Postulated mechanisms of petroleum accumulation have been divided into four categories, according to motion of the phases. These are as follows: I--both petroleum and water phases flowing; II--petroleum phase flowing, water phase stationary; III--water phase flowing, petroleum phase stationary; and IV--both phases stationary. Mechanisms III and IV require that petroleum migrate on a molecular scale through aqueous solutions.

A qualitative discussion of the pertinent factors involved in Mechanisms I and II is presented along with semiquantitative estimates of rates of accumulation for Mechanisms III and IV.

For Mechanism III, calculations are made of the water flow necessary to dissolve butane or natural gas from a source bed and to deposit this material in a given saturation at a higher elevation where the temperature and pressure and, consequently, the solubility are lower. In the case of Mechanism IV an estimate is made of the rate of diffusion of butane through the water phase to the location of accumulation. This migration results from the gradient in butane concentration caused by subsurface temperature and pressure gradients.

According to these estimates accumulation of petroleum by Mechanisms III and IV would be rather slow, and migration would be only in an upward direction. In addition, it is evident that considerable fractionation of petroleum would take place during migration. It appears that one must rule out substantial migration of petroleum on a molecular scale alone. The alternative is that migration of petroleum is accomplished by a mechanism involving bulk motion through stationary or moving water or perhaps through a combination of such phase flow and molecular migration.