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Comparison Between Manual and Computerized Petrophysical Interpretation of Rudeis Formation, Belayim Marine Field, Gulf of
Suez, Egypt El Kadi, Hassan & Gad Khalil
Al Azhar University & Belayim Petroleum Company
ABSTRACT
The petrophysical interpretation of Rudeis Formation was studied by using a
comprehensive interpretation of the available digital well – log data as resistivity (deep
and shallow), porosity (sonic, density and neutron), gamma-ray and composite logs and
for example nine scattered wells (113-M-03, 113-M-04, 113-M-08REP, 113-M-12, 113-
M-14, 113-M-17, 113-M-35, 113-M-45 & 113-M-56) have been studied, Corresponding
well-log data have been processed through a sequence of graphical relations (manual
interpretation) to evaluate the different petrophysical characteristics and also
computerized through available computer software. The manual interpretation was
started by Tri-porosity (M-N) cross-plots for mineral identifications are used to detect in
general the types of lithology. Lithological identification cross-plot is achieved through
cross-plots between RHOB, NPHI and DT to detect matrix density (ROH mat) and matrix
interval transit time (DT mat) for each facies. These charts show that sandstone and
shale represent the main components in Rudeis Formation. Mono-porosity cross-plots
are constructed to determine the water and hydrocarbon saturations (Sw& Shr),
formation water resistivity (Rw) and Sonic, neutron and density derived porosities (Øs,
Øn and Ød). Dia- porosity cross-plots were established to determine the shale volume
(Vsh) and effective porosity (Ø eff). All petrophysical characteristics, inferred from the
previous steps were represented vertically in the form of petrophysical data logs (PDL).
INTRODUCTION
Belayim Marine oil field is considered one of the large oil fields in the Suez rift. The
field was discovered in 1961 with the discovery well BM-1 that encountered over 200m
of oil bearing sandstone in Rudeis-Kareem Formations with established O.W.C. at -
2835m. Belayim Marine oil field is located in the central part of the Gulf of Suez, south
of Abu Rudeis town (Fig. 1).
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Belayim Marine oil field occupies the western side of the Belayim development
lease which is about 2600 square kilometers and contains two oil fields Belayim Marine
and Belayim Land. Based on vertical and lateral variations of petrophysical
characteristics, the main target of the present study is the evaluation of Rudeis
hydrocarbon reservoir at Belayim Marine Field. This target achieves, manually through
cross-plots and analytically through application of sophisticated computer software to
determine the petrophysical characteristics in the area under investigation. These
characteristics include; shale content, effective porosity, water and hydrocarbon
saturation by using of Density, Resistivity, Neutron porosity, Sonic and Gamma ray logs.
STRUCTURAL SETTING
Belayim Marine oil field is located in the central dip province of the Suez rift where
the dip direction is northeast and fault blocks are bounded by normal faults with down
throw toward the southwest direction (Robson, 1917; Moustafa, 1976; Garfunkel and
Bartov, 1977, Colletta et al., 1988, Patton et al., 1994; Bosworth, and McCaly 2001, and
Moustafa, 0224).
The Belayim Marine oil field occupies a northwest-southeast elongated rift block
with northeast tilt direction; it is bounded on the southwest by a major clysmic fault
called the Belayim Marine main bounding fault. It is bounded on the north by a north-
northeast oriented transfer fault with downthrow toward the west-northwest direction
and on the south by another transfer fault oriented NW-SE with downthrow to the
southwest direction (Fig. 2).
STRATIGRAPHY
The stratigraphic section of Belayim Marine oil field is shown in Fig. 3 which is also
the normal section of the central part of the Gulf of Suez stratigraphic succession except
for the absence of the Nukhul Formation (lower Miocene time) and the Oligo-Miocene
red beds in the Belayim Marine field area.
WELL-LOG ANALYSIS
Well log analysis represents the most important stage in the evaluation of
petrophysical characteristics, In this investigation, two techniques applied to evaluate
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the petrophysical characteristics of Rudeis Formation. The first technique is the manual
evaluation, which depends on the using of charts and cross-plots, while the second
technique is the using of computer software (computer processed interpretation).
Manual Formation Evaluation:
Manual evaluation for determination of petrophysical parameters (Vsh, Φeff, Sw and
Shr) obtained graphically to evaluate Rudeis Formation by exploration of its reservoir
possibility and petroleum accumulation. In this investigation, the manual evaluation
process passes through qualitative and quantitative interpretation techniques.
