Mike, the mud (Drilling mud) has many purposes. It lubricates the bit and drill string, carries drill cuttings to the drill deck for analysis and disposal, and has great weight (made greater (or lesser) by adding or subtracting Barite, made with Barium, among others) to hold the pressure of oil or gas in the formation, that is, in the sand and not in the conductor pipe. When a well is finished out, concrete is poured in to plug the bottom, and also to case the well. Once they reach the pre-set depth, they must run and cement the casing -- place casing-pipe sections into the hole to prevent it from collapsing in on itself. The casing pipe has spacers around the outside to keep it centered in the hole. The casing crew puts the casing pipe in the hole. The cement crew pumps cement down the casing pipe using a bottom plug, a cement slurry, a top plug and drill mud. The pressure from the drill mud causes the cement slurry to move through the casing and fill the space between the outside of the casing and the hole. Finally, the cement is allowed to harden and then tested for such properties as hardness, alignment and a proper seal. I'm told that BP management specified a lighter mud than the mudman would have liked. I have no idea why, maybe it is easier to clean the well if a lighter mud is used?