Geological Model for Jeribe/Euphrates Formation, Tertiary Reservoir in Qaiyarah Oil Field, North of Iraq

Abstract


Introduction
Geological modeling is considered one of the most important stages in making the decision to further develop the field, it will be visualizing and producing full knowledge about the subsurface structure.In newly developed fields, the geological model involves a number of processes from database creation, stratigraphic correlation, structural study, fluid contacts determination and ending with spatial distribution of various reservoir properties such as porosity, saturation and reserves estimation (Timur, 2022).
At the present time, and due to the development of technologies for handling and processing large amounts of reservoir data, which led to construct of more reliable geological models, as evidenced by the work on the automation of the construction of fluid contact surfaces in geological modeling (Abraham, 2019).
The rational model leads to faster acceleration of the routine data collection operation in order to build a 3D model with multiple sets of geological data for providing uncertainty estimates proposing various geological hypotheses and allowing timely updating of models, as well as minimizing the number of errors when creating 3D digital geological and reservoir simulation models of oil and gas fields.In the Republic of Iraq, there are mainly more than five heavy oil fields in the northwest of the country that have been neglected for more than five decades.This paper will be mainly focus on calculating Qaiyarah heavy oil field stock tanck tank oil intially initially in place (STIIOP) in a step to construct a reliable geological and petrophysical model for further field development and calculating the potential of the field.
Qaiyarah oil field have been discovered by the German in 1918 by appearance of the seepages at the surface, where the major portion of the field is located to the west of tigress river about 65 Km southern of the City of Mosul.The field dimension is about 15 km in length and 3.5 km in width.
Qaiyarah field / Tertiary Reservoir, is an anticline trap forming part of a bigger structure composed of 4 anticlines: Qaiyarah, Najmah, Jawan, and Qassab.The first three structures are successive individual culminations on a single fold axis, which trends approximately northwest from the western bank of the Tigris at Qaiyarah.Qassab is a parallel anticline, with two domes, which is offset to the northeast, and separated from Jawan by a broad, shallow synclinal saddle (Al-Mubarak, 1978), as shown in Fig. 1.Qaiyarah Field is an asymmetrical anticline consist of two domes separated by a saddle, the southeast dome is considerably wide and the northwest dome is considerably small and tighter.The southeast dome is structurally higher than the northwest dome by 45 meters at the upper limit of the Jeribe Formation (NOC, 2006).
Qaiyarah Structure has very gentle dips (3-5) in the axial area and for considerable distance on either flank, but along the southern outer boundaries the dips increase rapidly to 45° and just as sharply decrease further out (NOC, 2006).
Fatha Formation is exposed to the surface and in the axial areas is eroded, the erosion degree in some areas could reach the transition formation, this formation can be easily recognized by its coliseum index.The main reason for the seepages in the area where most of it is cover by bitumen and contains lots of sulfur springs due to leakages of H2S that is associated with the heavy crude in the field (MoO, 2020).
The source rocks of the tertiary reservoir may be from the Basinal Sargelu formation (Middle Jurassic) and the migration of oil may have happened along the fault.Systems from basinal radiolarian Upper Jurassic sediments, which lie down dip, a few tens of miles east of this field (Dunnington, 1958).The rocks of the tertiary reservoir in Qaiyarah field is consist of top Jeribe Formation to the depth of top of Shiranish (Upper Cretaceous Reservoir).The anhydrites, gypsum, marls and limestones of the Fatha Formation (previously Lower Fars) cap the Tertiary reservoirs. it includes below formations started from the newest to the oldest as it shown in Fig. 2 (NOC, 2006).

Jeribe Formation (Middle Miocene)
The facies of Jeribe Formation is distributed along the entire field.It consists of different types of facies of dolomitic limestone.The deposition happened in a supratidal to lagoonal environment.
It is considered one of the most important oil reservoir units in Qaiyarah Field; its highest thickness could reach 40 meters in well 39 and minimum thickness is about 26 meters in well # (90) that is located at the crest of the structure (Fig. 3).

Euphrates Formation (Lower Miocene)
Euphrates Formation continuously starts from bottom of Dhiban Formation and ends with appearance of the mixed facies of Euphrates/Serkagni in the age of Burdigalian.
As in the previous above formations, it covers the entire field and considers as the main oil reservoir like Jeribe Formation with thickness varying from 73 meters in well # (56) to 90 meters in well # (90).It is consisting on outer shelf to basinal deposits and is characterized by dolomites and dolomitic limestone with some gypsum towards the bottom (Fig. 5).

Fig. 5. Euphrates Formation thickness
Table 1 shows the minimum, maximum and average thicknesses for the interested formations in the tertiary reservoir in Qaiyarah field.  1 that the Dhiban Formation is very thin in thickness compared to the other two major reservoir formations and it has almost same properties like the bottom of Jeribe and top of Euphrates.Because of that, it will be considered as a continued formation in between Jeribe and Euphrates in the reservoir modeling.

