الفهرس 
Introduction and Geologic setting of Issaran FieldOnly 14 pages are availabe for public view
 |  | from | 178 |  |  |
| | | from | 178 | | |
Abstract
Issaran oil field is one of the few heavy oil naturally fractured carbonate unconventional reservoirs in the Middle East. It is located at the western side of the Gulf of Suez, in the central dip province of the gulf, a distance of 244 km southeast of Cairo city. The Miocene stratigraphic column of Issaran oil field represents a lagoonal to shallow marine facies, as compared to the conventional Gulf of Suez facies in deep parts. The produced hydrocarbon from Issaran field has a low API gravity, which is migrated from the Brown Limestone source rock, under reducing conditions at the early oil window (low maturity).
It has proved that, five main reservoirs called from bottom to top: Nukhul Dolomite, Gharandal Carbonates, Lower Dolomite, Upper Dolomite and Zeit Sand. Most of the carbonate reservoirs are deposited at very shallow depths. As a result, they are characterized by low pressure reservoirs. Each reservoir has its own characteristics, because of that each one of them required a special and different treatment, to improve the hydrocarbon productivity. The oil production from these reservoirs is mainly by using cyclic steam stimulation, due to the high viscosity of Issaran oil.
The Issaran field is characterized by its heterogeneous carbonates, with a complex structural history. This geologic complexity has a significant impact on the seismic data quality. The geological factors that influence the seismic data characteristics (velocity, amplitude, phase, and frequency) are referred to as geoseismic conditions. These geoseismic conditions are found in the studied wells data as low velocity layer, high velocity layers, thin beds, terminated units, truncated horizons and reservoir heterogeneity. These geoseismic conditions were not considered during the seismic data acquisition and seismic processing.
The main source for the velocity measurement is considered the sonic logs in the studied area. So, the conditioning for this log from the factors effecting on the reliability of readings, such as: the borehole rugosity and fluid invasion, stretching. Generally, all the four selected wells show that, the recorded sonic and density logs are highly affected by the bad hole conditions, with unusual low / high readings over several shale intervals of the Zeit and Gharandal formations. while the dolomite intervals have a good hole condition and the logs are not affected by the invasion, due to the presence of heavy oil in the formations.
The average velocity can be calculated from the time-depth relations, that extracted from the well velocity measurement. While the rms velocity can be computed from the velocity stacking analysis. There is a strong relation between the lithologic heterogeneity and the velocity complexity. This relation was noticed stratigraphically and structurally, the heterogeneity factor is much higher in the syn-rift sediments (sabkha deposits) than the pre-rift sequence sediments (marine to clastic deposits) in Issaran field.
Additionally, the heterogenty factor of the Upper Dolomite reservoir is higher than the heterogenty factor of the Lower Dolomite reservoir. Because of, the Upper Dolomite reservoir has anhydrite layer and anhydrite percentage more than in the Lower Dolomite reservoir. This Lithologic facies variation reflect on the heterogenty factor.
Furthermore, the heterogeneity factor affected by the structural truncation, which meaning that, the velocity is also affected by its occurrence. The Time drift between the time-depth relation curves, which are computed from the well velocity and derived from the stacking (rms) velocity, can be obtained, and plotted on the contoured maps. Hence, the sign and magnitude of the time drift are known at parts, in which lacking the well velocity data, the actual time can be properly calculated.
The unconventional seismic interpretation has played a vital role in enhancing the signal to noise ratio, either vertically or laterally, for the available seismic data. Furthermore, clarifying the structural framework and identifying small-scale faults, with vertical throws less than 50 feet. These micro faults could play an important role in hydrocarbon storage and production.
The implemented unconventional seismic interpretation workflow has been proved beneficial style of information, about the micro faults and layer heterogeneity in Issaran field. This workflow was accomplished, through several filters, whereas the lower frequency noises are eliminated by the bandpass filter. Moreover, the random noises are effectively removed, using a sophisticated filter called DSMF. Also, this filter removes the high tilting effect of reflectors on the seismic attributes. This effect results in masking the major and minor faults within the seismic data. As a result, the spatial and vertical seismic resolutions have been improved, after DSMF application. Then, the DSDF was applied for smoothing the continuous geologic features only and identifying the discontinuities (faults and fractures). As a result, The FEF is a powerful filter for clarifying and sharpening the highlighted discontinuous (faults and fractures). Besides that, the small-scale faults become more visible.
Moreover, several structural attributes were extracted, such as: similarity, dips and curvatures attributes. The FEF-similarity filtered data provided the best results and an idea about the structural configuration of Issaran field, before starting the structural interpretation of Issaran data. This attribute also provided the small faults with vertical displacements less than 50 feet. Also, the curvature attribute delineated the micro faults with vertical displacements less than 40 feet. Besides, the most positive curvature attribute is so beneficial for clarifying the high fracture density parts and small-scale faults. Furthermore, the polar dip and azimuth attributes are also useful for identifying the small-effect faults. The use of structural attributes has been extremely useful in helping the structural interpretation.
