Determination of Petrophysical Parameters using Well Logs for Mishrif Formation in F Oil Field Southern Iraq

Abstract

Due to its economic hydrocarbon accumulations, several studies have been conducted for the Mishrif Formation in Southern Iraq.These studies involve different disciplines including stratigraphy and petrophysics.For example, microfacies were used to interpret the environment of depositional and diagenetic processes of the Mishrif Formation in the Nassyriah (Handhal and Al-Shahwan, 2014), Faiha and Sindibad oil fields (Handhal et al., 2020).Facies analysis studies combine microfacies data with image logsdies (Ismail et al., 2021).Petrophysical studies include estimating the values of porosity and permeability by using conventional logs and nuclear magnetic resonance log (NMR) of the Mishrif formation as in the Buzurgan oil field (Al-Janaee and Al-shahwan, 2019).In addition, the output of well logs analysis was used in reservoir modeling of petrophysical properties in many oil fields as in Noor (Al-Najm et al., 2017), Tuba (Mohammed and Mahdi, 2019), and Majnoon (Abbas and Mahdi, 2020).The characterization of the reservoir and non-reservoir units of the Mishrif Formation were carried out by using different approaches, including geochemical and geophysical interpretation (Al-Mimar et al., 2018), chronostratigraphic-based facies distribution (Awadeesian et al., 2018), brine chemistry (Awadh, et al., 2018).Paleontological and paleoecological studies have been limited to foraminifera and rudists (Al-Dulaimy and Al-Sheikhly, 2013;Al-Dulaimi et al., 2013).F oilfield produces commercial hydrocarbons from the Mishrif Formation, and it was chosen for this study.The Mishrif Formation in F oilfield was first investigated as part of an Exploration Oil Company, geological-reservoir study in 1988 , and it was discovered that the Formation is made up of three main units: upper, middle, and lower units (South Oil Company, 1991).

Location and Geological Setting of the study Area
The F field is situated in the South of Iraq, in the Thi Qar Governorate (Fig. 1).It was discovered between 1976 and 1978.The main reservoirs are discovered in Mishrif and Yamama formations.Less important oil accumulations are found in Ratawi and Zubair formations (Oil Exploration Company, 1995) (Fig. 2).The surface geology of the study area is characterized by flood plain sediments consisting of clay, sand and sediments of marshes and swamps of the modern Quaternary era (Oil Exploration Company, 2010).In F oil field, 116 wells have been drilled since 1983.Two vertical wells (F-X1, F-X2) and one directional well (F-X3) were chosen for this study (Table, 1) (Fig. 3).

Aims of the study
The objectives of this research include analysis and interpretation of well logs data to predict and calculate the Petrophysical parameters such as porosity and water saturation.

Materials and Methods
The methodology of this research consists of the following: • The accessible open hole logs data was digitized so that they could be transferred into the proper program for analysis it and interpretation.The logs were digitized using Didger V.3.02 software.
The input data was interpolated to be in 0.5m sampling rate.The proper environmental correction (effect of Shale, conditions of borehole, invasion depth, etc.) were applied before determining the Petrophysical parameters from Neutron, gamma ray, resistivity and density logs.• (Paradigm Geology) well log analysis software was used to determine petrophysical parameters include of water resistivity (Rw), lithology, shale volume, porosity (total and effective), water saturation, oil volume, and produce Computer Processed Interpretation (CPI) for each well.

Petrophysical Properties
Petrophysical characteristics must be determined and assessed in order to infer reservoir properties of the Mishrif formation as following:

