ABSTRACT
Amplitude versus offset (AVO) analysis was carried out on MAR-1A of Montero field, in the Central Swamp Depobelt of Niger Delta ; using well log data. The well logs include; caliper, gamma ray, resistivity, bulk density, and Sonic compressional wave (Vp) respectively. Sonic shear wave (Vs) was derived using Castagna’s equation, while compressional and shear impedances were also derived as a product of the respective velocities and density. Four reservoir sands were identified from gamma ray and resistivity logs for analysis. These are; Sand A1 (195ft thick), A2 (70ft thick), B1 (50ft thick), and B2 (60ft thick). The AVO attribute analysis is based on the Zoeppritz equation. Two different cross plots from the acoustic and elastic properties of VpVs ratio against P-impedance, and Mu-rho against Lambda-rho; shows a clear lithological discrimination, between shale and sand, which confirms the already identified reservoirs from the well logs. Fluid discrimination was also made possible by the cross plots of VpVs ratio against P-impedance and Mu-rho against Lambda-rho, also distinguishes and re-establishes the various types of fluids present in the reservoir sands. In Sand A1, A2 and B1; the reservoir fluids present are identified as gas, oil and water, while Sand B2 only shows gas and water as the reservoir fluids. The AVO attributes of gradient and intercept were derived from generated synthetic gather with incident angles between 00-450. Amplitude reduces with angle of incident, and generally becomes unreliable between 350-450. The AVO cross plot of amplitude versus incident angle, and AVO gradient versus intercept plots, were used to classify the sands with respect to their hydrocarbon bearing potential. The sand tops were interpreted to belong to four different classes of sands, which are classes one to four.
