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Energy Sources, Part A: Recovery, Utilization, andEnvironmental Effects

ISSN: 1556-7036 (Print) 1556-7230 (Online) Journal homepage: http://www.tandfonline.com/loi/ueso20

Geochemical Characteristics of Crude Oils FromRas Gharib Oilfields in the Central Gulf of Suez,Egypt

M. M. Hammad, M. M. El Nady, F. S. Ramadan & N. M. Lotfy

To cite this article: M. M. Hammad, M. M. El Nady, F. S. Ramadan & N. M. Lotfy (2015)Geochemical Characteristics of Crude Oils From Ras Gharib Oilfields in the Central Gulf ofSuez, Egypt, Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 37:18,2029-2038, DOI: 10.1080/15567036.2012.717165

To link to this article: http://dx.doi.org/10.1080/15567036.2012.717165

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Geochemical Characteristics of Crude Oils From Ras GharibOilfields in the Central Gulf of Suez, Egypt

M. M. Hammad,1 M. M. El Nady,1 F. S. Ramadan,2 and N. M. Lotfy1

1Exploration Department, Egyptian Petroleum Research Institute, Cairo, Egypt2Geology Department, Faculty of Science, Zagazig University, Zagazig, Egypt

Four representative oil samples have been collected from productive wells, namely: Gharib-164,Gharib-163, Gharib-106, and Gharib-116, scattered within the Ras Gharib oilfield located in the centralGulf of Suez. Their chemical composition, API gravity, sulfur content, and asphaltene content weredetermined. Saturated hydrocarbon fractions were subjected to gas chromatography. Terpanes andsteranes biomarkers have been determined using gas chromatography-mass spectroscopy in order tocharacterize the type of organo-facies, oil classes, depositional environments, and to assess the thermalmaturity level for oil generation. The results showed that the studied oils are belonging to normal oilclasses with no heavy biodegradation. It has been found that mature oils are generated from marineshales and carbonates are enriched in marine algae with contributions from terrestrial organic matters.Biomarker analyses suggest that the crude oils are more mature and sources are derived mainly frommixed organic sources from terrestrial and marine inputs contribution to the biomass from algae andplankton in different saline environments.

Keywords: API, crude oils, Egypt, Gulf of Suez, isoprenoids, oilfields, Ras Gharib, steranes, sulfur,terpanes, tricyclic

1. INTRODUCTION

The study area covers a surface area of about 225 Km in the central part of the coastal strip ofthe western side of the Gulf of Suez, lies between latitudes 28° 23'–28° 24' N and longitudes33° 03'–33° 04' E (Figure 1). The lithostratigraphic units in the Gulf of Suez including study areacan be subdivided into three megasequences: a prerift succession (pre-Miocene or Paleozoic-Eocene), a synrift succession (Oligocene-Miocene), and a postrift succession (Pliocene-Holocene). These units vary in lithology, thickness, areal distribution, depositional environment,and hydrocarbon importance (Said, 1990).

Oil is a complex mixture containing a large number of related compounds. The compoundspresent and their relative amounts are controlled by the nature of organic matter in the source rock.Philp (1994) recognized that crude oils have a biogenic origin and are formed as a result of

Address correspondence to Dr. Mohamed El Nady, Egyptian Petroleum Research Institute (EPRI), ExplorationDepartment, Organic Geochemistry Unit, Ahmed El Zumer Street, Nasr City, Hai Al-Zehour, Cairo 11727, Egypt.E-mail: elnady1963@live.com

Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/ueso.

Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 37:2029–2038, 2015Copyright © Egyptian Petroleum Research InstituteISSN: 1556-7036 print/1556-7230 onlineDOI: 10.1080/15567036.2012.717165

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diagenetic and thermal conversion of organic matter deposited in different types of sedimentaryenvironments. The common methods for characterization of crude oils in terms of source rocksfacies and depositional environments as well as their maturity and alteration stage are an importantelement in exploration studies (Waples, 1985). The contribution has implications for the oilcharacterization using different parameters, which have been used for this purpose. Most ofthese parameters are based on the analyses of the American Petroleum Institute “API” gravityand sulfur compounds for the crude oil as well as saturated and aromatics hydrocarbons, pristaneand phytane ratio, isoprenoids, n-alkanes, and the biological markers distributions as steranes andtriterpanes in the crude oils (Waples and Michihara, 1991). These parameters are used forcomparison of oils to detect genetic relationships, depositional environments, and thermal matura-tion. They are classified into bulk parameters, including the API gravity, saturate content, con-centration of the main types of hydrocarbon and asphaltene content, and specific parametersincluding n-alkanes and isoprenoids distribution as well as biological markers obtained by gaschromatography (GC) and gas chromatography-mass spectrometry (GC-MS) analysis.

The purpose of the present study was therefore to determine the source (organic input anddepositional environment), thermal maturity level for oil generation, and the mechanisms control-ling their accumulation. The saturated biomarker distributions have been used in an attempt tocharacterize the type of organo-facies, oil classes, and depositional environments of the crude oils.

FIGURE 1 Location map of drilled wells in Ras Gharib oilfields, Gulf of Suez, Egypt.

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These are achieved through the geochemical analyses of oil samples, including sulfur content, APIgravity, C15+ saturated, and gas chromatography-mass spectrometry of biomarker compounds(steranes and terpanes). This study considers the complete extension for the evaluation of previousgeochemical study of crude oils in the Gulf of Suez.

The geochemical characteristics of crude oils in the Gulf of Suez are discussed by Zein El Dinet al. (1981) who reported that the oils are derived from similar marine source rocks. Roharback(1983) suggested that the oils have the same genetic family, highly similar in their source rocks, asthey reflect a marine origin and show no biodegradation. Shaltut et al. (1998) divided the oils intotwo groups based on porphyin distribution; the first group is characterized by the increase invanadyl porphyin distribution with the depth, while the second group shows inverse relation.Mostafa and Khaled (1988) concluded that the reservoir depth of the oils is not correlated with theoil maturity, based on sulfur percent, specific gravity, NSO compounds, asphaltenes, and carbonisotope ratios. Mostafa and Ganze (1990) revealed that the crude oils of the Rudeis oil field aresimilar in their origin; also they concluded that the Eocene Thebes Formation and the UpperSenonian Brown limestone may act as source for the Abu Rudeis oil. Barakat et al. (1996) dividedthe oils of the southern part of the Gulf of Suez into three types. Type I is sourced from carbonatesource rocks (mainly of marine origin), type II appears to be originated from Tertiary source rockswith contribution from high land plants, and type III is a mixture of type I and type II. Sharaf(1998) recognized two oil groups for the oils from October and Abu Rudeis fields. Group “1”heavy oil was derived mainly from hypersaline reducing environment, and group “2” moderate tolight oils formed under a low-salinity environment with a minor contribution from terrigenousorganic matter. Hammad and Barakat (2000) concluded that the oils were generated from mixedand algal organic matter deposited in transitional and reducing environments. El Nady (2001)recognized that the crude oils of the Gulf of Suez are sourced mainly from marine organic matters.Barakat et al. (2002) stated that there is a close genetic relation between the oils in the southern partof the Gulf of Suez. El Nady and Harb (2005) recognized that the crude oils are mature and derivedmainly from mixed organic sources (mainly terrestrial with marine input) under transitionalenvironments. El Nady (2006) reported that the crude oils in the south Gulf of Suez are mature,originated mainly from marine sources, and show good correlation with the Lower Miocene sourcerocks in the southern part of the Gulf of Suez. El Nady et al. (2007) divided the crude oils in theGulf of Suez into two groups: The first group includes moderately mature oils generated in astrongly reducing (marine). The second group comprises mature oils generated from source rocksdeposited in lacustrine palaeoenvironmental. Roushdy et al. (2010) reported that the crude oils aremature and derived mainly from mixed organic sources from terrestrial and marine inputs con-tribution to the biomass from algae and plankton in different saline environments. Harb and ElNady (2010) divided crude oils in the Gulf of Suez into: (1) Heavy oils characterized by lowmaturation and originated mainly from terrestrial organic sources. (2) Light oils of high maturitylevel orginated mainly from marine organic sources. Faramawy et al., (2012) classified the crudeoils into aromatic intermediate heavy oils of low waxy content characterized by a high maturitylevel and derived from mixed organic sources. belonging to the carbonate type, deposited intransitional environments under reducing-oxidizing conditions.

