Petrological and Geochemical Evaluation of the Paleocene-Eocene Lithofacies in Dahomey Embayment, Southwestern Nigeria

Abstract

The detail mapping and logging of the Ibese area within the Dahomey Basin has revealed the total thickness of the limestone to range between 12.0 m to 15.0 m. This study is based on qualitative lithologic, spectrochemical analyses via X-ray fluorescence and petrographic indices to understand the lithofacie, chemistry, viability of the sequences and depositional environment. This consists of milky to grey and nodular to massive fossiliferous limestone units while the sand content increases with depth. The composition of the major oxides in the thirty-seven (37) samples from six exploratory wells revealed that CaO (51% - 56%), SiO2 (2.6% - 10.56%), Fe2O3 (0.33% - 0.94%), MgO (0.78% - 1.02% and Al2O3 (0.72% - 0.98%) were the most abundant elements. The trend of the geochemical indices show increase in the percentage composition of CaO, and MgO down the sequences with high values which defined the depth of diagenetic re-distribution. The percentage composition of Al2O3, Fe2O3 and K2O are significant indicating continental influence during diagenesis and diverse depositional episodes. The three different microfacies recognized based on depositional texture are shelly biomicrite, shelly biosparmicrite and alga biosparmicrite. The predominance of micrite as the cementing matrix revealed that the rock was deposited in a quiet shallow inner shelf environment occasioned by storm waves.