Interpreting direct hydrocarbon indicators of low-API biodegraded oils — A case study from a deepwater South Atlantic Basin

Abstract

Strong seismic flat spots are typically associated with the oil/water contact (OWC) in many amplitude-supported deepwater South Atlantic Basin (SAB) discoveries, particularly in relatively shallow (∼2000 mbml), high-porosity unconsolidated Eocene-age reservoirs. These reservoirs contain biodegraded oils that have low API gravity, low gas-oil ratios, high viscosities, and fluid acoustic velocities between 4500 and 5000 ft/s. Often, a mismatch in amplitude is noted when comparing an observed seismic-amplitude response with a synthetic forward model generated using sonic logging data. These observations can be attributed to the high viscosity (low API gravity) of biodegraded oil that can lead to wave-induced heterogeneous pore pressure at logging frequencies, thus deviating from a key assumption in Gassmann's fluid-substitution theory. These dispersion effects can lead to significantly slower acoustic velocities of these oil-bearing reservoirs at seismic frequencies than those measured at logging frequencies. Using a generalized fluid/solid-substitution theory, we model this change in acoustic velocity and find that the theory can explain the observed bright-spot response.