The structural configuration of Issaran field is supported by the available seismic data, integrated with the four given wells and the constructed depth structure contour maps of Upper Dolomite, Lower Dolomite and Eocene section tops. These maps reflect that, the Issaran field has the same structural architecture of the Gulf of Suez. The structure of Issaran field is controlled by northeasterly tilted fault blocks and bounded by NW-SE oriented normal faults (parallel to the Clysmic trend) and NNE-SSW oriented normal faults (parallel to the Aqaba trend). The Aqaba trend faults are acting as transfer faults. These two trends are terminated on each other, formed trap door structures, which are favorable for hydrocarbon entrapment.
There are 8 faults (namely: master fault, F-1, F-2, F-3, F-4, F-5, F-6 and F-7) were mainly controlled the sedimentation and the hydrocarbon migration and entrapment in the Miocene carbonates section of Issaran field. Whereas the Issaran field can be divided structurally according to the main bounding faults in the field (master fault and F-1) into three blocks (Western, Central, and Eastern Issaran blocks). The Western Issaran block represents the hanging wall part of the master fault, while the Central Issaran block illustrates the footwall part of master fault and the hanging wall part of F-1, and the Eastern Issaran block is the footwall part of F-1.
Furthermore, the top of Thebes formation is dissected by several faults, some of them are rejuvenated during the rifting time, propagated upwardly cut the Issaran Miocene section. As a result, the Eocene sequence has higher fault density, as compared to the Lower and Upper Dolomite reservoirs. These faults play a significant role for the carbonate sedimentation platform formation in Issaran field.
The extracted physical attributes, such as: instantaneous phase, instantaneous frequency and RMS amplitude attributes are extremely sensitive to the changes of facies. The combined effect of the basin slope and structures controls the Issaran field’s carbonate platforms. In general, the Upper Dolomite reservoir has lithologic facies better than the Lower Dolomite. The Upper Dolomite facies in the central and eastern parts of Issaran field is better, compared to that in the West Issaran area. While the Lower Dolomite facies in the western part of Issaran field is better, compared to those in the central and eastern parts. The carbonate facies change radically toward the Gulf of Suez and the south directions, in the Upper Dolomite reservoir rather than the Lower Dolomite reservoir.
The structural complexity of Issaran field plays a significant role in the hydrocarbon migration, distribution and entrapment. It represents the main conduits for hydrocarbon migration pathways from the southeastern direction diagonally to the updip direction toward the shallow Miocene reservoirs in Issaran field. These Miocene reservoirs have been most likely charged from the South Belayim basin.
While the pre-Miocene sand reservoirs (Nubia Sandstone and Matulla formations) are water-bearing, in addition to that, neither oil nor gas shows were found, to prove their hydrocarbon migration. Moreover, the Brown Limestone source rock of Issaran field is above the depth of oil maturation window. As a result, there is no hydrocarbons generated or expulsed from Issaran field in-situ. Consequently, the accumulated hydrocarbons in Miocene reservoirs are of migratory origin.
Furthermore, the hydrocarbon trapping mechanism in Issaran field is mainly structural and combined traps, with pure stratigraphic entrapments may be occurred. Moreover, the Zeit and South Gharib formations provide the main sealing for the Miocene reservoirs, except Nukhul and Gharandal carbonates reservoirs, that are capped and laterally sealed by the thick Gharandal shale.
Recommendations
In areas of shallow heterogeneous carbonate reservoirs, with complex structural regimes, such as Issaran field, the prevailing geoseismic conditions of the study area, should be addressed first. This is in order to determine the appropriate treatment through specific and advanced procedures, during each phase of oil and gas exploration (Seismic acquisition, processing and interpretation).
I. During the seismic acquisition phase:
1. Broad frequency bandwidth for deeper imaging.
2. Large cable offsets for deeper imaging and anisotropic seismic interpretation. In addition, for fracture prediction and full solution of diffraction noises, particularly near to the fault planes and the stratigraphic boundaries.
3. Finer sampling for the source and receiver lines in the operated survey for increasing the resolution of seismic data.
II. During the seismic processing phase:
1. Dense velocity analysis is recommended, for heterogeneous reservoirs parts of the study area.
2. Eliminating the interbedded and surface-related multiples of the seismic energies, for better deep imaging.
III. Finally, during the seismic interpretation phase:
1. the seismic data conditioning is highly recommended, to carry out before interpretation phase, to enhance the signal to noise ratio. In addition, to extract the most of geologic information about the structural configuration and subtle geologic features of the studied area.
2. The implemented unconventional seismic interpretation in this thesis is highly recommended to apply, before starting the interpretation, especially in the complex structural and complicated stratigraphic fields, like Issaran field.