Fine pores, primarily slit-shaped micropores, in Upper Jurassic claystone source rocks were studied by gas adsorption. At 20°C, approximately 80% of the mesopore and micropore volume is penetrable to water only because of constrictions approximately 3 A wide. The pore surface is nonpolar and probably characterized by kerogen covering the clay mineral surfaces. Water is not structured in the mesopores and micropores, but readily forms a liquid phase without distinct monolayer formation. N-alkanes probably enter the pores "end on" and become adsorbed parallel with the surface. These results apply to all samples studied, irrespective of the degree of clay mineral diagenesis.

Three migration types proposed for these claystone source rocks are: (1) type A migration, which at experimental conditions permits entry and removal of n-alkanes, isobutane, neohexane, cyclohexane, and benzene in approximately equal amounts; (2) type B migration, which permits entry and removal of n-alkanes, isobutane, and neohexane in approximately equal amounts, but where only 20% of the pore volume and surface area is also open to cyclohexane and benzene; and (3) type C migration, which is intermediate between type A and type B.

The occurrence of the migration type is related to the reservoir oil composition. In the samples studied, tectonics and clay mineral diagenesis appear to have no influence on the migration type. Migration types may be related to past overpressured conditions, but do not appear to be related to current pressures.