Rock physics analysis and Gassmann’s fluid substitution for reservoir characterization of “G” field, Niger Delta

This study investigates the relationship between rock physics and petrophysical parameters in reservoir characterization. The data used for the analysis consist of suites of three wells, intercalated with sand and shale. Two reservoir sands were mapped, and petrophysical analysis was done to evaluate the hydrocarbon potential of “G” field. Elastic parameters such as compressional velocity (V_p), shear wave (V_s), acoustic impedance (AI), LambdaRho (λρ), MuRho (μρ), and bulk modulus (k) were generated and cross-plotted against petrophysical parameters to infer the Facies types. The sand lithology gave a low gamma ray, high resistivity, and low acoustic impedance in contrast to the shale. For the three wells, two reservoirs were observed each, at average depth ranging between 1546 and 1605 m and between 2010 and 2168 m. Cross plots such as V_p/V_s versus AI using gamma ray as indicator, λρ versus μρ using density as indicator, and volume of shale (V_sh) versus water saturation (S_w) using porosity as indicator were carried out for each reservoir to determine and discriminate their porosity and fluid content respectively. Well 2 was observed to be the most productive due to its high hydrocarbon saturation and reservoir characteristics. Gassmann’s fluid substitution was also carried out to understand the behavior of the dry rock; the pores were filled and superimposed on a rock physics template to extrapolate the general reservoir behavior of the field. It was observed that V_p and Rho decrease with increase in V_s for 100% gas saturation, while V_p and Rho increase as V_s remains constant with 100% water saturation.