[Willie Green]

Fart gas might not be the ideal gas,

That's a heckuva understatement!

Assuming intestinal gas production continues at the same rate at great depth as on the surface, after half an hour there will be a fair amount by weight but at small, insignificant volume.

I don't know that this is a valid assumption.
In my humble experience, fart gas production doesn't seem to be a continuous process, but in rather abrupt, discrete batches. (and sometimes indiscrete as well.)

I do, however, agree with what would happen if the whale tried to "hold it" until he/she reached the surface: it'd be like a humongous puffer fish blown up bigger than the Goodyear Blimp.

[RightWhale]

That's why whales are so big. A lifetime of deep diving coupled with habitual waiting until a discreet moment.

[Brett66]

I find it hard to believe a whale fart would make a "bloop" sound, should be more like a thundering "boom".