Well Logs Analysis and Petrophysical Properties of Shiranish Formation in the East Baghdad Oil Field, Central Iraq

The objective of this study is to characterize of the petrophysical properties of Shiransih Formation in East Baghdad oilfield. The analysis involves both of examination of Well Log data obtained from specific wells, namely EB-86, EB-84, EBSK-10-4, EBSK8-2, and EBSK-5-4. The Gamma Ray Log is utilized to quantify volume of shale within the formation, while Porosity Logs, including Density Log, Neutron Log, and Sonic Log, are employed to determine effective porosity and index secondary porosity. Water saturation, movable hydrocarbon, and residual hydrocarbon are calculated using Resistivity logs (LLd, MSFL, and ILD). Additionally, a cross-plot analysis is performed utilizing Density Log and Neutron Log to determine the lithology of the Shiransih Formation.


Introduction
Petrophysical log interpretation stands as a highly valuable and essential tool in the arsenal of a petroleum geologist.These logs play a crucial role in defining various physical properties of rocks, including lithology, porosity, pore structure, and permeability.By analyzing logging data, geologists it is possible to identify zones that have the potential for productivity, one can ascertain the depth and thickness of these zones through analysis, differentiate between oil, gas, or water within a reservoir, and estimate the storage of hydrocarbons present identifying clay volume from Gamma Ray log [1] 1. Estimating porosity formation from porosity logs.2. Differentiating between zones containing hydrocarbons and those containing water from resistivity logs.3. Calculating water saturation, residual Hydrocarbon and movable Hydrocarbon.4. Lithology identification from cross plot by Density-Neutron logs.
The formation holds economic significance as it is recognized as an oil reservoir in the Kurdistan region and northwestern Iraq.This is attributed to its abundant fractures, joints, and secondary porosity [2].The practice of well logging holds considerable importance in various applications, encompassing electrical imaging, mine mapping, and the exploration of hydrocarbon and water saturation.Its primary purpose is to acquire on-site data pertaining to the characteristics of potential reservoir rocks.Electric logs, in particular, occupy a significant role thanks to their ability to provide valuable insights into assessing fluid properties within the Formation [3].
Well logging plays a crucial role in the assessment of reservoirs, as it objective is to furnish measurements that can be correlated with the composition and hydrocarbons volume present in porous formations [4].East Baghdad oilfield represents a collection of oil fields, Situated east of Baghdad, it is positioned in the central region of Iraq, far about 20 kilometers from middle of Baghdad to the east.The entirety of the southern region encompassing the East Baghdad project is 120 kilometers.The East Baghdad oilfield covers in length about 120 km and about 25 km in width, with general direction NW-SE [5].

Geological setting
The Formation gradually passes into the Tanjero Formation to the NE.The Shiranish Formation is widely distributed in northern Iraq.Towards the west and southwest, the formation passes laterally and by intertonguing into the Hartha Formation.The Shiranish Formation, originally delineated by Henson (1940) in the High Folded Zone of Northern Iraq, proximate to the Shiranish Islam village, northeast of Zakho, exhibits remarkable similarity to the Qurna Formation, introduced by Rabanit in 1952, in Southern Iraq.The Shiranish Formation is lithology characterized by mudstone, marl, and intermittent limestone beds.In Southern Iraq, it commonly serves as the sealing unit overlaying the Hartha and Tayarat reservoirs.Conversely, in central and Northern Iraq, it can function as a reservoir in instances where it undergoes tectonic fracturing within the cores of anticlines.The Formation gradually passes into the Tanjero Formation to the NE [6].The region has been impacted by multiple faults extending in a Northwest-Southeast direction towards the primary structural element, which is Foredeep of Mesopotamia.While tectonically-derived surface formations of significance are infrequent, notable geological features within the foredeep encompass faults, folds and diapiric structures, which are predominantly concealed beneath the Quaternary sedimentary deposits [8].
Figure (2) illustrates the spatial distribution of the Shirianish Formation within an oilfield located east of Baghdad.Started from topes of the wells (EB-86 and EB-84( are positioned at the crest of the anticline, whereas the remaining wells (EBSK5-4, EBSK8-2, EBSK10-4) are situated on the flank of the anticline, towards the Nahrawan area.

Figure (2) (IP) program computer illustrate Shiranish Formation In Oilfield East Baghdad
The Formation's main areas of distribution are, however, the Foothill and the High Folded Zones.In the area of those units, the formation is also.Widespread on the neighboring Syrian and Iranian territories.Towards the north and northeast the, formation laterally 'passes into the flysch type sediments of the miogeosynclinal i.e. mainly into the Tanjero Clastic Formation.Extensive distribution across northern Iraq.In the western and southwestern regions, it transitions laterally and intertongues with the Hartha Formation (Fig. 1).The Shiranish Formation, as observed in its type development, represents a characteristic sedimentary formation associated with deeper open-sea environment [9].

