Print this pagePetroleum System in the Shushan Basin: A Mature Basin Leading to Future Exploration in the Western Desert of Egypt*
By
M.A. Younes1
Search and Discovery Article #10107 (2006)
Posted August 8, 2006
*Adapted from extended abstract for oral presentation at AAPG Annual Convention, Houston, Texas, April 9-12, 2006
1Geology Department, Faculty of Science, Alexandria University, Egypt, Alexandria, 21511, Egypt ( [email protected] )
Abstract
Shushan Basin (Figure 1) is one of the coastal basins in the northern Western Desert of Egypt that is characterized by its high oil and gas potentialities. Rock-Eval pyrolysis, biomarker properties and stable carbon isotopes of crude oils and related source rocks revealed two different types of extracts: namely, (A) Alam El-Bueib and Abu Roash-G and (B) Khatatba formations and two families of crude oils I and II of similar d13C carbon isotope composition. Fair correlation can be made between type (A) extracts and Bahariya crude oils, where similar biomarker properties, among them as C30 moretane ratio <10% and [20S/(20S+20R)] C29aaa sterane <0.5, suggest that these crude oils were generated from terrestrial-land-plant influence at low thermal maturity level. Meanwhile, type (B) extracts and Alam El-Bueib crude oils are genetically related and bear the same terrestrial source input generated at higher thermal maturity level than those of Alam El-Bueib and Abu Roash-G source rocks as evidenced from higher C30 moretane ratio >10% and [20S/(20S+20R)] C29aaa sterane >0.5. Organic-rich rocks with excellent potential to generate mainly oil are present in the Middle Jurassic Khatatba Formation, which entered the late mature stage of oil and gas generation window at vitrinite reflectance measurements between 1.0 and 1.3 Ro% during the Late Cretaceous. Meanwhile, good to fair source rocks of Alam El-Bueib and Abu Roash-G Member that are located within the early to mid mature stages of oil generation window between vitrinite reflectance of 0.5 to 1.0 Ro% at time varying from Late Cretaceous to Late Eocene. The similarities in biomarker characteristics of crude oils and source rock extracts in addition to the geologic occurrences are related to the stratigraphic as well as structrural entrapment elements that play an important role during the hydrocarbon accumulations in Shushan Basin.
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uAlam El-Bueib oil/Type B extract
uAlam El-Bueib oil/Type B extract
uAlam El-Bueib oil/Type B extract
uAlam El-Bueib oil/Type B extract
uAlam El-Bueib oil/Type B extract
uAlam El-Bueib oil/Type B extract
uAlam El-Bueib oil/Type B extract
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Source Rock EvaluationA potential source rock has the capability of generation and expulsion of thermally mature oil and gas that form accumulations. Source rock evaluation includes quantity and quality of organic matter in addition to thermal maturity or burial heating of organic matter buried in sedimentary succession. The source rock potential of, and the hydrocarbon generation in, the northern Western Desert of Egypt were studied by many authors; among them, Parker (1982), Taher et al., (1988), Abdel-Gawad et al., (1996), Khaled (1999), Ghanem et al., (1999), Sharaf et al., (1999), Waly et al., (2001), El-Gayar et al., (2002), Younes (2002), El-Nadi et al., (2003), and Harb et al., (2003). Accordingly, these studies concluded that the stratigraphic section of the northern Western Desert contains multiple source rocks of different degrees of thermal maturation. The dark shale of Khatatba Formation is considered mature source rock with an excellent capability for both oil and gas generation. Shale rocks of Alam El-Bueib and Abu Roash-G members are considered marginally to good mature source rock for oil generation during the Late Cretaceous.
The available Rock-Eval pyrolysis data of the studied rock units from the well Shushan-1X are represented in Figure 2. The results show that organic-rich intervals are present at three stratigraphic intervals: starting with the oldest, descriptions follows.
It consists of dark shale contains TOC between 3.60 and 4.20 wt.%, indicating an excellent source rock (Peters and Cassa, 1994). The source potential S1+S2 varies from 8.00 to 10.65 kg HC/ton rock, and the productivity index (S1/S1+S2) of these rocks ranges from 1.35 to 1.70. Therefore, the shale rocks of the Khatatba Formation have an excellent source rock potential.
Alam El-Bueib MemberThe shale section of Alam El-Bueib Member contains TOC that varies from 1.85 to 2.40 wt.%, indicating a good source rock. The source potential S1+S2 ranges from 3.60 to 4.50 kg HC/ton rock, and the productivity index (S1/S1+S2) of these rocks is generally less than unity. Therefore, the shale rocks of the Alam El-Bueib Member have a good source rock generating potential.
The organic richness of Abu Roash-G Member varies from 1.10 to 1.50 TOC (wt.%), reflecting a medium to good source rock. The source potential S1+S2 ranges from 0.85 to 1.10 kg HC/ton rock, and the productivity index (S1/S1+S2) of these rocks is generally less than unity. Thereform, the shale rocks of Abu Roash-G Member show fair source-rock-generating potential.