Qualitative Correlation:
It is the process of comparing one log with another of the same type or of a
different type enables the interpreter to obtain more information than can be derived
from log alone. The characteristics responses of Sonic, Gamma-Ray, and Neutron curves
to various subsurface formations found to be highly consistent over large area Certain
formations are easily recognizable from the unique shape of the response they produce
on the radioactivity curves and provide an accurate indication of the strata ( Mody,
1961 ).
Quantitative Analysis:
Quantitative log analysis based on a series of mathematical formulas or models. In
this investigation, the applied manual quantitative techniques are using of charts and
cross plots, which in fact based mathematically calculation by using the equations, for
example Archie's water saturation equation.
M-N (Tri- Porosity) Cross-Plot:
This technique depends on the fluid and log parameters which incorporated
together essentially in the three porosity logs; Sonic, Density and Neutron. In complex
lithology, the M-N Cross Plots or Tri-porosity Cross Plots used to identify mineral
mixtures. From the values of Sonic, Density and Neutron, two functions M and N
calculated, which independent of the primary porosity; therefore a cross-plot of these
two quantities makes lithology characteristics more apparent (Pirson, J. 1977). Different
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values of M and N can be plotted in the M-N plot and different proportions of pure
minerals forming the formations under investigation can be indicated on the plot
depending on their matrix and fluid parameters. Figure (4 ) represent M-N plots for
Rudeis formation.
Lithological Identification Cross-Plot:
Identification of lithology is particular importance in formation evaluation process.
Logs can be use as indicators of lithology. The most useful logs for this purpose are
Density, Neutron, Sonic and Gamma ray logs.
An important technique gives an accurate result for lithological identification is the
use of charts (Schlumberger charts) and cross plots between NPHI Vs RHOB and NPHI Vs
DT log reading to obtain the lithological characteristics. Different charts can be applied
to obtain the type of each lithological component of Rudeis Formation and to determine
the apparent Δtmat and apparent ρmat for Rudeis Formation.The main conclusion can be
summarized as follows:
Figure (5) represents NPHI-DT and NPHI-RHOB cross plots of Rudeis formation. It
shows that the major and predominant lithology is sandstone but it benching out in
some wells to shale. Matrix transit time (Δt) about 52 µsec/ft, while matrix density (ρmat)
is about 2.60 gm/cc
Mono-Porosity Cross-Plots:
Mono-Porosity cross plots carried out to determine the formation water resistivity
(Rw), water saturations (Sw) and derived porosities (Φs, Φd & Φn ). All water saturation
determinations from resistivity logs based on Archie's water saturation equation
(Schlumberger 1987).
Where:
Rw is the formation water resistivity
Rt is the true formation resistivity
F is the formation resistivity factor
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Resistivity-Porosity cross plots require that formation water resistivity is constant over
the interval plotted, that lithology is constant, and that
the measured porosity log parameters (ρb, Φn) can be linearly related to porosity.
In this investigation, water saturation for the different horizon determined by the
resistivity-porosity cross plots (Hingle, 1959). In addition, water resistivity and derived
porosity obtained.
Three values of water saturation ( Sws, Swd and Swn) and three derived porosities ( Φs,
Φn and Φd) corresponding to each horizon read directly from cross plots and tabulated.
The mean values of Sw and Φ have been obtained. The formation water resistivity (Rw)
can be measured directly from water sample and equal to 0.202 Ohm.m. It indicates a
saline water type.
Figure (6) represents mono-porosity cross-plots between deep resistivity and porosity
log data (ρb, Δt, Øn) for layer no. 0 in different wells.
The matrix parameters (ρb, Δt) of these plots selected as 2.6 gm/cc and 50 µsec/ft,
respectively.
The value or Ro equal to 0.4 ohm.m at F=20.
Water saturation line ranges from 6% to 20%. The derived porosity ranges from 16% to
28%.
Dia-Porosity Cross-Plots:
For the determination of effective porosity (Φeff) and shale volume (Vsh), a
combination of porosity logs can be used. Dia-Porosity cross-Plot is specific graphical log
analysis technique for actual petrophysical evaluation. Cross-Plot of NPHI and ROHB is
effective to determine Φeff and Vsh. The triangle which consisting the cross-plot is
defined by matrix point, water point and shale point.
