Abd El Gawad BNE

175Journal of Petroleum Geology, Vol. 30(2), April 2007, pp 175-188

THE USE OF WELL LOGS TO DETERMINE THERESERVOIR CHARACTERISTICS OF MIOCENE ROCKSAT THE BAHAR NORTHEAST FIELD,GULF OF SUEZ, EGYPT

E. A. Abd El-Gawad*

INTRODUCTION

The Gulf of Suez is the most important oil-producingregion in Egypt and accounts for more than 75% ofEgyptian oil production and more than 90% ofcumulative production (EGPC, 1996). The BaharNortheast field is located onshore on the SW marginof the Gulf, about 65 km north of Hurghada (Fig.1),and occupies an area of about 2 km2. The field wasdiscovered by General Petroleum Company (GPC) in1983 following the drilling of well BNE-1, and twelvewells have subsequently been drilled here. BaharNortheast may be considered as an onshore extensionof the Zeit Bay field.Total oil reserves are estimatedto be about 30 MM brl, and cumulative productionhad reached 21.7 MM brl by mid-1995 with an annualproduction rate of 136,000 brl. The oil’s gravity is31o API (EGPC, 1996).

*Geology Department, Faculty of Science, United ArabEmirates University, PO Box 17551, Al-Ain, United ArabEmirates. email: esam.abdelgawad @ uaeu.ac.ae

Key words: Gulf of Suez, Bahar NE field, Miocene, BelayinFormation, Kareem Formation, petrophysics.

The Bahar Northeast field lies on a SW-tilted faultblock which is aligned with the regional N35° E trend(Meshref, 1990). Major reservoir units includeMesozoic sandstones (so-called “Nubian facies”) andMiocene carbonates. The latter include the MiddleMiocene Hammam Faraun (72– 90m thick) and Sidri(40–105m thick) Members of the Belayin Formationand the Lower Miocene Kareem Carbonate (29–125mthick) (Figs 2 and 3).

Many stratigraphic complexities have beenobserved in the study area (e.g.pinch-outs, truncationsand onlap), and it was believed that these uncertaintiescould be clarified with the aid of detailed well log andfacies analyses of the reservoir units. The presentstudy attempts to integrate subsurface geologicalstudies and well-log characteristics in order to improveunderstanding of the hydrocarbon potential ofMiocene reservoir rocks at Bahar Northeast andnearby parts of the SW Gulf of Suez.

This paper is concerned with the petrophysical evaluation by means of electric logs ofMiocene reservoir rocks at the Bahar Northeast field, Gulf of Suez, Egypt. The reservoir rocksare assigned to the Hammam Faraun and Sidri Members of the Middle Miocene BelayimFormation and the Lower Miocene Kareem Carbonate.

Computer-assisted log analyses were used to evaluate petrophysical parameters such as theshale proportion (Vsh), effective porosity (φE), water saturation (SW), hydrocarbon saturation (Sh),flushed zone saturation (Sxo) and true resistivity (Rt).

Lithological compositions, effective porosity, and water and hydrocarbon saturations areillustrated on cross-plots of depth versus lithology and saturation. Isoparametric maps are usedto illustrate the spatial variation of petrophysical parameters and to show their relationshipswith the geologic setting of the study area.

Based on the results obtained, the Hammam Faraun and Sidri Members of the BelayinFormation and the Kareem Carbonate appear to possess promising reservoir characteristicswhich should be taken into consideration during future development of the field area.

176 Reservoir characteristics of Miocene rocks at the Bahar Northeast field, Gulf of Suez

MATERIALS AND METHODS

The subsea depths of the top crystalline Basement,top Pre-Miocene Nubia sandstone, and top Rudeis,Kareem, Belayim, South Gharib and Zeit Formationspenetrated by 13 wells together with seismic data (faulttrend maps) were used to construct a series of isopachand facies maps, structure contour maps and structuralcross sections.Complete sets of logs (MicroSpherically Focused Log, Latero Log Deep, LateroLog Shallow, Formation Density Compensated Log,Compensated Neutron Log, Bore Hole CompensatedSonic Log, Spontaneous Potential Log, Gamma RayLog and Caliper Log) for the reservoir units studied(Hammam Faraun and Sidri Members of the BelayimFormation and Kareem Carbonate) were digitized.Environmental corrections and depth matching wereapplied using particular subroutines for the logs priorto evaluation to compensate for poor boreholeconditions and for depth discrepancies for the varioustypes of logs. Formation evaluation was carried outfor the studied reservoirs by means of QLA2 software,

