Rock Physics as a Constraint on Seismic Inversion: a Carbonate case study : Seismic reservoir characterisation of the Miocene carbonates of SE Asia faces a number of challenges: the heterogeneous nature of the carbonates at many scales, the relatively low elastic property sensitivity of carbonate rocks to fluids and the limitations of the seismic data in terms of bandwidth, angle range and signal to noise. A recent well drilled into a carbonate reef in the Nam Con Son Basin offshore Vietnam provides a useful case study to assess these challenges.
Method
A seismic reservoir characterisation workflow has been followed. The well data are processed through a seismic petrophysics analysis. The petrophysics is calibrated to core and indicates complete dolomitisation of the reservoir interval; little terrigenous input; a very broad range of porosities (approximately 10-50pu); and very high gas saturations (>95%). The elastic properties can be adequately modelled with a simple inclusion based rock physics model using bimodal aspect ratios for the carbonate porosities. Examination of the elastic properties indicates that the seismic amplitudes will be dominated by the porosity variations of the carbonate rocks. In general, fluid effects will be subtle and require an accurate estimation of Vp/Vs for prediction from seismic data. The limited bandwidth and limited angle range of the actual seismic data suggest that sufficient accuracy of the Vp/Vs estimates from seismic for fluid discrimination is unlikely without additional constraints. Three methods of seismic inversion have been applied: elastic impedance of the near- and far-stack to relative elastic impedance; simultaneous inversion of partial angle stack data using a low frequency model based on a first pass interpretation of the top carbonate and GWC; a facies based inversion (Kemper and Gunning, 2014) solving simultaneously for elastic properties and the distribution of lithologies and fluids, where the same first pass interpretation is used to set prior probabilities.
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