ABSTRACT:The study was carried out to investigate the dissimilar seismic amplitude responses observed in sandstone reservoirs with the same fluid saturation. This challenge now informed the analysis of different amplitude responses from the 'Jay' Field in order to verify the reservoirs fluids around and away from well location based on the integration of Amplitude Variation with Angle (AVA) and seismic inversions. The well log data provided were used to identify hydrocarbon bearing zones and Poisson Ratio analysis. Anomalies from the AVA analysis were investigated using the elastic impedance inversion of the near and far volumes. Crossplots of Lambda-Mu-Rho inversion were produced to relate reservoir incompressibility and rigidity modulus for lithology and fluid determination. Sand E exhibited amplitude increase with angles at well point and also away from well location (red triangle). The approximate elastic impedances of the inverted seismics are almost similar to the log estimates, an indication of good correlation. Integration of incompressibility (Lambda-Rho) and rigidity (Mu-Rho) modulus analyses reveals the presence of gas saturation in reservoir Sand G because low Lambda-Rho coincides with high Mu-Rho from both log estimates and inverted seismics. In the case of Sand D, high Lambda-Rho coincides with high Mu-Rho because it is oil saturated. This study has helped to differentiate the fluids in Sands D and G despite having similar AVA responses. The methods adopted in this work can be useful in hydrocarbon detection from seismic data in fields with similar geological setting. Seismic inversion technique helps in the determination of reservoir properties away from well location (Simm, 2005). For instance, inverted seismic data enhances the identification of subtle properties in conventional seismics (Contreras et al., 2006). Reservoir properties between wells can be determined from inverted seismic results. This was demonstrated by Adekanle and Enikanselu, (2013) where the vertical and lateral extent of rock properties such are porosity, lithology and density were determined. However, seismic data have limited frequency content such that thin layers may not be resolved (Simm and Bacon, 2014).The integration of high frequency log information with seismic data in model based inversion was adopted to mitigate this effect. Moreover, model based inversion technique of iterative forward modeling and comparison procedure does not suffer from defective wavelet and over simplification of the subsurface associated with recursive and sparse spike inversions methods respectively (Cooke and Schneider, 1983;Veeken and Da Silva, 2004;Simm and Bacon. 2014).The integrated approach of Lambda-Mu-Rho inversion and Rock Physics modeling by Ekwe et al., (2012) was used to delineate hydrocarbon charged reservoirs in a Niger Delta field. In gas hydrates region where well information is most often sparse, Amplitude Variation with Angle (AVA) was the method used for free hydrate gas assessment as discussed by Javaherian et al., (201...
Quantitative Amplitude Variation with Offset (AVO) analysis of "Jay" Field, offshore Niger Delta, was carried out with a view to properly discriminating fluid and lithology using near, mid and far offset seismic and well data. Seismic and well data were interpreted and analyzed. Synthetic seismogram was generated using density (r) and sonic logs. AVO modeling, seismic AVO attribute analysis and AVO inversion were carried out and the results from well log interpretation using 70-API gamma-ray cutoff , neutron-density over lay and resistivity logs revealed that the field consists of intercalation of sand and shale with typical deltaic depositional environment log signatures. Four identified sand reservoirs (a, b, c and d) with high resistivity values and negative separation in the neutron-density overlay suggested that the field was hydrocarbon bearing probably containing gas or condensate. Two sand reservoirs showed good rock physics results, 'Sand a' at 11,632 ft TVD with 18% porosity (ϕ), 0.25 water saturation (S), decreasing ratio of compressional wave velocity to shear wave velocity (V /V) w p S and Poisson's ratio () relative to the background shale signified AVO response typical of a hydrocarbon bearing sand. 'Sand e' at 5,925 ft TVD, with 30% ϕ, Sw of 1, no change in V /V and relative to the background shale p S implied that an AVO response was unlikely. Gradient analysis result for the synthetic seismic at the top and base of the two sands agreed with Rutherford's classification scheme for class IV AVO for 'Sand a' and no AVO response for 'Sand e'. AVO attribute analysis and impedance inversion of the seismic volumes confirmed AVO result for the two sands. The study established that AVO technique could be effectively used for fluid and lithology discrimination in the "Jay" Field, Niger Delta.
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