Detailed Lithology and Fluid Mapping of the Asterix Gas Discovery Using Bayesian Inversion Methodology

Conventional AVO methods are in general effective for mapping the gas filled part of good reservoirs, while prediction of several other lithology and fluid classes (LFCs) may be much more challenging. For the Asterix gas discovery (Norwegian Sea), mapping of both the gas and the water-wet part of the reservoir (the aquifer) is important in the production planning. Standard AVO attribute cubes highlighting hydrocarbon presence are usually based on a kind of weighted stacking of the prestack data and represent deviation from a background trend for the water-bearing sediments. We define a set of multiple LFCs with elastic properties based on well logs and geological knowledge, and then predict the probability for each LFC simultaneously using a Bayesian inversion methodology. The approach is ideal for settings with limited well control, since the inversion does not require an accurate level of the elastic background model. Instead, it utilizes the relative elastic differences between the different LFCs. The results are presented as probability cubes for the different LFPs which simplify inter-disciplinary discussions and are easy to interpret for non-geophysicists. For the Asterix discovery, the results have impacted both the volume estimates and the production strategy.