1. n. 
The process by which complex molecules are broken down by micro-organisms to produce simpler compounds. Biodegradation can be either aerobic (with oxygen) or anaerobic (without oxygen). The potential for biodegradation is commonly measured on drilling-fluid products to ensure that they do not persist in the environment. A variety of tests exist to assess biodegradation.
2. n. 
The breakdown of medium-weight crude oil by microbial organisms into heavy and light components. When the light components, typically methane, escape to the surface, the heavy ends are left behind. Biodegradation gradually raises oil viscosity, reduces API gravity, increases asphaltene content, and increases concentration of certain metals and sulfur.