Well Logs Analysis and Reservoir Evaluation of Hartha Formation in the East Baghdad oil field central Iraq

This study objective of ascertaining various petrophysical properties, which encompass volume shale, porosity, and water saturation. Well logging assumes a pivotal role in the comprehensive evaluation of reservoirs by providing measurements that can be effectively correlated with the volume fraction and composition of hydrocarbons within porous geological formations.In the course of this research, the quantification of shale volume was achieved through the utilization of gamma-ray logs, while the determination of effective porosity was facilitated by employing porosity logs, including the density log, neutron log, and sonic or acoustic log. The Hartha Formation has been stratified into distinct reservoir units based on their petrophysical characteristics. The interpretation of the findings involved the direct examination of well log readings, complemented by the application of well-established relationships and profiles to discern the critical lithological and petrophysical attributes inherent to the Hartha Formation. The selected wells from the East Baghdad oilfield, namely EB-93, EB-97, EB-51, and EB-42.


Introduction
Well logs are widely acknowledged as indispensable and valuable instruments employed by geologists for the comprehensive analysis of petrophysical characteristics within a geological formation.They play a pivotal role in the determination and characterization of diverse physical attributes of rocks, encompassing lithological composition, porosity, pore geometry, and permeability.Moreover, well log data assumes a critical role in the identification of productive zones, assessment of their thickness and depth, differentiation between water, gas, and oil reservoirs, and the quantification of hydrocarbon content within the reservoir [1].The practice of well logging carries significant significance across a multitude of applications, including electrical imaging, mine mapping, and the exploration of hydrocarbon and water saturation.Its primary objective revolves around the acquisition of on-site information regarding the properties of potential reservoir rocks.In particular, electric logs hold a prestigious position due to their capacity to furnish valuable insights for the assessment of fluid properties within the formation [2].

Geological setting
Hartha Formation encompasses substantial carbonate reservoirs characterized by notable productivity in the central and southern regions of Iraq, a fact attributed to its favorable petrophysical attributes and hydrocarbon abundance [3].The area of focus within this study pertains to the East Baghdad oil field, a renowned major oil field situated in central Iraq.Its initial discovery can be traced back to seismic survey interpretations conducted by the National Oil Company between 1974 and 1975 [4].The primary hydrocarbon production in this field is derived from the Hartha Formation, characterized primarily by limestone and argillaceous limestone lithologies within the East Baghdad oilfield.In the eastern sections of the Mesopotamian Zone, where the formation transitions into the Shiranish Formation, argillaceous limestone becomes increasingly predominant [5].Variations in the thickness of the Hartha Formation are observed due to gradational changes and occasional overlap with the Shiranish Formation [6].In the southern region of Iraq, the thickness ranges between 250 to 200 meters, whereas in the northern part, it maintains an average thickness of approximately 350 meters [4].Within the wells of the East Baghdad field, depths of the Hartha Formation range from 1874 to 2194 meters, with an average thickness of 289 meters [7].The area was damaged by several faults that extended towards the main structure (Northwest-Southeast), the Foredeep of Mesopotamia Significant tectonic-derived surface formations are uncommon, nevertheless, there are several features in the foredeep, including faults, folds, and diapiric structures, which are largely hidden by the Quaternary deposits, as depicted in Figure 1 [8].

Fig (1 )
Iraqi tectonic map with location of area study [7].

Location of study area
The study area is situated within the prominent East Baghdad oil field, a strategically important oil field located 20 kilometers to the east of the central district of Baghdad [9].Specifically, this study encompasses the analysis of four selected wells, chosen for examination and evaluation.Four wells are selected in this study included: EB-97 (in the Suwayra region),EB 93 , EB 51 and EB 42 (in Rashidiya) , as shown in Figures (2&3).

