The Mishrif Reservoir Characteristics Utilizing Well Log Data Interpretation in the Fauqi Oilfield in Maysan, Southern Iraq

Abstract


Introduction
Zagros Fold Belt is an orogeny event controlled by flexural mechanisms, making it ideal for hydrocarbon generation and preservation.Multiple stacked sources in the Zagros zone ensure continual hydrocarbon generation, whereas stacked contingents promote a low tapering and massive wedge-top basin, The study area was limited in the southern part of the Zagros belt Basin in Maysan Governorate, which includes three promising oil fields, namely: Fauqi, Abu Ghirab, and Buzurgan (Aqrawi et al., 2010) (Fig. 1).
From the contour map of the Mishrif Formation top (Fig. 2 ) and the oil-water contact (OWC), it can be noticed it had two domes, northern and southern with striking bed elongation northwestsouth east and synclinal form between them (Fig. 1).The Fauqi oilfield was located in Maysan, about 50 km northeast of Amara city.Also, noticed that part of the field was placed in the Iranian lands regarding to the borders.The depth of the Mishrif Formation documented in the Fauqi oilfield is approximately (-4000 m), varied by (1000 m) from neighboring fields and (2000 m) from other fields in the Mesopotamian Basin in southern Iraq.The Mishrif Formation first figured in the southern Iraq by Owen and Nasr (1958), Alsharhan and Narin (1986).The Mishrif Formation petrophysical properties were prescribed by its content of an organic detrital limestone (wackstone and packstone) accompanied with rudist, algal, and colonial coral limestones (Al-Najm et al., 2022).In the study area, the formation is composed of dense, algal limestones, corals, and of detrital, porous, and foraminiferal limestones with rudist debris below (Aqrawi, 1998).Mishrif Formation was deposited in an environment of carbonate platform ramp graded from restricted to open marine (Kendall et al., 2020).Five facies have been recognized in the Mishrif Formation.These facies are lagoon, reef, shoal, open marine, and sub-basinal (Gili et al., 1995 andAqrawi, 1998).Shoal facies was initially composed of grain rocks, grain stones and packstones, whereas Rudist biostroms were dominated by autochthonous rudists within grain-stones and packstones.Lagoon facies were deposited in restricted areas within the platform of Mishrif Formation, and are characterized by a variety of benthonic foraminifera fauna composing miliolids, alveolinids and.The Mishrif Formation was divided into three rock units (A, B, and C) with seven stratigraphic units (MA, MB11, MB12, MB21, MB22, MC1, and MC2) (Beydoun, 1988 andReulet, 1970).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 (Al-Ameri et al., 2009); (Jun et al., 2016); (Liu et al., 2018); (Al-Khafaji et al., 2021); (Al-Aradi et al., 2022).The Mishrif Formation was identified as the main hydrocarbon reservoir in the Fauqi oilfield and some other fields in adjascent areas, it was withal labeled as the second fecund petroleum reservoir in the fields of southern Iraq (Awadeesian et al. 2018).Although, this formation lithostratigraphically consists mainly of limestone in most fields in the southeastern part of Iraq, it endorses as vertically stratified of carbonates and shale in the south of Iraq as in Basra that reflected in a large number of zonations in this region to reaches eleven units (Al-Mimar et al., 2018).
The method of investigation is based on the analysis of lithological properties and the determination of reservoir characteristics of the formation, such as porosity and oil saturation, obtained from log data interpretations analyzed with computer software and core data obtained from M. O. C. The study butt is to evaluate and recognize the Mishrif Formation reservoir characteristics in both north and south domes and identify the reason of being the south dome had most of the dag wells.

