In this paper the writer presents a hypothesis on the migration of petroleum and tries to examine its validity through its application to the elucidation of mode of occurrence of petroleum at some of the oil fidlds in Japan. The Hypothesis postuates that petroleum is generated quite early after the deposition of its source sediments, and that the newly-born petroleum or protopetroleum is asphaltic in nature, and, as its maturation proceeds, its non-hydrocarbon ingredients diminish and its hydrocarbons become less cyclic and more paraffinic. The migration of petroleum can be divided into following three successive stages, in concordance with the degree of maturation of petroleum. First stage: Little maturized oil, as well as protopetroleum, contains a considerable amount of surface active substances such as naphthenic and other organic acids and metalliferous organic compounds. Therefor, the interfacial tension between the oil and water is so low, that the oil can be said to be hydrophilic. The oil, than, in finely dispersed state in water, moves along with water, as the latter flows hrough the sediments as the sedimentray compaction current. The oil can pass through the finest pores of the sediments as long as water goes through then. Second stage: During the time-requiring travel of the oil its maturation proceeds, and it loose much of its content of surface active substances; it gradually turns to be less hydrophilic. At this stage the oil can still move along with water through larger pores of the sediments, but it can not enter into their finer pores, which only water can into. So, at the contact between fine-grained sediments and coarse-grained ones, the oil is screened out and remains in the latter. The sedimentary compaction current flows usually upwards, and when there is a layer of coarse sediments interalated in fine sediment, the current tends to flow along the layer up-dip-wards. Therefore, the discrete particles of the oil in water gradually cancentrate at the nearby the up-dip pinching-out end of the layer of the coarese sediments, that is to say, at and nearby the eventual reservoir trap. The concentration may also take place at and nearby the eventual anticlinal reservoir trap. Third stage: The oil is now suffciently maturized; it is now veritable oil and strongly hydro-phobic. The fine discrete particles of the oil at the site of their concentration coalesce with each other to form larger bodies of oil, which in turn coalesce with each other. Finally large bodies of oil are formed which have sufficient dimensions of phase continuity to be raised upwards by buoyancy through water-soaked reservoir rock. Thus, the accumulation of oil take place at the reservoir trap. The present hypothesis explains the usual occurrence of heavier and more asphaltic or cyclic oil in shallower pools and lighter and more paraffinic oil in deeper pools at many oil fields consisting of multiple superimposed pools, as follows: Less maturized, heavier oil, being more hydrophilic, could travel along with water farther upwards than more maturized, lighter oil, which should be screened out earlier and remain in the deeper reservoir rocks. The influence of downward percolating meteoric water upon the migration of oil also considered in the paper.
The course and level limit of upper migration are discusssed from the information of residual organic contents examined by liquid chromatographic method, saturation of oil and water and its salinity, which are contained in the core samples (mainly in the mudstone) of many exploration wells drilled on the various types of structures.