Diagenesis of Cambrian and Lower Carboniferous Sandstones, Southwest Sinai, Egypt

Antar A. Abdel-Wahab, Earle F. McBride, and Alaa M. K. Salem

Quartzose and feldspathic Cambrian and Lower Carboniferous sandstones (fluvial > marine > eolian) deposited on the Arabian shield in southwestern Sinai were not buried more than 1 to 1.5 km until Late Cretaceous and younger time, when the deepest rocks reached 2.5 km. Porosity was reduced by compaction to about 26 %. Grain rearrangement was the main mechanism of compaction; intergranular pressure solution and ductile grain deformation are insignificant.

Cementation by iron oxide (5%), quartz (3%), calcite (2%), kaolinite (2%) and K-feldspar (< 1%) reduced porosity to 12-15 % except in silcretes and some ferricretes where porosity was reduced to non-reservoir quality. Cements have no selectivity for certain textures or stratigraphic positions. Some secondary porosity was created (5.8 and 5.1 % for Cambrian and Carboniferous sandstones, respectively) chiefly by dissolution of feldspar. Feldspar dissolution was severe enough in some samples to convert them from subarkoses to diagenetic quartzarenites. Cementation by iron oxide,quartz, poikilotopic calcite, and kaolinite were dominated by meteoric water as inferred from burial history and oxygen isotopes. Both normal syntaxial quartz overgrowths and subsequent silica phases in silcret s formed at depths less than 300 m. Isotopic data show that micritic calcite cement precipitated from sea water and evaporative brines, and that poikilotopic calcite precipitated from meteoric water. Silica for quartz cement, CaCO₃, and iron were imported.Barite, gypsum and halite cements occur in outcrop samples.

The present-day porosity in these sandstones averages 19% and ranges from 1.5 to 32 %. Many sandstone samples (47 % of a total of 178 samples) have permeability values higher than 1000 md. The plot of porosity versus log of permeability has a good correlation, indicating that microporosity, even though locally important, does not significantly influence reservoir quality.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California