Methods and Materials
Data were obtained from the East Baghdad Oilfield in Las file type, field using the IP (3.5) interactive petrophysical properties program to interpretation logs.The methodology employed in this study involved several steps.Firstly, by the Gamma ray log clarifies the shale volume of the Shiranish Formation.
Secondly, Porosity logs were utilized to determine the total, effective, and secondary porosity of the formation.Thirdly, electrical resistance logs were used to differentiate between oil and water, enabling the calculation of water saturation, movable hydrocarbon, and residual hydrocarbon.The dominant fluid in the formation was identified through equations to determine water saturation.Lastly, a cross-plot analysis between Density and Neutron logging techniques were employed to ascertain the lithology of the formation.

Results and Discussions
1-Gamma-ray log (GR Log) Gamma ray logs are serve as lithological measurement tools that measure the radioactivity of a formation.Gamma ray logs are utilized for the purpose of identification lithological characterization, Formations correlation and determination volume of shale [1].

1)
IGR: Index Gamma-ray.GRlog: Gamma-ray recording by log (API).GRmin: Gamma-ray log for Minimum value (clean sand or carbonate).GRmax: Gamma-ray log for Maximum value (shale).Shiranish Formation is classified as ancient rock originating from the Cretaceous period, the shale volume formula employed in this study aligns with the Larionov (1969) methodology designed for old rock formations [1].
Table (1) shows the minimum and maximum values of the calculated volume of shale in the selected wells.In this table, distinguished in well (EBSK10-4) maximum recording of clay was in unit (SH2) and minimum recording was in unit (SH1).In wells (EBSK8-2) (EBSK5-4) (EB-86) (EB-84) the maximum recording was in unit (SH3) and minimum recording was in unit (SH1).

Porosity logs 6
Porosity serves as a fundamental rock characterization parameter, providing an estimate of the potential volume available for fluid storage [14].
Porosity of the rock can be determined using the sonic log, density log, or neutron log.These devices are influenced by the formation's porosity, fluid, and matrix.If the effects of the fluid and matrix are identifiable or can be determined, the tool's response can be correlated with the porosity.[12].
Porosity refers to the ratio of pores to the overall volume of the rock.It is typically represented as a decimal fraction or a percentage and sometimes expressed by Greek letter Phi (ࢥ) [1].

2-1 Neutron log (NPHI Log)
Gas zones can frequently be discerned through a comparative analysis of the neutron log in conjunction with either an additional porosity log or a core analysis.The utilization of the Neutron log with one or further supplementary porosity logs enhances the precision of porosity assessments and facilitates lithology identification, extending to the evaluation of shale content [13].
Neutron Log be specialized Porosity Logs that quantify the (H2) condensation in a formation.In clean formations (i.e., shale-free) where the porosity is occupied by either water or oil, the Neutron log is instrumental in mensuration the porosity full of with liquid substances.(ĭN, PHIN, or NPHI) [1].

2-2-Density log
Density log employs two distinct density values: the Bulk Density (ȡb or RHOB) and the Matrix Density (ȡma).Bulk Density corresponds to overall density of formation, encompassing both solid and fluid components, as determined by the logging instrument.Conversely, matrix density pertains specifically to the density of durable structural framework within the rock.The density log provides valuable support to geologists in the following ways [1].
Reveal minerals associated with evaporation processes.Locating zones containing gas.Determine the density of hydrocarbons.Assess reservoirs with a combination of shale and sand, as well as complex lithologies (Schlumberger, 1972) Where: ĳD = Density derived porosity ȡma = Matrix Density ȡb = Formation Bulk Density ȡf = Fluid Density Sonic log, a specialized porosity measurement tool, use to measure the duration transit time (¨t or DT) of a compressional sound wave as it traverses geological formation in line with the borehole axis.This duration transit time (¨t) is subjected by both lithological characteristics and porosity.Consequently, in order to calculate sonic porosity, it is imperative to possess knowledge of the formation's matrix interval transit time, as delineated in (Table 2), which can be [1].

3-Resistivity logs
Resistivity logs are electrical well-logging tools employed for several critical purposes, including distinguishing hydrocarbon-bearing zones from water-bearing ones, identifying permeable intervals, and assessing resistivity-based porosity.All these applications, the primary significance lies in differentiating hydrocarbon-bearing zones from water-bearing ones, owing to the non-conductive nature of rock matrices or grains.In such cases, the rock's capacity to transmit or carry an electrical current is primarily determined by presence of water in its pore spaces.The most commonly applied types of resistivity logs for these purposes involve induction logs, laterologs, and microresistivity logs [18].