Methods and Materials
Log data for the four designated wells (Wells 97, 93, 51, and 42) by Middle Oil Company.Subsequently, this dataset underwent analysis and interpretation employing the Techlog46 software and mathematical equations within the Microsoft Excel.
The current study includes evaluating the petrophysical properties of Hartha reservoir, such as shale volume, water saturation, and effective porosity, and also dividing the Hartha reservoir into several reservoir units.The litholgy formation was also identified through some of the given logs,

Results and discussion
The provided data underwent interpretation, leading to the derivation of various outcomes, which are as follows:

Gamma-ray log (GR Log)
The gamma-ray log is a measurement technique used to assess natural radioactivity emanating from radioactive elements like Uranium, Potassium, and Thorium within formations.Its key functions include zone correlation and shale volume calculation [10].Elevated gamma ray readings indicate higher shale content, while shale-free sandstone and carbonate formations exhibit lower readings [11].High shale content in reservoir rocks can distort porosity and water saturation readings [12].
The gamma-ray log is employed for: 1. Lithology diagnosis.
: A gamma-ray reading by log (API).
: Minimum value for a gamma-ray log (clean sand or carbonate).
: Maximum value for a gamma-ray log (shale).
After extracting the gamma-ray coefficient from the previous equation, the volume of shale is calculated using equation (2) [14]: : The volume of shale.
Elevated gamma ray log readings are notably present within the middle section of the Hartha Formation, and these readings exhibit a discernible correlation with elevated shale content, as depicted in Figure 4.These heightened gamma ray values exhibit a gradual diminishment as 1300 (2024) 012028 IOP Publishing doi:10.1088/1755-1315/1300/1/0120285 one proceeds towards both the upper and lower boundaries of the formation.An analysis of the well data.Table 1 indicates that the average shale volume across the studied wells reveals a maximum average value of 0.11.

Figure (4)
The shale volume of the Hartha Formation was determined by utilizing the gamma ray logs of the wells.

Porosity logs
Porosity represents a fundamental characteristic of geological formations, precisely defined as the proportion of void spaces relative to the entire volume of the rock.The computation of porosity holds significant significance, as it directly correlates with the capacity for hydrocarbon storage within reservoir formations [15].Porosity can be classified into two distinct categories: effective porosity, characterized by interconnected void spaces, and total porosity, which encompasses all voids, whether interconnected or not.The present study is dedicated to the comprehensive determination of both effective and total porosity within the examined geological formation.

Neutron log
The neutron log is a radioactive tool used to measure hydrogen ion concentration within formations by assessing neutron capture from a neutron source [15].Neutrons are generated by bombarding formations with sources like americium, beryllium, or other radioactive materials [16].The neutron log's response is highly correlated with porosity due to the presence of hydrogen in all formation fluids (water, oil, gas), which are confined to formation pore spaces [16].
Porosity calculation using the neutron log relies on neutron collisions with formation nuclei, resulting in neutron energy loss.When a neutron collides with a hydrogen atom, the maximum energy loss occurs because of their similar masses.Consequently, energy loss is primarily influenced by hydrogen concentration, establishing a direct link between energy loss and formation porosity [10].

Density log
The paragraph outlines the porosity log's function, measuring electron density in formations, expressed in grams per cubic centimeter.This tool proves valuable for identifying evaporite minerals, detecting gas zones, assessing hydrocarbon density, evaluating shaly sand reservoirs, and analyzing complex lithology [10] [13].The density log emits moderate-energy gamma rays into the formation, interacting with electrons and losing energy through collisions.The collision count directly corresponds to the formation's bulk density [15].A rock's bulk density depends on its lithology and porosity, with porosity calculable using the equation provided [16]. Where: : Porosity by density log.
: Density of the matrix.
: Density of the fluid.

Sonic log
The sonic or acoustic log measures porosity by analyzing the interval transit time of compressional sound waves as they travel through a 2-foot formation segment, with the unit of measurement in microseconds per foot (μsec/ft) [10].Interval transit time (Δt) is affected by both lithology and porosity.Wyllie et al. established a porosity calculation equation in 1958, specifically for the use of the sonic log [16]. Where: Φs= sonic-derived porosity.
Δtlog= interval transit time in the formation (recorded by log).

Δtma= interval transit time in the matrix
Δtf= interval transit time of the formation fluid (saltwater mud= 185 μsec/ft, fresh water mud 189 μsec/ft).

