Evaluation of surface Upper Eocene Rocks as reservoire, Fayum, Egypt: Sedimentary
petrophysical magnetic study
Mohamed A. Ragab, Khaled A. Khaled1
and Bassem S. Nabawy
Geology and Geophysical Sciences Dept., National Research Center, 1Geology
Dept., Helwan Univ., Cairo, Egypt
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Fayum depression is encircled to the north by a huge arc-like scarp made of several steps
marking the exposed more resistant hard fossiliferous limestone and dolostone,
intercalated with soft shales or friable sandstone beds of Upper Eocene age. These are
distinguished into Birket Qarun Formation (60.5 m) and upward Qasr ElSagha Formation
(180.0 m). The former is mainly built of claystones intercalated with Oyster banks and
sandstone deposited in shelf lagoons and winnowed platform edge sands with a fluvial
reflux.
The diagenetic processes, ex. tight cementation and pore filling deposition, obliterated
the pore spaces of the Oyster banks and sandstone beds and increased their tortuosity of
channels. Therefore the storage capacity of Birket Qarun Fm is confined to the friable
sandstone lenses. On the other hand, Qasr ElSagha Fm is mainly built of fossiliferous
Oyster and Turritella banks alternating with fluviatile deposits, ex. channel filling
sandstones and over bank claystones. Qasr ElSagha Fm was distinguished into Temple Member
and upward Dir Abu-Lifa Member.
Both members deposited in fluviatile environments at the base and passed upwards into
oscillations between fluviatile and marine environments. Effective porosity, permeability
and diameter of the channels of Temple Member was dishanced by followed cementation,
whereas storage capacity of Dir Abu-Lifa Member was enhanced by followed dolomitization.
In general, petrophysically the apparent electric resistivity values are mainly controlled
by the effective porosity, bulk density, diameter and tortousity of channels, true
electric resistivity, clay content, shaliness factor and the brine concentration; whereas
the true electric resistivity values are mainly controlled by the irreducible water
saturation and the grain density.
The magnetic susceptibility increases by decreasing both the apparent and true electric
resistivity values. The anisotropy of resistivity is reasonably accompanied with magnetic
foliations (P3); whereas the magnetic anisotropy (Pj) is mainly
affected by the MgCO3 content.
Paleomagnetically, it is achieved that unreliability of some data obtained from
sedimentary rocks is mainly related to biologic activities and diagenetic dolomitization
processes.
Therefore, only Dir Abu-Lifa Member and some friable sandstone lenses in Birket Qarun Fm
are suggested as good reservoirs for economic fluids.
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