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Seismic, Petrophysics, and Attribute Analysis to Evaluate the Tertiary
Reservoir in the High Folded Zone, Kurdistan Region-Iraq
ArticleinIraqi National Journal of Earth Sciences · July 2024
DOI: 10.33899/earth.2023.142239.1121
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Iraqi National Journal of Earth Science
www.earth.mosuljournals.com
Iraqi National Journal of Earth Science, Vol. 24, No. 2, 2024 (231-260)
131
Seismic, Petrophysics, and Attribute Analysis to Evaluate the
Tertiary Reservoir in the High Folded Zone, Kurdistan Region-
Iraq
Kakarash I. Gardi1’* , Bakhtiar Q. Aziz 2 , Ezzadin N. Baban 3
1’* Department of Geology, College of Science, University of Sulaimani, Sulaimani, Iraq.
General Directorate of Dam and Reservoirs, Kurdistan Region, Erbil, Iraq.
2 Department of Geology, College of Science, University of Sulaimani, Sulaimani, Iraq.
3 Department of Geology, College of Science, University of Sulaimani, Sulaimani, Iraq.
Article information
ABSTRACT
Received: 31- Jul -2023
Revised: 21- Nov -2023
Accepted: 20- Dec -2023
Available online: 01- Jul– 2024
The present study is intended to evaluate one of the oil-bearing
horizons in the (WN) oil field within the High Folded zone in the
Kurdistan Region of Iraq, which represents a carbonate reservoir of the
Tertiary units. Also, the picking and mapping of two other horizons of
interest from the top and bottom of the reservoir horizon are done
across the area. This work is achieved using 3D seismic data, check
shots, and well logs. The methodology involves several processes
including data loading, well seismic tying, horizon and fault
identification and interpretation, velocity modelling, time-depth
conversions, petrophysical analysis, and 3D properties modelling.
Isochron and depth maps for three horizons and two isopach maps are
constructed. An asymmetrical doubly plunging rollover anticlinal
closure with a length of 6.4 km and width of 3.5 km is identified on the
reservoir maps that trend in the East-West direction. Faults are
identified and extracted manually and automatically. A total eighteen
of minor reversal faults striking the northern flank of the anticlinal
closure are interpreted. Variance, Chaos, and Ant Tracking attributes
are selected and applied successfully that help to better visualize
fractures and automatic fault extraction. Petrophysical analysis and
cross-plots demonstrates that the reservoir consists of dolomite, lime
dolomite, and anhydrite limey dolomite. The petrophysical properties
reveal the average of each effective porosity, secondary porosity,
permeability, clay volume, and water saturation of the reservoir at
9.98%., 4.39%., 14.1 milli Darcy, 9.13%, and 47.8% respectively. The
study shows that the reservoir has moderate hydrocarbon prospects.
Keywords:
Seismic interpretation
Reservoir characterization
Attribute analysis
Petrophysical analysis
3D static modelling
Correspondence:
Name: Kakarash I. Gardi
Email:kakarashgeo@gmail.com
DOI: 10.33899/earth.2023.142239.1121, ©Authors, 2024, College of Science, University of Mosul.
This is an open-access article under the CC BY 4.0 license (http://creativecommons.org/licenses/by/4.0/).
Kakarash I. Gardi, et al…..
232
32 1 ةط
1
2
3
Introduction
The study area is located within the administrative boundaries of the Duhok province,
which lies to the northwest of Duhok City in the Kurdistan Region of Iraq (KRI) (Fig. 1.a &
b). The traditional 2D surveys with linear spread have limitations merely providing several
sparsely seismic cross-sections of the subsurface, whereas 3D surveys with a real spread
provide a complete picture of the subsurface (Alsadi, 2017). 3D seismic reflection techniques
31 2023
21 2023
20 2023
012024
WN
6.4 3.5
9.98%.4.39%.14.19.13%.47.8%
Email: kakarashgeo@gmail.com
DOI:10.33899/earth.2023.142239.1121, ©Authors, 2024, College of Science, University of Mosul.
This is an open-access article under the CC BY 4.0 license (http://creativecommons.org/licenses/by/4.0/).
Seismic, Petrophysics, and Attribute Analysis to Evaluate the Tertiary Reservoir in the High Folded Zone…….
