Many source rocks have argillaceous matrixes. Until recently, only chemical interactions between clay minerals and organic matter (catalytic effect) have been given serious consideration. It appears now that the physical or physico-chemical properties of clays also play a key role in the processes of primary migration of oil. Among these properties are (1) microstructure of clays which influences the porosity and permeability in both the water and oil phase, (2) mechanical behavior of clays which may contribute to the microfracturing processes thus allowing the expulsion of oil from source rocks, and (3) physico-chemical properties of adsorption and wettability of clays which act strongly because of the high specific area of clay minerals. This property may influence the ermeability of clay shales. In addition, adsorption is selective toward various components generated by organic matter, thus explaining major differences observed between the composition of source rock extracts and reservoir oils.

On a larger scale, such as anticlinal structures, clays are commonly the origin of fluid pressure anomalies. High pressures modify the flow system, since fluids are drained preferably by the tops of structures. The flow of water due to natural convection eventually may cause the alteration of clay minerals and enhance the permeability of the overpressured sections beneath the pressure seal. This process contributes to the modification of geothermal conditions in undercompleted rocks where poor thermal conductivity conditions exist.

These combined effects influence both the nature and spatial distribution of hydrocarbons trapped in reservoirs. Therefore, a better knowledge of the physical properties of clays should lead to a better understanding of the role of clays in the formation and migration of oil and gas and thus lead to better exploration practices.

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