Oil Saturation, Reflection Amplitude, and Frequency – Direct or Reverse Proportionality?

Oil saturation is assumed to cause higher intrinsic attenuation of seismic energy as compared to water saturation, especially at high frequencies. This notion is sometimes used to distinguish between oil and water saturated reservoir rocks. The widely known tool for attenuation estimation is spectral ratio technique aimed at measuring losses of high frequency content of the seismic wavelet propagated through an attenuating medium. The energy losses associated with reflections from interfaces between layers with different absorption were considered mainly theoretically with limited experimental confirmation. We show that time windows implemented in these two techniques shall be localized differently. Also, we present a new set of frequency-dependent attributes to be used in predicting storage capacity, permeability, and expected oil production rate from seismic reflections from within the reservoir. At a series of oil fields, improvement of these features has led to the increase of relative energy of high frequencies. At several other oil fields, the reverse trend is observed. Assumingly, this diversity is caused by the difference in the pore space microgeometry: the increase of high frequency amplitudes with the reservoir properties enhancement is inherent in reservoir rocks with dual porosity and dual permeability.