[Smokin' Joe]

Anerobes are responsible for the Hydrogen Sulfide in crude oil recovered from as deep as 15,000 feet, and reservoirs have been known to become contaminated with them and the oil become 'sour' (and less desireable as a result).

Who knows what critters are down there?

[hellbender]

Is any attempt made to decontaminate oil drilling equipment between uses? I know that aquifers can be contaminated by iron bacteria introduced by water well drilling. Could some of the bacteria in deep reservoirs have been conveyed from shallower levels by the drills? (Somewhat similar to drug addicts getting AIDS or hepatitis from reused needles)

[null and void]

Who knows what critters are down there?

The Shadow knows...