Novel Use of P- and S-wave Seismic Attenuation for Deep Natural Gas Exploration and Development

The well selected for the application of our attenuation theory and extraction ofattenuation attributes from seismic data is the Texaco well (API 177104132700) in Block313 of Eugene Island in the Gulf of Mexico (Well 2700). The rock physics diagnosticsindicates that the rock can be described by the uncemented (soft-sand) model. Thismodel is used to predict the S-wave velocity that was missing in the original well data.The Pand S-wave inverse quality factors are computed according to our theoreticalmodel. The ratio of these inverse quality factors (P-to-S) is small (on the order of one) inwet rock and large in the gas zone. The seismically-measured attenuation ratio mayserve, therefore, as an indicator of hydrocarbons. The synthetic seismic traces computedusing the well data and the ray-tracer with attenuation, specifically developed for thisproject, indicate that attenuation affects the seismic response and, therefore, can beextracted from real seismic data, including the P-to-P and P-to-S reflection amplitude. Rock Physics Diagnostics – Model for VelocityThe gas saturation in the well was calculated from the resistivity curve while the claycontent was estimated by linearly scaling the gamma-ray curve between its minimum andmaximum values. It was assumed that the formation water has the bulk modulus 2.85GPa and density 1.01 g/cc while the gas has the bulk modulus 0.14 GPa and density 0.26g/cc. The total porosity was calculated from the bulk density by assuming that thedensity tool samples the virgin formation with gas saturation as calculated fromresistivity.The measured impedance and P-wave velocity are compared to the curves due to theuncemented (soft-sand) model. The proximity of the data and model (Figure 1) indicatesthat this model is appropriate for the well under examination. This model was then use topredict the S-wave velocity (absent in the measured data) from the P-wave velocity.The in-situ impedance is plotted versus the total porosity and Poisson’s ratio (PR) inFigure 2 where the data are color-coded by gamma-ray and by water saturation. Similarcross-plots are shown in Figure 3 but for wet conditions where the elastic properties anddensity were calculated using the P-wave-only fluid substitution. The soft-sand modelcurves for water-saturated rock are superimposed upon the wet-condition data to furtheremphasize the relevance of this model. The curves are produced for varying porosity andeach for fixed clay content. The latter variable changes from one to zero with step 0.2.These model curves fully encompass the well log data.