The Albian – Cenomanian boundary on the southern Tethyan margin: Abu Gharadig Basin, Northern Western Desert, Egypt

Abstract

Lithostratigraphy and biostratigraphy sometimes provide contradictory interpretations, especially when trying to establish a chrono-lithological boundary between two reservoir formations from different paleoenvironments. One of these contradictions is the definition of the boundary between the Kharita and Bahariya formations and the Albian and Cenomanian Stages in the Abu Gharadig Basin of the Western Desert, Egypt. an integrated approach has been applied to this problem using borehole image logs, palynology, and conventional wireline logs. authors also calibrate the stratigraphic sections with global chronostratigraphic charts, using nannoplankton, planktonic foraminifera, and dinoflagellates to construct a better understanding of this basin. The abundances of pollen, spores, and dinoflagellates in the interpreted reservoir intervals suggest that hydrocarbons trapped in the Albian-Cenomanian reservoirs are composed of type II/III kerogen. The interpreted intervals are subdivided into seven depositional cycles: one in the Kharita Formation, five in the Bahariya Formation, and one in the Abu Roash Formation. The interpreted environments are, from bottom to top: I) fluvio-deltaic distributary channels, II) and III) intertidal to subtidal deposits, IV) offshore bar deposits, V) intertidal to subtidal deposits, VI) shoreface deposits, and VII) shallow marine carbonate deposits. Diagnostic palynomorphs reflect climatic change from arid to humid conditions and also enable changes in salinity to be determined.Much of the published work in the southern Tethyan margin, particularly in the Western Desert, is either related to stratigraphy, palynology, or reservoir evaluation with a focus on a narrow field. Tectonostratigraphic studies and analyses of petroleum system elements have often been very regional in nature. The work presented here is of an intermediate scale linking regional frameworks with local details to provide comprehensive information about the Albian-Cenomanian succession in the Northern Western Desert. The focus has been the Abu Gharadig basin but this integrated approach has implications for other basins in North Africa. A specific example is the similarity of Western Desert and Libyan Sirt basin Albian-Cenomanian key palynomorphs which would facilitate a similar integrated chrono-lithologic intepretation.