233
have become popular in hydrocarbon exploration, which offers the most precise, continuous
volumetric seismic coverage to map and delineate subsurface geology (Hart, 1999). The
reservoir is fully covered by a 3D seismic survey that makes it possible to fill in the spatial
gap between the wells (Varela et al., 2006). A combination of various data sets is necessary
for 3D reservoir characterization to infer and identify subsurface geology in more detail
(Johann et al., 2001). Integration of a 3D seismic data set with well-log data is incredibly
helpful in evaluating the reservoir because it can demonstrate the vertical and lateral
distribution of specific reservoir properties (Arifin, 2016). For reservoir characterization,
numerous studies have been implemented worldwide using the integration of seismic data and
well log data (e.g. Ashraf et al., 2019; Osinowo et al., 2018) and in Iraq (Khawaja and Thabit,
2021) among others. The Zagros thrust-fold belt is a rich hydrocarbon province. More than
5% of the global hydrocarbon reserves are located in this belt (Reif et al., 2012). The
Kurdistan region-Iraq forms part of this belt. Although over nearly two decades, 3D seismic
exploration in the region has been started, yet not been fully explored and no research has
been published. This research focused on using a 3D seismic cube and well-log data to
evaluate one of the Tertiary carbonate reservoirs in the High Folded Zone of the structural
division of Iraq (Fig. 1.a). In this study, besides tracking the targeted oil-bearing horizon, it is
also preferred to pick and map two other interesting horizons, and then using them to build 3D
static modelling for the reservoir. According to Aqrawi et al. (2010), Horizon-1 (H1) of
Middle Miocene forms a regional cap rock, Horizon-2 (H2) of Middle Miocene forms a
hydrocarbon reservoir, and Horizon-3 (H3) of Early Miocene can be a hydrocarbon reservoir
and/or local seal rock. Horizon-2 is widely distributed throughout Iraq and is significant as it
forms the oil reservoir. The Mesozoic source rocks are the most likely sources for this
reservoir.
Fig.1. (a) Tectonic map of Iraq showing the study area (from Edilbi et al., 2019), and (b) Tectonic map of
the Zagros Fold-Thrust Belt showing the main tectonic elements (modified from Koshnaw et al., 2020).
Kakarash I. Gardi, et al…..
234
Due to the complicated and heterogeneous structure, characterizing and modelling
carbonate reservoirs is a challenging task (Bueno et al., 2014). Despite this challenge, 3D
static modelling represents an effective technique in characterizing reservoirs, thus this study
is conducted to create a 3D geological model of this carbonate reservoir to evaluate and shed
light on it utilizing all available 3D seismic data, check shots, and well-log data.
The study area lies in the High Folded Zone, which represents a rugged topography
mountainous area characterized by high anticlines and narrow synclines. Due to tectonic
movements and uplifting, Cenozoic successions widely cropped out and covered the entire
area and surroundings (Fig. 2). In some parts, Quaternary and Holocene surface deposits
overlie the outcropped formations. A summary of exposed rock formations according to (Van
Bellen et al., 1959) is given in Table (1).
Fig.2. Surface geological map of the study area and its surrounds (from Bamerni et al., 2021).
The High Folded Zone is situated on the northern margin of the Arabian Plate (Fig.
1.b) and forms a part of extensive the Zagros-Taurus Fold and Thrust Belt which extends for
almost 2000 km and 200-300 km wide from the Strait of Hormuz in the southern part of Iran
NW-ward through the Kurdistan Region to the eastern part of Turkey within the Alpine–
Himalayan orogeny (Zainy et al., 2017).The belt and its associated foreland basin resulted
from the closure of the Neo-Tethys Ocean, and the subsequent plate convergence and
collision of the Arabian plate with the continental Eurasian plate (Iranian and Turkish), which
started from the late Cretaceous and continues to the present-day (English et al., 2015). The
structural features of the belt in the KRI mostly have two major trends, an NW-SE trend
paralleling the Zagros Mountains and an E-W trend paralleling the Taurus Mountains of
southern Turkey (Jassim and Goff, 2006). In the KRI, the Zagros-Taurus foreland basin was
filled with a thick sedimentary succession that varies from 7 km to 14 km (Al-Azzawi, 2013),
Seismic, Petrophysics, and Attribute Analysis to Evaluate the Tertiary Reservoir in the High Folded Zone…….
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and in particular, within the Duhok area ranges between 8 km to 11 Km (Doski and McClay,
2022).
Table 1. A brief description of the outcropped formations in the area.
Materials and Methodology
1- The 3D seismic cube consisting of 536 inlines (from 3500 to 4035) and 491 crosslines
(from 2150 to 2640) covers a semi-rectangular area of some 37 square km, and suites of
composite logs from four boreholes with their check shot data are used to evaluate one of
the Tertiary reservoirs of interest. Figure (3) is a seismic base map of the study area
showing inlines and crosslines coverage of the 3D seismic survey and well locations. The
length of each inline equals 5.880 km with 12 m intervals and the length of each crossline
equals 6.420 km with 12m intervals. Data are kindly provided by the International Oil
Companies operating in the region through the Ministry of Natural Resources of the
Kurdistan Regional Government. Schlumberger's Petrel Suite seismic interpretation
software 2017.4 and Senergy’s Interactive Petrophysics (IP) software version v4.5.5
windows- based are used for the geophysical and geological evaluation of the study area.
