Application of Spontaneous Potential and Resistivity Logs in Reservoir Characterisation of the Agbada Formation

This study applies spontaneous potential and resistivity well log analysis to characterise reservoir properties of the Agbada Formation in a Niger Delta well, South South Nigeria. The Agbada Formation constitutes the primary reservoir and source rock sequence of the Niger Delta petroleum system, and well log analysis remains the most cost-effective tool for characterising its reservoir quality and fluid content at the borehole scale. This study uses a composite well log dataset from a developmental well in Bayelsa State, provided by a Nigerian Petroleum Development Company database for academic research. The log suite includes GR, SP, caliper, deep induction, medium induction, shallow resistivity, sonic, neutron porosity, and density logs. Lithostratigraphic interpretation uses gamma ray cutoff of 65 API to define sand and shale intervals. Net reservoir sand is delineated and bulk volume water, effective porosity (averaging 23 percent), and hydrocarbon saturation (averaging 62 percent) are computed using the Archie equation with appropriate cementation and saturation exponents. Permeability is estimated using Timur and Coates equations. A total of seven sand units with net pay thickness averaging 8.4 metres and hydrocarbon columns ranging from 4 to 18 metres are identified. Comparison between SP and gamma ray logs confirms four sands with anomalously suppressed SP due to freshwater mud filtrate effects. The study provides a detailed reservoir characterisation workflow applicable to Agbada Formation development wells and recommends systematic log analysis as a prerequisite to all perforation decisions.

Keywords: well log analysis, Agbada Formation, Niger Delta, reservoir characterisation, petrophysics.