Shale Volume (Vsh)
To derive Shale volume (Vsh) from the Gamma ray log (GR), the index of gamma ray (IGR) must be calculated using the following equation (Schlumberger, 1974): (1) where: IGR = Index of gamma ray, GR log= Reading of gamma ray by log (API); GR min= minimum of gamma ray (clean carbonate or sand); GR max= maximum of gamma ray (shale).And the following Dresser formula (Dresser, 1979) used to calculate the volume of shale: (2) High values of shale volume are observed at the top of Mishrif formation which is a clear indicator on gamma ray log whom used as a marker for top of Mishrif formation.The other major zone of high shale volume is the Marl unite.The Marl unite is the cap rock of the reservoir separating the nonreservoir upper Mishrif unites (U1, U2) from the reservoir unites of middle and lower Mishrif unites (M1.2, L1.1, L1.2 and L2).
Total porosity (φt) computed from density-neutron porosities as following: φt = ( + )/ 2, (6) Effective porosity (φe) was computed by removing the shale-related pores from the total rock pores network using equation ( 7) (Schlumberger, 1998): where: φe = effective porosity; φt = total porosity; Vsh = volume of shale.At the study wells, high values of porosity (total and effective) are observed in the middle and lower unites of Mishrif formation (M and L), that indicates of good hydrocarbon storage capacity reservoir as Table 2. Maximum porosity values are observed in M1.2, L1.1, L1.2 units that occur at the middle and lower parts of the Mishrif Fm. as reservoirs beds, and can be distinguished in all studied wells.
Well F-X1 exhibits highest values of porosity in the main reservoir unite (L1.2) about 25%, while the lowest rate was recorded in the well F-X3, since showed the lowest value of about 22% for the same unite as Table 2.

3. Water and Hydrocarbon Saturation
The total water saturation Swt and the effective water saturation Swe were computed using Archi formula in equations 8 and 9 respectively (Archie, 1942): Swe = [(a × Rw) / (Rt × Ø  )] 1/n , (9) while of the invaded zone (Sxo) was calculated by using equation 9 through replacing of Rw with Rmf (resistivity of mud filtrate taken from headers of well logs), Rxo instead of Rxo (the invaded zone measured resistivity) and Øe with Øt (total porosity): where Rw = water formation resistivity which is calculated by well log analysis using cross and Pickett plots (Figs. 4, 5 and 6); a= factor of tortuosity; m= factor of cementation; n= exponent of saturation (Table 3).The total saturation of hydrocarbons (Sht) was determined by equation of (Asquith et al., 2004):

Well ID Rw
11) while, the saturation of moveable hydrocarbon (Shmov) was determined by the bellow equation: where Sht = total hydrocarbons saturation; Shmov = saturation of movable hydrocarbon; Sxo = invaded zone water saturation; Swt = uninvaded zone total water saturation.

Bulk volume (BV)
The uninvaded zone bulk volume of water (BVW) was calculated using the following equation: while, the bulk volume of hydrocarbons (Oil) can be calculated by the following equation: where: Bvo = hydrocarbon's bulk volume; Sht = total saturation of hydrocarbon; Øt =total porosity.

Results and Discussions
The interpretation of computer processing (CPI) for studied wells (F-X1, F-X2 and F-X3) which represent that has been deduced using GEOLOG software (Figs.7,8 and 9).Based on CPI results, the Mishrif Formation in F oil field could be subdivided into cap and reservoir units that are characterized by different Petrophysical properties.These include:

Cap Rocks
M1.1 and M2 are two cap units that have been identified at the middle part of the Mishrif Formation succession.The volume of shale values may be so high that the rocks of these units become marly.Unit M1.1 can have limited thickness (1.6m) as in well F-X3 (Fig. 9).Both units show low values of effective porosity (PHIE) values.Water saturation, on the other hand, is very high.As a result, the volume of oil is low.
Marl is also a cap unit has been identified at the middle part (above M1.1 unit) of the Mishrif Formation succession.This unit show high values of total porosity (PHIT) and low of PHIE values.In the study wells, the thickness of the Marl unit ranges between 4 to 6 m (Figs.7, 8 and 9), which makes it a good insulation layer in the upper part of the first reservoir unit M1.2.

Reservoir Units
Reservoir units of the Mishrif Formation with high quality have common petrophysical properties, most notably high effective porosity and low water saturation.The lithology of these units is clean limestone characterized by low gamma ray log response.These properties are distinguished in units M1.2, L1.1, L1.2.The least quality reservoir units include U1.1, U1.2, U1.3, U2, and L2 due to their high-water saturation (Table 2).

