An important difficulty in explaining primary migration from compacting source rocks concerns timing. The commonly accepted compaction curves for clay rocks imply maximum fluid expulsion early in a clay's compaction history, whereas most accumulations clearly have been formed later, after burial to depths of several thousands of feet.

It was shown recently that abnormally pressured clays have been abnormally pressured since burial to a shallow depth. It was concluded that the thicker clays in most sedimentary basins have been abnormally pressured to some extent.

If this conclusion is correct, the expulsion of petroleum becomes reconcilable with the time of entrapment because (a) thicker source clays, e.g., more than 200 ft, or 60 m, initial thickness) are probably more important than thinner source clays, and (b) mean fluid expulsion from the thicker clays is retarded.

In addition, most of the fluid in a thin clay is expelled under geothermal temperatures and essentially hydrostatic pressures (implied in early migration) over a small depth range, whereas fluid in a thick clay is exposed to geothermal temperatures and near-geostatic pressures over a much greater depth range.

The protopetroleum thus may be subjected to higher temperatures and much higher pressures for a longer time, thereby facilitating its solution in the pore water. The pressure decrease that necessarily accompanies expulsion may lead to the release of petroleum from solution in the outer, compacting layer of clay, and the accumulation of a slug that could be expelled by capillary force aided by the fluid potential gradient.