4-Porosity Estimation
The estimation of porosity can also be obtained from a solo log or integration of porosity logs, in order to account for impact of varying lithological influences in a complex reservoir.In carbonate Formations, minerals are mixtures include calcite, dolomite, quartz and may by anhydrite and gypsum also occur [1].

4-1 Total porosity
The determination of total porosity can be computed by employing the Neutron and Density logs, as depicted in following formula [9] and [19]: ߮t: Total Porosity ߮N: Neutron-deduce Porosity.߮D: Density-deduce Porosity.

4-2 Effective Porosity
It a proportion about interconnected network pore volume in relation to overall girth rock.In shalefree formations, this interconnected porosity facilitates fluid mobility.Effective porosity (ĭe) can be counting using equation employed in the present study [17].

4-3 Secondary Porosity
Secondary porosity develops after the deposition and burial of a geological formation and is considered more significant than primary porosity.Fracturing, solution, and chemical replacement are the primary mechanisms that contribute to the formation of secondary porosity.The variation between neutron-density porosity and sonic porosity serves as an index for secondary porosity.[11] as illustrate in following formula: SPI = (Ɏt -Ɏs) --------------- (11) Where: SPI = Secondary porosity index.Ɏt = Total porosity from (Neutron-Density log).Ɏs = Porosity derived by Sonic log.Water saturation (Sw) determination within the uninvaded zone of a reservoir represents a pivotal role of reservoir evaluation.Archie's equation serves as the essential parameter for this purpose.Archie's equation, formulated in 1942, holds paramount significance in the field of reservoir assessment [1].