2- Initially, the original 3D seismic data cube are in SEG-Y format of the study area, and the
check shots, well tops, and well logs of the four wells within the area are separately
imported into a Petrel. Then, the 3D seismic data are realized (converted) into the ZGY
bricked format which is regarded as a compressed physical copy that helps to handle and
manipulate large databases much quicker than the traditional SEG-Y format
(Schlumberger, 2010). Afterwards, using a frequency filter to suppress unwanted noise
and increasing the signal/noise ratio (Osaki, 2015) by selecting the Ormsby filter type and
Hamming taper because they demonstrated the greatest noise reduction with the least
amount of seismic energy loss (Grabeel, 2018). Next, automatic gain control (AGC) to
regulate and equalize the root mean square (RMS) amplitude decay with time over a given
window is used.
3- The Synthetic seismogram is created at the WN.1 well using calibrated sonic and density
logs for computing the acoustic and reflection coefficients with a check shot from the
same well. First, the interpretation of horizons on the vertical seismic sections at the well-
tie point was started and proceeds outward from there. All seismic sections are checked
for the presence of major faults. While eighteen of the visible subtle faults are detected
and marked on the horizons, the existence of such faults is not found.
Formation
Age
Description
Upper Bakhtiari
(Bai Hassan)
U. Pliocene
Conglomerate, siltstone, claystone, and sandstone
Lower Bakhtiari
(Muqdadiya)
L. Pliocene
Sandstone, mudstone, and siltstone
Upper Fars (Injana)
U. Miocene
Thin bedded sandstone and claystone
Lower Fars (Fatha)
M. Miocene
Anhydrite, gypsum, marl, and limestone
Pila Spi
M.-U. Eocene
Bituminous dolomitic and chalky limestone
Avanah Limestone
M.-U. Eocene
Limestone is usually dolomitized and recrystallized
Gercus
M. Eocene
Mudstones, sandstone, and sandy and gritty marl
Kakarash I. Gardi, et al…..
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Fig.3. Base map of the study area displaying the 3D seismic coverage and well locations.
4- Manually, horizon picking is performed on every fifth inline and crossline, then filled in
between by auto-tracking method. In addition, inserting an arbitrary intersection line or a
time slice in the situation where there is ambiguity regarding the continuity of the horizon
in question. Although, it is not necessary to track horizons on every inline and crossline,
due to the fractured zone in the northern part of the area and there is no properly working
auto-tracking technique in this zone, for precise mapping of the horizons further adjusting
to a denser grid is taken. Once, the picking of the horizons across the area is completed, the
data are directly converted to surfaces to acquire the time structure maps, and eventually,
an isochron map for each of them is constructed.
5- Converting the isochron maps to depth maps requires to construct a velocity model. Six
equations in Petrel 2017 can be used to create an advanced velocity model. The Eq. (1) for
the construction velocity model is selected, which incorporates the V0, K, and Z values and
uses the relationship of linear variation of velocity with depth. This model has been applied
by Ten Veen et al. (2019) and other researchers. This model has been applied by Al-Ridha
et al. (2018), Toba et al. (2018), and Ten Veen et al. (2019).
V=V0+K*Z ………. (Eq. 1) (Schlumberger, 2010)
Where; V is a velocity at any depth of the reflector, V0 is an initial instantaneous velocity in
ms-1 at the top of the reflector from the seismic reference datum, constant K is a vertical
velocity gradient (compaction factor) in s-1, and Z is actual reflector depth (Ogbamikhumi
and Aderibigbe, 2019).
The velocity V0 and velocity gradient K for each horizon are individually calculated using
check shots from all the wells WN.1, WN.2, WN.3, and WN.4. The velocity of horizons
H1, H2, and H3 are calculated and they equal to (2057 m/s, 1744 m/s, and 2253 m/s)
respectively; and K values equal (0.88 s-1, 1.3 s-1, and 0.65 s-1) respectively. The
constructed velocity model by implementing the moving average interpolation method is
Seismic, Petrophysics, and Attribute Analysis to Evaluate the Tertiary Reservoir in the High Folded Zone…….
237
used to convert the isochrone maps of all horizons to the depth maps. Two isopach maps
have been constructed between H1& H2 and H2 & H3 to show the variable thickness of the
cap rock formation (Fig. 10.a) and the thickness of the targeted reservoir formation
throughout the area (Fig. 10.b).
6- A structural framework of the static model is defined and created in the depth domain
which resembles a skeleton that allows for incorporating both structural and property
models in the unified model. After the manipulation and editing, eighteen of the picked
subtle faults on seismic sections across the area are used for building the structure
framework. A boundary of the target study area is determined and inserted into the model.
Then, using the horizon modelling process, all three depth maps of the horizons are
combined into the framework. To generate a 3D grid, the structural framework is converted
to a fault model, and a structural skeleton is produced through the pillar gridding process.
