Petrophysical and Reservoir Rock Property Analyses of Open-Hole Geophysical Logs for Afenmai Field, Eastern Niger Delta Basin of Nigeria

Abstract

Petrophysical attributes and reservoir rock properties were estimated and analysed for Afenmai Field located within eastern part of the Central Depobelt of the Niger Delta Basin in Nigeria. The reservoir rock properties comprised reservoir pore volume-to-grain volume ratio (Փz), reservoir quality index (RQI), reservoir flow zone (FZI) indicator, and reservoir deliverability index. The study was carried out with data-set obtained from Well 8 in the field. The data comprised subsea vertical depth to top and base of sand units, hydrocarbon-water contact depths, and a suite of gamma ray, resistivity and density logs. Total porosity estimated from the density log was corrected for shaliness effect using empirical relationship between shale volume and Gamma Ray shale index for Tertiary sandstones. The corrected porosity was utilized as effective porosity (Փe). Hydrocarbon saturation (Sh) was estimated using the Nigerian saturation equation. Matrix permeability (K) was estimated using the Timur equation. Reservoir pore volume-to-grain volume ratio (Փz), RQI and FZI were obtained from K and Փe. The well is a multi-reservoir well with Sh values ranging from 44 to 61%. Matrix permeability (K) varied between 187 and 583 md, in conformity with values commonly reported in literature for Niger Delta reservoirs. Matrix permeability (K) appeared to be influenced by degree of sorting, rather than by shale distribution within the sandstone matrix. D6000 sand has the best petrophysical attributes and highest Փe, Փz, RQI, FZI and reservoir deliverability index values. The results constitute a framework for recommending multiple completions option in D6000, D3000 and 5000 sands.