3D Seismic Reflection Study of Al-Akhadeir Area, Southwestern Iraq

This study includes structural and stratigraphic interpretation of 3D seismic reflection data for Zubair Formation (L. Cretaceous) within Al-Akhadeir area, southwestern Iraq (Karbala Governorate). Depending on the 3D seismic reflection interpretation process, and based on the synthetic seismogram and well logs data, two horizons were identified and selected (top and base Zubair reflectors). These horizons were followed up over the entire area in order to obtain structural and stratigraphic settings. TWT, depth, and velocity maps for the base and top Zubair Formation were constructed. From the interpretation of these maps and based on the seismic section, the study concluded that there are some enclosures that represent anticline in the NW of the horizon and syncline in the NE, while the nose structure appears in the middle of the horizon and trends N-S. The horizon represents a progradational with sigmoid configuration.. Other seismic structural phenomena were recognized in this part of the area, such as flat spot, down lap, and top lap, which give indicators of potential hydrocarbon accumulations .

study area is located within the stable platform [2].

Acquisition and processing of data
The 3D seismic reflection survey of Al-Akhadeir area used a specific design that ensures ease of execution with good quality of recorded data. Before the execution of the 3D survey, the pre-planning report of Al-Akhadeir area was prepared. A 3D symmetrical split spread with the vibrators source model Nomad 65 was achieved by the OEC in 2017 [17]. The survey in Al-Akhadeir area covered an area of 2042 K .
The data was processed by the OEC, the Iraqi Ministry of Oil, using OMEGA SPS version 2015.1 software. Noise attenuation process leads to a progress in the reflection continuity and an enhanced

Ali and Kadhim
Iraqi Journal of Science, 2020, Vol. 61, No. 11, pp: 3035-3046 3038 ability to calculate seismic attributes. The main steps in processing include editing, deconvolution, pre-stack analysis, muting, and gain recovery.

Interpretation of data and discussion
3D seismic surveys data of the EK-2, collected by the OEC, were interpreted by using the seismic program of petrel, through the following approaches: 1-Preparing time, velocity, and depth maps to explain the structural pattern elevation and identify the stratigraphic picture from Zubair Formation by using available 3D seismic data. 2-Applying seismic attributes (instantaneous phase, acoustic impedance, and alterative gain control (AGC) to investigate the hydrocarbon indicators within Zubair Formation. 3-Structural interpretation works.

Base map preparation
The base map of the study area was constructed by using seismic data which were loaded in an interactive workstation of interpretation in a SEG-Y format. Two types of perpendicular lines were assigned, termed as inline and cross line. Figure-2 shows the base map of the study area.

Generation of synthetic seismograms
The main step of the generation of synthetic seismograms is computing the acoustic impedance, which was achieved based on the following equation: = ×

Ali and Kadhim
Iraqi Journal of Science, 2020, Vol. 61, No. 11, pp: 3035-3046 3039 where is seismic velocity and ρ is density measured from density log. The reflection coefficients of the incident wave are computed on the reflector separating two series of time intervals, (i) and (i+1), that have values of acoustic impedance denoted by (ρi, vi) and (ρi+1, vi+1), respectively. According to an earlier study [18], the reflection coefficients could be calculated as follows: is the density at the interval (i), (i+1), whereas ( , +1) is the velocity at the interval (i), (i+1), respectively. The convolution process between the reflection coefficients and the experimentally selected wavelet is made to obtain the synthetic seismogram. The traverse seismic sections passing through the well location and synthetic traces of reflectors are displayed in Figure-3.
The picked reflectors' wavelets appeared as peaks and troughs on the synthetic trace (peak in the top and trough in the base of Zubair Formation).

Structural images of the picked horizons Two-way time maps Top of Zubair TWT Map
Two-way time (TWT) map of the top Zubair ( Figure-4) shows that the higher value of TWT is towards the NE direction and decreases in the W and NW directions of the area. The map indicates many enclosures, named A (represents anticline), B (syncline), and C (nose). The TWT value to the center of the anticline (A) is about 720 ms while it reaches to 940 ms at the center of the syncline (B). In the central part of the area, the nose is trending to the SE, as given in (C), and the well EK-2 is near this area.