2. SAMPLES

In the present study, four representative oil samples have been collected from four productive wellsscattered within the Ras Gharib oil field, namely: Ras Gharib-106, Ras Gharib-116, Ras Gharib-163, and Ras Gharib-164 (Figure 1). The main producing horizons of these wells are Watareservoir (Cretaceous) in Gharib-164 well, Basal Miocene Bed “Nukhul” reservoir in Gharib-163well, and Belayium reservoir in Gharib-106 and Gharib-116 wells.

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3. TECHNIQUES

1. API gravity is a measure of crude oil density at 60°F determined by Waples (1985).2. Sulfur content was determined by X-ray (sulfur meter model RX-500 S) according to

ASTM D-42914 procedure, and the results are expressed as weight percent.3. The crude oils were separated into saturates, aromatics, and resins by liquid column

chromatography. The column used was packed with 1:1 alumina overlying silica gel.Saturates, aromatics, and resins fractions were obtained by successive elution with n-pen-tane, toluene, and methanol, respectively.

4. Saturated hydrocarbon fractions were subjected to gas chromatography. The instrumentused was an Agilent 6890 Series equipped with flame ionization detector (FID) and splitt-aplittless injector. Oven temperature was programmed from 100 to 300°C at a fixed rate of3°C min–1. An HP-1 fused silica capillary column (60 m × 0.53 mm × 0.5 μm) was used forthe analysis. Nitrogen (free oxygen) was used as carrier gas at a flow rate of 2 ml min–1; theinjector and detector temperatures are 300 and 325°C, respectively.

5. GC-MS for saturated hydrocarbon was carried out using a Clarus-500 Perkin Elmer.Samples were injected onto a fused silica capillary column (Hp-5 MS; 30 m × 0.25 mmi.d. and film thickness 0.25 µm) coated with 5% diphenyl and 95% dimethyl poly siloxane.

These analyses were carried out in the Center Laboratories of the Egyptian Petroleum ResearchInstitute.

4. RESULTS AND DISCUSSION

4.1. Bulk Compositions

4.1.1. API Gravity

Waples (1985) reported that most normal crude oils have API gravities ranging from 20° to 45°,less than 20° are usually biodegraded, and above 45° are condensate oils. The oil samplesrecovered from the reservoirs in the study area have API gravity ranging from 22.3° to 24.3°(Table 1) indicating that these oils are normal oils. Furthermore, API gravity of crude oils is used asan indicator for the oil maturation, thus oils having API less than 20° are considered as immatureoils. Meanwhile, oils having API more than 20° are considered as mature oils (Tissot and Welte,1984). The studied oil samples are considered as moderately mature oils.

4.1.2. Sulfur Content

Tissot and Welte (1984) suggested that oils with low sulfur content (i.e., less than one) belong toparaffinic, paraffinic-naphthenic, or naphthenic classes with high wax content, while oils of high

TABLE 1Geological Information and Bulk Composition Data of the Studied Oil Samples

Oilfields Reservoirs Depth, m °API S, % Sat Arom. Asph. NSOa

Gharib-106 Belayium 468 22.4 3.6 68.0 9.0 4.0 19.0Gharib-116 Belayium 570 23.8 3.3 71.0 14.0 2.0 13.0Gharib-163 Nukhul 700 22.3 3.6 60.2 18.2 10.2 11.4Gharib-164 Wata 617 24.1 2.2 37.0 24.5 7.0 31.5

aNSO compounds (nitrogen, sulfur, and oxygen).