which combines a broad range of log interpretationtechniques such as Schlumberger Principles, 1990;Essentials, 1972; Charts, 1972; Applications, 1974 andHelander, 1978. A number of water samples werecollected from the Hammam Faraun Member at wellBNE-9 and from the Kareem Formation at well BNE-1, and were analyzed for salinity. Pickett plots(Schlumberger, 1990) were constructed to confirmthe results of water analysis. Neutron/density andapparent matrix / apparent grain density cross-plotswere constructed to show the lithology, density andporosity of the studied reservoir units.

Fig. 1. Location map of Bahar Northeast field,SW Gulf of Suez. Bahar Northeast is an onshoreextension of Zeit Bay field.

Fig. 2. Lithostratigraphic column for the BaharNortheast field. The synrift Belayim and KareemFormations, the subject of this paper, restunconformably on rocks of the pre-rift sequence.

Member LithologyFormationGroupAge

NUBIA

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The shale content (Vsh) was determined from thegamma ray, SP, neutron, neutron-density and neutron-resistivity logs. The minimum shale content given bythese shale indicators is likely to be close to the actualvalue of Vsh.

The corrected effective porosity was estimatedusing a combination of the density and neutron logsafter applying various corrections. The watersaturation (Sw) was calculated using the so-calledIndonesia equation (Schlumberger, 1990)

whose terms are defined on page 186.Determination of the fluid contents as water,

movable and residual saturation was also carried out.Litho-saturation plots and isoparametric maps for thestudied Miocene reservoirs (Hammam FaraunMember, Sidri Member and Kareem Carbonate) wereconstructed to illustrate the vertical and lateral changesin the petrophysical characteristics of these reservoirs.

GEOLOGICAL SETTING

The regional geology of the Gulf of Suez has beendescribed by many authors including Steckler et al.,

1988; Meshref, 1990; Lelek et al., 1992; Abdine etal., 1990; Schutz, 1994; Bosworth, 1995; EGPC,1996; Salah and Alsharhan, 1997; McClay et al. 1998;and El-Ghamri et al., 2002. The Gulf is generallydivided into three structural provinces (northern,central and southern) according to structural settingand regional dip direction. Bahar NE field is locatedin the southern province which is bounded to the northby the Morgan hinge zone. This extends from thenorthern end of Esh El Mellaha to Ras Shukheir, tothe north of well LL 87-1, then offshore north ofGebel Araba on the eastern bank. It is characterizedby the occurrence of surface outcrops of Mioceneand Pre-Miocene sedimentary rocks and basementrocks in the Gebel El Zeit and Esh El Mellaha ranges.The regional dip of strata is towards the SW and themain (“clysmic”) and cross-faults throw towards theNE and SE, respectively. The Galala and Morgan hingezones are shifted southwards on the eastern bank ofthe Gulf of Suez by some 45 km at Zeneima and 30km at Gebel Nakus, respectively.

According to EGPC and Beicip (1988), principalsedimentary and tectonic events in the Gulf of Suezcan be divided into pre-rift (pre-Miocene), syn-rift(Miocene) and post-rift (post-Miocene) phases. Atthe Bahar NE / Zeit Bay structure, peneplainedPrecambrian Basement is overlain by a Pre-Miocene

Fig. 3. Isopach and facies maps of the Belayim Formation (above, left) and Kareem Formation (above, right)at the Bahar Northeast field,Gulf of Suez.