Resistivity logs
Resistivity logs are electrical tools used to distinguish hydrocarbon-bearing from water-bearing zones [10] and to assess permeability and porosity.Resistivity measures a substance's resistance to electrical current flow, while conductivity is its reciprocal, measured in Ohm.m.Different materials have varying resistance levels, with hydrocarbons having high resistivity and saltwater-containing rocks having low resistivity.The primary methods for measuring formation resistivity include the normal log, laterolog, and induction log [16].
In the context of categorizing borehole zones into invaded (flushed and transition) and uninvaded areas, resistivity logs gauge resistivity in each zone.Specifically, Rxo measures the flushed zone, Ri quantifies the transition zone, and Rt characterizes the uninvaded zone.In this study, the Laterolog Deep (LLD) log is used to acquire Rt, the Laterolog Shallow (LLS) log provides Ri, and the Microspherically Focused (MSFL) log yields Rxo.

Porosity determination
As previously mentioned, porosity is assessed through porosity logs, including density, neutron, and sonic logs.This study focuses on two specific porosity types: total porosity and effective porosity.Their respective definitions and characteristics are elucidated below.

Total porosity
Total porosity encompasses the entirety of void spaces within the rock, including both connected and disconnected voids [11].Determining total porosity involves integrating neutron and density measurements, with the following equation illustrating the methodology for measuring total porosity: …………….. (7) Where: Φ N.D: neutron and density porosities combination (total porosity).

Effective Porosity
Effective porosity is characterized as the ratio of interconnected voids within the total bulk volume of the rock, facilitating fluid flow through the rock matrix.It specifically represents the void spaces that permit fluid passage.The calculation of effective porosity is accomplished using the following equation [17].

Water and Hydrocarbon saturation determination
Saturation analysis holds paramount importance in reservoir assessment.Once the clay content, porosity type, and proportions are determined within the rock formation, it becomes imperative to scrutinize the fluid content within the pore spaces.The objective of this analysis is to discern the nature of the fluids, whether they are water or hydrocarbons.The identification of hydrocarbon presence within the formation and the quantification of saturation levels as a percentage are pivotal for economic considerations.
Water saturation denotes the fraction of pore volume within a rock occupied by formation water, expressed as either a decimal fraction or a percentage [11].The water saturation in the studied wells exhibits significant decreases at both the upper and lower extents of the Hartha Formation, shown in Figure (6).Consequently, hydrocarbon saturation and production are notably higher in these segments of the formation.A gradual increase in water saturation is evident from well EB-42 to well EB-97, indicating a diminishing reservoir quality in the same direction.Well EB-93 stands out with a high water saturation, and minimal hydrocarbon saturation is observed in the upper part Figure (6), thus classifying EB-93 as a dry well.Table 3 provides an overview of the average water saturation across the study area.

Reservoir units of Hartha Formation
The well log data from four wells has been input into Techlog64 software to facilitate the generation of Computer Processed Interpretation (CPI) for the Hartha Formation.This CPI analysis has led to the subdivision of the Hartha Formation into seven distinct reservoir units, organized in descending order as Ha-A, Ha-B, Ha-C, Ha-D, Ha-E, Ha-F, and Ha-G.These units exhibit distinctive reservoir properties, which are elucidated as follows: • Unit H-A: Located at the uppermost section of the Hartha Formation and overlain by the Shiranish Formation, Unit Ha-A is distinguishable from the lower Unit Ha-B by a significant reduction in water saturation.It boasts good reservoir quality, primarily attributed to elevated values of effective porosity and hydrocarbon saturation.However, these attributes experience a decline owing to elevated shale volume at specific depths within wells EB-93 and EB-51 Figures (7 & 9).
• Unit H-B: Unit H-B sets itself apart from Units H-A and H-C through lower shale volume values, while still featuring a notable effective porosity.Reservoir quality within this unit exhibits enhancement in its upper segment due to heightened hydrocarbon saturation.In the case of well EB-93 Figure (7), reservoir quality is notably diminished due to elevated water saturation.The greatest thickness of this unit is recorded in well EB-42, reaching 82 meters Figure (10).
• Unit H-C: In • Unit H-F: Displaying the finest reservoir quality within the Hartha Formation, Unit Ha-F is marked by a thick uppermost interval distinguished by elevated effective porosity and hydrocarbon saturation.This interval can attain a thickness of up to 60 meters, as observed in well EB-42 Figure (10).The lower portion of this unit, however, exhibits reduced reservoir quality due to heightened water saturation despite having high effective porosity.
• Unit H-G: Situated at the lowermost stratum of the Hartha Formation, overlaying the Saadi Formation, Unit H-G registers negligible reservoir quality in comparison to Unit H-F.This disparity is primarily due to heightened water saturation and reduced porosity Figures (7-10).

