Subsurface Structural Image of Galabat Field, North East of Iraq Using 2D Seismic Data

Abstract


Introduction
The reflection method is concerned with the contrast in the elastic properties of subsurface media where seismic waves travel through, and particularly at the interfaces between different subsurface adjacent bedding planes.It provides a direct image about the details of subsurface geological features (Grana, 2013).Seismic interpretation is the last step after the processing of field data.The seismic data provides good visualization of the different properties and densities of subsurface rocks, through the determination of the acoustic impedance of the rock layers (Brown, 2011).In studies of seismic reflections, the determining of structural geology to subsurface accurately like folds, faults, and structural subsurface conditions, data representation, time, and velocity would be used to identify traps of structural, facies, depositional environment, stratigraphy traps, and direct hydrocarbon indication (Al-Sadi, 2017).The current study aims to image and interpret subsurface structural features related to the Fatha and Jeribe formations, particularly for the Galabat field area.

Location
Galabat field is located in the Diyala area northeast of Iraq, 18 Km east of Kifri town (Fig. 1).According to the Iraq map of tectonic (Al-Khadhimi, 1996), the field is surrounded by two anticlines, the first is Southern Hamrin mountain range which consists of Injana-Khashim Al-Ahmer-Minsuryia structural domes from the southwest side and Qumar-Naodoman structural domes from the northeast side.Table 1 shows the boundary coordinates of the studied area by U.T.M WGS.1984.

Reflectors Picking
The seismic section represents the end product of seismic work (fieldwork and data processing), so the accuracy of the interpretation depends on the accuracy of previous works.The geophysicist uses an interpretation workstation to map the structural and stratigraphic seismic features, correlating them with geological settings in the subsurface (Bacon and Redshaw, 2003).
Seismic interpretation work was achieved by using the Geoframe workstation.In the reflectors picking, Seismic section (QR-20) (Fig. 2), which links between the seismic lines and covers Galabat area with synthetic seismogram of the Galabat-3 well located in the middle of area, were interpreted and established as a key seismic section.Consequently, four horizons representing the Tertiary period were selected to be mapped and picked.Display of identified reflectors and their geological ages are shown in Table 1.

Seismic Structural Evidences
Recommended to pick and track affected faults on the seismic data then starting the reflectors picking.To achieve that, seismic instantaneous phase attribute technique was generated from the original 2D reflectivity data and displayed as seismic sections.The important section to detect structural and stratigraphic features is the Instantaneous phase section, (Taner et al., 1979 andSonneland et al., 1993).Aaliji Formation.
Eocene-Paleocene (Ypresian-Danian) It is independent of the amplitude and shows continuity and discontinuity of events (Al-Sadi, 2017).It is useful as the best indicator of lateral continuity of sequence boundaries and visualizes the details of bedding configuration.The seismic instantaneous phase attribute was applied and instantaneous phase sections were generated from the original 2D reflectivity data for conventional seismic interpretation (structural interpretation) on 2D seismic sections.This interpretation technique was widely used to detect faults during seismic picking and mapping.Interpretation observations showed that Galabat area was affected by a set of reversed faults.Fig. 3 illustrates the existence of the major reverse faults shown on dip seismic sections, and illustrates the operations folding in the Fatha Formation due to the deformation forces.
Based on interpretation of picked reflectors and data of the well drilled (GT-3), the study area was affected by two stages of tectonic forces, the first was during the end of the Cretaceous Movement because of Epiorogenic Movement, and the second was during the Late Tertiary Movement that led to Alpine orogeny, and the Galabat structure was affected by the horizontal compressive stress, the shape of the structure is longitudinal and narrow with direction of general axis NW-SE, Study area was affected by two stages of the effective force (Naji, 2021): • The first stage is represented by horizontal extensional stresses which produced Normal faults that affected the end of the Cretaceous and where normal faults have been identified in the study area with (Graben) below the picked reflectors.• The second stage is represented by Compressional Forces that were affected during the Tertiary Period and led to inverted faults.The result of the stresses of compression (NE-SW) into the inversion of positive to those pre-existing grabens, half grabens and positivity inverted structure formation (Mahdi, 2022).

Time Contour Maps
A structure contour map is one of the most important tools for structural interpretation because it represents the full two-dimensional form of a map horizon, and subsurface configurations must be understood in detail to effectively delineate the structures that are favorable for hydrocarbon accumulation (Coffen, 1984).
TWT values of top of Fatha and Jeribe formations were performed, then by using software Geoframe, the grid is processed, and it is mapped in time domain for central of interpretation.Above Sea Level is 100 m, and contour interval is 50 m, that has been to drawn for Fatha and Jeribe formations maps (Figs. 4 and 5).

Structural Model
3D modeling is a display where properties can be created by using pre-defined system variables, such as cell volume, seismic resampling, zone index, etc.Each cell will get a numerical value corresponding to the selected system variable.Geometrical modeling is not only restricted to simple geometrical properties; it also covers more complex property distributions such as random normal distribution, zone segments, fault segments, seismic connected volumes, and well index region (Schlumberger, 2013).
Therefore, the results of the interpretation of seismic sections depend on grids of depth and boundaries of fault of Fatha and Jeribe formations, in depth domain 3D structural model was constructed to determine of structural framework for the Galabat structure by Geoframe program.This model gives an initial strategy for the future of field work such as drilling wells or preparing planning for new 2D or 3D seismic surveys in the study area (Figs. 6 and 7).

Conclusions
Structural maps show that Galabat field appears as an asymmetrical anticline and two major thrust faults surrounding it from northeast-southwest limbs.
Galabat Field is a positively inverted structure, which represents a curved surface at depths that belongs to the Tertiary period.The structure axis is NW-SE trend and the major thrust faults.
The density of faults increases in the north than in the middle and south.In the middle and the south, there are two faults, but in the north, there are three faults as a result of the increase in forces.

Recommendation
Based on the conclusions, the Galabat Field is a complex structure.Thus, the most important recommendation is the implementation of 3D seismic scanning with pre-assembly time migration processing.

Fig. 3 .
Fig. 3. Instantaneous phase seismic section (QR-90) Illustrates Movement Inversion because of the compression process in the middle part of field

Fig. 4 .
Fig. 4. Two way time map of top Fatha Formation in the Galabat Structure

Fig. 6 .
Fig. 6.A 3D view of structural model of the Fatha Formation in depth domain

Fig. 7 .
Fig. 7.A 3D view of structural model of the Fatha Formation in depth domain