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sulfur content (i.e., more than one) belong to an aromatic intermediate class with low wax content.In the present study, the sulfur percent of the studied oil samples ranges from 2.2 to 3.6% (Table 1)indicating aromatic intermediate oil with low wax content.

4.1.3. Hydrocarbon Classes

The classification of crude oils is based on the content of the various structural types ofhydrocarbons, namely, paraffins, naphthenes, and aromatics. In addition, bulk composition of theresidual fraction should be considered (Waples, 1985). Results of the studied crude oil samples arepresented in Table 1. It has been found that the saturated hydrocarbons are the major type in thecrude oils range from 37 to 71% and aromatics hydrocarbons ranging from 9.0 to 24.5% (Table 1).These data indicate that the studied crude oils are classified as aromatic intermediate oils (Tissotand Welte, 1984).

On the other hand, the crude oils contain asphaltene and resins ranging from 2.0 to 11.0% and13.0 to 31.5%, respectively, and high sulfur contents ranging from 2.2 to 3.6 wt%) (Table 1) isindicative of marine origin (Gray et al., 1987).

4.2. Specific Compositions

4.2.1. Normal Alkanes

The fingerprints of the studied oil show that samples of Cretaceous reservoir (Wata Formation)in Gharib-164 well (Figure 2a) and Basal Miocene Beds Nukhul reservoir in Gharib-163 well(Figure 2b) are characterized by a high concentration of normal alkanes in the range of n-C12 ton-C20 and low to medium concentration of heavy normal alkanes with no odd or even carbonpreference index (CPI) ranging from 0.95 to 0.99 (Table 1) indicating algal/bacterial organicdetritus in the kerogen, typical for a marine source rock deposited under less reducing conditions(Collister et al., 2004), typical for a marine source rock deposited under reducing conditions(Lijmbach, 1975).

FIGURE 2 Gas chromatogram of saturated hydrocarbon fractions of (a) Wata oil, (b) Nukhul oil, and (c), (d)Belayium oil in Ras Gharib oilfields.

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The oil sample recovered from Belayium reservoir in Gharib-106 and Gharib-116 wells (Figures2c and 2d) show the predominance of normal alkanes in the range of n-C15 to n-C27 and moderateconcentration of heavy normal alkanes with no odd or even carbon preference index with CPI =0.98 to 0.99 (Table 1) reflecting the diagnostic of marine organo-facies sources (Collister et al.,2004). All of these parameters show that these crude oils are moderately mature oil originated frommarine organic sources and have the same genetic family.

4.2.2. Isoprenoids

The majority of isoprenoids are used in this work are pristane/phytane (Pr/Ph) and pristine/n-C17 and phytane/n-C18 ratio (pr/n-C17 and ph/n-C18). This ratio has been used as an indicator ofdepositional environment with low specificity due to the interferences by thermal maturity andsource inputs (Peters et al., 2005). The low values (Pr/Ph < 2) indicate aquatic depositionalenvironments, including marine, fresh, and brackish water (reducing conditions); intermediatevalues (2–4) indicate fluviomarine and coastal swamp environments; high values (up to 10) arerelated to peat swamp depositional environments (oxidizing conditions).

