--> Abstract: Petrography And Diagenetic History Of The Nubia Formation At Kharga Oasis, Western Desert, Egypt, by A. Abdel-Wahab; #90928 (1999).

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ABDEL-WAHAB, ANTAR
Geology Department, Faculty of Education at Kafr EI-Sheikh Tanta University, Egypt

Abstract: Petrography and Diagenetic History of the Nubia Formation at Kharga Oasis, Western Desert, Egypt

The Nubia Formation (Santonian) at Kharga at Kharga Oasis is almost entirly quartz with trace amounts of feldspar and rock fragments (Q94FIR5). 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, most of them filled with younger cements (5%), indicate the loss of feldspar grains 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 progresive burial depth, vast secondary porosity was produced through mesogenetic decabonatization of early calcite and dissolution of feldspar and ferromagnesian minerals. This event was followed by precipitation of little kaolinite, chlorite, illite , and mixed layer illite-smectite. The formation of iron-oxide cement (2.3%) which took place later, suggests an extensive invasion of meteoric water in the oxidizing zone. During teleodiagenesis, gypsum and barite were precipitated.

Pre-cement porosity versus cementation indicates that 76% of the destroyed porosity (avg. intergranular porosity = 19%) was lost by compaction and 24% by cementation. Pre-cement porosity (intergranular volume) values range from 18.5 to 34. 1 % (avg = 24. 1 %), then it losts an average of 27.4% porosity through compaction prior to cementation. Pressure solution has not been quantified because it is minor.

These sandstones have high reservoir quality. Present day porosity is 28% and the geometric mean permeability is 1213 md; most samples have more than 1000 md. The correlation between porosity and permeability is significant (r = 0.86).

The d18O for quarz cernent suggests silica was deposited by meteoric waters which probably was influnced by a hydrothermal source (intrusive). For calcite cernent the heavy d18O value (4.3 o/oo) suggests precipitation from evaporitic near surface water, while the depleted d18O value -3.311 o/oo is perhaps due to the envasion of meteoric water at shallow depth.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas