1. n. [Drilling Fluids]
Clays incorporated into a so-called native-solids mud when drilling shallow formations. Native clays are undesirable in muds that are (or will be) weighted with barite. The viscosity of weighted fluids can rise quickly with added native clays, making it difficult to control and pump the mud. Better mud properties result when the drilled solids level, including the level of native clays, is kept low.