5-Water and Hydrocarbon Saturation computation
The calculation of water saturation within formation's flushed zone (Sxo) also relies on the application of the Archie equation.However, in this context, two variables undergo modification: the mud pervade resistivity (Rmf) is employed of lieu the formation water resistance (Rw), and flushed zone resistivity (Rxo) replaces .uninvadedzone resistivity (Rt).The determination of water saturation within the flushed zone (Sxo) serves as a valuable index for hydrocarbon mobility.Comparing water saturation derived from shallow resistivity tools (Sxo) with that computed using deep-reading resistivity tools (Sw) to assessment of the quantity of movable hydrocarbons displaced by drilling fluid.If the value of (Sxo) significantly exceeds that of ( Sw), it suggests the possibility that hydrocarbons within the flushed zone, nearest to the borehole, have been displaced or flushed out due to the drilling mud [1].
According to schlumberger (1987) can estimate the hydrocarbon saturation utilizing equation: The calculation for Imprisoned hydrocarbon saturation (Shr) and mobile hydrocarbon saturation (Shm) be achieved through utilization of following equations, which are derived from the water saturation of the uninvaded zone (Sw) and the water saturation within flushed zone (Sxo) [1].Shr = (1-Sxo) ------------- (15)   Shm = (Sxo-Sw) - -----------------( The result In this study, the Shiranish Formation divides into three zones according to the variation in the sensor readings.These zones are SH1, SH2 and SH3 to facilitate determination the petrophysical characteristics of Shiranesh Formation in the East Baghdad oilfield in central Iraq.These zones are:-(SH1) the Caliber log reading for all wells is homogeneous.This means that the wells are homogeneous and peaceful from geological processes only in Well (EBSK10-4) showed the decrease of recording at the depth of the well in (1891-1895), because there is a swelling in rock.The increasing of Caliber recording from the depth (1900-1914) increased slightly, this means the presence of caverns in the shale rock.
The porosity logs show a decrease in the neutron log (NPHI) and the sonic log (DT) with an increase in the density log (RHOB) that mean compacted Rock.Within wells (EBSK10-4, EBSK8-2, EBSK5-4 and EB-86), the MSFL recording increased, along with the values of Rxo and Ri.However, LLD recording decreased, indicating a reduction in RT as well as water saturation (SW).In well (EB-84), the MSFL recording decreased in comparison to LLD, indicating the presence of movable hydrocarbon and a decline in water saturation (SW), particularly when Rxo was lower than RT, suggesting the presence of movable hydrocarbon.
. The matrix, from the log some recorder are 2.71 and other more than so its between Limestone and Dolomite.Water saturation began to decrease significantly.
There is an increase in the total (PHIT) and effective (PHIE) porosity, while the Index secondary porosity (SPI) decreases.The type of fluid was water dominated.The Gamma Ray log indicates a decrease in shale volume as recorded by the log, although there are some depths where slightly higher readings are observed in the well (EBSK10-4).
(SH2) The caliber log recording only in well (EBSK10-4) is homogeneous, meaning the well is free from geological processes.As for the remaining wells, recognized an expansion in recording of the caliper log, this means the presence of cavitation.
The porosity logs show an increase in the neutron log (NPHI) and acoustic log (DT) but they indicate a decrease in conjunction with the density log (RHOB).In well (EBSK10-4), both LLD and MSFL recording exhibited similar and close values, resulting in high water saturation (SW).In wells (EB-84, EBSK8-2, EBSK5-4, EB-86), a decrease in MSFL reading relative to LLD indicated the presence of movable hydrocarbon with a significant decrease in water saturation (SW).This effect was most pronounced in wells (EB-84 and EBSK8-2), which differed notably from the other wells.In some depths, the MSFL recording in these wells exceeded the LLD reading, indicating the presence of residual hydrocarbon.
The matrix, the wells (EB-86) (EB-84) (EBSK 10-4) (EBSK5-4) were distinguished that they have vales of matrix above 2.71 or equals in some depths so they are between Limestone and Dolomite.As for well (EBSK8-2) log recording were ringing from 2.71 to 2.65 so it is between sandstone and limestone.In water saturation, decrease in water saturation, corresponding to an increase.Oil saturation, and the total and effective porosity is the same consistency and homogeneity with a decrease in secondary porosity.Gamma ray log, slightly increasing in volume of shale so lethargically, the limestone has high clay contain and more pore space.
(SH3) Regarding the caliber log, the well (EBSK10-4) that distinguished was the same zone above (SH2), which has not geological processes that affect the recording caliber log.As for the rest of the wells, the log recordings are not homogeneous because the wells contain cavitation.
The porosity logs, depths where the recordings increased and where they decreased according to the petrophysical characteristics.Recognize at depths where (NPHI) and (DT) logs recordings increased and density (RHOB)log recordings decreased which is the rock is porous , as they were depths with high porosity and contain fluids with low density.There appears a decreasing in neutron log(NPHI) recordings with the sonic(DT) logs and increasing Density(RHOB) log in some depths because these depths have low porosity and few fluids with high density its mean the rock is compacted.
At the initial depth, all wells showed a high shale volume (5m) along with increased resistivity logs recording, which mean high compact of beds due to diagenetic processes.Well (EBSK10-4) also exhibited similar and close LLD and MSFL recording, leading to high water saturation (SW).In contrast, the other wells (EB-84, EBSK8-2, EBSK5-4, EB-86) experienced a decrease in MSFL readings compared to LLD recording, which signified the presence of movable hydrocarbon and a substantial decrease in water saturation (SW).This trend was particularly prominent in wells (EB-84 and EBSK8-2), which differed significantly from the other wells.
The matrix, log recording has increased in wells (EB-86)(EBSK10-4) (EMSK5-4) more than 2.71 due to diagenetic process occur that altered limestone into Dolomite.As for wells (EBSK8-2) and (EB-84), there are depths more than 2.71 and lower depths from log recording so it ¶s between Limestone, Sandstone and Dolomite.Water saturation, a decrease in water saturation, compared to an increase in oil saturation.PHI, (PHIT) and (PHIE) porosity are similar behavior in all wells, while there was a decrease in the index secondary porosity (SPI) in which oils are present.
Only (EB-84) have high percentage of oil, while the rest of the wells, according to records in this zone, included water.Gamma ray log, illustrated the clays are prevalent in this zone in all the wells, according to the recorder of the log with high porosity and limestone.The below figures from 3 to 7 will show the petrophysical properties in all wells of studied area.(SH3) in well (EBSK5-4) limstsone most dominated, well (EBSK8-2) between limestone and sandstone, (EB-84) between limestone and sandstone with dolomite in some depth and in wells (EB-86, EBAK10-4) limestone with dolomite.The below figures from 8 to 12 will show the lithology in selected wells.Based on the cross-plot analysis, our findings indicate that the first zone primarily consists of compressed limestone resulting from diagenetic processes.This zone also contains dolomite and exhibits limited pore space.In contrast, the second and third zones exhibit a combination of limestone and significant pore space, some wells within these zones also contain layers of sand.

Conclusion
According to the petrophysical analysis of the Shiransh Formation that can deduce the first zone exhibits high density due to diagenetic compaction processes.Additionally, it displays minimum porosity and highest water saturation.In contrast, second & third zones demonstrate favorable petrophysical properties.While there is evidence of oil presence in certain wells, it is highly saturated water contain.

Figure ( 4 )
Figure (4) Petrel program printing the distribution of selected wells in East Baghdad oilfield.