Petrophysical Evaluation of a Shaly Sandstone Reservoir and the Effect of Clay Minerals on Reservoir Quality: A Case Study from the Barremian Mengo Sandstone, Kouilou Basin, Republic of Congo

The growing global demand for energy is driving specialists to reevaluate and exploit fossil fuel deposits previously considered economically unviable. This study addresses the challenge of evaluating the Mengo shaly sandstone reservoir, a fan sandstone that is affected by diagenesis and that has heterogeneous porosity and low permeability and was abandoned due to its poor petrophysical properties, in the Kouilou Basin, Republic of Congo, particularly the impact of clay minerals on reservoir quality. The significance of this study lies in addressing the complex challenges associated with evaluating and exploiting shaly sandstone reservoirs, which are increasingly important for meeting global energy demands. The study involved integrating well logs, core data, and well testing data from 14 wells, with a focus on seven, using techniques such as the Schlumberger Techlog software; gamma ray, density, neutron, sonic, and resistivity logs; core analysis to measure porosity, permeability, and lithology; Xray diffraction; and scanning electron microscopy for mineral identification. The results indicated a reservoir composition of poorly sorted coarse and fine-grained lithologies with a significant clay content. Chlorite was identified as the primary clay mineral affecting porosity and permeability, with porosity values ranging from 8 to 15% and permeability values averaging from 0.16 to 1.68 mD. The study identifies diagenetic processes, such as the presence of clay minerals, as the primary cause of the contrast between porosity and permeability by reducing reservoir quality. Despite the challenges posed by the high clay content and radioactivity, integrating log and core data into a conceptual geological model is crucial for accurate reservoir characterization. The Midfan area around the MEN-105 well is pinpointed as the most favorable zone for future drilling.