<b>Contrast in clay mineralogy and their effect on reservoir properties in sandstone formations</b>

Abstract

Adigrat sandstone formation in the Blue Nile Basin is dominated by quartz arenite and subarkosic arenite, and cemented by carbonate, clay minerals and quartz overgrowths. Clay minerals in the Adigrat sandstone formation are dominated by kaolinite, illite and chlorite. Illite is the common grain-coating clay mineral. The common pore-filling minerals are kaolinite, illite and chlorite. The formation reaches a depth of 3km, which facilitated the change of kaolinite to illite. Clay minerals are the major reducer of porosity and

permeability in the Adigrat sandstone formation.

                    The Permo-Triassic sandstones in the Ulster Basin were deposited in NE-SW trending fault-bounded asymmetrical graben. The Permo-Triassic sandstones reaches a burial depth of 1/2 km in the marginal areas and 4.5 km in the basin center. The basin underwent multiple episodes of uplift and erosion in the Upper Jurassic, and Mid-Cretaceous, and uplift and inversion in the Upper Tertiary. The difference of the clay mineralogy in the Permo-Triassic in the Ulster Basin depends upon the depositional environments, source area geology and burial history of the basin. Illite-smectite is grain coating, whereas smectite, illite-smectite, smectite-chlorite, chlorite,

illite and kaolinite are the common pore-filling clay minerals in the Permo-Triassic sandstones in the Ulster Basin. Clay minerals reduced the porosity and permeability of the Permo-Triassic sandstones.

KEY WORDS: Adigrat sandstone, Authigenic mineralization, Clay minerals, Diagenesis, Permo-Triassic sandstones, Reservoir characteristics

Bull. Chem. Soc. Ethiop. 2008, 22(1), 41-65.