Energy Sources, Part A: Recovery, Utilization, and Environmental Effects
ISSN: 1556-7036 (Print) 1556-7230 (Online) Journal homepage: http://www.tandfonline.com/loi/ueso20
Geochemical characteristics of crude oils dependent specific and biomarker distributions in the central-southern Gulf of Suez, Egypt Mohamed A. Younes, Mohamed M. Afife & Mohamed M. El Nady To cite this article: Mohamed A. Younes, Mohamed M. Afife & Mohamed M. El Nady (2017) Geochemical characteristics of crude oils dependent specific and biomarker distributions in the central-southern Gulf of Suez, Egypt, Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 39:2, 191-200, DOI: 10.1080/15567036.2016.1208306 To link to this article: http://dx.doi.org/10.1080/15567036.2016.1208306
Published online: 10 Feb 2017.
Submit your article to this journal
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ueso20 Download by: [Dr Mohamed El Nady]
Date: 10 February 2017, At: 08:18
ENERGY SOURCES, PART A: RECOVERY, UTILIZATION, AND ENVIRONMENTAL EFFECTS 2017, VOL. 39, NO. 2, 191–200 http://dx.doi.org/10.1080/15567036.2016.1208306
Geochemical characteristics of crude oils dependent specific and biomarker distributions in the central-southern Gulf of Suez, Egypt Mohamed A. Younesa, Mohamed M. Afifeb, and Mohamed M. El Nadyc a
Faculty of Science, Geology Department, Alexandria University, Alexandria, Egypt; bGeology Department, Faculty of Science, Benha University, Benha, Egypt; cExploration Department, Egyptian Petroleum Research Institute, Cairo, Egypt ABSTRACT
Specific geochemical analyses included API gravity, sulfur content, gas chromatography, and the advanced gas chromatography-mass spectrometry for biomarker distributions, in conjunction with stable carbon isotope geochemistry, to characterize the depositional environmental conditions and maturation assessment of the studied crude oils. For this purpose ten crude oils were collected from Miocene, Upper Cretaceous, and Nubia Sandstone representing the central-southern Gulf of Suez. The results showed that two different oil types identified as Miocene oils are characterized by a dominant oleanane and low gammacerane biomarkers, suggesting an overall angiosperm land plants input of terrigenous organofacies source rock. Pre-Miocene oils are distinguished by a dominant gammacerane and low oleanane biomarker distributions, which may be generated from fully mature marine carbonate source rocks. The higher sterane isomerization parameters, C29αßß/(αßß + ααα) and C29 ααα 20S/(S+R), further support the higher thermal maturation level for the Pre-Miocene oils rather than the Miocene type.
KEYWORDS
Crude oil; Egypt; Gulf of Suez; saturate biomarker
1. Introduction The Gulf of Suez occupies the northern end of the Red Sea rift (Said, 1962). It is a northwestsoutheast-oriented faults forming basin development that provided good sites for hydrocarbon generation and maturation (Dolson et al., 2000). The Gulf of Suez province has been producing oil since 1908 and is reported to have 1.35 billion barrels of recoverable oil reserve. The intensive exploration has resulted in the discovery of more than 120 oil fields share with more than 50% of the overall daily oil production in Egypt. Numerous papers investigated the petroleum potential and hydrocarbon characteristics of the Gulf of Suez (El Nady, 2001; El Gayar et al., 2002; Younes, 2003, El Nady et al., 2007; El-Sabagh et al, 2015). Many authors such as Roharback (1983) concluded that both Miocene and pre-Miocene sediments are sufficiently mature to generate oil while the Eocene and Lower Miocene source rocks correlate closely with produced crude oils, which appear to be related to a single source rock. Mostafa (1993) recognized that oils could be classified into three genetic groups (I, II, and III) derived from marine carbonate source rocks, and that sulfur-rich Group II oils in the central province, including the Belayim fields, closely correlate with source rock intervals in the Brown Limestone and Thebes Formation. Bakr and Wilkes (2002) studied carbazoles and benzocarbazoles in crude oils from the Gulf of Suez. They found that the oils could be divided into three groups according to their geographical origin from the northern, central, and southern structural sectors of CONTACT Mohamed M. El Nady
[email protected] 1-Ahmad El-Zomor St., 11727, Nasr City, Cairo, Egypt. Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/ueso. © 2017 Taylor & Francis Group, LLC