TWT Map of the Base of Zubair
The TWT map of the Base Zubair presents a higher value of TWT in the NE, which indicates that the slope of the reflector is towards the NE. The value of TWT in the W and NW directions is about 880 ms, while it reaches to 1200 ms in the NE, as shown in Figure- Figure-6 shows that the velocity value increases toward the NW direction and decreases toward the NE and S directions, while the contour interval is 10m/s. Figure-7 shows that the velocity value increases toward the NW direction and decreases toward the S and NE directions, while the contour interval is10m/s.

Depth maps Depth map of top Zubair
The higher area appears in the west and is about 1880 m in depth, which represents the anticline (A). There is a syncline structure (B) in the NE of the study area and a nose structure (C) in the middle of the area, trending NW-SE (Figure-8).

Depth map of base Zubair
The depth reached to 1975 m in the E direction and decreased to 1500 m in the W direction of the study area. Figure-9 shows the anticline structure (A) in the W of the area and the synclinal structure (B) in the NE area, those are the same structure features which appear in the top Zubair reflector. There is a nose structure (C) trending SE in the study area and a closure structure (D) in the SW of the study area.

Stratigraphic Interpretation
Seismic stratigraphy aims to enhance the understanding of the depositional environments by attempting to interpret seismic data supported by the geological information. This includes understanding the types of traps, accumulation of hydrocarbons, system of migrations, etc. All these information and details of the subsurface are the aim of the seismic section interpretation [19].
The type of reflection configuration within the Zubair reflectors is progradational, with fundamental type of sigmoid configuration. Figure-10 illustrates the sigmoid formed through the progradation high stand (HST), which is deposited during some parts of a relative sea level rise cycle. The structural and stratigraphic pictures of the studied horizon (Zubair Formation) could be described as follows; the strata units are mainly horizontal with some stratigraphy and structure phenomena .There are some enclosures represented by an anticline in the NW of the horizon and a syncline in the NE, while the nose structure appears in the middle of the horizon and trends N-S (Figures- (4,5,8,9) Stratigraphically, some indications suggest a depositional channel which represents the trend of sediment influx from the basin toward the continental shelf, whereas others pattern from the HST to the margin of the shelf insulting of sigmoid within the transgression stand tract deposited on the surface boundaries (SB) within the Zubair Formation. Two high-stand tracts were recognized in the left side of the horizon as well as two transgression stand tracts in the middle which represents the delta front environment of the Zubair Formation. It is ended by the sea level rise and deposition of Shuaiba Formation which represents a marine environment [3]. The nose structure in the seismic structure map appears in this part of the study area. This part of the horizon could be described as having hydrocarbon indicators. Other phenomena could be shown in this part, such as flatspot, down lap, and top lap, which give indicators of potential hydrocarbon accumulations (Figure-10). The final picture of the horizon in this area could be described as a structure-stratigraphic reservoir. Conclusions the study concludes the following 1. Two reflectors of top and base Zubair Formation were defined by using synthetic seismogram in the time domain for well EK-2. Zubair wavelet appeared as peak /trough due to the difference in reflection coefficient. 2. The TWT, average velocity, and depth maps were used to interpret the structure and stratigraphy pictures of Zubair horizon. The TWT maps demonstrated a higher value of TWT in the NE, which indicates that the reflector is slopping towards the NE of the area. Also, the TWT maps show closure structures (anticline and syncline) in NW and NE and a nose structure in the middle of the area. The structural features seem to be parallel to the boundary between Iranian and Arabian plates, which is caused by their collision.

3.
Velocity values increase toward the NW direction of Zubair Formation, according to the velocity maps. These maps of the studied Zubair reflectors have velocities that ranged between 3280-3500 m/s. 4. The depth maps of the studied horizon showed the structural configuration of Al-Akhadeir area. Depth maps illustrated that the depth increases gradually from W and NW toward the NE and E. 5. The seismic section shows that the Zubair Formation represents clastic facies with a delta front environment that is represented by two transgressed stand system tracts (TST). The presence of fluvial channels refers to the end of this stage. This case is repeated again in the upper part of Zubair Formation which is represented by HST and ended by the rise of the sea level. The Shuaiba Formation began to be deposited in this stage as carbonate rocks, which represents marine environment. Facies of Zubair Formation have progradational configuration (sigmoid) subjected to truncation erosion of the stream onlap surface of the carbonate platform. 6. The middle part of the Zubair Formation, which is labeled as sigmoid in the seismic section, may represent good hydrocarbon indicators.