Vs estimation in mudrocks - Implications about mineralogy and grain size

This study investigates the implications of clay minerals and grain size for Vs estimations in mudrocks. A series of experimental compaction test of well characterized synthetic mudstones helps to understand the velocity development in mudrocks as a function of stress or depth of burial. Comparison of experimental and literature data illustrated that abetter understanding of mineralogical and textural relationships can significantly improve the Vs estimation. The scatter found in empirical Vs regression prediction models reflects the lack of mudrock understanding. Mudrocks are composed of various amounts of clay minerals, sands and silts resulting in complex lithologies and textural relationships. Clay minerals, the most important constituents of mudrocks, interact differently with pore fluids due to their individual physicochemical properties and this is a significant control on petrophysical, acoustic and seismic properties. The empirical Vs regressions are reliable if the mineralogy and textural relationships of the mudrocks are well known. This study demonstrates that velocity development in mudrocks is function of clay mineralogy and grain size but in addition several other factors like sorting, fabric, packing, stress, cementation and pore pressure could potentially also be important. However, the discrimination between the different effects is at present difficult to quantify.