Petrology and Staining Analysis of Carbonate Rocks of A1 Well, Eastern Dahomey Basin, Southwestern Nigeria

Abstract

Limestone, a soft rock, is known to be highly susceptible to various post-depositional diagenetic effects whose imprints are visible in hand specimens and more expressly in thin section studies. This study aims to provide a diagenetic assessment of the limestone in A1 well penetrating the Tertiary sediments of the Eastern Dahomey Basin through an integrated approach combining staining and petrographic analyses. A total of fourteen thin sections obtained from seven limestone intervals of the A1 well core samples were studied. The petrographic analysis reveals that the limestone is of biosparitic and biomicritic microfacies; and dominantly packstone classification with the imprints of certain diagenetic processes such as physical and chemical compaction, micritisation, neomorphism, and dolomitisation delineated. The staining analysis using the Alizarin Red-S and Potassium Ferricyanide composite test reveals the presence of calcite, ferroan calcite, dolomite and ferroan dolomite. The incorporation of iron into the carbonate minerals indicate that the A1 well has been subjected to late stage chemical diagenetic alterations and are not so pure for industrial use. Although ferroan dolomite is more expressed in the upper section, ferroan calcite occurs in all the sampled limestone intervals with scarce occurrence of pure calcite. Overall, the imprints of dolomitisation are delineated revealing greater dominance at the base of the column.