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sedimentary sequence typical of the southern Gulf ofSuez, with strata ranging in age from Palaeozoic toEarly Tertiary (Eocene). In common with the rest ofthe southern Gulf of Suez, the Zeit Bay / Bahar NEstructure was then uplifted and tilted about 20° towardsthe SW and underwent intense erosion. The deepesterosion occurred along the up-dip edge of the faultblock towards the eastern side. Well data shows thatupthrown and downthrown blocks were levelled atthis time as a result of the erosion.

Erosion products from uplifted and exposedstructural highs filled accommodation space in down-thrown blocks to the west of fault FA (Fig. 4). Thesediments pinch out towards the west before reachingthe location of well BNE-l. This is confirmed by thepresence of thick Early Miocene sandstones of similarfacies to the Nubia sandstone, with intercalations ofmarine shales in well BNE-12 within the interval 1,750to 1,885 m. After this thin “Basal Miocene” sandstonewas deposited, marine carbonates of the RudeisFormation were deposited along the downthrown blockwest of fault FA, thus again levelling the block withthe eastern upthrown block of fault F2.

Well BNE-1 encountered Rudeis carbonates at 1,871– 1,995 m MD; they are thought to be of Eocene age(EGPC, 1996), although Suez Oil Company geologistsconsider this interval to be Miocene. Rudeis carbonateswere deposited to the east and on the downthrown

block of fault F4. During the Early-Middle Miocene,the siliciclastics of the Shagar Member of the KareemFormation were deposited on the NE flank of theBahar NE / Zeit Bay structure. On the deeper westernflank, a more typical Kareem Formation sequence,including shales of the Shagar Member and anhydritesof the Markha Member, were developed, asencountered in well BNE-1. In the centre and onthe western flank, a build-up of carbonates took place(“Kareem Carbonate”).These carbonates werekarstified and fault F3 became active.

During deposition of the Baba evaporites at thebase of the Belayim Formation, fault F1 wasrejuvenated as shown by the different thicknessesof the Belayim evaporites across the fault.

A thin layer of Sidri carbonates was then depositedthroughout the area except for the central highcorresponding to the Kareem Carbonate build-up.The absence of Sidri carbonates in the central highis therefore due to non-deposition.

Following the deposition of the Sidri carbonates,a thin anhydritic layer (Feiran Member) wasdeposited, covering the Sidri carbonates and the Babaanhydrites where the Sidri was not deposited. Thishas led to the term “Belayim evaporites”, which refersto the undifferentiated Baba-Feiran evaporitic section.This is followed by the thin shales and carbonates ofthe Hammam Faraun Member. The Zeit Bay / Bahar

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Fig. 4. Structure contour maps, top-Belayim Formation (above, left) and top-Kareem Formation (above, right),Bahar Northeast field, Gulf of Suez. Wells indicated by open circles with well-number marked above.


NE structure was defined by the end of Belayim time.Following the deposition of the Hammam Faraun

clastics, marine regression occurred and thickevaporites (salt and anhydrite) of the Middle-LateMiocene South Gharib and Zeit Formations weredeposited. These evaporitic sequences represent thecap rock overlying the structure.

The Hammam Faraun and Sidri Members of theBelayim Formation together with the KareemCarbonate represent the most important targets forexploration in the Gulf of Suez. An isopach map ofthe Belayim Formation (Fig. 3a) shows that itsthickness increases towards the central and south-central parts of the field area. The formation consistsmainly of limestones and anhydrites with minor shalesand sandstones. The Hammam Faraun Memberconsists of shales in the upper part and of limestonesin the lower part with minor sandstones. The SidriMember is mainly composed of carbonates. Thelithofacies and faunal content of the Hammam Faraunand Sidri Members reflect a littoral depositionalenvironment, while the Feiran and Baba Members weredeposited under lagoonal conditions.

An isopach map of the Kareem Formation (Fig.3b) shows that its thickness increases towards thewest and SW of the field area. The Kareem Formationconsists of (i) relatively deep-water shales andsandstones with some limestones, which occur in thewest and SW of the field area - these are divided intothe Shagar Member above and the Markha Memberbelow; and (ii) thick carbonates which are mainlyrepresented by a dolomitic, highly pyritic limestones,which occur as a depositional wedge along the centraland western flank of the field. The Kareem Formationis absent on the crest of the structure (e.g.at wellBNE-7l) (Fig. 5).

