الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
 |  | from | 443 |  |  |
| | | from | 443 | | |
Abstract
The region of Nile Delta deep water is rapidly emerging as a major gas province. The area of the study covered an area of about 1200 Km2 which is part of North El-Amriya area in the western part of the offshore Nile Delta. The key reservoirs in the study area are related to the Neogene time i.e. Miocene and Pliocene sequences.
Seven wells were selected to be used in the thesis study, they are namely; A, B, C, D, E, F and El King-1X, they include complete well data in addition to two 3D seismic surveys (NEA-West and NEA-NA ). from the seismic data acquisition and processing perspective, the objective was to apply advanced seismic data processing techniques aimed to get amplitude preservation seismic data.
The study motivations is mainly to explain the reasons behind the negative results of three dry holes, one of them along the Pliocene which is Well-F and the other two are Messinian wells (Well-A and Well-B). Although they are, in term of amplitude responses, they are quite similar to the other nearby successful wells. This required to check the possible reasons for the change in amplitude in the study area, which can be accomplished by building updated rock physics models, investigating the sensitivity of the elastic properties and by suggesting different kind of qualitative and quantitative seismic interpretation workflows for get a robust reservoir characterization results which can be used to decrease the associated geological risks in any upcoming exploration programs.
Synthetic seismograms generated in the seven wells using statistical wavelet with fair to good match and in later stage, the wavelet extraction from the seismic using the deterministic approach was done to get better wells to seismic match.
Four seismic horizons identified and interpreted in the time domain, those are; upper Pliocene, middle Pliocene, top Messinian and top Serravallian. The main scope of the seismic mapping is to assess the potentiality of the study area and to identify potential leads, either structural and/or stratigraphical traps by using the nearby discoveries as analogues, and trying in the same time to understand the failure reasons of the dry wells.
Seismic interpretation workflow resulted two-way time (TWT) structure surfaces which merged together from two different seismic 3D surveys, during surfaces gridding, fault polygons were incorporated, and three horizons are considered as prospective objectives in the study area i.e. upper and middle Pliocene and top Messinian. 3D Velocity model using VoK method was created and used for depth conversion. The final output are the three structure depth maps.
After seismic interpretation and by screening in the Pliocene section, some striking amplitude anomalies and some preliminary opportunities highlighted as stratigraphic Pliocene leads, they named Harara-1X, Harara-2X and Harara-3X leads and three structural closures mapped in the Messinian level, they named Zwail-1X, Zwail-2X and Zwail-3X.
Post stack attributes calculation and extraction were applied such as Graphic Equalizer, RMS Amplitude, Sweetness, Variance and Relative acoustic impedance. The applied multi-trace post-stack attributes are successful and yielding good and positive results in the wells and in the identified potential leads. The good attribute results involve filtering out some frequencies increasing the signal-to-noise ratio and improving data quality (graphic equalizer attribute), emphasizing some potential sweet spots and relative low impedance zones (RMS Amplitude, sweetness and relative acoustic impedance attributes) and aided in faults mapping appropriately (variance attribute).
Before starting the frequency decomposition and to optimize the input data for further frequency studies, a geological expression workflow is applied which involves two steps of data conditioning; noise cancellation and spectral enhancement. Standard and high definition frequency decomposition methods applied in the study, each of them has different embedded algorithms, within all the defined leads, the spectral decomposition results are supporting the reservoir presences as channel fairways, multi-channel complexes and paleo- incised valleys which therefore, diminish the reservoir presence risk.
Consequently, by using both of the post stack seismic attribute and the various methods of spectral decomposition, the six identified leads are well defined qualitatively.
Checking the possible reasons for the change in seismic amplitude is mandatory, which achieved by building rock physics models and investigating the sensitivity of the elastic properties in all wells of the study.
Cross-plot analysis of all the studied wells performed and supported by the rock physics templates as constraints, this successfully distinguished fluids and lithology effect. The gas presence tends to significantly decreases the compressional velocity (VP) and it slightly increases the shear velocity (VS), thus using the VP versus VS relation is quite valuable to highlight hydrocarbon reservoirs.
Fluid substitution was applied using Gassmann’s equation for different Pliocene - Messinian sequences to predict the possible well-log responses in different scenarios and via the seismic modeling, the seismic amplitude in different cases can be simulated. The fluid replacement study showed the hydrocarbon saturation influences on seismic amplitude and also on the elastic properties of sand intervals of the studied wells and therefore it realized which properties could be inverted to be used as hydrocarbon indicators.
AVO modelling is done through two approaches, the first is the simplest via half-space interface (blocky) modelling to simulate the seismic response across such lithological interfaces as a function of incidence angle.
The second approach using Gassmann’s algorithm for fluid replacement modelling to the log data in order to predict new velocity and density curves, which used via seismic modelling to generate synthetic gathers for in-situ, brine, oil and gas cases.
It comprehended from modelling results that Abu Madi sand, as one of the target reservoirs in the study area, has been masked by top thick anhydrite section (Rosetta Formation), which is quite harmful to reveal the genuine AVO signature however by applying the fluid replacement in the lower “thicker” brine sand, we can expect to see distinguishable AVO effect .
Middle and lower Pliocene reservoirs are known as amplitude driven in both of onshore and offshore Nile Delta, nevertheless the low saturation gas (fizz water) still a key problem and challenge as one of the main causes of failure of AVO procedure like Well-F case in our study.
Using AVO analysis, several different AVO attributes extracted, mapped and analyzed. The most important two are the zero-offset reflectivity or intercept (R˳) and AVO gradient (G) based on Shuey’s approximation.
