Defining the volume of hydrocarbons in a reservoir is a key aspect of resource estimation. Subsequently, determining the likelihood of this volume is critical for reserve evaluation. We outline a geophysical framework using multicomponent (3C) 3D seismic data, well logs, and geostatistics to assist in this assessment. The use of converted-waves (P-to-S) is a significant and integral part of the process, especially in estimating the reservoir sand thickness and its porosity. This procedure is applied to data from the Blackfoot oil field, Alberta. The predicted original oil in place and its likelihood (90% probability of 4.5 MMbbl computed from 1996 geophysical data) compares reasonably well with that inferred from actual cumulative production up to 2011 (5.5 MMbbl)