Seismic stratigraphy of the Messinian Nile Delta coastal plain: Recognition of the fluvial Regressive Systems Tract and its potential for hydrocarbon exploration

Abstract

The Upper Miocene strata of the Nile Delta (Egypt) record the dramatic events of the Messinian opening and closing of the Mediterranean Sea. Furthermore, the complexity of the associated stratigraphic relations within the Lower Messinian Qawasim and the Upper Messinian Abu Madi formations contribute to present challenges in their effective gas exploration and production. Through recognition and delineation of the Regressive Systems Tract on the Messinian Nile coastal plain, a new understanding of fluvial responses to changes in sea level as observed in the sedimentary record and to its optimal hydrocarbon reservoir exploration may be obtained.Seismic stratigraphic analysis of 1800km of 2D seismic with eighteen boreholes of these two formations reveals the dynamic interplay between fluvial downcutting and fill as a response to global and Mediterranean sea level oscillations. The observation of fluvial channel terraces on the delta plain during downstepping relative sea level falls reveal correlatable RST (Regressive Systems Tract) terraces incised by LST channels and then subsequent flooding by TST delta incised back stepping channel fills culminating in HST (High Systems Tract) deposition. While the Qawasim fluvial downcutting and fill are in response to global sea level changes, the Abu Madi responses are antithetic to global sea level changes.Specifically, the RST channels of the Qawasim Formation are represented by fluvial terraces of lateral accretion units in response to downward stepping base levels owing to a slowly falling global sea level base and shale channel fill during a global sea level rise. The RST channels of the Abu Madi Formation are represented by incised valley fluvial channels which eroded most of Qawasim Formation deposits especially in the northward of Nile Delta in response to rapidly falling local sea level as the Mediterranean Sea was cut off from the global ocean then are capped by a rapid transgression when the Mediterranean suddenly reopened. The RST and LST in both the Qawasim and Abu Madi fluvial channels exhibit potentially good reservoir continuity, sand quality, and charging potential. Productive boreholes confirm the hydrocarbon potential of the RST and LST in the Qawasim and the LST in the Abu Madi.Proper understanding and interpretation of the coastal fluvial RST stages on sand deposition as a response to sea level changes are a key issue in assessing Messinian reservoir quality of the Nile Delta and for coastal fluvial deposits elsewhere in the world.