Using of Polynomial and THD Methods to the Depth Map of Shurau Formation that Obtained from 2D Seismic Interpretation to Structural Map Construction in Ismail Region, Northern Iraq

Abstract


Introduction
The seismic reflection was intensively applied in exploring subsurface oil-traps, due to its great efficiency in the exploration work, (Al-Sadi, 2017).A lot of seismic studies were carried out in Iraq and in the world for hydrocarbon exploration (Dobrin and Savit, 1988;Veeken, 2007;Khorshid et al., 2017;Al-Ridha and Al-Khafaji, 2019).The result of the seismic interpretation aims in its first stage to obtain the depth maps of some interesting formations.The polynomial is an analytical technique like the trend surface method tend to produce the regional trend of certain data (Dobrin and Savit, 1988).The simplest regional surface is an inclined plane with many orders.The subtraction of the regional surface from the original map produces which may called residual map (represent the difference between them (Nguimbous-Kouch et al., 2017;Ibrahim et al., 2019).The Total Horizontal Derivative (THD) is also a mathematical technique deal with the determination of certain variable variations with the distance.The Technique usually used in the interpretation of gravity and magnetic data to detect the edges or fault's locations, (Verduzco et al., 2004;Al-Banna and Al-Khashif 2019).This study attempts to plot the regional and residual maps of the depth map of Shurau Formation at the study area.Also, it is attempted for the first time to determine the Total Horizontal Derivative (THD) of the residual depth map of Shurau Formation in order to detect precisely the faults in the study area.

Location of the Study Area
The study area is located within zone 38 in northern Iraq within the administrative boundaries of Arbil governorate, specifically to the far northwest of Kirkuk governorate, with a distance of 55 km and to the northwest of the Bai Hassan field at a distance of 1 km (Fig. 1).

Tectonic of the Study Area
The study area is located within the unstable shelf (Foothill Zone) within the low folded zone, specifically the Butmah-Chemchemal subzone, and in Deir Alzor-Erbil block (Fig. 2) (Jassim and Goff, 2006).The prominent features of the subzone are long folds often not associated with longitudinal faults with the exception of the Kirkuk structure.The studied area is characterized by a broad and deep syncline in the subsurface (Jassim and Goff, 2006).The updating of the geological and geophysical survey is useful for getting the best geological map (Sissakian et al., 2021).

Surface and Subsurface Geology of the Study Area
According to the surface geological map of the study area (Sissakian and Fouad, 2015), shows that most parts of the region are covered by gravel, and sand deposits belonging to the Pleistocene age (Fig. 3).Most parts of the area are Flat and valleys, while the other parts are mountain chains in which the dominant rocks belong to the Bai Hassan Formation (Upper Bakhtiari) and Muqdadiya (lower Bakhtiari) belong to the Pliocene epoch (Sissakian and Fouad, 2015).
The Shurau Formation, which is considered in this study, belongs to the lower sequence of Oligocene, defined by Bellen in 1956 in well K-109 of the Kirkuk structure (Bellen et al., 1959) It is found consists of 18 m of coralline limestone in the lower (thickest) part and dense grey limestones in the upper part.The contact of the formation with the underlying Sheikh Alas Formation is conformable, while the upper contact with the Baba Formation is unconformable.The thickness of Shurau Formation is 74 meters in Ismail well, which was drilled in the study area (OEC, 2019).The stratigraphic and geological column in the study area was obtained from Ismail well (Is-1), which penetrated to a depth of about 2880 m (OEC,2016).The relatively shallow structures subdivided the Hamrin anticline into bocks like structure (Al-Banna, 1998).Fig. 4 shows the stratigraphic column of Ismail well (Is.1).

