A computer implemented method for modeling fluid effects in a subsurface reservoir, in order to estimate the economic potential of exploration prospects and to plan well design and placement, the method comprising: a. receiving, at a computer processor, a plurality of well logs from a subsurface reservoir and conducting a petrophysical evaluation to determine in-situ porosity, fluid saturation, and lithology; b. deriving dry frame properties including bulk and shear modulus and Poisson's ratio from the well logs and evaluation results via a Gassmann equation based method; c. aggregating the dry frame properties and plotting the dry frame properties versus porosity and by lithology and dividing the plots into bins covering limited porosity and lithology ranges; d. calculating a median porosity-dry frame Poisson's ratio pair for each bin and joining the medians with splined fits to derive a mathematical model to interpolate between medians for any arbitrary porosity-lithology pair to determine interpolated dry frame Poisson's ratios, the interpolation being between adjacent splined fits; e. using the combination of a shear modulus from density and shear sonic logs from the plurality of well logs and the interpolated dry frame Poisson's ratio to estimate a modeled dry frame bulk modulus; f. using the modeled dry frame bulk modulus to estimate the difference via the Gassmann equation-based method between dry frame compressional and shear velocity and density at a plurality of modeled fluid saturations; and g. adjusting recorded compressional and shear velocity and density logs from the plurality of well logs according to the difference between properties at in-situ fluid saturation and the plurality of modeled fluid saturations, and transforming velocity and density to other modeled elastic properties. a. receiving, at a computer processor, a plurality of well logs from a subsurface reservoir and conducting a petrophysical evaluation to determine in-situ porosity, fluid saturation, and lithology; b. deriving dry frame properties including bulk and shear modulus and Poisson's ratio from the well logs and evaluation results via a Gassmann equation based method; c. aggregating the dry frame properties and plotting the dry frame properties versus porosity and by lithology and dividing the plots into bins covering limited porosity and lithology ranges; d. calculating a median porosity-dry frame Poisson's ratio pair for each bin and joining the medians with splined fits to derive a mathematical model to interpolate between medians for any arbitrary porosity-lithology pair to determine interpolated dry frame Poisson's ratios, the interpolation being between adjacent splined fits; e. using the combination of a shear modulus from density and shear sonic logs from the plurality of well logs and the interpolated dry frame Poisson's ratio to estimate a modeled dry frame bulk modulus; f. using the modeled dry frame bulk modulus to estimate the difference via the Gassmann equation-based method between dry frame compressional and shear velocity and density at a plurality of modeled fluid saturations; and g. adjusting recorded compressional and shear velocity and density logs from the plurality of well logs according to the difference between properties at in-situ fluid saturation and the plurality of modeled fluid saturations, and transforming velocity and density to other modeled elastic properties. <EOS>