THE EFFECT OF OIL UNDER INSITU CONDITIONS ON THE SEISMIC PROPERTIES OF ROCKS

Direct hydrocarbon indicators (DHIs) on seismic sections are commonly thought to be diagnostic only of gas. However, oil sands can also generate DHIs such as bright spots and flat events since oils under in-situ conditions can contain large amounts of solution gas. This dissolved gas substantially decreases the velocity of sound and the density of the oils as compared to measurements of these properties at surface conditions. Hydrocarbon indicators caused by oil sands are investigated by first measuring the elastic properties of an oil as a function of gas-oil ratio, next, calculating the elastic properties of additional oil compositions under in-situ conditions using standard pressure-volume-temperature (PVT) measurements, and then calculating the compressional velocity in oil-saturated rocks for several typical oils using Gassmann's equation. The potential for seismic anomalies caused by oil-saturated rocks is higher than thought because the properties of oil under reservoir conditions can differ significantly from those of surface oils. Specifically: 1) The properties of oil depend on its composition: the higher the API gravity and the gas-to-oil ratio (GOR), the lower the density and velocity of sound (adiabatic bulk modulus) and the lower the velocity of a rock saturated with the oil. 2) Calculations of oil-sand velocities using the insitu properties of oils show that areas having light oils and/or poorly consolidated rocks are the most likely areas in which to encounter oil DHIs. Since overpressured areas can have both poorly consolidated rocks and high GOR oils, they are especially prone to large oil responses.