**3.1. Bulk geochemical characteristics of natural gas**

The abundant composition of the natural gases is dominated by methane ranging between 92.72% and 99.96%. The natural gas wetness denoted by the formula C2+/∑(C1\_ C5 ×100 and cumulative contents of the higher hydrocarbons from ethane up to pentane range from 0.05% to 6.76%. According to Schoell [14] the natural\_ gas may be classified as wet if it exhibits a wetness index greater than 5%. The natural gas samples examined from the western Nile Delta can therefore be classified as wet gas, while the major number of gas samples from the eastern Nile Delta is classified as dry gas except the samples produced from the two wells PSE-1and DEN-1. Wet gas is thermogenic in origin but associated with condensate in most cases while the dry gas may have either a microbial or late mature thermal origin [14] or may be related to that microbial alteration occurring within the reservoirs through the preferential removal of the C3+ components [15].

**Figure 4.** Relationship between isoprenoids Pr/*n*-C17 versus Ph/*n*-C18 showing source and depositional environments of the natural gas samples from the two provinces of the Nile Delta from Shanmugam[17]. All the study gas samples are located within the terrestrial to peat coal source depositional environments.

The predominance of pristane over phytane (Pr/Ph) ratio was used as an indicator of source rock depositional environments [16]. The ratio of Pr/Ph was ranged between 2 and 3 for the natural gas produced from the eastern province indicating fluviomarine and coastal swamp environment. The same ratio was found to be in the range between 6.4 and 9.4 which indicates peat swamp depositional environment. Figure 4 summarizes the achieved results concerning the nature of the source rock depositional environments is strongly supported by the plot of the isoprenoid ratios Pr/*n*-C17 versus Ph/*n*-C18 [17]. It can be seen from this figure that both the natural gases produced from the eastern and western provinces of the offshore Nile Delta and Mediterranean Sea are located within the region of terrestrial to peat coal source depositional environments. The high odd even carbon preference index (CPI > 1) for the studied natural gas samples from the two provinces reflect the generation from source facies dominating terrigenous and wax-rich components [18].

Measurements of δ 13C and δ D on the hydrocarbon gases were conducted with a Thermo Finnigan Delta plus XL mass spectrometer. Gas components were separated on a gas chro‐ matograph and injected into the mass spectrometer. Isotope values are reported in the δ

Data of analysis including gas chromatography, gas chromatography-mass spectrometry were

The abundant composition of the natural gases is dominated by methane ranging between

cumulative contents of the higher hydrocarbons from ethane up to pentane range from 0.05%

wetness index greater than 5%. The natural gas samples examined from the western Nile Delta can therefore be classified as wet gas, while the major number of gas samples from the eastern Nile Delta is classified as dry gas except the samples produced from the two wells PSE-1and DEN-1. Wet gas is thermogenic in origin but associated with condensate in most cases while the dry gas may have either a microbial or late mature thermal origin [14] or may be related to that microbial alteration occurring within the reservoirs through the preferential removal

**Figure 4.** Relationship between isoprenoids Pr/*n*-C17 versus Ph/*n*-C18 showing source and depositional environments of the natural gas samples from the two provinces of the Nile Delta from Shanmugam[17]. All the study gas samples are

located within the terrestrial to peat coal source depositional environments.

C5 ×100 and

gas may be classified as wet if it exhibits a

92.72% and 99.96%. The natural gas wetness denoted by the formula C2+/∑(C1\_

notation in per mil (‰) relative to the common PDB and SMOW standards.

conducted through Stratochem and Corex Laboratories (New Maadi, Cairo).

**3. Results and discussions**

32 Advances in Petrochemicals

of the C3+ components [15].

**3.1. Bulk geochemical characteristics of natural gas**

to 6.76%. According to Schoell [14] the natural\_

**Figure 5.** Genetic characterization of natural gases using carbon isotopes and C1 / C2 + C3 ratios modified after Ber‐ nard et al.[29] and Faber and Stahl [30].

### **3.2. Source and maturation dependent biomarker distributions**

The results of organic geochemical analyses of twenty gas samples representing the western Nile Delta province namely, Abu Qir and North Abu Qir fields and the eastern Nile Delta province, namely Temsah, Wakkar and Port Fouad fields. The results of gas chromatogram mass fragmentogram of the study gas samples show a predominance of C29 stereoisomers > 50% over C27 regular steranes between 20% and 27% suggestings a significant input of terrestrial organic matter [19, 20]. Also, the achieved result is strongly supported by the relative abundance of diasterane index that ranges between 55% and 69% of the study gas samples.

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].

Biomarker maturity parameters, include the sterane isomerization ratios\_ C29 ααα20S/(S+R), 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 early stage of source rock maturation from type-III kerogen [26].

**Figure 6.** Carbon and Hydrogen isotope ratios of natural gases. Genetic fields according to Schoell[27, 31] and Whiticar et al. [32].