Each corner of the resulting grid cell will have one pillar, which is set in between the faults.
Once faults are modelled and adjusted, pillar editing, and all other necessary processing is
done. Then, for zonation, two thickness isochore maps are constructed and used as inputs
for zone modelling by which two zones between the already modelled horizons were
produced. The next step, for increasing assurances of petrophysical modelling by layering
(sub-dividing the grid) process, the first zone and second zone are divided into sixty and
fifty layers respectively.
7- Preconditioning the seismic data is necessary to improve their quality before any
interpretation, especially when applying Ant Tracking attributes. Frequency filter and AGC
filtering and then structural soothing for further improvement were applied to the realized
seismic data cube. However, while manually interpreting and determining faults is a time-
consuming task, tedious, and lacks accuracy (Pedersen et al., 2002), eighteen different
minor faults are identified, marked, and labelled F1 to F18 on the interpreted seismic
sections to define the appropriate positions of their surfaces and then used in the 3D static
model construction. Besides manual picking, the fascinating fault detection and automatic
extraction technique-based Ant Tracking algorithm can be implemented to extract fault
surfaces from the seismic data in a 3D volumetric attribute (Pedersen et al., 2002) based on
Variance (Donahoe and Gao, 2016), Chaos (Aliouane and Ouadfeul, 2014) and both
attributes (Kozak, 2018) and several others. The Variance and Chaos (Jiratitipat, 2020) are
the two most often attributes utilized to enhance fault and discontinuity in seismic data. To
isolate the area of interest, the cube is cropped into a sub-volume. Afterwards, the Variance
and Chaos attributes are applied to the cropped cube. In the next step, they are used as
inputs to precondition the seismic cube for applying the Ant Tracking attribute.
8- The available log data for analysis and estimation of the petrophysical properties from
wells WN.2 and WN.3 are imported to the interactive petrophysics software IP. Suites of
composite logs from wells WN.2 and WN.3 including gamma-ray (GR), calliper, bit size
(BS), resistivity, bulk density, and neutron porosity logs. As the well-log reports are not
available for this work, therefore any corrections for the environmental conditions are not
implemented. In addition, the formation temperatures which are necessary for estimating
properties also unavailable. Therefore, an average temperature gradient in the Kurdistan
Region of 21°C /km (Abdula, 2017) with a reference surface temperature of 25°C is used
to estimate the essential petrophysical parameters such as total, effective, and secondary
porosity, permeability, clay (shale) volume, and water saturation (Sw). These analyzed
properties are loaded into the Petrel software for building a 3D static geological model.
Effective porosity, secondary porosity, water saturation, and clay volume are calculated by
applying and selecting an appropriate equation provided by the IP software. For porosity
determination, the volume of clay is taken into account. A computer-processed
Kakarash I. Gardi, et al…..
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interpretation of well logs is performed for wells WN.2 and WN.3. by a combination of the
Gamma Ray Neutron-Density cross-plot method and M-N lithology cross-plots which
allowed for determining porosity and lithology.
9- Upscaling well logs can be done in various ways including arithmetic, geometric, and
harmonic methods. The arithmetic averaging method has been used for upscaling
porosities, clay volume (VCL), and water saturation (Sw), while the harmonic averaging
method for permeability has been used. The petrophysical model which is a geostatistical-
based algorithm includes a process for determining petrophysical log properties and their
distribution throughout the reservoir. The sequential Gaussian simulation (SGS) algorithm,
which is one of the popular techniques (Ortiz, 2020) has been used to produce this model.
Five upscaled petrophysical properties such as effective porosity, secondary porosity,
permeability, water saturation (Sw), and clay volume (VCL) have been incorporated into
the previously constructed 3D structural framework to visualize their spatial distribution
within the 3D grid. The Buckles plot is constructed just for Horizon-2 at wells WN.2 and
WN.3.
Results
Seismic-well data tying are roughly perfect at the well locations, so any stretch and
squeeze of the synthetic seismogram to force matching or adjusting are not performed (Fig.
4.a). The targeted horizon-2 (H2) and two other interesting horizon-1 (H1) and horizon-3 (H3)
are identified and marked on the seismic section with high certainty (Fig. 4. b).
Fig.4. (a) The synthetic seismogram generated from the well WN.1 showing a good match with the seismic
section, and (b) The synthetic seismogram and the picked horizons on the seismic section of the Inline No.
3876.
Seismic, Petrophysics, and Attribute Analysis to Evaluate the Tertiary Reservoir in the High Folded Zone…….
