3D Seismic Geophysical Evaluation

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2D geophysical evaluation

To verify that the up-dip structuring in the sub-surface observed on 2003 reprocessed data exists only on one line or extends through the area, two separate interpretation exercises were carried out, and time contour, velocity and depth maps were generated at reservoir horizon (Top Eocene level).
The detailed procedure of the geophysical work is explained below:

2D geophysical interpretation

Seismic interpretation was carried out on PreSTM time volume of 2D seismic data. SeisWorks® 2D seismic interpretation software application of LandMark suite of softwares developed by Halliburton was used for data interpretation purposes.
All seismic lines pertaining to the Dhurnal field were interpreted on workstation. Top of Miocene, Eocene, Paleocene, Permian and Infra Cambrian formations were identified on the basis of VSP data available on Dhurnal-1 well (Figure 2.1).
Top Eocene level was interpreted on seismic profiles of the study area after mistie adjustment. Below the Miocene group, the Eocene stratigraphic unit can be recognized throughout on the basis of distinctive seismic signatures having strong, continuous reflector of moderate to high amplitude. Figure 2.1: Vertical seismic profile (VSP) of Dhurnal-1 well used for horizon identification. The vertical panels above show the different processing levels. The red circle indicates the Eocene level which is the main reservoir in the Dhurnal field (Source: OPL).
This reservoir horizon was then interpreted on each and every line present in the study area.
Figures 2.2, 2.3, 2.4, 2.5, 2.6 and 2.7 are a few examples of horizon and fault interpretation.
Figure 2.2: Horizon interpretation on seismic line NP84-13 (1987 data). Yellow color shows Eocene reflector and the rest of colors indicates fault interpretation.
Figure 2.3: Horizon interpretation on seismic line NP84-13 (2003 data). Yellow color shows Eocene reflector and the rest of colors indicates fault interpretation.
Figure 2.4: Horizon interpretation on seismic line NP84-10 (1987 data). Yellow color shows Eocene reflector and the rest of colors indicate fault interpretation.
Figure 2.5: Horizon interpretation on strike line NP84-52 (2003 data). The Eocene reflector is marked and fault interpretation is also visible in this section. The inset map shows the location of this line highlighted by red.
Figure 2.6: Composite line passing through D-3 and D-6 wells (1987 data). This display clearly indicates downthrown block at the reservoir level on line NP84-13. Eocene level is displayed by yellow color. The inset map shows the location of these line highlighted by red.
Figure 2.7: Composite line passing through D-3 and D-6 wells (2003 data). This display clearly indicates the presence of anticlinal feature at reservoir level on line NP84-13. Eocene level is displayed by yellow color. The inset map shows the location of these lines highlighted by red.
Two independent maps at Top Eocene level have been generated (Figures 2.8 and 2.9).
Figure 2.8: Time structure map at Top Eocene level based on 1987 reprocessed PSTM data. The black-colored time contours show that a downthrown block exists between D-1 and D-6 wells. The fault polygons are represented as grey where the fault arrows indicate a thrust regime and purple lines are the Dhurnal mining lease.
Figure 2.9: Time structure map at Top Eocene level based on 2003 reprocessed PSTM data. According to black-colored time contour values, a pop-up anticlinal structure separates D-1 and D-6 wells. The fault polygons are represented as grey where the fault arrows indicate a thrust regime and purple lines are the Dhurnal mining lease.

Comparison of results of 2D interpretation based on 1987 and 2003 processed data

Figure 2.8 indicates lack of presence of any new structure between D-1 and D-6 wells. There is a downthrown block separating the two wells and the faults trend shows that the wells have been drilled at the optimum locations, and that there is no more structural anticlines undrilled in this area. So according to this interpretation, Dhurnal oil field lacks any further drillable hydrocarbon potential.
Figure 2.9 reveals that a pop-up anticlinal structure exists between D-1 and D-6 wells and has an aerial closure of 174 acres (0.704 sq. kms). Attic hydrocarbons may be present at this up-dip area that can be recovered by side-tracking D-1 well or drilling a new well.
The interpretation results clearly verified the major difference in the structural configuration of the two different seismic processing. The up-dip potential mapped on 2003 data confirmed that the anomaly observed on one seismic line does actually extend in the study area and had significant importance that cannot be neglected.
Therefore, in order to identify the reason behind this major discrepancy, a detailed and comprehensive analysis of the two different processing/reprocessing was carried out.