Structural configurationIn order to understand the structure in the study area,a series of structure contour maps were compiledfor most of the penetrated formation tops using thesubsea depths of the drilled formations together withseismic data. For example, the structure at top-Kareem Formation level (Fig. 4) is an elongated SW-tilted fault block dissected by a number of normalfaults (FA, FB, F2, F3 and F4). Faults F4 and FB

Hamman Faraun Member

Kareem Carbonate

Fig. 5. SW-NE structural cross-section through wellsBNE 3, 2 and 7, Bahar Northeast field, Gulf of Suez.

Fig. 6. Pickett plots (Schlumberger, 1990) for Rwdetermination, Hamman Faraun Member (above)and Kareem Carbonate (below) Bahar Norteastfield, Gulf of Suez.


trend NW-SE, and are down-thrown to the NEforming a step structure. Faults FA, F2 and F3 trendNNW-SSE and are downthrown to the SW. Faults F3and F4 form a horst structure in the northern part ofthe field area. Top-Kareem Formation varies from –1,334 m at well BNE-4 to –1,704 m at well BNE-1,with dips generally to the SW. The structure contourmap at top Belayim Formation level shows almost thesame structural configuration as at top-KareemFormation level, except that it is not affected by faultF3 (Fig. 4).

To study the structure further, a series of palaeo-tectonic and structural cross-sections wereconstructed. These show almost the same structuralconfiguration as the structure contour maps. Inaddition, they show that the faults bounding the studyarea are most likely of Pre-Miocene age, but most ofthem were rejuvenated on a number of occasions(Fig.5).

RESULTSWELL LOG ANALYSIS

The NaCl content of water samples from the HammamFaraun Member and Kareem Formation varied from210,000 to 240,000 ppm, which was equivalent to awater resistivity (Rw) ranging from 0.023 to 0.025ohm.m at 130° F. Pickett plots were constructed andshowed that Rw values range from 0.025 to 0.035ohm.m (Fig. 6).

Neutron/density and apparent matrix/apparent graindensity cross-plots show that the Hammam Faraun

Member is principally composed of sandy dolomiticlimestones with bulk density RHOB ranging from 2.20to 2.79 gm/cc, and an average porosity of about 20%(Fig. 7). Calcite and dolomite are the main constituentstogether with quartz and some heavy minerals (Fig.7).

The Sidri Member is dominated by sandy dolomiticlimestone and minor anhydritic calcareous dolomitewith bulk density RHOB ranging from 2.30 to 2.90gm/cc and an average porosity of about 10% (Fig.8). Dolomite and calcite are the major matrixconstituents together with quartz and some K-feldspar (Fig. 8).

The Kareem Carbonate is almost entirely composedof dolomitic limestone and/or calcareous dolomite,with RHOB ranging from 2.15 to 2.83 gm/cc and anaverage porosity of about 19.5% (Fig. 9). The matrixis composed of dolomite, calcite and quartz withminor heavy minerals, anhydrite and illite (Fig. 9).

RESERVOIR ROCK EVALUATION

Litho-saturation logs and isoparametric maps for theHammam Faraun and Sidri Members and KareemCarbonate were constructed in order to illustrate thevertical and lateral changes in the petrophysicalcharacteristics of these reservoir units.

Litho-saturation logsThe Hammam Faraun Member at Well BNE-1Fig. 10a shows a litho-saturation log for the HammamFaraun Member at well BNE-1 (depths 1420 to

Fig. 7. Neutron/density and apparent matrix/apparent grain density cross-plots for the Hammam FaraunMember, Bahar Northeast field, showing main and minor lithological constituents, bulk density and averageporosity.


Fig. 9. Neutron/density and apparent matrix/apparent grain density cross-plots for the Kareem Carbonate,Bahar Northeast field, showing main and minor lithological constituents, bulk density and average porosity.

Fig. 8. Neutron/density and apparent matrix / apparent grain density cross-plots for the Sidri Member, BaharNortheast field, showing main and minor lithological constituents, bulk density and average porosity.


