OK, do I understand this right?

Fill the well bore with mud. The weight of the column of mud creates pressure and counteracts the pressure of the oil in the reservoir, reducing the pressure at the top of the well bore.

I think this is basic petroleum engineering 101.

I've always assumed on most land wells, that when you fill the well bore with mud, the pressure at the top of the well bore drops to zero. i.e. When the bore is full of mud, wouldn't you normally be able to chop the well off at the surface and not have anything flow out of the well? When you're drilling, pulling pipe, etc, isn't the top of the well bore basically open, allowing mud to basically flow freely out the top of the pipe? Isn't the weight of mud the thing that allows you to drill into a pressurized reservoir in the first place?

What I wonder is will the mud produce enough pressure that the gauge pressure at the top of the well bore on the Macondo well is zero, or does it just reduce the pressure? Is the weight of 10,000 feet of mud enough to counteract the reservoir pressure of 7000+ PSI?