Rock Physics Guided Inversion of Anisotropic 3D Seismic Reflection Data for Characterization of an Onshore Gas Field of the Nigerian Delta

Abstract

The combined use of rock physics modelling, AVO modelling and seismic inversion successfully helped to address exploration challenges in the gas field studied. While each technique provided unique insight into the prospect and potential drilling risks, integrated interpretation of all the data was necessary to demonstrate that the seismic reflections are indicative of high-impedance, gas-bearing sand reservoirs. Collectively, the information obtained using these techniques confirmed that the sands are of good quality. Thus, integration of the results reduced uncertainties associated with the petroleum system and increased the likelihood of successful exploration. The main advantage of the study arose from the simultaneous use of rock physics modelling, AVO analysis and prestack seismic inversion. Rock physics modelling was used to evaluate the effect of fluids on seismic responses; seismic inversion was used to validate reservoir distribution; and AVO analysis was used to locate hydrocarbon occurrences at the wells. The drilling outcomes were consistent with the results predicted by the inversion process, as the high-impedance zones corresponded to the presence of hydrocarbons within the sand reservoir. Similarly, the AVO analysis identified positive anomalies consistent with the high-acoustic-impedance gas-bearing sands. For the target seismic anomaly with a positive AVO response, the reservoir depths predicted using seismic inversion were 1,579 m and 1,682 m, whereas drilling indicated the presence of high-impedance gas-bearing sand at 1,578 m and 1,681 m, respectively, indicating good agreement between the predictions and drilling.