The first migration, that is, migration of petroleum from the mother rock to the reservoir is more important in Japan than the second migration, because the regional migration is not possible on account of locally limitted distribution of reservoir beds.
Assumed that the first migration is caused mainly by compaction of mother rock according to its burial, the quantity of migration can be given by reduction of porosity of the mother rock.
Therefore, the conception of “Potential of Migration (PM)”has been defined as the volume of fluids flowing from mother rock into the unit pore space capacity of reservoir rock
The value of PM is determined by the volume & porosity of both reservoir and mother rocks, and the porosity of the mother rock before its compaction or burial.
After computing each value of PM in various exploration wells, the following fact has been found.
In oil productive areas, PM values are larger than 50, whereas in nonproductive areas, they are less than 50.
From this fact, the assumption has been recognized as valid that the presence or absence of oil pool is mainly controlled by the condition of first migration.

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