Four main groups of parameters influence the seismic response in carbonate rocks: (1) petrophysical properties, such as porosity, pore compressibility, fluid saturation, and fluid type; (2) lithologic parameters, such as mineralogy and shaliness; (3) measured conditions, such as effective stress, temperature, and frequency; and (4) geometric factors such as bed thickness and anisotropy. Knowledge of porosity variations over any carbonate reservoir has significant engineering and economic implications for production development activities. Because porosity is the most important factor determining the seismic response in carbonate rocks, quantification of its variation over a reservoir becomes a critically important task. Thus, before any velocity change is used for quantifying reservoir porosity, relative influence of other factors must be identified and accounted for.
An integrated approach to achieve this objective starts by building a rock physics model using core data, validating it using log data, and then applying it to the seismic interpretation. A laboratory study of selected cores from several wells over a field should be carried out to determine P- and S-wave velocities, porosity, permeability, and petrophysical parameters. This information is then used to build a rock physics model that accounts for the influence of porosity, pore shape, mineralogy, saturation, and measured frequency on seismic responses. The next step is to calibrate the rock physics model to log data which involves interpretation of logs. Finally, this rock physics model, established for core data and validated by logs, is applied to the seismic interpretation.
A case study of a Tertiary carbonate reservoir confirmed the quantitative interpretation of seismic impedance traveltimes in terms of total reservoir thickness. It also confirmed that the validity of the method strongly relies on three factors: (1) an accurate relationship between seismic attributes and rock properties; (2) coherency among data acquired by cores, logs, and the seismic model; and (3) high-quality seismic data, reliable seismic interpretation, and adapted seismic processing and inversion techniques.