Measured pressures in Pleistocene strata of the Eugene Island block 330 area of offshore Louisiana reach approx. nine-tenths of the lithostatic pressures below 2 km depth; three-fourths of these geopressures are due to compaction disequilibrium. We show the relation between effective stress and porosity for compacting sediments to be exponential in shallow, normally pressured strata, then use the relation to calculate fluid pressure at depth in geopressured strata. Measured pressures below 2 km exceed our predicted values. A plot of effective stress vs. porosity demonstrates that compaction disequilibrium accounts for about three-quarters of the overpressures. We infer that the remainder must be due to pore-pressure generation at depth that occurred after the rocks reached their present porosity.