Oil Characteristics and Source Rock Potential of Abu Rudeis-Abu Zenima Area, Central Province, Gulf of Suez, Egypt

Oil characteristics; Source rock potential; Abu RudeisAbu Zenima area; Gulf of Suez Abstract The Gulf of Suez region is one of the most important petroleum provinces in Egypt and the Abu Rudeis-Abu Zenima area (ARAZ) located in the central province of the Suez Gulf on the eastern side. The scope of the current work is to evaluate the oils in the ARAZ area to identify oils characteristics and source rock potential using the data of whole oil gas chromatography and gas chromatography-mass spectrometry (GC-MS), in addition to stable carbon isotope analysis and vanadium and nickel contents. The bulk oils composition in ARAZ area characterized by moderate saturates and aromatics contents, high sulfur and NSO &asphaltene contents and heavy and normal API gravity. The composition of oils differs primarily due to their degree of thermal maturity. The oils generated from algal marine material with little terrestrial materials deposited under anoxic depositional conditions suggested by a low Pristane/Phytane (Pr/Ph) ratio. Moreover, the oils generated from carbonate source rocks of Thebes and Duwi rock units indicated by high C31-C35 Hopanes (45-50%) and C35/C34 extended Hopane ratio (0.99 – 1.12). The oils reached the peak-oil generation stage (0.85% Ro) indicated by the sterane isomerization ratio (20S % C29) that is less than 55 %. The oils of ARAZ area are matched the oils of the Gulf of Suez.


Introduction
Abu Rudeis-Abu Zenima area (ARAZ) is located in the central province of the Gulf of Suez on the eastern side. October Field and Ras Budran Field surrounded the ARAZ from the west and north directions respectively (Fig.1).
Many authors studied the characteristics of oils and source rocks in the Gulf of Suez and they concluded that the source rocks of Duwi and Thebes formations characterized by good to excellent organic content and kerogen of type II and mixed II/III with maturity ranging from immature to peak oil window (%Ro < 0.9) based on the analyzed pyrolysis data. Based on the carbon isotopes of biomarker data, the source rocks deposited under anoxic conditions with mainly marine organic matter and minor terrigenous higher plants. The chemical composition of the Gulf oils are variable in API gravity and sulfur contents mainly due to different thermal maturity levels (Alsharhan and Salah 1997 1;Wever, 1999Wever, 2, 2000Alsharhan 2003 4;Younes and Philip, 2005 5;El Nady and Mohamed, 2016 6;Mohamed and El Nady, 2016 7;El Nady et al., 2016 8;Younes et al, 2017 9;Mohamed and El Nady, 2019 10;El Diasty et al., 202011, Elmaadawy et al, 2021. Little published works studied the oil characteristics and source rock potential of the Abu Rudeis-Abu Zenima (ARAZ), therefore, the current work aims to assess the oils of selected wells in the ARAZ area to recognize the oil characteristics, organic matter types, depositional environments and source rocks potential. Moreover, study the impact of the oils geochemistry on the hydrocarbon migration and entrapment and determining if the oils geochemistry in the ARAZ area consistent or not with the oils of the Gulf of Suez basin.

Geological and tectonic setting
The interplay of Arabian and African plates during Late Oligocene -Early Eocene resulted in the Suez rift in Northwest-southeast direction (Garfunkel and Bartov 1977 13). Large-scale zones of normal faults surrounded both margins of the Suez rift and the rift subdivided into three main tectonic domains attributed to the polarity changing of the block faults (Patton et al. 1994 14;Moustafa, 1996 15;Younes and McClay 2002 16). Three main basins of northern Darag, central Belayim and southern Amal-Ziet recognized with a Zaafarana accommodation zone between northern and central basins and a Morgan accommodation zone between central and southern basins (Colletta et al. 1988 17).These accommodations resulted from half grabens and horsts related to large-scale normal faults (Bosworth 199518, 2015Bosworth and Durocher 2017 20).

Dataset and methodology Geochemical and biomarker data
Seven oils from different reservoir intervals analyzed by the whole oil gas chromatography and gas chromatography-mass spectrometry (GC-MS). The geochemical data of six wells of AZSE-1, ARN-2, AR-8, ARS-1, ARS-3 and S-9. The oils came from different reservoir intervals at different elevations such Nubia-A, Matulla, Thebes and Nukhul reservoirs (Table 1). Page|3 Figure 2 Lithostratigraphic column of the study area based on the available studied wells. The data of bulk oil composition include saturates % and aromatics %, NSO % compounds, asphaltene %, API gravity and sulphur % content (Table 2). Whole oil gas chromatography data of Normal alkane distribution (n-C18-C38), Pristane/Phytane (Pr/Ph) ratio and C7-derived expulsion temperature. GC-MS data comprise triterpenes (m/z 191) and steranes (m/z 217) distributions, biomarker isomerization, organic facies and depositional environment ratios (Tables 3 & 4). In addition to stable carbon isotope analysis of saturate and aromatic fractions with vanadium and nickel contents for well Sidri-9 (Fig.3).