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The isochron maps have been constructed for the horizons as displayed in (Fig.5.a),
(Fig.6.a), and (Fig.7.a). The two-way-time (TWT) values of the top of the H1, H2, and H3
range from (467.50 to 1135.15), (570.54 to 1272.45), and (581.57 to 1292.33) milliseconds
(ms) respectively. The depth maps of the top of the H1, H2, and H3 have been generated as
shown in (Fig.5.b), (Fig.6.b), and (Fig.7.b), their depth range from (558.98 to 1309.75),
(732.10 to 1529.58), and (774.35 to 1554.71) m respectively.
Fig.5. (a) The isochron of the Horizon-1, and (b) The depth map of the Horizon-1.
Kakarash I. Gardi, et al…..
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Fig.6. (a) The isochron of the Horizon-2, and (b) The depth map of the Horizon-2.
Seismic, Petrophysics, and Attribute Analysis to Evaluate the Tertiary Reservoir in the High Folded Zone…….
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Fig.7. (a) The isochron of the Horizon-3, and (b) The depth map of the Horizon-3
The depth maps are constructed using velocities extracted from the velocity model
multiplied by their isochrone surface grids. It is necessary to take into account that while
using the equation (Eq.1), the computed depths from the seismic and well data should
Kakarash I. Gardi, et al…..
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completely match together at the wells (Al-Shuhail et al., 2017). Indeed, a perfect match
between the produced depth maps of the horizons and well depths at the wells is obtained
which is clearly shown in the 3D rendering (Fig. 8) and (Fig. 9). The depth maps for all the
horizons seem to be similar to their corresponding isochron maps that depict the same
structural features.
Fig.8. (a) 3D visualization of the depth of the horizon-1 with its depth at the wells, and (b) 3D visualization
of the depth of the horizon-2 with its depth at the wells (arrow points northward).
Seismic, Petrophysics, and Attribute Analysis to Evaluate the Tertiary Reservoir in the High Folded Zone…….
243
Fig.9. (a) 3D visualization of the depth of the horizon-3 with its depth at the wells, and (b) 3D visualization
of the depth of all horizons with their depth at the wells (arrow points northward).
All the isochronous and depth maps display an asymmetrical doubly plunging anticlinal
closure trending in East-West direction (clearly depicts Taurus style), with steeply dipping
beds on the northern limb and gentle strata on the southern limb. The crest of this structure is
in the eastern part at approximately 559 m, 732 m, and 744 m and extends to the1310 m,
1529.5, and 1554.7 m at the flanks at the level of the depth of the top of each of H-1, H-2, and
H-3 respectively. The approximate length and width of the anticline structure are 6.4 km and
3.5 km respectively. At the top of the reservoir horizon (H-2), two domes are overlined by one
anticlinal closure of the top of the cap rock horizon (H-1) which is looked upon as a structure
of interest for hydrocarbon exploration. Isopach map represents a true sedimentary thickness
between horizons (Osaki, 2015) rather than the horizon depth that is perpendicular to the
bedding plane. The isopach map of the cap rock (Fig. 10.a) shows gradual thickening from the
south and southwest towards the north and northeast, from 110m to 239m. The targeted
Kakarash I. Gardi, et al…..
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reservoir isopach map (Fig. 10.b) shows irregular variation in thickness throughout the study
area, from less than 5m in the southern part to 82 m in the northeastern part.
Fig.10. (a) Isopach thickness map between Horizon-1 and Horizon-2 that is regarded as the cap rock for
the targeted reservoir, and (b) Isopach thickness map between Horizon-2 (the reservoir) and Horizon-3
Seismic, Petrophysics, and Attribute Analysis to Evaluate the Tertiary Reservoir in the High Folded Zone…….
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Although perfect pillar gridding for fault modelling in the faulted area is obtained
without cell distortion, it can be seen (Fig. 11.a) that there is no coincide with the horizon
surfaces everywhere due to the influence of uneven topography that all the horizons have
undergone folding. Thus, it is perceived that such a grid model is not suitable for use to build
actual property modelling. Creating a new 3D grid for horizon modelling and then bringing all
the faults from the previous fault model is a possible way to tackle this situation (Fig. 11.b).
Fig.11. The grid skeleton (a) Fault-based modelling, the top of the skeleton and contours depth map of the
Horizon-1 with the fault patch surfaces, and (b) Horizon-based modelling, the top of the skeleton and
contours depth map of the Horizon-1 with the fault patch surfaces in their positions (arrows point
northward).
Kakarash I. Gardi, et al…..
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Seismic conditioning is performed by applying frequency filtering (Fig. 12.b), AGC
filtering (Fig. 12.c), structural smoothing (Fig. 12.d), and then picking faults and horizons on
the section manually (Fig. 12.e).
Fig.12. The seismic section of inline No. 3936 and Faults no. 7 and 9 (a) before any filtering, (b) after
applying the frequency filter, (c) after implementing the AGC filter, (d) after employing structural
smoothing, and (e) shows picked faults and horizons on the seismic section after running the AGC.
Eighteen picked faults in such a way are put in a 3D grid framework as shown in (Fig.
