AMPLITUDE-VERSUS-OFFSET ANALYSIS USING VERTICAL SEISMIC PROFILING AND WELL-LOG DATA

Craig A.Coulombe

ABSTRACT

A multioffset P(vertical seismic profiling) geometry is used in this thesis to acquire data for AVO (amplitude-versus-offset) analysis of a reservoir zone. The VSP geometry consists of ten surface source positions and eleven downhole receiver positions. The downhole receivers are placed immediately above the reservoir zone. A zero-offset and and offset VSP were also acquired and analysed.

A three-component processing flow is developed, and tested here using synthetic data, to extract the true seismic amplitudes from VSP data. AVO analysis of the multioffset VSP data suggests a lower P- wave velocity in the reservoir zone based on forward modeling. Well-log interpretation indicates that the reservoir zone is a gas-bearing dolomite with porosities as high as 18%.

Generalized-linear-inversion methods are developed to invert the zero-offset VSP corridor stack for the acoustic impedance, and to invert the multioffset VSP P-P and P-SV reflectivity traces jointly for the elastic parameters Vp , Vs , and ρ . The zero offset inversion resulted in an inverted impedance curve that matched the higher frequency trends but failed to match the low frequency trends of the initial-guess impedance curve. The joint P-P/P-SV inversion is found to update the elastic parameters in a realistic manner; however there is some residual error. The resulting P- wave velocity in the reservoir is higher than the initial guess which is the opposite of the forward modeling results.

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