Oils characterization
Whole oil gas chromatograms (Fig. 4) show that the light hydrocarbon composition of oils indicates a high n-Heptane (1.5-2.1) attributed to water washing or maturation based on the plot of heptane versus isoheptane indices of Thomson (1987 40 ) (Fig.5 a) and the shallow oil of AR-8 well is less affected (1.2) than those of the deep oils. The bulk composition of the most oils characterized by a medium saturates and aromatics (54.7-73.8 wt %), relatively high NSO &asphaltene contents (24.5 -35.1 wt %) (Fig.5 b), a heavy API gravity (API > 12.8 and <25.3) and a relatively high sulfur content (1.4-3.7 wt %) ( Table 2). In contrast, AR-8 well marked by the highest saturate/aromatics ratio (2.6) and the lowest NSO & asphaltene contents (24.5 wt %). In addition, the two oils of wells ARS-3 and ARS-1 have lower NSO & asphaltene contents (26.2-28.2 wt %) than those of AZSE-1 and ARN-2 (31.2-35.1 wt %)-suggesting minor compositional differences among the oils in the ARAZ area.
The variations in bulk oils composition not primarily related to the API gravity and sulfur contents. However, the oils of wells ARS-1 and ARS-3 characterized by the highest sulfur content (3.5-3.7%) are relatively heavier than the other oils and have medium NSO and Asphaltene contents, whereas the oil of AZSE-1 well has a high API gravity content (25.3). The oil of well ARN-2 has the lowest contents of API gravity content (12.8) and sulfur content (1.4 %), while the oil of AZSE-1 well has the highest API value (25.3) with a relatively high sulfur content (2.9%). The oil of AR-8 well has moderate contents of sulfur (2.5%) and API (21.1), while the rest of oils have low NSO & asphaltene contents and a high saturate-aromatics ratio.
The results of the bulk oils composition in ARAZ area matched the results of the Gulf of Suez in that the API gravity values vary from heavy to moderate and are inversely proportional to the sulfur contents. The API increases from north to south attributed to the increasing of thermal maturity of the source rocks Stable carbon isotopes for saturates and aromatics range between (30.20-24.50) and (29.40 -22.60) respectively, which indicate that the oils of the Gulf of Suez mainly generated from marine organic materials with minor terrestrial contribution (Sofer, 1984 44;Collister and Wavrek 1996 45;El Diasty et al. 2020 11). In ARAZ area, the stable carbon isotopes for saturates and aromatics are 29.2 and 28.7 respectively for well S-9 suggesting good correlation with the Gulf of oils (Fig.3).

Terpanes and Steranes distributions
The biomarker ratios attributed to terpanes and steranes are important in identifying the organic matter origins, depositional conditions, thermal maturity level and the age of the source rock (Hunt 1996 46;Peters et al. 2005 47). The terpanes (m / z 191) distribution suggests the oils mainly derived from algal organic matter and from minor terrestrial organic matter that indicated by the low C19 tricyclic / C23 tricyclic terpanes, high hopane and high C23 tricyclic / C24 tetracyclic terpane (Fig. 6). Moreover, the source rocks deposited under anoxic conditions indicated by high C35 homohopane.

