Grain size distributions and petrographic analyses of the outcropped Eocene-Miocene sedimentary successions, southern Nigeria: Depositional environments and petroleum reservoir quality predictions

This investigation presents a case study in which an integrated approach involving petrographic analyses was used, employing grain size tests, to obtain robust insights into the constraints on depositional environments, and contributing to reservoir quality estimation using outcrop and thin section data on the three main sedimentary formations of the Eocene-Miocene (Nanka, Nsugbe, and Ogwashi), southern Nigeria. Fine-grained, clay-poor sandstones, medium-grained, clay-poor sandstones, and coarse-grained, clay-poor sandstone facies were delineated on the basis of field data. The depositional environments of the Eocene-Miocene sediments stretch from fluvial/aeolian to shallow agitated marine environments. Facies, depositional environments, petrographic compositions, grain size distributions, probe permeameters, and empirical equation models were used to evaluate porosity and permeability levels. The fine-grained, clay-poor sandstone facies have the greatest reservoir quality potential, the medium-grained, clay-poor sandstone facies have better reservoir quality, and the coarse-grained, clay-poor sandstone facies have the least reservoir quality potential. The outcropping units of the Eocene-Miocene succession, in southern Nigeria have very good porosities and permeabilities levels to hold and transmit hydrocarbons and other fluids. A geologic reservoir model of the reservoir in the 3-D differential elucidation of the reservoirs capable of holding and transmitting hydrocarbons and other fluids in the study area within the Eocene-Miocene succession was developed. This study will facilitate the prediction of subsurface lateral equivalents of the Niger Delta Basin and other basins in the world with similar lithological networks. The reservoir model will also enhance effective, efficient, and optimal reservoir management.