Lithology and fluid estimation in 3D based on seismic inversion and geostatistics: the Rubiales field case

Seismic data and well-logs are the major sources of information for lithology discrimination and determination of the spatial distribution of reservoir sands and fluids. In Rubiales and other Llanos fields, conditions are adequate for accurate mapping of lithology based on elastic inversion of seismic data and petrophysical estimation of shale volume fraction. In general, sandstones are more rigid and lighter than shale at the basal Carbonera formation that comprises the reservoir. Spatial distribution of sands derived from seismic information is shown to be heterogeneous with layers and lenses of varying reservoir quality and communication across stratigraphic levels. The description obtained from the elastic inversion corresponds very well with the acquired well-log information and the geostatistical characterization of shale fraction derived from well-logs. This morphological complexity accentuates the importance of the 3D estimation of lithology and fluid saturation in the area. Seismic sensitivity to fluid saturation in the Rubiales field is smaller than lithology sensitivity, due to the density similarities between the heavy oil and brine. However, the spatial information provided by the large number of wells in the field, combined with the seismic information on lithology (which conditions the oil accumulation) provides an effective mapping for oil-bearing sands in 3D. We achieve an adequate combination of this information by geostatistical means, conditioning the estimation of reservoir properties to well data and seismically derived shale fraction. The study has been relevant for the interpretation of the complex sand morphology, water-oil contacts, the location of horizontal wells that increase the productivity of the field and the definition of the static model for the area.