In the present study, the pristane/phytane ratio for Wata reservoir is 0.62 and 1 for BasalMiocene Beds “Nukhul” reservoir while for Belayium reservoir it ranges from 0.74 to 0.75(Table 2), indicating marine organic source. The studied oil samples have pr/n-C17 and ph/n-C18

ratio ranging from 0.56 to 0.9 and 0.63 to 0.87, respectively (Table 2), showing that these oilsamples were derived from a marine organic matters source (mainly algae) deposited underreducing environment, except for the oil sample of Lower Miocene reservoir, which has pr/n-C17

and ph/n-C18 ratio that is 0.9 and 0.87, respectively (Table 2), indicating mixed organic matter withthe source rocks being deposited under reducing conditions and receiving significant clastic input(Hunt, 1996).

TABLE 2Geochemical Parameters Derived from Gas Chromatographic and Gas Chromatography-Mass Spectrometry

Analysis of Studied Oil Samples

Oilfields

Gharib-106 Gharib-116 Gharib-163 Gharib-164

Reservoirs Belayium Belayium Nukhul WataAge Middle Middle Lower Cretaceous

Miocene Miocene MioceneCPI 0.99 0.98 0.95 0.99Pristane/Phytane 0.74 0.75 1.00 0.62Pristane/n-C17 0.56 0.63 0.90 0.61Ts/Tm 0.63 0.53 0.43 0.60Homohopane index 0.01 0.11 0.08 0.98Gammacerane index 0.263 0.23 0.08 0.45C29/C30 Hopane 0.07 0.08 0.18 0.05C27 (%) Steranes 32.5 42.6 89.82 25.83C28 (%) Steranes 15.7 23.3 3.28 27.97C29 (%) Steranes 15.11 34.1 6.90 46.37Diasterane index 0.51 0.71 0.21 0.11(20S)/20(S + R) 0.40 0.50 0.60 0.10

Note: Homohopane index: (C35 homohopane S + R)/(C31 + C32 + C33 + C34 + C35 homohopanes S + R); Diasteranesindex: (C27 diasteranes S + R)/[(C27 diasteranes + R) + C29 steranes S + R)]; Gammacerane index: gammacerane/(gammacerane + C30 hopane).

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4.2.3. Tricyclic Terpanes

The concentration of tricyclic terpanes in crude oils is more sensitive to the specific paleoenvir-onments. In addition, it has been used as a qualitative indicator of maturity. In high mature oils, thetricyclic terpanes are dominated more than in low mature oils (Waples and Machihara, 1991).

The studied oil samples of Upper Cretaceous and Lower Miocene “Nukhul” reservoirs arecharacterized by low predominance of C22 tricyclic terpanes (Figures 3a and 3b). Meanwhile, thestudied oil sample of Belayium reservoir is characterized by high concentration of C22 tricyclicterpanes (Figure 3c) indicating that Belayium oil is more mature than Upper Cretaceous andNukhul oils and sourced mainly from carbonate source rocks.

4.2.4. Homohopanes

The homohopanes (C31 to C34) and homohopane index in crude oils can be used as an indicatorof the associated organic matter type, as it can also be used to evaluate the oxic/anoxic conditionsof source sediments (Peters and Moldowan, 1993). Low C35 homohopanes is an indicator of highlyreducing marine conditions during deposition, whereas high C35 homohopane concentrations aregenerally observed in oxidizing water conditions during deposition, consistent with the oxicconditions as suggested by high pristane/phytane ratios.

The studied crude oils have low concentration of C31–C35 homohopanes (20S and 20R)(Figure 3a, 3b, and 3c), which are more significant to hypersaline marine oils. Moreover, thehomohopanes index for the studied oil samples has values 0.08 to 0.98 (Table 2) less than unity,reflecting a reducing marine environment.

4.2.5. Gammacerane

Gammacerane is associated with environments of increasing salinity, both marine and lacustrine(Waples and Machihara, 1991). Gammacerane (Figure 3a, 3b, and 3c) is only detected in relativelylow amounts in the studied oils indicating input of marine organic matter in different saline

FIGURE 3 Gas chromatogram-mass spectrometry of triterpanes (m/z 191) of saturated hydrocarbon fractions of (a)Wata oil, (b) Nukhul oil, and (c) Belayium oil in Ras Gharib oilfields.

