Hydrocarbon vertical micro-migration is a complicated geological-geochemical process.It is strictly controlled by boundary condition

possesses stages in geological history.The mineralization degree of ground water shows gradient variation and ionic combinatiom changes due to the action of ground water and hydrocarbon during the vertical micro-migration.Hydrocarbon vertical micro-migration can change the ion combination

causes regular variation in component of light hydrocarbons and the fluorescent intensity of aromatic hydrocarbons.That is to say

light hydrocarbons with small molecular weight and small radius relatively increases from reservoir to the caprocks

but the fluorescent intensity of aromatic hydrocarbon decreases distinctly

and the carbon isotopic value of methane of both deep seated reservoir and groud surface are basically the same.Since hydrocarbon vertical miero migration is periodical and intermittent

the geochemical index usually in cycles from high to low or from low to high on geochemical exploration index colunm.