Materials and Methods
Data from five wells in the Fauqi oilfield were employed in this paper to estimate and evaluate the main reservoir characteristics and lithology.The wells were FQ-4, FQ-6, FQ-19, and FQ-20 placed in the south dome, and FQ-14 in the north dome (Fig. 2).The initiative logs used in this study were gammaray, neutron, density, resistivity, sonic, and self-potential logs.Software applications that helped to final erect were Didger 5, Surfer, Microsoft Excel, Corel Draw, and interactive petrophysics.Didger software with Microsoft Excel program at the first stage of the work was used to digitize the image log as well as the time structural map and export it to text file readable by Surfer software to gain the wanted file.The second step was to import these files using Interactive Petrophysics software version 3.5.12for interpretation.An interactive petrophysics software plots and cross plot were used to interpret the lithology, shale volume, porosity, and water resistivity .
The estimation of shale volume in the Fauqi Mishrif Formation was calculated depending on Gamma-ray log interpretation with a standard function dealing with old rocks that have an age of pre-Tertiary.The standard equation of Schlumberger, 1972 was used to calculate gamma-ray index (IGR) through (Schlumberger Ltd. 1984): = (  −   )/(  −   ) (1) Where: IGR represent the Gamma ray index, GR log record for Gamma ray log of the zone of interest totally about (3900 -4280 m), GR min, GR max are minimum and maximum reading for gamma ray respectively of shale volume plot over (API) unit.Later, as a second step to final estimate of shale volume (Vsh) of the Fauqi Mishrif Formation by calibrating the gamma ray index value depended on 0.33 as a constant for the rock with old age of Tertiary rock (Larionov 1969): (2) Where: V sh represents the decimal fraction of shale volume, and IGR is the gamma ray index obtained from equation 1.
Mishrif Formation porosity was calculated from different log readings, but the total porosity was estimated from both neutron and density logs, as a combination in (%) unit.The porosities were øN of the neutron reading porosity and øD is the porosity recorded for the density log.The secondary porosity index (SPI) was calculated relating to Schlumberger (1989): (3) Where: øND is the combination porosity of Neutron and density logs (%), and øS is the primary porosity recorded for sonic log (%).The last step of porosity calculation was to estimate the effective porosity using the Schlumberger, equation (Schlumberger 1972): ) Where: øeff represents the effective porosity, øND is the combined porosity gained from the Neutron and Density logs, and Vsh is stand for the shale volume obtained from the equation ( 2), all in decimal fraction.To estimate the exact value of porosity, the measured log values were correlated with the core poro sity analysis values of wells FQ-6 and FQ-19, by making a cross plot between the two readings from the two sources in linear-linear equation to identify the relationship type.
Another parameter to be calculated was water saturation SW depending on Archie's equation which is measuring the relation between porosity and resistivity of the formation (Archie, 1950): Where: SW is the water saturation in decimal fraction, (a, m, and n) are Archies's parameter stand for: (a) is the tortuosity, (m) is saturation exponent, and (n) is for cementation exponent with value of (1, 1.77, 2) respectively, Rw is the water resistivity of formation recorded from Maysan Oil Company (M.O.C) with value of (0.0217 ohm.m), and Rt is the true resistivity of formation whereas the calculated Rw from Pickett plot was (0.0214 ohm.m) (Fig. 3).The (a, m, and n) values calculated with interactive petrophysics software with the logarithmic scale Pickett plot, and the true resistivity was measured directly from the resistivity log (Pickett, 1966).

Results and Discussions
This study focused on the principal characterisitics to discriminate the importance of zones in the formation, and these characterisitics are: shale volume, porosity and permeability, water saturation, and lithology.

Shale Volume Estimation
The volume of shale in the formation was considered a reflectance of the porosity and water saturation, and of that it takes an important place in petrophysical calculations when studying the reservoir characteristics to evaluate the hydrocarbons in the targeting layers (Beydoun, 1989).Considering the values of total and effective porosity took from neutron and density logs for two wells and the cute changes in Gamma ray reading were employed to limit the formation units.The chosen wells were FQ-4 and FQ-19 from the south dome (Fig. 4-a), and the values of shale volume according to this calculation were in range of 10% -28 % of the bulk volume.The lower value was at the MB21 unit and may increase up and down ward away from this unit (Table 1).

