**3.3. Relation between isotopic composition and origin of natural gas**

The isotopic composition of natural gas depends on the type and maturity of source rock [27, 28]. Stable isotope geochemistry helps to differentiate the original gas derived from a single source rock. The methane carbon isotope compositions range between \_ 65.6 ‰ and \_ 41.1‰. Methane hydrogen isotope data ranges from \_ 193 ‰ to \_ 149‰. The isotopically lightest δ 13C sample was obtained from the SEM-1 well, while the heaviest δ 13C value was received from AQ-16 well. Many of the δ 13C values have intermediate positions between these end members, which are consistent with what is commonly interpreted as mixed microbial and thermogenic gas [14, 28]. A general increase in the δ 13C values with depth from Miocene to Plio-Pleistocene reservoir is consistent with the increasing thermal maturity and the decreasing relative concentration of biogenic gas.

All the study gas samples that exhibit higher oleanane index ranging from 19% and 42% strongly support the enrichment of angiosperm higher land plants input to the siliciclastic source kerogen which is thought to be derived from Late Cretaceous to Tertiary age [21, 22]. The medium concentration of moretane index in all the study samples ranges between 11% and 16% and the absence of gammacerane index strongly supports a terrestrial input [23, 24].

and C29 αßß/(αßß+ααα) which according to Seifert and Moldowan [25] are genetically related to the effect of thermal maturity processes. The average sterane isomerization ratios C29ααα 20*S*/(*S+R*) and C29αßß/(αßß+ααα) of natural gas are 0.6 and 0.5 respectively indicating a medium stage of thermal maturation equivalent to the main peak of oil generation window (0.85 Ro%) [18]. The medium sterane isomerization ratios may reflect the rapid rates of subsidence and sedimentation in the Nile Delta and appear to have been generated during the

**Figure 6.** Carbon and Hydrogen isotope ratios of natural gases. Genetic fields according to Schoell[27, 31] and Whiticar

The isotopic composition of natural gas depends on the type and maturity of source rock [27, 28]. Stable isotope geochemistry helps to differentiate the original gas derived from a single

sample was obtained from the SEM-1 well, while the heaviest δ 13C value was received from

193 ‰ to \_

**3.3. Relation between isotopic composition and origin of natural gas**

source rock. The methane carbon isotope compositions range between \_

Methane hydrogen isotope data ranges from \_

III kerogen [26].

C29 ααα20S/(S+R),

65.6 ‰ and \_

149‰. The isotopically lightest δ 13C

41.1‰.

Biomarker maturity parameters, include the sterane isomerization ratios\_

early stage of source rock maturation from type-

34 Advances in Petrochemicals

et al. [32].

The wet gas (C2+) components also display a significant variability in their isotopic composi‐ tions. The δ 13C of ethane (δ 13C2) ranges from \_ 37.5‰ to \_ 27.9‰ but that of propane (δ 13C3) ranges from \_ 32.2‰ to \_ 11.1‰.

Natural gases become isotopically heavier and contain relatively more methane with increas‐ ing thermal maturity. Thermogenic methane is generally enriched in δ 13C compared with microbial methane [27].

Figure 5 represents the Bernard plot in which the molecular ratio C1/ (C2 + C3) is plotted versus methane's stable carbon isotope ratio according to Bernard et al.[29] and Faber and Stahl [30]. The relationship between these two parameters allows distinguishing microbially generated methane from thermogenic hydrocarbon gases. The figure provides a strong indication for gas mixing of thermogenic and early microbial methane.

The δ 13C methane versus δ D methane plot (Figure 6) represents a scheme for the carbon and hydrogen isotope ratios of methane and their genetic implications. Although this diagnostic plot was initially established for determining the thermogenic origin of methane [31] it is useful for the discrimination of microbial methanogenesis, i.e. carbon dioxide (CO2) reduction versus acetate fermentation [32]. The δ 13C values between \_ 65.6 ‰ to \_ 41.1‰ and δ D values between \_ 193 ‰ and \_ 149‰ obtained for the methane of the samples from this study classify methane as mixtures between thermogenic and biogenic origin. Figure 7 gives a good matching with the published regional gas data. The relatively heavy hydrogen isotopic compositions (Figure 7) suggest that the microbial gas proportions are probably generated through CO2 reduction [31, 32].

In the offshore Nile Delta and Mediterranean Sea, the Oligocene to Early Miocene sedimentary sequence is considered as the primary source of natural gas and condensate characterized as mixed type II/III kerogen [8, 26].

The isotopic composition of the individual gas components is a function of thermal maturity of the generated source rock kerogen. Stahl [33] proposed an empirical relationship between methane carbon isotopic composition and vitrinite reflectance. Berner et al. [34] and Berner and Faber [35] illustrated a model between the carbon isotopic compositions of methane, ethane, and propane and source rock maturities represented by vitrinite reflectance measure‐ ments. In this study we used the isotope maturity models developed by Berner and Faber [35] to estimate the type and maturity of the precursor (source) material (Figures 7and 8). The geochemical characteristics of natural gas from the Nile Delta indicating a derivation from source rocks rich in terrestrial precursors [8, 36, 37]. Consequently, the admixture of light microbial methane to the thermogenic hydrocarbon gas takes place during secondary proc‐ esses, such as migration, microbial oxidation, or mixing. These processes usually affect the gas properties within the reservoir [28, 40].

**Figure 7.** Isotopic composition of methane and ethane in natural gases of the western and eastern Nile Delta. The ma‐ turity line is according to Berner and Faber [35]. The terrestrial organic matters indicate the maturity of the source rock in vitrinite reflectance measurements

**Figure 8.** Isotopic composition of ethane and propane in natural gases of the western and eastern Nile Delta. The ma‐ turity line is according to Berner and Faber [35].

Isotopic compositions of ethane and propane from the west and east Nile Delta samples proved their mixing origin and derivation from terrestrial organic matter with a source rock maturity between 0.1% and 2.0% Vitrinite Reflectance measurements.
