An improved gas-reservoir delineation from seismic-derived impedance and density interpretation at Dragon field, Venezuela

Abstract

Dragon field of Norte de Paria, offshore eastern Venezuela, was discovered in the 1980s and offers a good opportunity for production of gas methane. Since 2008, several development wells have been drilled for production of gas reserves. After analysis of exploratory well E1 and development well D1, gas was found in the former and water in the latter. A recent study showed that analysis and interpretation of seismic amplitudes can differentiate fluids that saturate rocks and thus can optimize the production of hydrocarbons. Seismic amplitudes show significant differences in the same stratigraphic level, depending on the presence of different fluids -brine, gas, or a mixture of the two. Differences are associated with density and compressibility of fluid under the pressure and temperature conditions of the reservoir. After computing Poisson's ratio and acoustic impedance from analyzed wells, it was observed that those values were lower at the gas-bearing sands than for the encasing rock. Based on that result, an acoustic-inversion exercise was performed over the existing seismic data to interpret lateral heterogeneities associated with gas saturation. However, the well-known effect of low gas saturation on seismic velocities introduces an appreciable uncertainty in interpretation. To overcome this issue, it is proposed to estimate densities from an elastic inversion of the seismic data and to interpret fluid distribution in the reservoirs at Dragon field using this attribute.