ABSTRACT

 This study investigates the applicability of Amplitude versus Offset (AVO) techniques including modelling and multiple inversion in a Niger Delta oil field. Well bores in the area reveal unadorned lithologic sequence of sand and shale. Seismic data for this study came from 780 mImes and 496 cross-lines selected from available 3-D seismic data over the area. The geologic data used for this study was provided by the wireline logs from 3 wells provided by Shell Petroleum Development Company (SPDC), Nigeria. For a meaningful study, the applicability of Castagna velocity relationship was investigated and it became obvious that they could not be applied in this field hence locally calibrated trend was necessary. Using fluid substitution equations, AVO curves for different fluids were examined. Shear wave velocity information from Well A was used in the modelling studies. Finally, field data were inverted and the results were compared with the model results for the best match. The correlation of the inversion results with those of rock physics analysis observed at the wells provided confidence in our interpretation of hydrocarbon potential away from the wells. Crossplot analysis revealed distinct clusters for the gas sand. The gas sand depth is anticipated between 8870ft (2703.58m) and 8950ft (2727.96m) with an effective porosity range between 26 and 39%, which suggest moderate to good porosity. Litho analyses show that the best AVO indicator in the study area is the LI = Zp — 1.324 * Z and far elastic inversion. From the enhanced AVO method, various combinations of rock properties have been proposed as hydrocarbon indicators such as F! = Zp — 1.296 * Z, 1tp and 2/j which showed greater sensitivity to pore fluids as well as lithology variations. Modelling shows that zero offset reflectivity at the shale-gas sand interface is a marginally negative intercept and negative gradient computed from the model derived from log data. Synthetic gathers generated from the model show good correlation with real seismic gathers. On the other hand, the zero offset reflectivity at the shale-brine interface show no AVO effects. Generally, for the reservoir studied, hydrocarbon saturated sands show a class 3 AVO anomaly. AVO analysis and inversion techniques has shown they are strong and suitable tools in exploitation of gas zones using AVO attributes, especially the intercept and gradient, far angle impedance and fluid factor attribute that optimized the area of gas zones approved by well logging process. This is the central objective of this study. This research has shown that the combination of methods including Poisson Impedance, Range-limited stack, elastic impedance, intercept/gradient analysis, impedance extraction inversion and lambda-mu attribute analysis methods can be used for processing attributes. Using similar capture technique as applied to mapped gas sand, a possible prospect away from the well location was identified as an exploration interest.