Seismic inversion: comparison of acoustic and elastic impedance inversion models for rock property prediction

Niger Delta is one of the major hydrocarbon producing basin in the world. This basin has a quite complex geology which makes routine seismic interpretation a challenging task for understanding the reservoir properties such as lithology and fluid content. Seismic inversion has proven to be a reliable tool for detailed understanding of the reservoir especially for lithological identification.In this study, effort was made to compare acoustic and elastic impedance volumes with regards to litho-fluid discrimination in an offshore field in the Niger Delta. For this purpose, five horizons were interpreted to determine geological inputs for the impedance model building. Well log data was tied to a near post stack seismic volume and this was used in creating an initial acoustic impedance model. Thereafter, an initial elastic impedance model was created using well log data tied to a far post stack seismic volume. The initial elastic impedance model was created with an elastic impedance log generated at 360 which conforms to the incident angle for the far offset stack. Following analyses of the initial models at the well location, a full model-based acoustic and elastic impedance inversion was carried out separately for the entire area, using the interpreted horizons as controls.The inverted results reveal reservoir tops and show lateral variations in lithology away from the well location. In particular, the elastic impedance inversion gave superior results only in areas where the acoustic impedance log used in inverting the near seismic volume is near constant through top reservoir transition. In areas where the acoustic impedance log could clearly distinguish the reservoir top, the acoustic and elastic impedance volumes gave comparable results. In comparison to the individual input seismic volumes, the inverted results would greatly improve reservoir property interpretation with possible integration with seismic stratigraphy.