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Seismic Property Relationships for Characterization of a Giant Carbonate Reservoir, Grosmont, Canada


Shell's leasehold within the giant Grosmont platform in Canada holds ~90 Bboe. Core observations confirm a complex triple porosity system ranging from 5% to 37% porosity with oil filling the fracture porosity, the vuggy porosity, and the matrix porosity. Hydrocarbon volume and producibility are influenced by diagenesis effects that are difficult to capture with conventional subsurface data analysis outside of well control. Quantitative seismic interpretation is one approach which can help to fill this gap. Successful interpretation requires integration of core and log data to put the elastic properties in the context of reservoir performance drivers. We identify field specific controls on the elastic rock properties to range from fractures, to porosity and pore types, and to fluid types. We illustrate how elastic rock property trends from sonic logs are derived by calibration with relevant core and thin-section observations. Then we use a carbonate rock property model to predict the effects of different pore fluid type and matrix properties (composition, porosity and pore type). Initial observations show a wide range of sonic p-wave velocities in the Grosmont platform, from 2800 m/s to 5600 m/s, indicating various degrees of brittleness. The fastest velocities at a given porosity are observed within Nisku, Upper Grosmont (A, B, C), Middle Upper Ireton (UIRE) B and UIRE A5 units. The widest range of p-wave velocities at a given porosity are observed within the Nisku and Upper Grosmont A & C stratigraphic units (>1000 m/s). UIRE zones show smaller variations of velocity (<600 m/s). Highest water saturations are observed in lower porosity intervals and show an overall lower velocity-porosity trend compared to the oil saturated intervals. Two stratigraphic units show distinct groupings in the velocity-porosity space: (a) the Nisku stratigraphic unit is dominated by vuggy and matrix porosity and shows generally high p-wave velocities; (b) the UIRE stratigraphic unit is dominated by microcrystalline matrix-porosity and low p-wave velocity. Using field specific, calibrated rock physics model parameters as input for our XStream Probabilistic Seismic Inversion (PSI), we improved the porosity predictions by successfully executing a “3D Close-the-Loop” workflow. Successful interpretation of the inversion requires the understanding of complex effects of pore fluid type, fracture intensity and matrix properties (composition, porosity and pore type) on elastic properties.