Hydrocarbon columns from 443 oil and gas pools in California are compared with columns from 364 pools in the Rocky Mountain area of Colorado-Montana-Wyoming. Maximum columns for oil are 2700-5500 ft (823-1676 m) long and for gas are 1500-1800 ft (457-549 m) long. Seals for these accumulations range in age from Ordovician to Neogene. Most seals are mudstone and shale, but some evaporitic seals occur in the Paleozoic beds of the Rocky Mountain area. Comparisons of buoyancy pressures and seal thicknesses show that the highest pressures generated by the longer columns are retained by seals 50-100 ft (15-30 m) thick. Calculated displacement pressures for leakage through the interconnected pore throats of the mudstone and shale seals are in accord with the buoyancy pressures ge erated by the 5-10% of the accumulations with the longest hydrocarbon columns. Shorter column lengths with their lower buoyancy pressures are probably not controlled by leakage through the pore throats of their seals, but rather by lack of hydrocarbon charge, by leakage at structural or stratigraphic spill points, or by leakage related to faulting or other types of weakest point leaks in the sealing beds.