Maturity related biomarker ratios
The chromatograms of the whole oil-gas show a normal alkane distribution in the range of n-C10 to n-C38 except the AR-8 oil of shallow reservoir indicating a slightly higher maturity than that of deeper reservoirs. Thebes and Duwi source rocks characterized by low Pr/Ph ratio indicating algal and terrigenous organic matter deposited under reducing conditions (Mostafa, 1993 48). In ARAZ area, the Pristane/Phytane (Pr/Ph) ratio is low and ranging between 0.66 and 0.73 in wells ARN-2 and AR-8 respectively suggesting a marine organic material deposited under moderate reducing depositional conditions (Fig. 8). Pr/n-C17 and Ph/n-C18 ratios are useful in recognizing organic material types, maturity level and biodegradation (Connan and Cassou 1980 49;Peters et al. 1999 50).The oils were come from nearly the same origin of marine organic material suggested by small variations in Pr/Ph, Pr/n-C17 and Ph/n-C18 ratios. These oils have low moderate maturity (Ro  0.8%) suggesting by low Pr/n-C17 (0.5) and Ph/n-C18 (0.9) values, in addition to low ranging between 110 and 115 ℃ and AR-8 oil has the lowest C7-derived expulsion temperature of 103 ℃ (Fig.9).  Figure 4 Whole oil gas chromatograms in ARAZ area. The 20S % C29 sterane isomerization ratio as a maturity indicator is lower than 55% indicating that the oils reach approximately to the peak oil generation stage (0.85% Ro), which consistent to the maturity derived from C7 (0.7-0.8 % Ro).
Trace elements Vanadium and nickel contents in in well S-9 oils are 96.4 ppm and 41.5 and the Vanadium / nickel ratio is 2.32. These results suggest different source rocks with different thermal maturity levels for oils (Moldowan et al. 1986 51;Hunt 1996 46;El Diasty and Peters 2014 38).

Organic facies and depositional environment
GC-MS biomarker parameters for oils of the ARAZ area indicate organic facies, thermal maturity and alteration degree (tables 3 &4). 7 Figure 6 Triterpanes (m / z191) in ARAZ area.
The oils show little higher land plant and high algal/bacterial derived organic material indicated by triterpanes distributions showing a high Hopane content (64-72 %), low C19 and C20 Tricyclics / C23 Tricyclics ratios (0.20-0.27) and low C27, C28 & C29 sterane (Fig. 10). The Hopane/steranes ratio is low ranging from 1.5 to 3.1 suggesting that the algal source contribution is more than the microbial contribution, while the bacterial contribution is greater in the oils of wells ARS-3 and ARN. The organic facies deposited under marine conditions indicated by the C30 sterane (5%) and the high amount of C31-C35 Hopanes (45-50%) and the relatively high C35/C34 extended Hopane ratio (0.99 -1.12) suggesting that the source rocks are carbonates.

The impact of oils characterization on hydrocarbon migration
The oils of the ARAZ area are low-moderately mature, sour derived from pre-Miocene marine, carbonate rich sources of Duwi and Thebes limestones. Rohrback (1983 52) stated that the oils of the Gulf of Suez approximately have the same origin and the variations in chemical composition attributed to the level of maturity. API gravity, asphaltene and NSO contents of the oils approximately are similar, while AZSE-1 and ARN-2 oils have higher asphaltene content increasing the viscosity of both wells than the other wells. The maturity of oils is relatively similar and with the depth, the oils are progressively heavier and richer in asphaltene fraction (Fig. 11), which consequently is affecting on the hydrocarbon migration. The oils rich in NSO and asphaltene fractions are heavy and migrate difficulty, while the oils rich in saturates and aromatics migrate more easily. The oils of northern AZSE-1 and ARN-2 wells are heavier than the other wells indicating low maturity level, deep reservoirs and short migration path (Fig. 12).

Conclusions
In ARAZ area, the geochemical and biomarker data are available for seven wells AZSE-1, ARN-2, AR-8, ARS-1, ARS-3, ARS-6 and S-9. ARS-6 and AZSE-1 wells are similar in the geochemical analysis, and AZSE-1 well selected to represent both wells. Seven oils collected from different reservoir intervals such Nubia-A, Matulla, Thebes and Nukhul reservoirs. The bulk composition of the most oils characterized by a medium saturates and aromatics (54.7-73.8 wt %), relatively high NSO &asphaltene contents (24.5 -35.1 wt %), a heavy API gravity (API > 12.8 and <25.3) and a relatively high sulfur content. The variations in bulk oils composition not primarily related to the API gravity and sulfur contents, but attributed to the thermal maturity. The oils of ARAZ area generated from marine organic matter with little terrestrial materials indicated by the stable carbon isotopes of saturates and aromatics in oil of well S-9. The biomarker ratios of terpanes and steranes indicate algal marine organic materials with little land plants. The Pristane/Phytane (Pr/Ph) ratio is low (< 1) suggesting a marine organic material deposited under moderate reducing depositional environments. The generation stage of the oils reached the peak-oil indicated by the sterane isomerization ratio (20S % C29) is less than 55 %. The oils generated from different source rocks with different thermal maturity levels indicated by vanadium / nickel ratio. The organic facies are carbonates deposited under marine conditions indicated by the C30 sterane (5%) and the high amount of C31-C35 Hopanes (45-50%) and the relatively high C35/C34 extended Hopane ratio (0.99 -1.12).

Funding sources
This research received no external funding.