Lithology and Pore-fluid Discrimination Using Rock Physics Analysis in the “Kola” Field, Niger Delta

Abstract

Background: The ultimate goal of any Rock Physics analysis is to gain insights into the physical properties of a reservoir. These consist of properties such as lithology, porosity, and permeability, or dynamic properties like fluid content or pressure. A geophysical Rock Physics analysis will make use of the measured elastic properties from seismic data to generate attributes that yield relevant information about the reservoir rocks/sand. Aim: This study aimed to predict reservoir behaviour with respect to rock physics, elastic properties and presence of fluid within the study area “KOLA” Field, Niger Delta. Methodology: The study is an offshore field located within latitude 05051’55N to 05052’03N and longitude 05041’27E to 05042’05E in the Niger Delta. The Niger Delta basin is one of the seven sedimentary basins in Nigeria where active petroleum exploration activities are carried out.  The study took place between August 2023 and August 2024. Seismic attributes, e.g. Vp/Vs ratio, Lambda-Rho, Mu-Rho, and P-Impedance, were estimated. Rock physics cross-plots of the attributes were used to discriminate fluid and lithology in the reservoir sands of the ‘KOLA’ Field Offshore Niger Delta. The seismic attributes were colour-coded with various reservoir properties from well logs such as gamma ray and resistivity, in order to differentiate the lithology and zones charged with hydrocarbon from zones charged with brine. Results: The results of the analysis showed that hydrocarbon sands have low P-Impedance, VP/VS, Lambda-Rho and Mu-Rho values. P–Impedance and VP/VS are sensitive to both fluid and lithology, whereas Lambda-Rho is only sensitive to fluid, and Mu-Rho (rigidity) is only sensitive to rock matrix. The results showed that both mu-rho and density are lithology discriminators, with density also being a fluid discriminator. Cross-plot of Vp/Vs and Acoustic impedance was used to differentiate the reservoirs into hydrocarbon-charged zones and water-charged zones. Cross-plot of Lambda-rho (incompressibility) and velocity ratio differentiate the reservoir lithology into sands and shale units. Lambda-rho (λρ) and mu-rho (μρ) cross-plots classified the reservoir unit into three zones, namely water saturated sand, oil saturated zone and gas saturated zone, confirmed by density and neutron cross-plot. Conclusion: Having realized that the porosity in the reservoir unit is moderately good and the water saturation is low. The models explained the general behavior of the rock in terms of poor, moderate and good qualities. This is helpful to note that the reservoir is good and economical.