit model appears to be a useful tool for separating nonproductive core samples from productive core samples and offers a good explanation of the porosity permeability relationship. It appears that the notion of petrophysical flow unit type, with their capacity to identify porosity-permeability connections, will be utilized more frequently in engineering applications, particularly those in petroleum engineering, which are frequently characterized by a high inherent complexity. During the application phase, the flow unit types produced positive results and may be used with new wells. Flow zone indicator results, Rockfabric numbers (RFN) that were divided into three classes, characteristics of rock types to identify the reservoir zones in the Mishrif Formation, and microfacies types.