Molecular and isotopic evidence for the origin of light oils and associated gases in the onshore northeast Nile Delta

The Nile Delta is a prolific hydrocarbon province in Egypt and the eastern Mediterranean region, particularly for gas resources. However, the origin of discovered hydrocarbons from the onshore Nile Delta has not been comprehensively studied. Fourteen condensates and 10 natural gas samples retrieved from Oligocene–Pliocene pay zones in the onshore northeast Nile Delta were studied for their molecular and isotopic composition to infer origin, source lithology, organic facies, depositional environment, thermal maturity, and reservoir secondary alteration. The isotopic compositions of the analyzed condensates indicate non-marine waxy oils. Results show that these condensates have high Pr/Ph ratios (1.86–6.46), abundant C19 and C20 tricyclic terpane contents relative to the C23 homologue, elevated oleananes, paucity of gammacerane, high hopane/sterane ratios, very low abundance of homohopanes, low dibenzothiophene/phenanthrene ratios and high C29/C27 sterane ratios. Clay-rich source rocks with abundant Type-III terrigenous organic materials deposited in an oxic fluvio-deltaic setting are suggested for these condensates. Light hydrocarbon (C7) compositions indicate that all investigated condensates, except the Allium-1 sample, suffered evaporative fractionation within their reservoirs. Chemometric analysis based on 14 biomarker and isotopic results suggests 3 genetic oil families for these condensates. These oil families have geochemical characteristics that indicate various contributions of terrigenous and marine organic matter and different levels of thermal maturity. The molecular and isotopic results indicate that the onshore Nile Delta natural gases are wet-thermogenic in origin with no signs of microbial biodegradation. These gases were generated by secondary cracking of associated oils derived from Type-III and Type-II/III or Type-II kerogen. The condensate and associated gas samples from Oligocene pay zones have different geochemical signatures than those from Miocene reservoirs, suggesting derivation from different source rocks with variable levels of thermal maturity or the presence of multiple charge systems from a common source in the onshore Nile Delta.