Porosity calculation
The porosity considers one of the most important properties of the fluid (including hydrocarbons) preserves potential for a good reservoir (Ehrlic et al., 1984).Neutron and density logs readings were dependent in porosity estimation values, and the best value recorded was in the zone of MB21 unit which was about 16% with a range of unit's porosity values 4.5% -16% (Fig. 4-b).
The porosity value represents both primary and secondary values, where the primary porosity developed during the period of sedimentation, where it is calculated from the sonic log interpretation, while the secondary porosity post developed over tectonic or/and diagenitic processes (van Golf-Racht 1982).The secondary porosity was distinguished character for the carbonate rocks more than in the clastic one, and in this study, the evaluation of the secondary porosity for the Mishrif reservoir formation was calculated from the variance between the total and primary porosity as a secondary porosity index (SPI) (Ghafoori et al., 2009).The secondary porosity was graded between 1.5% and 5.5 %, and the values showed that the best developing secondary porosity overall was recorded in the MB21 unit.
For all registered porosities, an effective porosity was considered the more important one because it was representing the interconnection of pores that permit fluid to be moved through.The modifying porosity with the values recorded to the core samples endorsed the estimation of effective porosity and it was in the averages in the MB21 with a grade of 16%.
More precise porosities of rock formations are usually recorded from the core sample with some difficulties, but the symbiosis between core porosity measuring and log reading is essential to reach the true values.The core porosity for this paper was gained from Maysan Oil Company for two wells .
The porosity estimation from the well log data was shown in the (Fig. 5) below, where a primary recorded was in low values of 4% to 8.9% , and the secondary porosity was at low to fair decimal fraction in values about 5% to 12%.All the recorded data for the well logs and the obtained data that belong to the core measuring porosity showed the MB21 unit of the Mishrif Formation in the Fauqi oilfield had the high value of secondary porosity, but with less than MB11, and MB12 in the primary porosity.
The type of the diagenesis processes that responsible of secondary porosity in the MB21 reservoir unit are not evaluate in this paper according to the lack of the data involved with petrographic study, but by sighting the previously registered data at the library of reservoir directorate and reviewing the last studies, all of it indicate to the dissolution process especially in the rudist facies layer of MB21 reservoir unit (Albahadily and Nasser, 2017), and compared this information with the studies on the Mishrif equivalent formation in south Iran, Sarvak Formation, the results tend to the same fact ( Ghafoori et al., 2009).The lonely well in the north dome was FQ-14, and the data recorded for this well was week regarding (Fig. 5) another type of diagenesis processes affecting in this region representing in the cementation and compaction which need special tool to make oppositely treatment.The noticeable thing was that most of the productive oil wells took place at the south flank of Jabal Fouqi oilfield just like the most oilfields in the Maysan Governorate zone.

Permeability calculation
The calculation of permeability (K) depended on Timur's method to estimate the value where its calaculation without aiming to limit the type of fluids in reservoir voids (Timur, 1968).The equation considers the ratio of porosity to the irreducible water saturation in manner of Timur constant, and the equation: Where: K is the permeability in millidarcy, ø is the porosity; SWi represents the irreducible water saturation water in the formation, and a, and c are constants.
The results showed that the best values were in the MB21 units and by comparison with values obtained for porosity, the best linear relationship was also noticed in the Mb21 unit (Fig. 6).

Water Saturation Evaluation
The parameter of water saturation is important as a petrophysical character to limit the approximate hydrocarbon content in a reservoir (Pickett, 1973).The main dependent function to measure the water saturation in the formation is resistivity.Regarding the hypothesis of considering all the voids in rock formation will be filled with fluids of water and hydrocarbons (oil and gas), it may contains one phase or two and in special cases filled with three phases of fluids (Kamel and Mabrouk, 2002).The most important target is the volume of the hydrocarbon in the reservoir, but with no easy way to estimate its quantity, and where the volume of water could be estimated with more than a method, the hydrocarbons volume gained from the percentage of water saturation from the equation: Where: hydrocarbons saturation is (Sh), and water saturation is (Sw).The water saturation was calculated for in the first time depending on electric resistivity log for the clean formation (non-shaly), because for the formation that contain a high value of shale (30% and more) there was an additional conductivity came from the mud that caused an error in the calculation.Another formula to measure the water saturation for the clastic rocks with high shale percentage had been neglected in this study as we deal with a carbonate formation (Archie, 1942).
Where: Sw is stand for water saturation, Rw is the water resistivity that may calculate directly from SP log or from the oil company, where in this study the value obtained from the Maysan Oil Company, ø is the porosity in decimal fraction estimated from the main porosity logs such as neutron, density or sonic logs, Rt is the total resistivity of formation measured from resistivity log, and a, m, n are constants, where: a is constant number pends on formation type and the magnitude of tortuosity of fluids path in the rock body in range of 0.6 in unconsolidated clastic rocks to 1 in carbonates, m is an exponent stand for cementation index in range of 1.6 -2.4, and 2 in general, and n as a saturation exponent in values of 1.7 -2.2 (Archie, 1950).By using Archie's equation with standard constant parameters (a=1, m=2, and n=2) and depending on the value of water resistivity obtained directly from the MOC., which was about 0.0217 ohm.m, with little fractions different from well to another, the water saturation value in total was about 15% to 85 %, and the hydrocarbons saturation estimated according to equation 6 above.The lower value of water saturation recorded in MB21 unit, and the larger was recorded value in the Mb11 and MC2 units (Fig. 7) and (Table 2).

