Hydrocarbon fluids identification using rock properties crossplots

Abstract

Rock properties are those physical properties of a rock which affect the way in which seismic waves travel through the rock. They include p-wave velocity Vp, s-wave velocity Vs, density ρ, porosity Ф, and their attributes (simple combination of the rock properties) which include p-impedance Ip, s-impedance Is, Poisson impedance PI, Vp/Vs ratio, Poisson’s ratio σ, incompressibility modulus λ, and shear modulus μ. In oil and gas exploration, the petrophysical properties and attributes can, due to their robustness on the crossplot domain, help in no small measure in reservoir characterization. In particular, this includes the identification and discrimination of fluid types in reservoirs (reservoir characterization) and separation of hydrocarbon sands from non-hydrocarbon background trend. In this study, we have carried out crossplots of Vp against Vs, Ip against Is, Vp/Vs against σ, and Vp/Vs against depth to help us identify the hydrocarbon fluids in reservoirs, using well data obtained from a field in the onshore central Niger Delta. The results of the crossplotting show that Vp/Vs ratio for sand formations containing hydrocarbon fluids ranges from 1.50 to 1.85 and Poisson’s ratio ranges from 0.17 to 0.27. The Vp for reservoir sands is from 2261m/s to 3200m/s, that is, the hydrocarbon sands indicate low velocities. Ip for the hydrocarbon sands is low and ranges from 4300 (g/cm³ x m/s) to 8200 (g/cm³ x m/s) and Is from 2500 (g/cm³ x m/s) to 4200 (g/cm³ x m/s). The values of Vp, Ip,Is, Vp/Vs and σ (Poisson’s ratio) obtained from this analysis for the hydrocarbon– saturated sands are consistent with the values published in the literature. The sand reservoirs expectedly showed lower values of Vp/Vs and Poisson’s ratio, while velocity Vp and impedance Ip were also observed to be low from our analysis. In a sand reservoir characterized by water, oil and gas contacts, the water–wet portion exhibits higher Vp/Vs and Poisson’s ratio than the oil and gas portions. The gas portions or high GOR oil shows the lowest Vp, Ip, Vp/Vs and σ. The oil sand containing high gas content behaves seismically and shows bright spots at the interface separating the gas layer and an encasing shale (Class III AVO), while low GOR oil is highly incompressible and is biased towards relatively higher Vp, Ip, Vp/Vs and σ than the gas portion. The general result of our crossplotting here shows Vp,Vs, Ip, Is, Vp/Vs and Poisson’s ratio to be sensitive to hydrocarbon fluids within a reservoir. The quantitative values of rock parameters obtained from crossplot analyses can thus be used as a powerful tool to discriminate hydrocarbon fluids in wells and on inverted seismic sections.