Heavy oil is defined as viscous fluid having an API gravity of less than 22°. Seismic velocities are sensitive to heavy oil properties and especially to reservoir properties, such as temperature and pressure. Laboratory studies have shown that heavy oils with much lower API values can act as a semi-solid, and therefore support shear wave propagation. In such cases, using a conventional Gassmann fluid substitution model (Gassmann, 1951) to understand the fluid scenarios is not applicable for seismic reservoir characterization.
From the different published approaches for modeling fluid behavior in heavy oil reservoirs, the extended Gassmann (Ciz and Shapiro, 2007) is normally used as a lower bound (for low frequency and moderate fluid shear modulus change) for fluid substitution sensitivity and the Coherent Potential Approximation (Berryman, 1999) is used as an approximate upper bound (for high frequency). The true fluid substitution sensitivity for heavy oil is somewhere between these two bounds. A recently developed non-Gassmann rock physics workflow, in principle, provides a consistent approach to estimate (from calibration data, e.g., log data, core measurements) and model the data within these two elastic bounds.
A pilot study was undertaken to understand heavy oil's elastic properties in a Mesozoic carbonate reservoir located in Saudi Arabia. Due to the lack of core-fluid measurements, a range of heavy oil fluid parameters, such as gas-oil ratio (GOR), temperature, pressure and API range were tested to understand their effects on the elastic properties.