Early compaction of terrigenous mud is controlled primarily by particle size, initially in reverse relation to porosity but changing to direct below a depth of 3,000 feet. This relation should be more meaningful if density and porosity of shales can be determined at their natural depth by use of formation density and sonic logs. Fluids expelled during compaction are important as agents of diagenesis, and possibly carry hydrocarbons; however, such migration was recently considered temperature controlled, rather than of compaction origin. Supratenuous folding, draping over reefs, and contemporaneous faulting are major compaction structures; minor are compacted laminae around nodules and distorted shells within compacted shale. Laboratory experiments show that carbonate muds also compact readily; field observations indicate little or no compaction in carbonate rocks, probably due to early cementation.