Unit U1.1
This unit is located at the uppermost part of the Mishrif Formation overlain by the Khasib Formation.The transition from unit U1.1 to Khasib Formation shows sharp changes in the values of well logs due to the unconformable occurrence of shale rocks of the Khasib Formation above the limestone rocks of unit U1.1.The effect of dolomitization occurs in wells F-X2 and F-X3 (Figs. 8 and  9).The low reservoir quality of this unit is attributed to low effective porosity and high-water saturation.

Unit U1.2
The unit U1.2 is confined between units U1.1 and U1.3.It is differentiated by higher total and effective porosity.However, this unit does not represent an important reservoir due to the high-water saturation.It can be traced in all wells and its thickness decreases significantly in well F-X1 (Fig. 7).Dolomitization effect is prominent in wells F-X2 and F-X3 (Figs. 8 and 9), so that the lithology is changed into dolomite.

Unit U1.3
The unit U1.3 shows a noticeable increase in the shale volume and a decrease in the porosity compared to the units U1.2 and U2.Dolomite in this unit appears less compared to unit U1.2.As with the unit U1.2, the unit U1.3 is totally saturated with water (Fig. 7), which loses its reservoir quality.

Unit U2
The shale volume of unit U2 is largely reduces, which makes it easy to distinguish from the unit U1.3.This change is accompanied by a marked increase in porosity and water saturation (Fig. 7).As is the case with the aforementioned units, the reservoir efficiency is low in this unit.The volume of dolomite is little in unit U2.

Unit M1.2
The M1.2 represents the uppermost reservoir unit that has good reservoir quality.It is capped by unit M1.1, and characterized by increase of effective porosity and decrease of water saturation associated with good oil volume as observed in well F-X3 (Fig. 8).These reservoir properties decrease in other wells.The lithology of this unit is limestone with irregular changes in shale volume.The average of effective porosity and water saturation reach to 15% and 45%, respectively.Thickness of the unit decreases in wells F-X1 and F-X2 (Figs. 7, 8).

Unit L1.1
This reservoir unit does not appear in all wells, which makes it less important.Furthermore, the unit is characterized by irregular changes in Petrophysical values (Fig. 7).The average of effective porosity (PHIE) is 14%, and average of water saturation is 40%.

Unit L1.2
The L1.2 unit is considered the most important reservoir unit of the Mishrif Formation in the F oil field due to its large thickness, and high values of porosity and low water saturation.The average of PHIE is 26%, and average of water saturation reaches to 16%.The unit is capped by unit M2 where unit L1.1 is non-existent (Fig. 8).The Petrophysical properties are gradually enhanced towards the top of the unit.Such trend in the Mishrif Formation was explained by (Mahdi et al., 2013;Mahdi and Aqrawi, 2017) as a result shallowing upward cycles that are accompanied by the increase of porosity resulted from dissolution.

Unit L2
The L2 represents the lowermost reservoir unit.Although this unit has a high porosity, it has low quality due to the high-water saturation.Limited increase of oil volume is observed at the uppermost part of the unit in well F-X1 (Fig. 7).This unit has a large thickness and the lower boundary may not appear due to incomplete drilling to reach the lower boundary of the Mishrif Formation.

Conclusions
The Mishrif Formation in the F oil field could be divided into several units of reservoir that differ in terms of a reservoir characteristics, depending on the results of the well logs analysis.Unit L1.2 is considered the most important due to the high effective porosity values and the decreasing water saturation.Other units show lower quality reservoir properties due to increased water saturation as in units M1.2, L1.1 and L2.The cap units are characterized by increased shale volume and low effective porosity as in marl units and limestone units M1.1 and M2.The stratigraphic extent of the reservoir and the cap units can be traced in the study wells, except for the unit L1.1 which does not appear in the wells F-X2 and F-X3.

Fig. 2 .
Fig.2.Stratigraphic column of the F oil field based on observation from F-1, F-2 and F-3 combined with F-X1 wells.

Fig. 3 .
Fig.3.Top Mishrif depth structure map shows location of study wells

Table 2 .
Porosity and water saturation values of the Mishrif units in study wells