Seismic Information
The 2D processed seismic data for 27 lines with a total length of (227) km and a total area of (405) km2 were obtained from the Iraqi Oil Exploration Company (O.E.C).The Basic map of the study area was built using these seismic data loaded in an interactive interpretation workstation in SEG-Y format.All necessary processing to get a good seismic section is done.The accurate horizons picking on the seismic sections for the Shurau Formation leads to accurate interpretation results.The general analysis of the seismic sections introduces the time map of Shurau Formation (Fig. 5).The more accurate velocity to be used in time to depth conversion is the average velocity, which can be computed from the check-shot survey of the wells (IS-1), (BH-16).Average velocity maps were acquired from the velocity model for Shurau Formation, with a contour interval (20 m/s) Fig. 6.The depth maps values from the velocity and one way time were obtained Fig. 7. Depth = (Average velocity × (TWT(/2)).Where TWT is two-way traveltime.The depth map of Shurau Formation indicates presence of enclosure trending northwest-southeast extended in length to about 10 km at the southwest part of the study area.Many small uplifts (anticlines) and subsidence (synclines) can be detected in Fig. 7.

The Polynomial Technique Application
One of the most versatile analytical strategies for calculating regional gravity is polynomial fitting.The observed data are used to create the mathematically describable surface that provides the best match to the gravitational field within a given degree of detail, usually using least squares.The difference between the gravitational field as mapped and the regional field as calculated by this surface is the residual.(Dobrin,1976).The polynomial fitting is a mathematical tool used to describe the main trend surface of certain data, with specific orders depending on the complexity of the original data.The aim of using the polynomial technique in this study aims to obtain the regional trend of the depth map of Shurau Formation using first order (simple planner surface).The depth map of Shurau Formation was digitized manually on the contour lines Fig. 8.The polynomial technique was applied to the depth map of Shurau Formation, and the regional trend of depth map was obtained with the 1st order surface, and plotted using surfer program (Fig. 9).The regional surface of the depth map of Shurau Formation was found to be dipping toward the east-northeast with a slope equal to about 42.5 m/km.The regional depth map also shows the formation dipping toward the southeast by about 25 m/ km slope.By subtraction of the regional trend depth map from the original depth map of Shurau Formation, the difference(residual) depth map was obtained (Fig. 10).The residual depth map of Shurau Formation shows that the central part of the study area represents by subsidence area relative to the northeast and southwest sides.The area includes many parallel anticlines (Redline) and synclines (Blue line).Some parts in the study area are characterized by a steep gradient which may be indicated a fault location.

Total Horizontal Derivative method (THD)
The THD method is calculated according to the following equation : The maximum THD values usually coincide with the maximum gradient which is considered as an indication of boundary, edge or faults.The THD was applied to the depth map of Shurau Formation (Fig. 11).The maximum values of the THD traced in black lines which may indicate a possible fault's location, and it is also indicates a possible tectonic boundary in (orange) (Fig. 11).

Final Structure Map
The location of anticlines, synclines, and faults in the study area was used to construct the structural map of the study area (Fig. 12).It is observed on this map some changes in the trends of the structural features (Anticlines, Synclines, and Faults) or discontinuities.The map which considered as a transverse fault (Orange lines).The study area seems to be divided into many blocks.Al-Banna (1998) Concluded that shallow structures in Hamrin region subdivided the area into blocks like structures (Al-Banna, 1998).

Conclusions
• The seismic interpretation of Shurau Formation was achieved in the study area.
• The polynomial technique was used to obtain the regional trend of the area which is found to be inclined generally toward the east-northeast direction.The local structures Anticlines and Synclines were found mostly trending NW-SE coincide with the main trend of the structure in folded Zone.• The THD method was applied to the depth map of the Shurau Formation, and good results were obtained concerning the tracing of the fault lines in the study area.• According to the result of this study, a detailed structural map was obtained.

Fig. 1 .
Fig.1.The location map for the study area (Modified from United Nations, 2014)

Fig. 2 .
Fig.2.The location of the study area on the tectonic map of Iraq(Fouad,2015)

Fig. 6 .
Fig.6.The velocity map of Shurau Formation in the studied area

Fig. 7 .
Fig.7.The depth map of top Shurau Formation in the study area

Fig. 8 .
Fig.8.The depth map of Shurau Formation shows the digitized points

Fig. 10 .
Fig.10.The residual depth map of Shurau Formation in the study area horizontal distance in X-axis and Y-axis, respectively.

Fig. 11 .
Fig.11.The Total Horizontal Derivative of the depth map of Shurau Formation of the study area