Pre-Stack Simultaneous Inversion for Delineation of the Lower Wilcox Erosional Remnant Sandstone Beneath the Texas Gulf Coastal Plain: A Case Study

The Lower Wilcox lowstand sand deposits encased between two sequence boundaries along the Texas Gulf Coastal Plain are of good reservoir quality and usually gas productive. However, the sedimentation is sparsely scattered within such depositional environment and it is hard to predict by qualitative interpretation methods. Simultaneous inversion of elastic parameters such as P-impedance, S-impedance, and density by the integration of pre-stack data and well logs allows us to quantitatively characterize the reservoirs, and to distinguish them from the surrounding rocks. In this study, we use pre-stack simultaneous inversion of the elastic parameters for delineation of the gas reservoir in an active field with limited log availabilities. For wells that are missing sonic and density logs, we estimate the parameters using the time average equation and Gardner’s equation, respectively. The shear wave velocity estimation methods are tested and compared using measured log value. The estimation results are verified using well log correlations in adjacent wells. Rock physics analyses on wells are conducted to find the optimal elastic parameters for characterizing the gas bearing sand. We successfully delineate the reservoir using the crossplot of VP/VS versus S-Impedance values. The inversion results are quality controlled by a producing well in the reservoir zone, and probability maps of each lithology are calculated by the probability density function. Our results from the Lower Wilcox Formation indicate that simultaneous inversion based on the estimated parameters using the time average equation is feasible, and the gas bearing reservoirs can be recommended with high confidence.