192
M. A. YOUNES ET AL.
the Gulf. The geochemical characteristics of oils from the central sector indicated derivation from an anoxic marine carbonate source rock. Alsharhan (2003) differentiated these oils into three main groups and related the central-southern Gulf of Suez crude oil to group III. However, the biomarker distributions and stable carbon isotope composition to the crude oils indicated a great variability in their saturate and aromatic biomarker characteristics, and the carbon stable isotope composition reinforces the further subdivision of group III oil into two subfamilies consistent with the syn-rift (Miocene) and pre-rift (Upper Cretaceous to Nubia Sandstone) lithostratigraphic sequences of the Gulf of Suez. Roushdy et al.(2010) suggested that the crude oils are mature and derived mainly from mixed organic sources from terrestrial and marine inputs contribution to the biomass from algae and plankton in different saline environments. El Nady (2012) classified crude oils in the Gulf of Suez into heavy oils, which are low mature originating mainly from terrestrial organic sources, and light oils, which have a high level of maturation and orginate mainly from marine organic sources. Faramawy et al. (2012) classified the crudes in central Gulf of Suez as aromatic intermediate oils and heavy oils of low waxy content characterized by high maturity level and derived from mixed organic sources (mainly marine with few inputs from terrestrial origin), belong to the carbonate type, and deposited in transitional environments under reducing-oxidizing conditions. El Nady (2012) recommended that the molecular and polycyclic aromatic sulfur compounds play an important and powerful role in the characterization and evaluation of crude oils. El Diasty and Peters (2014) concluded that most of the oils in the Gulf of Suez were generated from marine source rocks, which supply the overlying reservoirs in the Kareem, Rudeis, and Belayim Formations. Two genetic oil families were recognized, tied to a carbonate-dominated source rock in the central sector of the Gulf of Suez and a mixed marine-terrigenous source rock in the southern sector. The present study describes an application of a geochemical non-biomarker (molecular and sulfur compounds) and biomarker approach in the interpretation of source organic matter input, depositional environment, and the maturation conjunction with stable carbon isotope geochemistry to assess the thermal maturity of source rock responsible for oil generation the centralsouthern Gulf of Suez. This approach is achieved through the analysis of ten crude oil collections of various ages from Miocene, Upper Cretaceous, and Nubia Sandstone representing the centralsouthern Gulf of Suez.
2. Samples and methods Ten crude oil samples were recovered from the major producing fields in the central-southern Gulf of Suez namely: Belayim marine, Ras Bakr, Ras Gharib, Ras Fanar July, Ramadan, El-Morgan, East Zeit, Sidki and Shoab Ali (Figure 1). These oil samples were undertaken from Miocene, Upper Cretaceous, and Paleozoic Nubia Sandstone reservoirs (see Table 1 for detailed sampling information). The following analyses were carried out: (1) Crude oil sample was distilled up to 200°C at atmospheric pressure. The residual fraction (>200°C) was deasphalted according to IP-143 standard procedure. (2) Density (API gravity) and Sulfur (S%) of the crude oil were measured according to ASTM D-4294 and ASTM D-4052 procedures, respectively. (3) The collections of oil samples were fractionated using high performance liquid chromatography (HPLC) into saturated aromatic hydrocarbons and resins. (4) Saturate fractions were treated with a molecular sieve (silicate) to remove the n-alkanes. Saturate fractions were analyzed on a Hewlett Packard 5890 Series-II gas chromatograph equipped with a Quadrex 50 m fused silica capillary column. The gas chromatograph was programmed from 40°C to 340°C at 10°C/min with a 2 min hold at 40°C and a 20 min hold at 340°C. The saturate and aromatic fractions were also analyzed using gas chromatographymass spectrometry coupled to a Hewlett Packard 5971A Mass Selective Detector (MSD).
ENERGY SOURCES, PART A: RECOVERY, UTILIZATION, AND ENVIRONMENTAL EFFECTS
193
Figure 1. The Gulf of Suez map showing the distribution of oil sample collections from the different fields of the central-southern Gulf of Suez province.
(5) The saturated hydrocarbon fraction was examined by capillary gas chromatographic analyses on a Perkin-Elmer Clarious 500 gas chromatograph equipped with a split/splitless injector, a flame ionization detector (FID), and a HP-1 fused silica capillary column (60 m x 0.25 mm). The temperature was programmed from 60 to 300°C min−1 at 5°C min−1 with a final hold of 30 min. (6) Stable carbon isotope analysis δ13C was carried out for the saturate and aromatic fractions and the whole oils using a Micromass 602 D Mass-Spectrometer. Data are reported as δ13C (‰) relative to the PDB standard. All the organic geochemical analyses for the studied crude oil collection were carried out at the organic geochemical laboratories, Oklahoma State University, USA.
Miocene
19.2 3.32 1.37 0.72 0.58 1.02 0.80 0.79 26.42 9.44 0.32 0.51 0.63 −26.42 −25.2 −25.31 −0.754
Reservoir age
APIº gravity Sulfur (wt%) Pr/Ph Pr/n-C17 Ph/n-C18 CPI Ts/(Ts+Tm) C35/C34 Homohopanes Oleanane index% Gammacerane index% C29ααα 20S/(S+R) C29αßß/(αßß + ααα) Diasterane Index δ13C Saturate (‰ PDB) δ13C aromatic (‰ PDB) δ13C whole oil (‰ PDB) C.V.
