On the effectiveness of using quantitative avo analysis in fluid and lithology discrimination in an offshore Niger Delta Field, Nigeria

Abstract

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 cut-off, 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_w), decreasing ratio of compressional wave velocity to shear wave velocity (V_p/V_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, S_w 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.

Keywords: Amplitude Variation with Offset (AVO), Seismic and Well data, Rock Physics, Reservoir Sands