13). Next, run each of the Variance (Fig. 14.a) and Chaos (Fig. 14.b) attributes separately.
Subsequently, the outputs of the last two attributes are used as inputs to the application of the
ant track attribute (Fig. 14.c & d). Finally, an automatic fault extraction algorithm is applied
to the Ant Tracking cube and the results were displayed in (Fig. 15) and (Fig. 16). All faults
of the reverse type are small with throws ranging from 6 m to 12 m. Faults no.1, 2, 3, 4, 5, 6,
and 8 cut only horizon (H1), while faults no. 7, 9, 10, 11, 12, 13, 14, 15, 16, 17, and 18 cut all
the three horizons. The majority of faults almost trend in the East-West direction and few of
them have an ENE-WSW orientation.
Seismic, Petrophysics, and Attribute Analysis to Evaluate the Tertiary Reservoir in the High Folded Zone…….
247
Fig.13. A 3D grid skeleton with minor fault surfaces picked on the horizons manually. (a) 3D side view
(arrow points northward), (b) 3D front view (arrow points northward).
Kakarash I. Gardi, et al…..
248
Fig.14. Inline no. 3936 and the results of different attributes (a) illustrates applying the Variance, (b)
presents employing the Chaos, (c) displays the Ant Tracking of the Variance cube, and (d) shows the Ant
Tracking of the Chaos cube.
Fig.15. A 3D grid skeleton with fractures and fault patches (minor and major discontinuities) extracted
automatically from a Variance-based Ant Tracking cube.
Seismic, Petrophysics, and Attribute Analysis to Evaluate the Tertiary Reservoir in the High Folded Zone…….
249
Fig.16. A 3D grid skeleton with fractures and fault patches (minor and major discontinuities) extracted
automatically from a Chaos-based Ant Tracking cube.
The well-log analysis is one of the substantial means that helps to better understand
reservoir characterization (Cannon, 2016). A computer-processed interpretation (CPI) analysis
for reservoir formation (Horizon-2) and Horizon-3 of composite logs data from wells WN.2
and WN.3 are shown in (Fig.17.a & b). Tracks represent depths of horizons, gamma ray (GR),
calliper, clay volume (VCL), water saturation (SW), permeability (Timur KT; Morris Biggs
oil KMBo; Schlumberger (KSchl), total porosity (PHIT), secondary porosity (PHISEC),
effective porosity, Limestone volume (Vlime), Dolomite volume (VDol), and sand volume
(VSand). The permeability (KT) curve estimated by Timur formulae coincides with the
(KSchl) curve estimated by the Schlumberger technique, while the (KMBo) curve by the
Morris Biggs oil method with both. The KSchl permeability has been used for 3D modelling.
The magnitude of porosities, VCL, and SW are represented as a percentage or a decimal
fraction, whilst the permeability is expressed in millidarcy (mD) or 1/1000 of a Darcy.
Kakarash I. Gardi, et al…..
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Fig.17. Computer-processed interpretation (CPI) for (a) Well WN.2, (b) Well WN.3 showing depth,
horizon names, gamma ray-calliper-bit size (BS), the volume of clay, permeability, effective and secondary
porosity, porosity comparison, total porosity and lithology in tracks 1,2,3, 4, 5, 6,7,8, and 9, respectively.
The combination between the neutron-density cross plot and M-N cross plot is used
for determining the lithology of the target reservoir (Horizon-2) in the wells WN.2 and WN.3
(Fig. 18 and Fig.19). According to these plots, the rock formation of the reservoir is composed
of dolomite, lime dolomite, and anhydritic limey dolomite.
Seismic, Petrophysics, and Attribute Analysis to Evaluate the Tertiary Reservoir in the High Folded Zone…….
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Fig.18. (a) The neutron-density cross plot, and (b) The M-N cross plot for the targeted reservoir (Horizon-
2) in well WN.2.
Fig.19. (a) The neutron-density cross plot, and (b) The M-N cross plot for the targeted reservoir (Horizon-
2) in well WN.3.
Kakarash I. Gardi, et al…..
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A Buckles plot is a graph of water saturation (Sw) versus porosity including a set of
diagonal curves consisting of points of equal Bulk Volume Water (BVW) (porosity multiplied
by water saturation) that can be used for permeability prediction and determine lithology type
of the reservoir. The estimated permeability using this approach (Fig. 20) approximately
ranges from 0.01 crossed over to 1000 mD.
Fig.20. The figure represents the Buckles plot for the targeted reservoir (Horizon-2) in (a) Well WN.2, and
(b) Well WN.3.
The porosity is the ratio of the pore volume to the total bulk volume of that rock (AL-
Tool, et al., 2019). Effective Porosity is the degree of interconnected pore volume that
controls the transmission of fluids (Ma et al., 2020). The effective porosity model of the
Seismic, Petrophysics, and Attribute Analysis to Evaluate the Tertiary Reservoir in the High Folded Zone…….
