Ann: Zapato-1ST Drilling Update, page-97

guessing this would be a very similar scenario to water bores.In that even if you have drilled through other "bad" (high sodium levels) aquifers, when the corect aquifer table is reached, and is cased down to that level to ensure only the selected water is pumped out.
so what im trying to say is that once they cased the hole (and pretty sure sealed it with cement) the drilling mud is now only traveling up,in the caseing and not on the outside of it,where it would potentially carry foreign substances (like oil) to the surface.