Various patterns of hydrocarbon migration would result in different fractionation effects of methane carbon isotopes.When hydrocarbons migrate through diffusion

methane carbon isotopes tend to get heavier along with increasing migration distance.For example

methane carbon isotopes in different wells in Tabei area change from-46.98‰—55.98‰ in the deep Triassic to-31.47‰—37.51‰ in the shallow Quaternary.In Forties oilfield in North Sea

the methane carbon isotopes are lighter in the near surface sediments over the oilfield(the value of δ

13

C

1

being-40‰)

and they also tend to get heavier along with the increasing migration distance to the periphary(the value of δ

13

C

1

being-30‰).When hydrocarbons migrate through filtration

methane carbon isotopes do not change along with migration distances.For example

the methane carbon isotopes of natural gas in the Ordovician source rocks in Akekumu structure

Tabei area

is in the range of(-34.85‰)—34.98‰.The methane carbon isotopes measured through surface geochemical exploration is in the range of-34.62‰-36.56‰

which is near or equal to that in the subsurface Ordovician

indicating that the surface and subsurface methane carbon isotopes have cogenetic relationship.The reason that causes apparent and regular variation of methane carbon isotopes in various hydrocarbon migration patterns is that the methane molecules with light carbon isotopes would be dissolved during diffusion migration.As a result

the carbon isotopes detected with acidolitic hydrocarbon method would be fractionated

or become heavier upward along the stratigraphic section.While during filtration migration

negligible dissolution of methane in water would not change the methane carbon isotope features in extensive migration

due to the large scale and rapid migration of natural gas.