Velocity logs are the most important data used to evaluate rock, fluid, and geotechnical properties of hydrocarbon reservoirs. As a complementary physical property, P-wave attenuation (Q −1 ) can be used as an indicator of lithology and fluid saturation in oil and gas reservoir characterization. We implemented an inversion selfconsistent rock physical model to predict P-and S-wave velocities in two old wells near a new well containing a complete suite of logs at the Waggoner Ranch oil reservoir in northeast Texas. We selected a training data set from the new well to test the algorithm that was subsequently applied to predict velocity data in the two old wells. We used an attenuation log from the new well to perform data analysis via the Gamma test, a mathematically nonparametric nonlinear smooth modeling tool, to choose the best input combination of well logs to train an artificial neural network (NN) for estimating Q −1 . Then, the NN was applied to predict attenuation logs in the old wells. The Q −1 logs detected oil-saturated sand that was modeled with a rock physical model. This is a significant result that revealed for the first time that oil, gas, and water saturations of sand can be quantified from an attenuation anomaly estimated from full-waveform sonic data. In addition, water, oil, and gas saturations of the sand were determined from Q −1 anomalies observed in the old wells. This confirms the productivity of the Upper Milham oil-saturated sand intercepted by the three wells. The velocity, density, and Q −1 logs were used to generate synthetic seismograms to calibrate seismic data to verify and evaluate the work flow for predicting velocity and attenuation logs in older wells. This demonstrated that attenuation logs can discriminate between anomalies due to lithology and those due to oil and gas saturation.
IntroductionThe characterization of oil and gas fields relies on knowledge of the distribution of rock-physics properties of the reservoir based on well log data. P-and S-wave velocities are the most important data used to evaluate reservoir rock and fluid properties, as well as geotechnical rock properties. Similarly, P-wave attenuation (Q −1 ) can be used as an indicator of lithology; pore structure; clay, sand, and fluid content; and hydrocarbon saturation. For most old wells, P-and S-wave velocity logs and attenuation data are missing. When new wells are drilled at such sites, service companies provide complete suites of logs to small oil and gas producers. The producers can then integrate new data with existing data to better delineate the reservoir and to estimate new reserves. To facilitate data integration, several investigators have developed regression analysis methods to predict P-and S-wave data at the vicinities of new wells where the complete suite of logs is available (Augusto and Martins, 2010). These authors extend