Reservoir heterogeneity in the Cambrian sandstones: A case study from the Araba Formation, Gulf of Suez Region, Egypt

The reservoir quality of the Araba sandstones that are exposed west of the Gulf of Suez, northeastern Egypt, exerts a considerable vertical heterogeneity owing to variations in detrital composition, depositional facies and post-depositional alterations induced during diagenesis. Meteoric water played a significant rule in the destruction of unstable detrital grains such as feldspars and other less-stable rock fragments. Eight facies assemblages are recognized in the Araba Formation, stacked from base to top: LST braided fluvial stream, TST flood plain, LST upper shoreface-foreshore and coastal plain, TST tidal flat, HST offshore-lower shoreface, FSST upper shoreface, and HST subtidalintertidal sand-flat. Three diagenetic realms are differentiated: 1) eodiagenesis under humid climate, that resulted in iron-oxide pigmentation, early formed quartz and calcite cementation, and limited grain dissolution; 2) mesodiagenesis during burial depth less than 3000 m that resulted in mechanical compaction, intergranular quartz and unstable grain dissolution and secondary porosity creation, precipitation of quartz, kaolinite, and calcite cements, and clay authigenesis; 3) telodiagenesis during uplifting, which led to precipitation of iron-oxides, kaolinite, gypsum, halite, and secondary porosity development. Variations in textural parameters such as grain size, sorting, and grain fabric, as well as abundance and distribution patterns of diagenetic modifications, had a great impact on the deterioration of porosity and permeability and thus led to reservoir heterogeneity of these sandstones. Accordingly, two main types in the Araba sandstones, A and B, are recognized. The first type (A) is the submature sandstones that are characterized by a lower reservoir quality (av. φ = 21.41 % and K = 26.6 mD). This type is represented by the upper shoreface-beach LST sandstones that are characterized by a relatively heterolithic grain size and sorting and strong compactional effect (COPL up to 33.5 %), as well as the lower shoreface HST sandstones that dominated by extensive iron-oxide and kaolinite cementation (CEPL up to 39.0 %). In contrast, the second type (B) is the mature to supermature quartzarenite that shows a reasonable reservoir quality (av. φ = 23.0 % and K = 171.75 mD) owing to better sorting and fairly quartz and kaolinite cementation as a result of incursion of meteoric water which also enhanced grain dissolution and porosity evolution. This type includes the uppermost part of SQ2 and the entire SQ3 of the Araba Formation, and it is represented by the FSST upper shoreface and subtidal-intertidal HST sandstones.