Gullfaks is a structurally com-plex field with sandstone reservoir units of Cretaceous to Triassic age. The reservoir quality is generally very good, with porosities in the range of 30–35% and per-meability up to several darcies in the most important Brent Group reservoir. The baseline streamer survey was acquired in 1985, one year before the start of production. A simultaneous seismic AVO in-version of the baseline streamer survey was performed in 2006 to obtain acoustic impedance (AI) and Poisson's ratio (PR). AI and PR contain valuable information about rock and fluid properties, and one goal of this project was to create empirical relationships based on well-log data between AI/PR and porosity (ϕ), differential pressure (Pd), lithology (Vsh), and fluid bulk modulus (Kf). Kf is directly linked to brine saturation. The next step was to estimate rock and fluid properties as functions of AI and PR from the seismic inversion. This approach depends on good agreement between modeled and well-log data and also agreement between well-log data and the inverted seismic. This paper first describes the methodology, and then shows some preliminary results.