Elastic seismic inversion and reservoir characterization in the Llanos Basin, Colombia

Abstract

The contribution of seismic inversion to lithology and fluid characterization in oil reservoirs is geology and data dependent. Several studies of reservoir characterization based on elastic seismic inversion in the Llanos Basin in Colombia have proved to be successful for accurate description of the siliciclastic lithology of lower members of the Carbonera Formation. Mass density estimated from the inversion has been an important lithology discriminator because of compaction of shale, which exhibits larger densities than sand. In the analysis of properties calculated from well-log data, the separation in property space of oil sands and brine sands is moderate to small. Oil sands likely are distributed toward the zones of lower VP/VS ratio, Poisson's ratio, density, and bulk modulus, compared with other lithofluid types. The theoretical effect of fluid substitution in elastic rock parameters and density, although small because of the proximity of heavy-oil density to water density, is nevertheless present in these reservoirs. Successful exploitation of the seismic information associated with the discrimination of oil-bearing and brine-bearing sands has been achieved via fine calibration of the elastic inversion and derived reservoir indicators and by combining this information with relative two-way time (TWT) location of the strata to interpreted horizons. With statistical analysis and direct analysis over crossplots, indicators for oil sands have been formulated that are consistent with information available from wells, although slightly less accurate than lithologic (Vshale) estimation. After lithology estimation is performed from the seismic-inversion results, further improvement of the lithology description is achieved by combining the seismic-derived lithology with the localized well-log lithology profiles using geostatistical methods.