Clay minerals distribution in the pre-, syn-Messinian salinity crisis sediments of the onshore Nile Delta, Egypt: Mineral origin and implications on the reservoir quality

Abstract

Abstract The Messinian Qawasim and Abu Madi formations are the main exploration targets for hydrocarbons in the onshore Nile Delta. However, their diagenetic history and the burial attributes which impacted their reservoir characteristics are poorly studied. Among these are the preferential chlorite authigenesis and complete illitization of kaolinite and feldspars in the syn-MSC (Messinian salinity crisis) Abu Madi sediments. SEM-EDS analyses revealed two different chlorite pseudomorphs in the MSC Abu Madi sediments. The first (chlorite I) has Si/Al ratio similar to the ratio in smectite (∼2.2:1), and the other (chlorite II) has a ratio very similar to kaolinite (∼1.3:1). These pseudomorphs were formed due to progressive chloritization of both smectite and kaolinite. Chlorite I formed by chloritization of smectite, and its absence in the pre-MSC Qawasim facies is due to the absence of smectite itself. Moreover, the comparatively enhanced burial of the MSC Abu Madi sediments in Fe-rich system prompted the chloritization of kaolinite and formation of chlorite II. The incomplete illitization, and formation of intermediate illite-kaolinite phase in the shallower pre-MSC facies confirms the prominent depth control. The Messinain facies were subdivided into three rock types of descending reservoir quality and different clay distributions (RT I, II, and III). The pre-MSC facies contain abundant diagenetic kaolinite which has minimal impact on the facies reservoir quality (RT I). The MSC facies are dominated by depositional clays (RT III) and diagenetic chlorite (RT II) which have the most detrimental impact on the reservoir quality, and caused high degree of permeability anisotropy.