Type of Organic Matters (Kerogen Types) Kerogen types are distinguished using the Hydrogen Index (HI) versus Oxygen Index (OI) on Van Krevelen Diagrams. Analyses of the shale source rock intervals of Khatatba, Alam El-Bueib, and Abu Roash-G show that the three stratigraphic units contain mixed kerogen types II-III, of mixed vitrinite-inertinite derived from land plants and preserved remains of algae (Peters et al.,1994). Mixed kerogen type characterizes environment containing admixture of continental and marginal marine organic matter that has the ability to generate oil and gas accumulations (Hunt, 1996).
Thermal Maturity of Organic MatterThe thermal maturation of organic material is a process controlled by both temperature and time (Waples, 1994). The vitrinite reflectance is used to predict hydrocarbon generation and maturation. Vitrinite reflectance measurements (Ro%) for the well Shushan-1X were plotted against depth to indicate the phases of hydrocarbon generation and expulsion (Figure 3). The burial history model of the different hydrocarbon-bearing rock units indicates that the shale source rock of Khatatba Formation entered the late mature stage of oil and gas generation window, between vitrinite reflectance measurements between 1.0 and 1.3 Ro%, during the Late Cretaceous. The shale source rock of Alam El-Bueib Member entered the mid mature stage of oil generation window, between vitrinite reflectance measurements between 0.7 and 1.0 Ro%, during the Late Cretaceous while shale source rock of Abu Roash-G Member entered the early mature stage of oil generation, at vitrinite reflectance values between 0.5 and 0.7 Ro%, during Late Cretaceous to Late Eocene. Hydrogen Index (HI), Maximum Temperature (Tmax), and Total Organic Carbon (TOC) indicate that the shale source rocks of Khatatba Formation are located within the oil and gas generation window and are considered to have excellent source rock potential. Meanwhile, the shale rocks of Alam El-Bueib and Abu Roash-G members are considered good source rock for oil generation, having a less degree of thermal maturation than the shale source rock of Khatatba Formation.
Source Rock ExtractsTwo types of extracts can be identified in this study on the basis of the saturate/aromatic and the pristine/phytane ratios. Type (A) characterizes Abu Roash-G and Alam El-Bueib whereas, type (B) characterizes Khatatba Formation. The GC and GC-MS chromatograms of type (A) extracts have a predominance of saturate compounds rather than aromatic, with the ratios of saturate/aromatic of about 2.5, pristane/phytane around 0.6, and Ts/Tm 0.5. The plotting of isoprenoids/n-alkanes shows that the organic matters in the shale source rocks of Alam el-Bueib and Abu Roash-G are of mixed sources, with significant terrestrial contribution as indicated by the low ratio of C30 moretane less than 10%. The predominance of C27 regular steranes and diasteranes indicates the greater input of terrestrial organic matter. The low ratio of [20S/20S+20R] C29aaa steranes of about 0.4 indicates a low maturity level of hydrocarbon generation of Alam El-Bueib and Abu Roash-G shale source rocks. Type (B) extract of Khatatba Formation is of a waxy type, with the ratio of saturate/aromatic of 1.2, pristane/phytane of 0.7, and Ts/Tm 0.7. The ratio of isoprenoids/n-alkanes suggests that the organic matter was derived from terrestrial sources (Moldowan et al., 1985). This conclusion is further supported by the relatively higher ratios of C30 moretane of 14%. The predominance of C27 regular steranes indicates higher land plants input of terrestrial of sources (Huang and Meinschein, 1979). The high ratio of [20S/20S+20R] C29•••steranes 0.59 indicates a higher maturity level of hydrocarbon generation of Khatatba shale source rocks than the shale source rocks of Alam El-Bueib and Abu Roash-G members.
Crude Oils CharacteristicsTaher et al., (1998),Y ounes (2002 and 2003), El-Nadi et al., (2003), Harb et al., (2003), and El-Gayar (2003) used the geochemical fingerprints of crude oils produced from different basins of the northern Western Desert to assess the genetic relationship between hydrocarbon generation and their source rock depositional environments.
Family (I): Bahariya Crude Oils Family (I) represents Bahariya crude oils, which have a wide range of API gravities--between 32.6o and 43.3 o, corresponding to a high variation of sulfur content, which was found to be ranges between 0.05 and 0.13 wt.%. The saturate/aromatic ratios were found to be more than 2.30. The isoprenoids suggest that these oils were derived from peat coal source environment of terrestrial origin (Shanmugam, 1985). GC-MS fragmentograms of triterpane (m/z 191) and sterane (m/z 217) show ratio of C30 moretane to be <10%, further suggesting that the Bahariya crude oils may have been derived from mixed source rocks dominated by terrestrial organic matters (Moldowan et al., 1985; Zumberge, 1987). The regular C27 sterane distribution further suggests their derivation from higher land plants input of terrestrial and estuarine environments (Huang and Meinschein, 1979). The ratio of [20S/(20S+20R)] C29aaa sterane was found to be around 0.4, which suggest that these crude oils were generated at low level of thermal maturation (Peters and Fowler, 2002).
