It is controversial about the origin of the natural gas in Minfeng area.A set of indexes for differentiating kerogen-pyrolysis gas and oil-cracked gas is presented based on geologic analysis and thermal simulation experiments on source rocks and crude oil samples from the Es

4

.In mature phase

the kerogen-pyrolysis gas features in rapid generation of methane and low contents of cyclane and benzene.While in high mature phase

both kerogen-pyrogenic gas and oil-cracked gas are characterized by high contents of cyclane and benzene.The condensate oil and natural gas in Mingfeng area show a larger scope of ln(C

1

/C

2

) ranging from 2 to 6 than that of ln(C

2

/C

3

) varing from 1 to 1.5 and have relatively high contents of cyclane

isoalkane and aromatic hydrocarbon

revealing the characteristics of kerogen-pyrolysis gas in high mature phase.Therefore

it is believed that the natural gas in Minfeng area is of kerogen-pyrolysis gas.