Petrography, Fabric Analysis and Diagenetic History of Upper Cretaceous
Sandstone, Kharga Oasis, Western Desert, Egypt
Antar Abd ElWahab
Dept. Geology, Fac. Education, Kafr ElSheikh, Tanta Univ.,Egypt. |
The sandstones (Santonian) at Kharga Oasis are composed almost entirely of quartz with
trace amounts of feldspar and rock fragments (Q94F1R5). Although these sandstones were
buried to at least 3 km, the effect of compaction was minimized because of early
cementation. The presence of 8% oversize pores (5% with cement) indicates loss of feldspar
by diagenetic processes. Quartz overgrowths and calcite cements precede most other
diagenetic events. Quartz cement is either normal overgrowth (1.7%) or meteoric silcrete
(27%).
With progressive burial depth, vast secondary porosity was produced through mesogenetic
dissolution of early calcite, feldspar and ferromagnesian minerals. This was followed by
precipitation of little kaolinite, chlorite, illite , and mixed layer illite-smectite. The
Fe-oxide cement (2.3%) taking place later, suggests an extensive invasion of meteoric
water in the oxidizing zone. During telediagenesis, gypsum and barite were precipitated.
The  18O
for quartz cement suggests silica was deposited by meteoric water which probably was
influenced by a hydrothermal source (intrusive). For calcite ceIment the heavy 18O
value (4.3%) suggest precipitation from evaporitic near surface water, while the depleted 18O
value -3.3% is perhaps due to the envasion of meteoric water at shallow depth. To quantify
processes of compaction and to assess the maximum burial depth, several packing parameters
were determined: Types of contacts, number of contacts, contact index, consolidation
factor, contact strength, packing proximity, packing density, and weighted contact
packing.
Pre-cement porosity values (18.5 to 34.1% av. = 24.1%) lost an av. 27.4% porosity through
compaction prior to cementation. These exposed sandstones have high reservoir
quality. Present day porosity is 28% and the geometric mean permeability is 1213 md. The
correlation between porosity and permeability is significant (r= 0.86).
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