Family (II): Alam El-Bueib Crude Oils Family (II) represents Alam El-Bueib crude oils, which have low range of API gravities--between 40.0 o and 42.9 o, with a corresponding low variation of sulfur content, which was found to range between 0.03 and 0.07 wt.%. The plotting of isoprenoids/n-alkanes suggests that these oils were derived from peat coal source environment, derived from terrestrial sources. The ratio of C30 moretane was found to be >10%, further suggesting that Alam El-Bueib crude oils may have been derived from source rocks with higher input from terrestrial organic matters. The ratio of [20S/(20S+20R)] C29aaa sterane was found to be >0.5, which suggests that the Alam El-Bueib crude oils were generated at relatively high level of thermal maturation rather than the lower maturation for Bahariya crudes.
Stable Carbon Isotope CompositionTaher et al., (1988), Ghanem et al., (1999) Sharaf et al., (1999) and Younes (2002) used the stable carbon isotope composition in the aromatic and saturate fractions of the Western Desert crude oils and extracts to characterize waxy from non-waxy oil sources. Sofer (1984) distinguished the crude oils derived from marine and nonmarine sources for crude oils from different areas of the world, including Egypt, depending on the stable carbon isotope d13 C composition in the saturate and aromatic fractions. He applied a mathematical relation to conclude the canonical variable parameter that differentiates between the source of crude oils and their depositional environments. Although there are two types of extracts and two types of crude oils, they are isotopically similar (Figure 4) and genetically related. This may be attributed to slight differences in the degree of thermal maturity. The stable carbon isotopes of the saturate fraction in the extracts range between -26.2 and –24.7 ‰PDB); whereas those in aromatic fraction range between -24.3 and –22.6 ‰PDB. The stable carbon isotopes of the saturate fraction in the crude oils range between -25.4 and –24.7 ‰PDB); whereas for the aromatic fraction the range is between -23.1 and –21.5 ‰PDB. The data reveal that the studied crude oils are of terrestrial origin and the organic matter responsible for hydrocarbon generation in shale source rock of Khatatba, Alam El-Bueib, and Abu Roash-G were probably originated from terrestrial sources. This conclusion is also supported from the calculated canonical variable parameter which was found to be >0.47 for all the studied crude oils and source rock extracts, indicative of waxy oils type rich in terrigenous organic matter.
Inferred Oil - Source Rock CorrelationBahariya Oils-Type (A) Extracts The organic geochemical characteristics of the type (A) extract have close similarities to the crude oils reservoired in the Bahariya Formation; similar biomarker characteristics for both the oils and extracts. They both show a similar C30 moretane ratio, found to be <10%, suggesting terrestrial-land-plant influence and similar ratio of [20S/(20S+20R)] C29aaa sterane, found to be <0.5, reflecting the generation of these crude oils at low level of thermal maturation.
Alam El-Bueib Oils-Type (B) Extracts Gas chromatograms show that both crude oils of Alam El-Bueib Member and type (B) extracts are of waxy type. They show identical C27 regular sterane distributions and similar C30 moretane ratio of >10%, further suggesting a higher terrestrial land plants input. The identical ratios of [20S/(20S+20R)] C29aaa sterane, >0.5, reflect that these crude oils were generated from shale source rocks of Khatatba Formation at higher level of thermal maturation than those of Alam El-Bueib and Abu Roash-G source rocks.
ConclusionsThe organic geochemical characteristics of crude oils and related source rock extracts in Shushan Basin of the northern Western Desert of Egypt reveal two types of extracts (A) and (B) and two families of crude oils. Fair correlation can be seen between type (A) extracts of Alam El-Bueib and Abu Roash-G source rocks and Bahariya crude oils, with similar biomarker properties, such as C30 moretane ratio <10% and [20S/(20S+20R)] C29aaa sterane <0.5, suggesting that the Bahariya crude oils were derived from terrestrial-land-plant influence at low thermal maturity level. Alam El-Bueib crude oils and type (B) extracts of Khatatba Formation are genetically related and bear the same terrestrial source input, but they were generated at higher thermal maturity level than those of Alam El-Bueib and Abu Roash –G source rocks, as indicated by higher C30 moretane ratio >10% and [20S/(20S+20R)] C29aaa sterane >0.5. Organic-rich rocks with excellent potential to generate mainly oil are present in the Middle Jurassic Khatatba Formation, which entered the late mature stage of oil and gas generation window at vitrinite reflectance measurements between 1.0-1.3 Ro% during the Late Cretaceous. Meanwhile, good to fair source rocks of Alam El-Bueib and Abu Roash-G Member, within the early to mid and mature stages of oil generation window (with vitrinite reflectance measurements 0.5 and 1.0 Ro%), developed at a time varying from Late Cretaceous to late Eocene.
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