Characteristics of pre- and syn-rift sequences in the central Gulf of Suez: Implications for hydrocarbon generation and paleoenvironment

Abstract

The October sub-basin is one of the largest oil fields in Egypt, situated within the central Gulf of Suez petroleum province. Four wells were studied to investigate the petroleum system using integrated geochemical and petrographic techniques. This gives valuable information on the source rock potentialities, which may help in new hydrocarbon discoveries in the October sub-basin. Organic-rich pre-rift (Late Cretaceous-Eocene) and syn-rift (early-middle Miocene) sequences were analyzed for TOC, Rock-Eval pyrolysis, vitrinite reflectance (R_o%), and lipid biomarkers. 1D basin models were built to assess hydrocarbon generation timing and remaining potential. This analysis provides valuable insights into source rocks characteristics, including quantity, quality, thermal maturity, and paleoenvironmental implications. Pre-rift strata show fair-very good organic richness, while syn-rift strata demonstrate poor-good organic richness. Pyrolysis shows that the NW wells predominantly contain Type III dispersed organic matter and organic petrography shows the presence of vitrinite in the form of collotelinite and collodetrinite) in both sequences. The same analysis also shows that the SE wells contain Type II-III and II kerogen while organic petrography shows the presence of dispersed liptinitic organic matter (e.g., telalginite, lamalginite, sporinite, liptodetrinite) and also low inertinite content. This indicates a greater marine organic matter input, which is also supported by biomarker data, with the latter suggesting low-oxygen conditions in the SE parts of the sub-basin. Pre-rift source rocks range from immature to the upper oil/wet gas window (R_o% of 0.50–1.35 % in the NW wells, 0.41–0.82 % in the SE wells), but mostly within the oil window. The NW wells show higher thermal maturity, particularly in well NO124–1, likely due to elevated geothermal gradients from rift-related Oligocene-Miocene igneous activity. As expected, syn-rift strata are immature or in the early oil window (R_o % 0.37–0.61 % in the NW, 0.36–0.61 % in the SE). Thermal maturity modeling indicates that pre-rift strata are effective source rocks entered the early oil window ∼20–15 Ma, earlier onset in the NW wells. Present-day pre-rift transformation ratios are higher in the NW (60–88 %) than in the SE (17–35 %) wells, suggesting greater remaining hydrocarbon generation potential in the SE region.