Integrated Rock Physics, Elastic Properties, and Geomechanical Characterization of Unconventional Reservoirs in the O-Field, Niger Delta Basin, Nigeria

Abstract

This study presents an integrated rock physics, elastic properties, and geomechanical characterization of unconventional reservoirs in the O-Field, Niger Delta Basin, Nigeria, using well logs (gamma ray, density, resistivity, neutron, and porosity) alongside derived elastic and geomechanical parameters, including Young’s modulus, bulk modulus, Poisson’s ratio, Lambda Rho, Mu-Rho, unconfined compressive strength (UCS), and brittleness index, to enhance lithology and fluid discrimination. Rock physics cross-plot analysis effectively reduces uncertainty, with Lambda-Rho versus Mu-Rho plots, colour-coded by gamma ray and density, clearly distinguishing lithologies and fluid types. Lower λρ and μρ values correspond to gas- and oil-bearing sands, intermediate values to brine sands, and higher values to shale, while sand clusters are characterized by lower to moderate density and rigidity. The brittleness index versus gamma ray cross-plots, colour-coded by porosity, show an inverse relationship, separating brittle, porous sands at lower gamma ray values from less brittle, low-porosity shale intervals at higher gamma ray values. Geomechanical property relationships are consistent across both wells, with Young’s modulus increasing with UCS, indicating that stiffer rocks are also stronger. Bulk modulus shows an inverse relationship with UCS, particularly within shale intervals, while Young’s modulus decreases with increasing Poisson’s ratio, clearly distinguishing brittle sands from ductile shales. The results confirm reliable litho-fluid and geomechanical discrimination across the field. Brittle zones are identified as favourable for hydraulic fracturing, while ductile zones act as effective seals. This integrated approach provides a robust framework for evaluating unconventional reservoir quality and geomechanical behaviour.