Sensitivity of reflection seismic data to oil‐column height in high‐porosity sandstones

Abstract

Tuning is the effect of interference between the reflections from the top and bottom of a thin layer on the amplitude of the composite reflection. For a homogeneous sandstone reservoir containing an oil column overlying brine, interference between the reflection from the top reservoir and the oil/water contact is a function of the height of the oil column. If the properties of the sandstone do not vary across the oil/water contact, the SS, PS and SP reflection coefficients from the oil/water contact are small in comparison to the PP reflection coefficient. This allows analytic expressions for the effective PP and PS reflection coefficients from the reservoir to be derived that include all P‐wave multiples within the oil column. For a given source/receiver offset, the component of the wavevector inside the oil column normal to the interface is larger for the PPPP reflection than for the PPPS reflection, due to the asymmetry in the raypath for the PPPS reflection. The PPPS reflection is therefore useful for determining oil‐column heights larger than that discriminated by the PPPP reflection, especially when used at wider offsets. A convenient classification of the AVO response of hydrocarbon‐bearing sandstone reservoirs overlain by shale is the scheme of Rutherford and Williams. Class 1 sands have higher acoustic impedance for normal incidence than the overlying shale, Class 2 sands have nearly the same acoustic impedance as the shale and Class 3 sands have lower acoustic impedance. Synthetic shot gathers calculated for these three classes as a function of oil‐column height show that a combination of the PPPP and the PPPS amplitudes can be plotted as a tuning trajectory, which can be used to determine the oil‐column height. This method is most sensitive for reservoirs that belong to AVO classes 1 and 2, and therefore may be useful in AVO analysis of Class 1 and 2 reservoirs where the traditional AVO indicators (developed for Class 3 reservoirs) do not work very well. These results demonstrate the usefulness of shear waves recorded in the marine environment at wide offsets.