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environments, e.g., lakes, lacustrine, ponds, etc. In addition, the studied oil samples are character-ized by Gammacerane index ranging from 0.08 to 0.45 (Table 2) less than unity indicatinghypersaline marine oils deposited in reducing environments.

4.2.6. Ts/Tm

The ratio of Trisnorhopane/trisnorneohopane (Ts/Tm) is considered as a facies and depositionalenvironmental parameter of the relevant source rocks (Hunt, 1996). The results show that theTs/Tm range from 0.43 to 0.6 (Table 2) for oil samples revealing marine organic facies ofdepositional environmental. It is also considered a maturation parameter due to the greater thermalstability of Ts than its counterpart Tm. Ts/Tm ratios for the crude oils are generally less than unity(Table 2), indicating an anoxic marine depositional environment (Petersen et al., 2000).

4.2.7. C29/C30 hopanes

C29/C30 hopanes ratios are generally high (>1) in oils generated from organic rich carbonatesand evaporates. The studied oil samples are characterized by hopanes ratio ranged from 0.05 to0.18 (Table 2) less than unity indicating the marine organic sources.

4.2.8. Steranes

The resulting mass chromatograms for the representative samples are shown in Figures 4a, 4b,and 4c. It is agreed that the relative amounts of C27–C29 steranes can be used to give an indicationof source differences (Lijmbach, 1975).

The studied crude oils are characterized by low predominance of C27 steranes and slightly highabundance of C28 and C29 (Figures 4a–4c; Table 2). Also the relatively high concentration C27

FIGURE 4 Gas chromatogram-mass spectrometry of steranes (m/z 217) of saturated hydrocarbon fractions of (a)Wata oil, (b) Nukhul oil, and (c) Belayium oil in Ras Gharib oilfields.

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diasteranes (Figures 4a–4c) indicate input of marine organic source. Oils with all of these featuresare believed to be generated from both pre-Tertiary marine shales and carbonates enriched inmarine algae with more contribution from terrestrial organic sources, deposited under salineconditions and oxic setting (Peters and Moldowan, 1993). Predominance of C27 steranes in theoil samples of Nukhul and upper Cretaceous reservoir suggests marine sources with input fromterrestrial organic matters.

The amount of C27 diasteranes, diasteranes index, and concentration of (20S) and (20R) isomersinfluences the maturity level of crude oils. Peters and Moldowan (1993) recognized that thematurity level of oils increases with the increase of these parameters (i.e., high concentration ofC27 diasteranes, >0.1 diasteranes index, and >0.5 C29 20S/20S + 20R). Thus, it is obvious that thestudied oils have a slightly high concentration of C27 diasteranes with diasteranes index rangingfrom 0.11 to 0.7 and C29 20S/20(S + R) from 0.1 to 0.6 (Table 2). These data reveal that the studiedoils are characterized by high maturity level.

5. CONCLUSIONS

The oil samples were subjected to routine geochemical analysis as °API gravity, sulfur content,crude oil composition, gas chromatography, and gas chromatography-mass spectrometry forsaturated hydrocarbon fractions. The results revealed the following:

1. The determined API gravity, sulfur content of the studied oil samples reflected that thecrude oils are normal oils, aromatic intermediate class with low wax content.

2. The distribution of the saturate, aromatics, asphaltene, and NSO showed the oil sampleslocated within the normal oils with no heavy biodegradation.

3. The study of normal alkane distribution, isoprenoid (pristane and phytane), isoprenoid/n-alkanes (pr/n-C17 and ph/n-C18) indicates that the studied crude oils are mature and areoriginated from marine organic matter under reducing conditions.

4. Biomarker analyses suggest that the crude oils are more mature and sources derived mainlyfrom mixed organic sources from terrestrial and marine inputs contribution to the biomassfrom algae and plankton in different saline environments.

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