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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

Bosworth, W., and McCaly, K., 2001, Structural and Stratigraphic evolution of the Gulf of

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

Israel Bulletin, 71, 41 p.

Hingle, A.T.,1959, "Use of Logs in Exploration Problems 29th Annual International SEG

Meeting, Los Angeles, California.

Mody, G.B.,1961, "Petroleum Exploration Handbook", Mc Graw Hill Book Company, New

York. Chapter 19 (Electrical Logging) by H.G. Doll, pp. 1-19 to 19-41.

Moustafa, A.M., 1976, Block faulting in the Gulf of Suez: Proc. 5th Egyptian General

Petroleum Corporation Exploration Seminar, Cairo, 35 p.

Moustafa, A.R., 2004, Geologic maps of the Eastern side of the Gulf of Suez rift (Western

Sinai Peninsula), Egypt: AAPG/Datapages, Inc. GIS Series (Geologic maps and

cross sections in digital format on CD).

Patton. T.L., Moustafa, A.R., Nelson, R.A. and Abdine, A.S., 1994, Tectonic evolution and

structural setting of the Gulf of Suez rift. In: S.M. Landon (editor), Interior Rift

Basins: AAPG Memoir, v. 59, p. 9-55.

Pirson, J. S., 1977, "Geologic Well Log Analysis", Gulf Publishing Company. Huston,

London, Paris. Taka pp.165- 169.

Robson D.A., 1971, the structure of the Gulf of Suez (Clysmic) rift, with special reference

to the eastern side: Journal of the Geological Society, v 127, p. 247-276.

Schlumberger,1987, Log interpretation Principles /Application Schlumberger Educational

Services, U.S.A, p 168.

Sobhy, H., and Moustafa, A. R., 2012, Tectonic evolution of the Belayim marine oil field,

Annals Geol. Surv. Egypt. V. XXXI, P. 99-107

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Fig.(1): Location Map of Belayim Marine Oil Field

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Fig.(2): Structure Contour Map on top Rudeis Formation

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Fig.(3): General Stratigraphic Column of Belayim Marine Field

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Fig.(4): M-N Plot for Mineral Identification of all Studied Layers of

Rudeis Formation

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Fig. (5): Lithological Identification Cross Plots of all Studied

Layers of Rudeis Formation.

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Fig. (6): Mono-Porosity Cross-Plots of layer No. 2

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Fig. (7): Dia -Porosity Cross-Plots of Layer No. 2

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Fig. (8): Manual and Computerized Cross-Plots of 113-M-12 Well