Wide‐angle inversion for density: Tests for heavy‐oil reservoir characterization

Abstract

Mass density, due to its linear relationship with porosity, has long been recognized as a potential seismic indicator of fluid saturation. Given its dependence on mineral composition density can also be diagnostic for lithology. In this paper we discuss some key aspects of a wide‐angle processing and density inversion workflow, and apply it to a heavy‐oil reservoir. In this field intra‐reservoir shales typically have higher densities than surrounding reservoir sands. This wide‐angle workflow yields stable density estimates, from reflected P waves alone, at a resolution suitable for mapping the intra‐reservoir shales. Our field test indicates that data from an additional 15°–20° of reflection‐angle aperture, beyond the conventional 40° far‐angle limit, act as an effective constraint on the inversion density solution. However, these wide (60°) reflection angles cannot be successfully processed and inverted with conventional workflows. Some of the key wide‐angle considerations include anisotropic prestack imaging, a wavelet stretch correction and regularization of the inversion solution based on appropriate statistical constraints. Anisotropic imaging is required to position reflections from beyond 40° consistent with their corresponding small‐angle image time. However, imaging introduces about 50% wavelet stretch in a 60° angle stack. A subsequent wavelet‐stretch deconvolution reduced the stretch to about 13% (equivalent to stretch at 30°) in this test. These stretch‐corrected angle gathers form the input data for a constrained linear prestack inversion. The constraining term characterizes statistical correlations between elastic properties measured in wellbores local to the field. This improves scaling and continuity of the density amplitudes, over and above the stability achieved by inverting wide‐angle data.