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1438m).In this interval, the shale content in thelimestone is 50% - 60%, and the effective porosityvaries from 10% to 16%. Water is present throughoutthe unit and oil is also present in significant volumes.

The Sidri Member at Well BNE-1Fig. 10b shows a litho-saturation log for the interval1,610 to 1,624m, which has a shale content rangingfrom 29 to 53%. The effective porosity varies from4 to 15% and generally increases downwards. Water

is dominant and movable hydrocarbons occur in smallquantities.

The Kareem Formation at Well BNE-1Fig. 10c illustrates a litho-saturation log for the intervalbetween 1804 and 1838 m. The Kareem Formation isrepresented by limestones intercalated with shales andthin sandstones in the lower part. The shale contentvaries from 22% to 60%, while the effective porosityranges from 50% to 17%. Oil is present in significant

Fig. 11. Isoparametric maps for the Hammam Faraun Member, Bahar Northeast field (a) shaliness,(b) effective porosity, (c) water saturation, (d) hydrocarbon saturation.

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volumes in the lower part of the unit and water is presentthroughout except in the oil zone

Isoparametric maps of theHammam Faraun Member (Fig. 11)An iso-shaliness map of the Hammam Faraun Memberat Bahar NE (Fig. 11a) shows that the shale contentvaries from 0% at wells BNE 5, 7, 8, 9, 11, to 12-27% at well BNE-10. This SW-NE trend of

decreasing shaliness appears to correlate withvariations in effective porosity.

Fig. 11b shows that effective porosity in theHammam Faraun Member varies from 11% at wellBNE-1 to 28% at well BNE-10, generally increasingfrom SW to NE. This trend approximately matchesthe direction of decreasing shaliness within themember (Fig. 11a). Also, compared to the structurecontour map on the top of the Belayim Formation

Fig. 12. Isoparametric maps for the Sidri Member, Bahar Northeast field (a) shaliness, (b) effective porosity,(c) water saturation, (d) hydrocarbon saturation.

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(Fig. 4), it may be noted that the highest-porosityareas occur on relative structural highs in horsts andstep blocks. This suggests that the effective porosityof the Hammam Faraun Member was affected by bothsedimentological factors (e.g. shaliness) and thestructural setting.

The map of water saturation (Fig. 11c) showsthat water occurrence within the Hammam FaraunMember varies from 15% at well BNE-3 to 50% at

well BNE-9, generally increasing to the SW. Thisdistribution can roughly be correlated with thestructural setting of the field area. Thus, in generalareas with low water saturation occur in horst blocks(e.g. the areas around wells BNE 2, 3, 4, 5, 6, 7 and13), reflecting the migration of oil up and into theseblocks; while more highly water-saturated areas occurwithin step blocks. Moreover, the less water-saturatedhorst blocks coincide with sectors in the field area

Fig. 13. Isoparametric maps for the Kareem Formation, Bahar Northeast field (a) shaliness, (b) effectiveporosity, (c) water saturation, (d) hydrocarbon saturation.

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with elevated porosities, while the other step blocksrepresent lower porosity areas. This suggests that thedistribution of water saturation in the Hammam FaraunMember was controlled both by the structural settingand the effective porosity of the member.Finally, Fig.11d shows that hydrocarbon (oil) saturation variesfrom 51% at well BNE-1 to 85% at well BNE-3. Thismap resembles the map of water saturation, althoughthe trend of overall increase is to the NE.

Isoparametric maps of theSidri Member (Fig. 12)The shale content of the Sidri Member varies from8% at well BNE-6 to 31% at well BNE-11, withgeneral trend of increase towards the NW (Fig. 12a).

Porosity in the Sidri Member (Fig. 12b) rangesfrom 10% at well BNE-6 to 25% at well BNE-3, andin general increases eastward.In wells BNE-2, 4, 5and 7, the Sidri Member cannot be differentiated, andmay be represented by anhydrite intercalated with thinbeds of limestone.

Water saturation in the Sidri Member varies from10% at wells BNE 3 and 10 to 81% at well BNE-1(Fig. 12c). It generally increases towards the SW.