Materials and Methods
Carbonate reservoirs are generally heterogeneous and characterized by complex pore structures showing multimodal pore-type distribution.This complex porosity system strongly affects the petrophysical properties and determines the reservoir rock quality (Morgan and Gordon, 1970;Tiab and Donaldson, 2004;Ali et al., 2020).This implies that detailed knowledge of pore-type distribution for reliable pore-system characterization is critical to ensure accurate prediction of reservoir parameters (Montaron 2009), in particular the m factor that has been suggested to greatly influence the quantification of fluid saturations used for evaluating reservoir formation (Ragland, 2002;Rezaee et al., 2007;Li et al., 2013;Soto et al., 2015;Qin et al., 2016).
In the Qaiyarah field, petrophysics provided the vital reservoir property attributes required in building the geological model and calculating the volumetric.The data from core analysis are available for 4 wells (39, 40, 55, and 76), however, logs and CPI results are analyzed for the same four wells.All the data indicates that the tertiary reservoir in Qaiyarah Field which is mainly (Jeribe / Euphrates) limestone is vuggy and fractured.
Because of the limited availability of core analysis in all drilled wells in the field, which led to generalize the results from these mentioned core for the entitre model.

Petrophysical Properties
To determine and assist the tertiary reservoir properties in Qaiyarah oil field, the petrophysical characteristics must be determined as follows (Ali, 2022).

Shale Volume
Shale volume calculation have done 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) In the Tertiary reservoir, total gamma variations are not directly related to the caly elements of potassium and thorium but have a more direct association with uranium.Lithological information from cutting and core indicates that the carbonates in general have low caly content.The reservoir is considered to be clean and no clay correction was required.Fig. 5, showing Net to Gross (N/G) of the tertiary reservoir of Qaiyarah field.In the Jeribe Formation, gamma ray reflections are due to uranium-bearing minerals, such as zircon, epidote, apatite, etc. present in the (hot) dolomites which formed under sabkha conditions (Murani, 1985) (Fig. 7).

Porosity
The total porosity is calculated by using of the Neutron-Density logs.The Neutron porosity was corrected of shale effect using equation 3 (Tiab and Donaldson, 1996). (3) Where: Φn corr.= shale effect corrected neutron porosity; Φn = raw neutron porosity; Vsh = shale volume of gamma ray (fraction); Φnsh = neutron porosity of shale (fraction).Density derived porosity was determined by equation 4 (Wyllie et al., 1985). (4) Where: ØD = shale effect corrected density derived porosity (fraction); ma = density of matrix (2.71 gm/cm 3 ) for limestone, (2.87 gm/cm 3 ) for dolomite; b= formation bulk density recorded by density log (g/cm 3 );  f = fluid Density (mud filtrate) (1g/cm 3 ) for fresh water or 1.1 g/cm 3 for salt mud.Density derived porosity although was corrected of shale effect using the following equation 5 to remove the effect of shale from the calculation of porosity in intervals with a shale volume more than 10%. (5) Where: Total porosity computed from density-neutron porosities as following. (6) Effective porosity was computed by removing the shale related porous from the total porous network using equation ( 7). ( 7) Where; = effective porosity; = total porosity; = shale volume.In Qaiyarah oil field (tertiary reservoir), estimates have been made of the total/effective reservoir porosity and reservoir matrix porosity.Since no correction was applied for Vcaly as it is mentioned in section (1.3.1)above, the total porosity and effective porosity estimates are the same (Fig. 8).

Fig. 8. Total nad effective porosity interpretation
Porosities from the sonic log are expected to be consistent with or lower than those from the total porosity logs (density and neutron).In an overlay of porosity reading from density log and porosity from sonic log, the secondary porosity index (SPI) particularly in the Ephrates formation is small, which suggest that the sonic is reading not just matrix but nearly total porosity.This means either an inter-crystal sucrosic, or a highly vuggy and micro-fractured intergranular porosity fabric (Fig. 9).