Several AVO attributes have been used for hydrocarbon detection, like the partial angle stacks (Far stack and Far minus near-stack attributes), R(0)*G, RP-RS and the Scaled Poisson’s reflectivity
The intercept vs. gradient cross-plots have been used to confirm the AVO class, The Pliocene leads; Harara-1X, Harara-2X and Harara-3X showed positive AVO class III.
The fluid factor applied and found very useful in the study which clearly is showing gas intervals in both of the Pliocene and Messinian opportunities and distinguished amplitude anomalies with clear and possible flat spots.
Flat spot AVO modelling has been applied in two wells to confirm the presence and check validity of observed flat spot which could coincides with gas water contact (GWC) in seismic data, AVO anticline models performed to simulate different saturation cases, using different wavelets
with various frequency in the modelling, in Well-C case , by adding 90 % gas and 80% oil leg, the results of the modelling confirm the observation of flat spot, even with adding thick anhydrite section on top, while Well- B, the results of this simulation attempt doesn’t support the modelling of the GWC or the flat spots.
Seismic inversion have employed using several approaches and algorithms, this involves the following types of seismic inversion; recursive “bandlimited”, sparse spike, model-based inversion types (Post- stack “acoustic” inversion, Angle-limited stacks inversion, Pre-stack “simultaneous” inversion), colored inversion, Elastic AVO “independent” inversion and the Extended Elastic impedance “EEI” inversion.
The recursive and sparse spike inversion demonstrated very good results at the drilled wells and in the potential leads in which the gas reservoirs in both of the Pliocene and Messinian fit with low “absolute” impedance values.
Seismic colored inversion results showed considerably lower impedance values along the Pliocene and Messinian leads which increase the chance of finding potential gas bearing sand.
Three types of model-based inversion was applied:
1. Post-stack (acoustic) inversion which generated good lithology indicator acoustic impedance (ZP),
2. Angle-limited stacks inversion performed for both near and far angle
stacks highlighting the discovered and potential reservoirs which have booming amplitude (lower impedance anomalies) in the inverted far impedance compared with the inverted near ones,
3. Pre-stack simultaneous inversion which uses angle dependent
wavelet and it considered the best in terms of vertical resolution and in fluid and lithological discrimination, the outputs are incidence (acoustic) impedance (ZP) and shear impedance (ZS) which manipulated to create other useful volumes such as VP/VS and Poisson’s ratio, ZP/ ZS and Lames’s constants (LMR ) λρ and μρ.
3D volume cross-plotting performed; P-impedance versus Vp/Vs, λρ versus μρ and ZP versus ZS which discriminated lithology effect away from the fluid one, which helped to preciously delineate and confirm the presence of the gas at the defined leads; Harara-1X, Harara-2X, Harara-3X, Zwail-
1X, Zwail-2X and Zwail-3X
The Elastic AVO “independent” inversion performed, using Fatti’s approach from the seismic gathers, the inversion is performed for each of Rp and Rs independently (not simultaneously), The independent inversion results shows, the direct hydrocarbon indicator of using both of Zp and Zs when the hydrocarbon bearing reservoir have low Zp and high Zs.
The Extended Elastic impedance inversion applied; in our study, EEI P- impedance volume at (χ = 59°) shows excellent definition of the gas sand reservoir. The EEI inversion results don’t show any low impedance values at Well-F location which is dry Pliocene hole, which support the EEI results while the (EEI) inversion results are well defined and can delineate gas sand reservoirs in the three Pliocene lead and also in the three Messinian leads.
Reservoir characterization workflow implemented by incorporating both of seismic data, “internal generated” seismic attributes, inversion results and the well logs, this is done by using statistics to analyze the logs together with available seismic and attribute data, to find proper relationships to ”predict” or estimate a 3D volume of the log properties.
Quantitative results of the performed reservoir characterization produce the following 3D property volumes; density, porosity, gamma ray, and water saturation. For examples, the porosity volume shows that the reservoir level at Well-D has porosity zone ranging 0.25 – 0.28 %, while the potential reservoir section at Zwail-3X lead has higher porosity zone ≈
0.3% While, at Harara-3X lead, the Gamma ray property 3D volume showed potential sand reservoir with lower gamma ray ≈ 35-38 API. On the other hand, the density property 3D volume showed lower density interval ≈1.8-1.9 g/cm3 at Harara-1X lead, “Zwail-3X lead showed potential gas reservoir with lower water saturation ≈ 0.3-0.4 %. Using the
same quantitative approach, the essential reservoir properties can be characterized in all the potential leads.
The subsurface risks is quantified by using geological possibility of success (PoS) which is mainly concerned of the presence and efficiency of both of trapping, source and reservoir rocks. By integrating the reservoir characterization results together with the entire study results, after application of numerous qualitative and quantitative workflows, which emphasizing the role of the rock physics, which support in reducing the uncertainty and the associated interpretation risks and increase the possibility of success for the defined six Pliocene and Messenian leads by range between 50 - 300%
The de-risking results can be summarized for the six lead as following:
1. Harar-1X lead has been initially PoS =14% and upgraded to 40%
2. Harar-2X lead has been initially PoS =9 % and upgraded to 35 %
3. Harar-3X lead has been initially PoS =9 % and upgraded to 37 %
4. Zwail-1X lead has been initially PoS =19% and upgraded to 40%
5. Zwail-2X lead has been initially PoS =30% and upgraded to 45%
6. Zwail-3X lead has been initially PoS =20% and upgraded to 42%.