Hydrocarbon saturation within the Sidri Memberranges from 19% at well BNE-1 to 90% at wells BNE-3 and 12 (Fig. 12d). The hydrocarbon saturationgenerally increases towards the NE.

Isoparametric maps of theKareem Formation (Fig. 13)The shale content in the Kareem Formation variesfrom 0% at wells BNE- 2, 9 and 11 to 23% at wellBNE-1 with a general trend of increase towards theSW and the central parts of the study area (Fig. 13a).

Porosity varies from 11% at well BNE-10 to 30%at well BNE-6, and increases towards the northernand eastern parts of the field area (Fig. 13b).

Water saturation in the Kareem Formation increasesfrom 20% at well BNE-2 to 90% at well BNE-12towards the SW of the study area (Fig. 13c).

The hydrocarbon saturation of the KareemFormation (Fig. 13d) increases from 10% at wellBNE-12 to 85% at well BNE-3 towards the NE (i.etowards the structurally higher parts of the field area).

DISCUSSION

The results show that the Hammam Faraun Memberis mainly composed of sandy dolomitic limestoneswith an average porosity of about 20%. The shalecontent is low and the average water saturation isabout 30%. An inverse relationship between porosityand shaliness is indicated by the iso-parametric maps.Areas of high porosity and low water saturation occur

on horst blocks (e.g. around wells BNE 2, 3, 4, 5, 6,7 and 13), reflecting the migration of oil into theseblocks; while areas of low porosity and high watersaturation occur within step blocks.

The Sidri Member is dominated by sandy dolomiticlimestones and minor anhydritic calcareous dolomiteswith an average porosity of about 10%. Minor shalesalso occur. The Member’s water saturation is about37%. It cannot be differentiated in the eastern marginof Bahar NE field where it may be represented byanhydrite intercalated with thin beds of limestone.

The Kareem Carbonate is dominated by dolomiticlimestones and/or calcareous dolomites with anaverage porosity of about 19.5%. The shale contentis minor and correlates closely with porosity values.The water saturation is about 45%, and decreasestowards structurally high areas.

CONCLUSIONS

The best reservoir characteristics of the Miocenereservoir units studied (the Hammam Faraun and SidriMembers of the Belayim Formation and the KareemFormation) occur in the NE of the study area, instructurally high areas, where the units have low shalecontents, high effective porosity and low watersaturations.

The effective porosity of these reservoir units isaffected by both sedimentological factors (e.g.shaliness) and the structural setting, i.e. whetherlocated on a high or a low structural block.

The distribution of water and hydrocarbonsaturation in the reservoirs was strongly affected bythe structural setting and elevation and by the effectiveporosity; areas of high porosity and low watersaturation occur on horst blocks.

The shale content in the reservoir units is minorand shows a good correlation with the porosity values.

abbreviations:

Rt: true resistivity;Rw: formation water resistivity;Rcl: clay resistivity;Vcl: clay volume;Sw: water saturation;φ: corrected effective porosity;a: coefficient in the Archie Equation;m: cementation (porosity) exponent;n: saturation exponent.

ACKNOWLEDGEMENTS

The author acknowledges with thanks the ExplorationDepartment of the General Petroleum Company


(GPC), who provided the raw data upon which thepresent work was based. Review comments on aprevious draft by Martin Keeley and David A. Pivnikare acknowledged with thanks.

REFERENCES

ABDINE, A.S., MESHREF, W., SHAHIN, A.N., GAROSSINS, P.,and SHAZLY, S., 1992. Ramadan field-Egypt, Gulf of Suezbasin. AAPG Treatise of Petroleum Geology, Structural Traps VI,113-139.

BOSWORTH, W., 1995. A high strain rate rift model for theSouthern Gulf of South, Egypt. In: Lambiase, J.J. (Ed.),Hydrocarbon habit in rift basins. Geol. Soc. Lond. Spec. Publ.80, 75-102.

EGPC (EGYPTIAN GENERAL PETROLEUM CORP-ORATION), 1996. Gulf of Suez oil fields (A comprehensiveoverview). EGPC, Cairo, 736 p.

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