Hydrocarbon reservoir definition and delineation using log analysis, petrophysics and rock physics in Y-Field, offshore Nigerian Niger Delta

Purpose This study aims to characterize the reservoirs through well log analysis, intensive elastic log studies and cross-plotting (rock physics studies) of the Y-Field reservoirs to define the reservoirs hosting hydrocarbons in the basin with structural complexity. Poor evaluation of sand body has been part of the problem leading to wet wells and these above methods accounted for this gap. Design/methodology/approach The basic parameters needed for the characterization are P-wave (Vp), S-wave (Vs) and density (ρ) and understanding the relationship between these parameters with the pore-fluid and lithology is of utmost importance in the evaluation of sand body. This was achieved through elastic rock physics studies. Important among these elastic logs include P-impedance (Zp), S-impedance (Zs), Vp/Vs ratio, Poisson’s ratio, lambda-rho (λρ) and mu-rho (µρ), and are critical. Rock physics diagnosis involving elastic log generation and analysis accompanied by cross-plotting of the elastic logs color-coded with petrophysical properties were employed. Findings The findings from the elastic logs indicated that hydrocarbon-charged units are characterized by low Zp, Zs, Vp/Vs ratio, Poisson’s ratio, λρ and µρ resulting from a significant drop in P-wave and density because of hydrocarbon presence. The results also showed that some of the reservoirs with hydrocarbon accumulations exhibit high Vp/Vs and Poisson’s ratio as against their usual low values in hydrocarbon intervals which results from the presence of shale with high plasticity. The elastic logs cross-plots of Vp/Vs with Zp, Poisson’s ratio with Zp and λρ with µρ successfully distinguished between the hydrocarbon-charged zone and the brine-saturated units. All these outcomes are the benefits of this research. Originality/value It can be shown that the analysis/results have distinguished hydrocarbon-charged units from brine-saturated intervals and the originality lies in the integration of various sophisticated fluid identification templates.