253
reservoir (Fig. 21) displays porosity ranging from 2.02% to 35.23%, with an average of
9.98%.
Fig.21. The 3D effective porosity model of the reservoir and wells.
Secondary porosity is formed after deposition as a result of the impact of the post-
depositional fracturing and the interaction of water formation (Ali et al., 2010). The secondary
porosity in carbonate rocks is considerably more significant than the primary porosity (Tiab
and Donaldson, 2015), and values of the secondary porosity model (Fig. 22) lie between
0.00% to 30.34% and an average of 4.39%. According to Glover (2000), the permeability
values for the rocks can vary considerably, from 1 nano-darcy, nD (1*10-9 D) to 1 microdarcy,
mD (1*10-6 D). The permeability of reservoir rocks may generally range from 0.1 to 1000 mD
or more (Tiab and Donaldson, 2015). The permeability (KSchl) model shown in (Fig. 23)
displays a wide range of permeability values ranging from 0.00 mD to 1000 mD, with an
average of 14.1mD. According to permeability reservoir classification by Tiab and Donaldson
(2015) this reservoir is a moderate quality (Table 2).
Table 2. Classification of the reservoir qualities based on permeability according to Tiab and Donaldson
(2015).
Permeability (K) in milliDarcy (mD)
Reservoir quality
k < 1mD
Poor
1mD < k < 10 mD
Fair
10 mD < k < 50 mD
Moderate
50 mD < k < 250 mD
Good
k > 250 mD
Very good
The pinkish patches roughly represent minimum permeability or impermeable area,
while a tiny red patch represents maximum permeability in the southwestern part. The
permeability almost has a great relationship with effective porosity, thus the low effective
porosity resulted in low permeability of the reservoir. Water saturation (Sw) is the proportion
of water volume present in the pores of a rock formation (Kennedy, 2015). It shows the
Kakarash I. Gardi, et al…..
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existence of water-saturated zones and aids in estimating the hydrocarbon saturation of the
reservoir. Figure 24 displays a 3D perspective view of the water saturation model of the
reservoir. The water saturation ranges between 12.53 and 100% with an average of 47.8%.
Fig.22. The 3D secondary porosity model of the reservoir and wells.
Fig.23. The 3D permeability (KSchl) model of the reservoir and wells.
Seismic, Petrophysics, and Attribute Analysis to Evaluate the Tertiary Reservoir in the High Folded Zone…….
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The hydrocarbon saturation is estimated by using the equation:
Sh = 100 – Sw% (Asquith and Gibson, 1982)
where, Sh=hydrocarbon saturation; Sw= water Saturation. So, the mean value of the
hydrocarbon saturation for the reservoirs is 52.2%.
Fig.24 The 3D water saturation model of the reservoir and wells.
The clay volume model (Fig. 25) shows that the clay volume ranges from 0.00 % to
40.63%, and has an average of 9.13%.
Fig.25 The 3D clay volume (VCL) model of the reservoir and wells.
Kakarash I. Gardi, et al…..
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Discussion
The depth maps show subsurface structural features that are almost the same pattern of
structural elements that can be observed in their corresponding time maps (Fig. 5), (Fig. 6)
and (Fig. 7). The asymmetrical double-plunging anticline with the E-W trend fold axis is
controlled by listric faults that made it verged towards the north. It represents a rollover
anticline that is bordered to the north by the main thrust fault. At the time of the closing of the
Neo-Tethys Ocean and subsequent continental plate collision, tectonic compression resulted
in significant thrusting and basin inversions along the northeastern Arabian plane edge
(Abdulnaby et al., 2013; Sharland et al., 2001), so faulting in this structure resulted in
producing several reverse secondary faults on the northern limb.
The majority of foreland folds and thrust belts reveals the presence of complicated
basement relief, which is typically characterized by clear current sedimentary sequence uplift
expression (Al-Kubaisi and Shakir, 2018). The East-West orientation of the anticlinal closure
is consistent with the known pattern of the Taurus mountain structural style within this part of
the Kurdistan Region. However, it is worth noting that an erratic to the general style can be
seen in all maps that the northern limb is steeper than the southern limb. According to
(Ameen, 1991) this discord can be interpreted as related to the existence of deep-seated faults
that typically have no surface indication.
The collision between the Arabian plate with the Iranian and Anatolian plates started after
the final closing of the Neo-Tethys Ocean in the Miocene (Abdulnaby et al., 2013). The
uplifting, folding, thrusting, and dominant deformation events of the Zagros fold-thrust belt
associated with this collision (Csontos et al., 2012). The belt experienced compressions due to
the collision resulting in a reversal of movement on the previously formed normal faults and
turning them into up thrusts (Abdulnaby et al., 2013). However, the faults have a tiny throw
and do not have a great impact on the contouring of the horizons, it is preferred to mark their
locations on the depth map with red sticks as displayed in (Fig. 5.b), (Fig. 6.b), and (Fig. 7.b).
