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
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 <
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 >
Stable Carbon Isotope Composition
Taher 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 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
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 <
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 >
Conclusions
The 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 <
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