Over the past decades, overpressure zones have been routinely identified in the Niger Delta by means of resistivity logs, dc component logs, direct measurements (using pressure level detectors attached to BHA) and penetration rate of the drilling bit. The present research, however, applies porosity as an alternative method for overpressure zone detection. The application of porosity as a tool for overpressure detection is demonstrated using plots of porosity versus depth for two wells in an X-field in the Niger Delta. Results show that between 5000ft and 7000ft depth, (for well-1) there is continuous decrease of porosity values with depth. At 7000ft, however, there is an abrupt shift to the right side of the plot, indicating abnormally high porosities (at least for the delta), with values ranging from 25% to 35%. Such abnormally high porosities suggest that the pore fluids support a disproportionately large part of the overburden (from the Benin Formation mainly), leading to an overpressure scenario. Geologically, overpressure zones indicate restricted vertical and horizontal permeability, (possibly by a high density of growth faults common within the Agbada Formation), high shale-to-sand ratios, deposition of impermeable sediments and montmorillonite diagenesis among other factors. In deed, results from the computation of sand-to-shale values indicate a marked change of ratios from the previously 2:1 to 3:5 in the formations, underscoring the association of overpressure zones with thick undercompacted shales. Montmorillonite diagenesis here is confirmed by high GR values of between 74 - 94 API. When compared with the resistivity-depth plots for same wells, the identified overpressure zone lies at the same interval, thus lending credence to the authenticity of porosity-depth plots as reliable indicators for overpressure zone in the Niger Delta.