The Variance (Fig. 14.a) and Chaos (Fig. 14.b) attributes successfully helped in fault and
fracture capture. The Ant Tracking attribute puts out non-discontinuity events and enhances
the edge structures like faults and fractures that also can aid in manual fault interpretation
(Fig. 14.c & d). The produced Ant Track cubes from each of the Variance and Chaos
attributes served as input for automatic fault extraction. Plenty of subtle faults and fractures
are extracted from the Ant track cubes, almost trending in east-west directions (Fig. 15) and
(Fig. 16).
The 3D model of the effective porosity allows for the prediction of future production and
injection planning. The quantitative values of this model (Fig. 21) range between 2.02% and
35.23%, also the secondary porosity model (Fig. 22) ranges from 0.00% to 30.34%, revealing
that the lithology of these carbonate rocks are not homogeneous throughout the reservoir.
Buckles plot is often utilized for analyzing various reservoir parameter values (Riazi, 2022),
and (Singh, 2019) selected the Buckles model to estimate and show permeability. The
permeability model (Fig. 23) demonstrates a broad range from 0.00 mD to 1000 mD, and the
presence of a few permeability values over 1000 mD are depicted in the Buckles model in the
Well WN.3 (Fig. 20.b) also suggests an intricate nature of the pore structure of this reservoir
carbonate rocks.
The water saturation shown in Fig. (23) shows values ranging from 12.53 to 100%, with
an average of 47.8%, and the hydrocarbon saturation (Sh) values range from 0% to 87.47%,
with an average of 52.2%. These results suggest that the wells drilled in the area (the crest of
the structure) relatively have an intermediate to high water saturation and low to moderate
hydrocarbon saturation. The clay volume model in Fig. (25) displays low clay content
Seismic, Petrophysics, and Attribute Analysis to Evaluate the Tertiary Reservoir in the High Folded Zone…….
257
between 0.00 % and 40.63%, with a mean of 9.13%. This low average of the clay (shale)
content, which is less than 10%, suggests that the reservoir is a clean formation (AL-Tool, et
al., 2019), and dolomite to lime dolomite rocks are the predominant lithology.
Relatively moderate effective porosity (Fig. 21) in blue-green colour, a relative
intermediate permeability (Fig. 23) in yellow-green colour, relatively low water saturation
(Fig. 24) in yellowish green colour, and low clay volume (Fig. 25) in orange colour,
especially in some parts of the crest zone of anticline closure suggests that this field has a
moderate prospect for hydrocarbon exploration and production.
Conclusions
The study has shown the effectiveness and versatility of integrating and using 3D seismic
data, check shots, and well logs with attribute analysis in mapping subsurface features,
estimation of petrophysical properties, characterization, and building a 3D static model for the
targeted reservoir. The main conclusions are as follows:
• The isochronous and depth maps show a doubly plunging anticlinal closure trending
in the East-West direction (Taurus style) with an area of 6.4 km length and 4.5 km
width. Its northern limb is steeper than the southern limb, which is inconsistent with
the general structural style in this region. This abnormal situation may be due to the
local tectonic activity generating stress toward the north direction.
• The northern part of the anticlinal closure (rollover anticline) is dissected by eighteen
minor reversal faults with a tiny throw range between 6m and 12m oriented in E-W
and ENE-WSW direction. After manually detecting and interpreting the faults, they
were used for building 3D static modelling.
• The top, mid, and bottom of the produced skeletal framework by pillar gridding
process from the fault modelling has shown inconsistency with the top of horizons
everywhere outward from the faulted area, building a new 3D grid by horizon
modelling and then incorporating the faults into it can be one way for amending the
grid.
• The attribute analysis enhanced and improved fault interpretation in the 3D seismic
data set, and showed their effective role in automatic fault and fracture extraction.
• The analysis performed on the reservoir zone, cross-plots show good capabilities to
determine and delineate porosity and lithology in well logs data.
• The cross-plots show that the reservoir consists mainly of dolomite, lime dolomite,
and anhydritic limey dolomite.
• The 3D models of petrophysical parameters reveal that the reservoir has moderate-
quality reservoirs.
Acknowledgments
Sincere thanks to the Ministry of Natural Resources (MNR) of the Kurdistan Region
Government (KRG) and oil companies operating in the region for providing necessary 3D
seismic data and well logs. Grateful appreciation goes to each of Schlumberger and Senergy
companies for making free licenses of their valuable software (Petrel Suite seismic
interpretation and Interactive Petrophysics IP) for academic research.
Kakarash I. Gardi, et al…..
258
Conflict of Interest
The authors declare that there is no conflict of interest
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