Alaa M. Salem¹, S. Morad², Raafat R. Abdel-Razik³, Ihsan S. Al-Aasm⁴
(1) Tanta University, 33516 Kafr El-Sheikh, Egypt
(2) Institute of Earth Sciences, Uppsala University, Villavägen 16, Sweden
(3) Belayim Petroleum Company, Nasr City, Cairo, Egypt
(4) University of Windsor, Windsor, ON

Abstract: The origin and influence of grain-coating chlorite on water saturation in gas fields: Evidence from the Abu-Madi gas reservoirs (Upper Miocene), Nile Delta Basin, Egypt

The Abu-Madi fluvio-deltaic sandstones (Upper Miocene) form a major natural gas reservoir ( 3350-m depth) in the Nile Delta Basin, Egypt. The reservoirs are characterized by very high water saturation which is attributed to the presence of Fe-rich, grain-coating chlorite. These coatings ( 15%) occur as well-developed crystals ( < 5 m across) rich in micropores and arranged perpendicular to grain surfaces. Additionally, chlorite rims blocked pore throats causing substantial permeability deterioration. Chlorite coatings formed mainly by mesogenetic transformation of infiltrated clays. Chlorite occurs also as authigenic pore-filling rosettes and as replacement of volcanic fragments, mud intraclasts and biotite.

Despite the negative effects, the chlorite coatings have enhanced reservoir quality by inhibiting mesogenetic quartz cementation ( < 2%), and thus preservation of primary intergranular porosity. Clay minerals that further contribute to the increase in microporosity and, hence high water saturation include kaolin. Other cements that impose important control on reservoir quality include eogenetic and mesogenetic calcite. Petrographic and geochemical evidence indicate that mesogenetic processes are governed by diffusive mass redistribution within the sequence and not by influx of external fluids.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana