Elastic Properties of Silica-Rich Mudrocks: Woodford Shale, Anadarko Basin, Oklahoma

Applications of seismic methods in mudrock exploration are limited due to a lack of well developed rock physics models that can relate mudrock matrix and fluid properties to seismic velocities. We propose a first principle based rock physics modeling method for predicting P- and S- wave velocities in mudrocks accounting for silica (quartz), porosity, and free gas. Using sonic and shear-sonic logs we apply the proposed method to estimate silica, porosity, and free gas depth profiles in the Woodford Shale of the Mcneff 2-28 well, Grady County, Anadarko Basin, Oklahoma. Our modeling suggests that silica and free gas decreases (90-50% and 90%-85% respectively) and the porosity increases (10–20%) in the Woodford from the top to its base. The estimated silica and porosity depth profiles are validated by comparison with the photoelectric and the density-porosity logs. X-Ray Diffraction analysis of a rock sample from the Upper Woodford in the Cambell 1-34 well located ~1 km to the southeast additionally supports high silica content in the Upper Woodford. This study strongly suggests that the rock physics method, such as that proposed here, can be potentially used as a guide for relating seismically-derived properties of the subsurface to key parameters such as silica, porosity, and gas saturation; the former two especially important for inferring brittleness and fracability and the latter for the production potential.