General elastic inversion, Orange Basin RSA case study

Timothy B. Berge, Forest Oil Corp, 1600 Broadway, Suite 2200, Denver, CO 80202, phone: 303 812-1593, fax: 303 812-1632, tbberge@forestoil.com and George C. Smith, University of Cape Town, Cape Town, South Africa.

General Elastic Inversion is a specific inversion process that is designed to combine both P and S impedance volumes in a straightforward and meaningful way to emphasize rock pore fluid type. Like the Fluid Factor (Smith and Gidlow, 1987) and Rp-Rs (Castagna and Smith, 1994) it combines both volumes by summation of Vp and Vs. The calibrated results are simple and values non-arbitrary.

In essence the process has 2 steps, proper execution of both is required to get good results. The first step is generation of the pseudo-shear volume in a way that produces real pseudo-shear values, the second step is the combination and re-projection of the P and S volumes.

In the pre-stack domain the objective is to obtain converted s-wave shear amplitudes at different offset angles. We employ a rigorous new ‘full Zoeppritz’ solution.

The inverted P and S volumes are combined and re-projected to produce a ‘fluid’ volume and a ‘porosity’ volume. Examples are from the 3D surveys over the giant Ibhubesi gas fields offshore the Republic of South Africa in the Orange River Basin.

Geologic Interpretation of Seismic Attributes and Amplitude Anomalies
2004 AAPG International Conference and Exhibition Technical Program