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Seismic data processing/reprocessing

About 352 L.km of seismic data of two vintages by AMOCO (1972), OXY (1984) was processed up to PSTM level, at Golden Geophysical in 1987. The same 2D data set with addition of 17 L.km of OPI (2001) was reprocessed up to PSTM by PGS, Cairo in 2003. The reprocessing objective was to enhance/improve the clarity of minor faults, which had indicated compartmentalization.
The processing/reprocessing sequences of these two campaigns were analysed in detail by me. It was important to find out why this reprocessing (2003) differed from the old (1987) processing. Comparison of the two processing parameters showed that the difference in structural configuration was actually caused by the velocity modeling.
These reprocessing results were interpreted and demonstrated in the figure below: Figure 2.10: Comparison of old 1987 and new 2003 processing/reprocessing of PSTM data showing the presence of pop-up anticline in 2003 data. However, a downthrown block is observed exactly at the same shot point location on 1987 data.

Comparison of the two processing/reprocessing

Detailed review of the two processing sequences reveals that the major factor which may affect the sub-surface geometry was the dense seismic velocity analysis carried out on 2003 data. Apart from this, there was no significant difference in the processing parameters.

Velocity analysis

As mentioned earlier, velocity analysis is probably the most critical and important stage. On the 1987 data, velocity analysis was done on a 1 km grid (Figure 2.11), whereas, grid spacing used on 2003 data was 0.5 km (Figure 2.12). In my view, the closed grid interval enhanced the structural configuration and clarification of fault geometry. The 2003 PSTM time data showed some changes in fault interpretation (Figure 2.12) and further improved the structure significantly.
Figure 2.11: Sparse velocity picking on old 1987 data showing vertical velocity panels at the top of the seismic section. In this case, the velocity grid is 1 km apart displaying the zone of interest as a downthrown block between D-1 and D-6 wells.
Figure 2.12: Detailed velocity picking on 2003 data showing vertical velocity panels at the top of the seismic section. In this case, the velocity grid is 0.5 km apart displaying a pop-up anticlinal structure (an up-thrust block) between D-1 and D-6 wells.
The velocity model is generated utilizing the root mean square (RMS) velocities provided at fixed CDP intervals on PSTM hard copy sections of the two data sets. These velocities were calculated and computed at the interpreted two way time of Top Eocene horizon. This velocity information was then imported to OpenWorks® and velocity maps were constructed using ZMap Plus™ software (Figures 2.13 and 2.14).
Figure 2.13: Velocity map based on 1987 data showing a velocity anomaly in the south of D-1 and D-6 wells. Black contours represent velocity trend (meter/second) in the area and purple lines are the Dhurnal mining lease.
Figure 2.14: Velocity map based on 2003 data showing a velocity anomaly over the apex of Dhurnal structure. Black contours represent velocity trend (meter/second) in the area and purple lines are the Dhurnal mining lease.

Comparison of results of velocity analysis of 1987 and 2003 data

Figure 2.13 shows that the velocity anomaly was observed in the south of D-1 and D-6 wells. This shift in velocity anomaly from the apex of the Dhurnal structure was contradicting the structural configuration observed on Time structure map as flank of Dhurnal structure can be clearly seen on seismic lines and time map.
Whereas Figure 2.14 reveals the development of a velocity anomaly over the apex of Dhurnal structure which was due to dense velocity grid. This velocity anomaly seems to be fairly consistent with the geology of Dhurnal structure.
The velocity grids of 1987 and 2003 data were further merged with respective time grids to construct depth structure maps (Figures 2.15 and 2.16). Both single and dual grid operations were performed in ZMAP Plus™ software.

Table of contents :

Chapter 1
1.1 Introduction
1.2 Study area
1.3 Stratigraphy of the area
1.3.1 Geological and geophysical data
1.3.2 2D Seismic data
1.3.3 Well data
1.3.4 3D Seismic data
1.3.5 Software used
1.4 Problem identification
1.5 Objectives
Chapter 2 2D Geophysical Evaluation
2.1 2D Seismic interpretation
2.1.1 Comparison of results of 2D interpretation based on 1987 and 2003 proc. data
2.2 Seismic data processing/reprocessing
2.2.1 Comparison of two processing sequences
2.3 Velocity analysis
2.3.1 Comparison of results of velocity analysis based on 1987 & 2003 data
Chapter 3 3D Seismic Geophysical Evaluation
3.1 3D seismic data interpretation
3.2 Results of 3D geophysical analysis
Chapter 4
4.1 Discussion
4.2 Conclusions
References

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