Lithology Distinction
The lithology was estimated with the interactive petrophysics default function of M-N in a cross plot between density and neutron logs which appears as the main lithology.In this study, the main lithology was limestone, which is the dominant lithology in the MB21 (Fig. 8-A) with fair abundance of shale and lesser presence of dolomite in the lower units (Fig. 8-B).The total computer calculation and interpretation of the CPI plot gave integrated results that gathered the most characteristic logs that gave the wider inteptretion view by comparing the logs reflecting.Also, it could be used to diclusure the relationship between the varies of the borehole with the change of the Gamma ray logs as an indicator of shale presence (Fig. 9).

Discussion
Through the results obtained after completing the interpretations of log data of the studied wells and reviewing the distribution of the wells in the field of study, it was noted that the formation of Mishrif, especially in Unit MB21, have high oil saturation, but most of the wells were located in the southern fold of the Fauqi field, and also the explanations led to the fact that the northern part of the field is affected by the diagenetic processes, especially cementation, which reduced the effective porosity level in this zone, and it was possible to develop this part economically for oil production purposes by increasing the number of wells drilled in this area and to do well stimulation conducting acidizing fracturing in drilled wells.

Conclusions
The Mishrif Formation was considered the main reservoir in Fauqi oilfield, was prescribed and simulated with a computer software based on log data analysis and core analysis for five sellective wells in the field.The shale volume varied in both lateral and vertical directions, but in total, it was nearly 22% of the formation bulk volume.Evaluation of porosity is well estimated by means of consequences of both logging and core analysis.The gained results clarified that the prevalent porosity of the formation was characterized by values ranging from low to medium for total porosity.The best recorded porosity value was in the MB21 unit of the Mishrif Formation especially in manner of effective porosity to be the best reservoir unit in the formation which was about 17%.The permeability estimation regarding Darcy's law and Timur's equation considering to the porosity and irreducible water saturation gave the best results in MB21 unit.The lowest ratio of water saturation and the highest hydrocarbon accumulations was in the MB21 unit.The good porosity and high hydrocarbons saturation, led firmly to the result that the MB21 unit had the best reservoir characteristics and represented the main reservoir unit in the Mishrif Formation in this field.The default lithology equation of density/ neutron cross plot was employed to discriminate the main lithology of the formation which was limestone.

Fig. 1 .
Fig. 1.Iraq regional map with extension of Zagros Belt Basin, pointed on the area of study, modified after, (Le Garzic.et al 2019).

Fig. 2 .
Fig. 2. Top of Mishrif Formation structural map in the Fauqi oilfield showing on it the study wells.

Fig. 3 .
Fig. 3.The Pickett plot of density-porosity relation to find the value of formation water resistant.

Fig. 4 .
Fig. 4. A comparing of data analyzed with interactive petrophysics software for two chosen well (FQ-4 and FQ-19).a) the shale volume, and b) the total and effective porosity.Total porosity shown in the dark blue color whereas the effective porosity shown in the light blue.

Fig. 5 .
Fig. 5.The correlation of porosity for the studied wells warding (NW -SE), in all wells appeared the MB21 unit with good value of porosity.The well FQ-14 shows poor data reading in order to the poor log reflectance.

Fig. 6 .
Fig. 6.An integration cross plots of porosity and permeability.Overall relations the best value showed in the MB21 unit as linear relation of two wells (Fig 7 A and B).Fig 7 C shows the k log in the golden color and the porosity log in light green color, and obviously the best integer was in the MB21 unit.

Fig. 7 .
Fig.7.The estimated water saturation and hydrocarbons contents.Water saturation in the (med blue) color, and hydrocarbon saturation is in green color.

Fig. 8 .
Fig. 8. Neutron log (X axis)/ density log (Y axis) cross plot to define the main lithology of Mishrif Formation.The interpretation obvious the main lithology is limestone especially in the MB21 unit as in (A), whereas the other units may contain sand or dolomite as in (B).

Fig. 9 .
Fig. 9. CPI plot clarifies the main characteristics of the Mishrif Formation of: lithology, shale volume, porosity, permeability,water saturation, and hydrocarbon saturation by well log interpretations.

Table 1 .
The average of the shale volume in the Mishrif Formation's units for chosen wells of FQ-4 and FQ-19