Lithology determination
Lithology models were developed to represent reported lithology based on rock cutting descriptions and composite logging data.The widely accepted method for determining formation lithology in the subsurface domain is the neutron-density cross plot.However, the accuracy of this analysis depends on log quality, the selection of lithology zones for interpretation, and consideration of factors like gas and fluid content, which can affect neutron and density measurements.In Figures (11 to 14), most cross plot points align with the limestone line, with others closely following the dolomite line.This indicates that the primary lithology of the Hartha Formation in the studied wells is limestone, confirming findings from lithologic data and microfacies studies that highlight the dominance of limestone facies.

Conclusion
The interpretation of the computer-processed data (CPI) from four wells in the East Baghdad oilfield was conducted utilizing Techlog.64 software.
1.The CPI analysis revealed that the Hartha Formation can be categorized into seven distinct reservoir units, namely H-A, H-B, H-C, H-D, H-E, H-F, and H-G.Based on the analysis conducted through the computer-processed interpretation (CPI), it has been determined that the (H-F) reservoir unit stands out as the productive unit within the Hartha Formation.
2. This particular unit (H-F) exhibits favorable reservoir properties, including high effective porosity, low water saturation, and low shale volume.In contrast, the remaining units display relatively less efficient properties due to their lower petrophysical characteristics.
4. The lithology of the Hartha Formation is determined by analyzing the cross-plot relationship between total density and neutron measurements, which enables the identification of limestone.

Figure ( 2 )
Figure (2) The location map displays the locations of the wells studied in the Hartha Formation.

Figure ( 3 )
Figure (3) The contour map displays the locations of the wells studied in the Hartha Formation, along with the fractures present in the area (Middle Oil Company).
Φ t: total porosity (Φ N.D).Vsh: volume of shale.Within the study wells, elevated porosity values (both total and effective) are evident in the upper and lower segments of the Hartha Formation, signifying a favorable reservoir capacity for hydrocarbon 1300 (2024) 012028 IOP Publishing doi:10.1088/1755-1315/1300/1/0120289 storage.Conversely, the mid-section of the formation exhibits diminished effective porosity values, closely associated with elevated shale volume readings.There is an observable increase in effective porosity in the central part of the formation.These variations in porosity values are consistently evident across all examined wells, Fig.5.Table2presents the average effective porosity values for the study area's wells.

Figure ( 5 )
Figure (5) Porosity calculations of Hartha Formation by utilizing the well logs data.
The shale volume of the Hartha Formation was determined by analyzing the gamma ray logs of the wells under study. (1)ble(1)

Table ( 2
) The average values of the effective porosity, calculated as a percentage, for the studied wells.

Table ( 3
) The mean value of the water saturation, calculated as a percentage, for the studied wells.
contrast to Units H-B and H-D, Unit H-C showcases elevated shale volume values, resulting in lower reservoir quality.However, at certain depths, this unit presents increased values of effective porosity and hydrocarbon saturation, a pattern consistent across all studied wells except well EB-93 Unit H-E is characterized by a marked reduction in effective porosity and hydrocarbon saturation.It can be considered a cap unit overlying Unit H-F, which offers significantly superior reservoir quality.The thickness of Unit H-E remains relatively consistent across the studied wells Figures (7 to 10).