Figures (7) represent Dia-porosity cross-plots for layer (2) which represent to
Rudeis formation. As shown in this figure, the effective porosity ranges from 18.5% to
27.5 % while shale content ranges from 6.5% to 20%. The matrix-water line and shale-
water line are linearly divided into effective porosity (Φeff); while the matrix-shale line
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and water shale line are linearly divided in to shale volume (Vsh) percentage. Some
permeable horizons are shifted outside the triangle.
Vertical Variations of Petrophysical Characteristics:
The vertical distribution of hydrocarbon occurrence can be explained and
presented through the construction of the litho-Saturation cross-plots. Litho-saturation
cross plot is a representation, zone-wise, for the contents of fluids include water and
hydrocarbon saturation.
As mentioned before, the vertical distribution of petrophysical characteristics is
represented both manually in the form of petrophysical data logs (PDL) and
computerized representation in the form of litho-saturation cross plots (CPI).
The following figure show (PDL) & (CPI) 113-M-12 well.
As shown in this figure, it composed of two tracks (from left to right).
1-Petrophysical data log (PDL) obtained from manual interpretation.
2-Litho-saturation cross plotted obtained from computer software (CPI).
Litho-Saturation Cross-Plot of 113-M-12 Well:
Figure (5) represents the combined litho-saturation cross-plot of 113-M-12 well. As
shown in the figure, this well characterized by the predominance of shale layers
separated by sandstone in both layers no. 2, 4, & 6.
(CPI) track shows, the effective porosity of layer no. 2 is about 25%, layer no. 4 is
about 00% & layer no. 6 is about 20%. While PDL shows, the effective porosity of layer
no. 2 is about 28% , layer no. 4 & 6 is about 25 %.
(CPI) track shows, the shale content of layer no. 2 is about 10%, layer no. 4 is
about 5% & layer no. 6 is about 10%. While PDL shows, the shale content of layer no. 2,
4 & 6 range from 0-10%. As shown in (CPI) and PDL tracks of 113-M-12 well, it has oil
reservoir in layers no. 2, 4 & 6. The hydrocarbon percent from CPI in layer no. 2 is 85%,
layer no. 4 is 90% & layer no. 6 is 85% . The hydrocarbon percent from PDL in layer no. 2
is 89%, layer no. 4 is 94% & layer no. 6 is 88%.
CONCLUSIONS:
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Oil production in Rudeis Formation comes from six main sandstone reservoirs :
layer 2, 4, 6, 8, 10 and 12 or sometimes classified as R1, R2, R3, R4, R5 & R6 . The shale
of Rudeis Formation considered being the secondary source rock for the field.
The petrophysical characteristics of Rudeis Formation show that, the shale content
ranges from 5 to 45% and increases in two accumulations, the smaller found around
113-M-35 well at the East of the area under investigation and the largest one around
113-M-04, 113-M-03 and 113-M-08 wells at the west of the area under investigation.
The effective porosity of this Formation ranges between 6 to 27.5% and increases in two
accumulations, the smaller found around 113-M-35 well at the East of the area under
investigation and the largest one around 113-M-04, 113-M-03, 113-M-08 and 113-M-56
wells at the west of the area under investigation. The hydrocarbon saturation for this
Formation ranges from 0% to 95% and generally increases from East toward the West of
the channel around 113-M-35, 113-M-04, 113-M-03, 113-M-08 & 113-M-56 wells.
From the previously mentioned facts, the petrophysical characteristics of Rudeis
Formation (Layers 2, 4, 6, 8, 10 & 12) reflect the ability of these Reservoirs to store and
produce hydrocarbon.
It is worth-mentioning that, also the litho-saturation cross-plots inferred from
computer processed interpretation are to be correlatable with the manual ones.
As a recommendation, more exploration activities are needed along the axis of
Rudeis channel especially in the western part of it.
REFERENCES
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Suez Rift, Egypt: a synthesis, in P.A. Zeigler, W. Cavazza, A.H.F. Robertson, and
S. Crasquin-Solea (eds), Prei-Tethys Memior 6: Prei-Tethyan Rift/Wrench
Basins and passive margins. Mem. Mus. Nat. Hist., p. 567-606.
Colletta, B., Le Quellec, P., Letouzey, J. and Moretti, I., 1988, Longitudinal evolution of
the Suez rift structure (Egypt): Tectonophysics, v. 153, p. 221-233.
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Garfunkel, Z. and Y. Bartov, 1977, the tectonics of the Suez rift: Geological Survey of
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Hingle, A.T.,1959, "Use of Logs in Exploration Problems 29th Annual International SEG
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