17.9 4.23 1.12 0.57 0.58 1.01 0.56 0.77 28.43 8.62 0.31 0.48 0.61 −27.26 −26.92 −26.96 −1.407
Miocene
Ras Bakr 14.8 4.39 1.15 0.52 0.48 1.01 0.78 0.81 23.15 8.66 0.36 0.53 0.58 −27.63 −26.98 −27.31 −1.645
Miocene
RasGharib 18.9 4.28 1.13 0.54 0.56 1.00 0.67 0.87 21.4 7.56 0.34 0.52 0.62 −28.36 −28.04 −27.56 −2.148
Miocene
Ras Fanar
July 35.8 0.97 0.81 0.37 0.46 0.99 0.64 1.15 3.38 21.70 0.50 0.59 0.71 −28.76 −27.68 −28.15 −0.339
Cretaceous
El-Morgan 34.8 0.89 0.91 0.37 0.42 0.98 0.77 1.04 4.45 25.53 0.51 0.60 0.69 −28.46 −28.43 −27.78 −2.76
Cretaceous
Ramadan 36.8 0.98 0.93 0.52 0.47 0.97 0.61 1.05 3.09 23.71 0.49 0.58 0.73 −28.78 −27.57 −28.24 −0.045
Cretaceous
E-Zeit 41.3 0.88 1.01 0.49 0.48 0.99 0.92 1.00 6.30 29.19 0.59 0.67 0.75 −26.31 −26.25 −26.98 −3.365
Nubia S.S
Sidky 37.2 0.83 0.94 0.33 0.35 0.99 0.81 1.25 5.01 21.99 0.54 0.65 0.81 −28.96 −28.69 −28.5 −2.073
Nubia S.S
Shob Ali 43.2 0.78 1.03 0.53 0.56 1.01 0.72 1.07 6.03 25.00 0.59 0.71 0.78 −28.16 −28.06 −27.76 −2.698
Nubia S.S
Pr/Ph: Pristane/Phytane ratio; Pr/n-C17; Pristane/n- alkane ratio; Ph/n-C18; Phytane/n- alkane ratio; CPI: Carbon preference index = Odd carbon atom/Even carbon atom, Ts/(Ts+Tm)= Trisnorhopanes/Trisnorhopanes +Trisnorneohopanes, Oleanane Index%= oleanane index [oleanane/(oleanane+hopane)], Gammacerane index = gammacerane/(gammacerane+C30 hopane); Diasteranes index = (C27 diasteranes S+R)/[(C27 diasteranes +R) + C29 steranes S+R)]; C29 20S/20S+20R, C29αßß/(αßß + ααα) Sterane ratios, C.V.: Canonical variable relationship = −2.53 δ13C saturates + 2.22 δ13Caromatics– 11.65.
Belayim M
Field name
Table 1. Specific, biomarker distributions and stable carbon isotope composition of crude oils from the central-southern Gulf of Suez.
194 M. A. YOUNES ET AL.
ENERGY SOURCES, PART A: RECOVERY, UTILIZATION, AND ENVIRONMENTAL EFFECTS
195
3. Results and discussion 3.1. Specific properties API gravity of crude oils is used as indicator for oil maturation. Thus, oils having less than 20°API are immature, and those having more than 20°API are mature (Tissot and Welte, 1984). Sulfur content was used by Waples (1985) as indicator for the source origin. Thus, oils originated from marine source have sulfur (1% S). Furthermore, the maturity of oils is more sensitive to sulfur content; it increases with the decrease of sulfur percentage (Gray et al., 1987). API gravity of the crude oils produced from Miocene reservoirs ranging from 14.8° to 19.2° and sulfur contents between 3.32 wt% and 4.39 wt% (Table 1) indicate immature oils of a naphthenic and non-waxy type, originated mainly from non-marine origin. Meanwhile, the crude oils that occur in Pre-Miocene is mature oils belonging to paraffinic and waxy of marine origin, where API gravities ranges from 34° to 43.2° and sulfur content ranges between 0.78 wt% and 0.97 wt% (Table 1). High sulfur oil to the first type indicates a clastic predominant source rock, while the low sulfur concentration suggests generation from carbonate lithofacies source rock of high grade of thermal maturity (Gransch and Posthuma, 1974).
3.2. Biomarker distributions Biological marker molecules are compounds that characterize certain biotic sources and that retain their source information after burial in sediments (Meyers, 2003). Figure 2a shows the gas chromatograms of some n-alkanes of representative oils, and the great variability of the saturate and aromatic biomarker indices is listed in Table 1. We show the predominance of n-alkanes and acyclic isoprenoids in the long chain region of the gas chromatograms range of C11 to C35 that is produced from a variety of biological precursors and is diagnostic to marine organofacies sources (Collister et al., 2004). The predominance of pristane over phytane ratio >1 and the strong odd carbon preference index CPI>1 for the Miocene oils ranging from 1.12 to 1.37 and 1.0 to 1.02, respectively (Table 1), is typical of crude oils generated from source facies containing terrestrial, wax-rich components (Peters et al., 2000). PreMiocene oils show less predominant of pristane over phytane ratio
