Oligocene source-rock characteristics and hydrocarbon generation modeling of the eastern Nile delta, Egypt

Abstract

This study investigates Oligocene source rock development in the eastern Nile Delta, integrating TOC, Rock-Eval pyrolysis, as well as vitrinite reflectance (Ro%) measurements from over 9 onshore and offshore wells. 1D basin models, constructed for 2 wells onshore and 2 wells offshore, investigate the burial, thermal histories, and the stage and timing of hydrocarbon generation. The present-day data indicate that the Oligocene section encompasses Type III and rarely Type IV kerogen onshore, whereas offshore Type III and Type II-III kerogen dominate. The back-calculated original TOCo and HIo values indicate a significant hydrocarbon generation (i.e. effective to relic source rock) for Rupelian section in Tayifah Deep-1x and Satis-1 wells. The different maturity indicators used here show significant differences in some intervals. The variability may be attributed to natural impregnation of migrated hydrocarbon in permeable strata, in-situ expelled hydrocarbons where S2 contamination is rather likely for samples within the oil window, or retarded Ro% and/or Tmax caused by overpressure. Therefore, thermal and generation models are calibrated with measured bottomhole temperatures, which match quite well with δ13C values of gases C1-3, and indicate that the onshore Oligocene basal section comes in the early stage of hydrocarbon generation throughout the middle-late Miocene, and was in the main stage of hydrocarbon generation since the early Pliocene. The transformation ratio indicates an in-situ expulsion (25–50%) since the Pleistocene time. Offshore models indicate the basal Oligocene section entered the late phase of hydrocarbon generation during the early to middle-late Miocene, where its transformation ratio exceeds 50%. This indicates the main hydrocarbon expulsion from the late Oligocene to the early-middle Miocene time. Further integrated approaches to the Oligocene geochemical analysis after solvent extraction are vastly required. Higher resolution overpressure and maturity indices (Tmax and Ro) profiles for some deep sections are essential to strengthen concepts about the mechanism(s) and magnitude of their retardation/suppression.