Facies-based Reservoir Characterisation through the Asset lifecycle

Seismic lacks low frequencies, so for absolute seismic inversion a so-called Low Frequency Models (LFM) is required. Starting from an empty LFM, we would like to post Sand values where there is Sand, Shale values where there is Shale, etc. Typically we don’t know precisely where the various facies are located in the subsurface; after all, understanding the facies distribution is one of the main aims of seismic inversion. So populating the LFM as outlined in the previous paragraph is not normally possible. The LFM’s constructed to date are therefore compromised (e.g. well log interpolation leads to averaging, resulting in impedance values unrepresentative of the facies present). Importantly, during the inversion the seismic cannot ‘fix’ a compromised LFM as – we come full circle here – seismic lacks low frequencies! We introduce a facies-based approach that overcomes this issue. For each facies expected (e.g. Shale, Water-Sand, Oil-Sand), a LFM is constructed, and all are input to the inversion (i.e. the low frequency information is over-specified). The inversion can then decide which LFM is used where, based on the facies estimate, which is one of the quantities inverted for. So the LFM ultimately used in the inversion is an output, not an input.