Fig. 9. Density and neutron porosity interpretation
Note that in the Jeribe formation, the porosity from density log reads silghtly less, probably because the grain density of the anydritic dolomite present here is higher than the 2.81 gm/cc value used in the computation.This phenomenon may be in fact due to the presence of gypsum in the Jeribe formation (Al-Samarrai, 2016).In reservoir zones, the sonic-derived porosity proved to be more compatible with core porosity, a composite porosity was computed in many wells from both density and sonic logs.In wells with only a sonic log available for interpretation, the match is less than good.
After upscaling the prosity data from the well logs and from available core data analysis, it is very clear that the Jeribe and Euphrates have good to very good porosity as is shown in Figs.(9,10 and 11),

Water Saturation
Water saturation have been computed using Archie formula in equation ( 8) (Archie, 1942).( 7) The parameters required for the calculation of water saturation were established as outlined below.
• Formation Resistivity "Rw" Salinities of recovered water samples are available from several wells.Some reported salinities vary across a significant range, which reflects the varying degrees of contamination of the samples by invading mud filtrate, water cushions, or spent acid.In the bulk of water sample collected, the Total Dissolved Solids (TDS) lie in the range of 14,000-20,000 ppm.In this analysis, the value of 16000 ppm has been considered and this yields a formation water resistivity (Rw) of 0.28 ohm.m at 110 °F (considered as approximate reservoir temperature).

• Cementation Factor "m"
The cementation factor "m" is a direct reflector of tortuosity, which is determined largely by pore structure and pore size distribution.A characteristic of carbonate reservoirs is that the pore geometry is complex and variable and very dependent on the mix of pore types that occur at any one location, and the value of "m" therefore can be found to vary from below 1.5 to in excess of 2.5 (Mahmood Akbar, 2008).
Several SCAL (Special Core Analysis) of "m" as shown in Table 2, clearly indicated that porous and fractured, dolomitized grain stone facies defined as good and very good facies in the Jeribe and Euphrates formations yield as low value for "m" as 1.57.
On the other hand, in very upper part of Jeribe Formation, the dense muddy wackestones and strongly cemented grainy facies, which are regarded as poor reservoir facies, with non-connected vuggy porosity and complex pore geometry, yield a value higher than 2. Some intermediate values are recorded for variable and fractured and/or vuggy dolomites and dolomitic limestone.
Significant errors can occur in the calculation of fluid saturation if the wrong value of "m" is used, then the computed water saturation (Sw) will show erroneously high saturation.A high cementation factor of 2.4 is indicative of poor communication between pore spaces and an expected low permeability.This is in direct contrast to the SCAL data and available core descriptions.
In this literature, the Archie equation constants used are cementation factor "m" = 1.8, saturation exponent "n" = 2 and constant "a" = 1.0 and average water saturation is calculated at 31.5%

• Net Reservoir
Since very minor or no clay volume was considered, the net reservoir intervals were identified by apply the porosity cut-off values only.
A porosity cut-off of (13%) has been taken as it predicted from porositypermeability plots for many wells considering maximum value of cut-off as a worst scenario, specially this study has not taking fracture modelling in consideration due to the lack of information of geophysical data (Fig. 13).

Results and STIIOP Calculations
For reliable volume calculation of the original oil place, a combined result of the petrophysical analysis, mapping and reservoir layering studies is used to build the geological model to be used for the STIIOP calculations.The following data is used for the calculation: • Porosity: the computed data of core analysis and CPI were Upscaled and results is shown in Table 3 and Fig. 14.  • Average Water Saturation: The results of logs interpretations and upscaled data is shown in Table 4 and Fig. 15.Net thickness: Net pay intervals have been limited to lay only above the established OWC that was determined in several well tests to lie at around (120 m BSL) in the NE region of the southern dome and around (135 m BSL) in SW region of the southern dome as it showed in Table 5 and Fig. 16.A series of volume calculations was carried out to solve varied issues, and by using the formation volume factor (Bo) of 1.06, the STIIOP of the full field is calculated to be 6.519*10 9 Barrels of oil.

Conclusions
Gamma ray variations are not directly related to clay elements of potassium and thorium but have a more direct association with uranium, so the reservoir is considered to be clean and clay correction was required.Temperature data are plotted against measured depths and a temperature gradient of 1.2 °F/100 ft. was established.From test data a value of 16000 ppm has been taken for TDS, and this yields a formation water resistivity (Rw) of 0.28 at 110 °F (approximately reservoir temperature).Minimum porosity cut-off of 13% had been applied, a water saturation of 31.5% was used.

Recommendations
Standardize and run full logging suites, including FMI, Image Log, etc., for clearer property evaluation.Run Spectral Gamma Ray (SGR) tools to evaluate the impact of uranium-bearing minerals on total gamma ray measurements.Formation water salinity and resistivity need to be accurately recorded.A 3-D seismic survey is essential, not least to determine the fault pattern and associated fractures.A detailed study of the intensity and distribution of fractures is a requirement to understand the reservoir performance particularly in this environment.The presence of gypsum, together with anhydrite in the Jeribe Formation is interesting and requires investigation.This is because the gypsum here is likely to be a diagenetic product, the result of meteoric waters and the addition of nH2O to CaSO4.This may entail that some of the limestone is in fact "Dolomite" which would impact the textures and reservoir properties.

Table 2 .
SCAL Analysis Results

Table 5 .
Upscaled net to gross statistics