[zarq]

Petroleum and gasses should be lighter than the rock surrounding them right, so if you move those up, the rock above those layers will be moved closer to the earth's center. So I would expect that that will cause the earth to spin faster, not slower.

[omgwtfbyobbq]

That's a good point, so it could be both. I'm not sure how high the average distribution of CO2 would have to be to offset that. The petroleum/gas could also be compressed under pressure, which might offset that effect to some degree.