Methane carbon isotopic composition can provide information on origin

maturity and secondary modification of natural gas.Therefore

it can serveas a useful geochemical clue for gas prospecting.The focus of this study isconcentrated on discussing the variation of methane carbon isotope in thermo-genic natural gas.1.It is the temperature that influences the maturity of organic mattersand determines isotopic fractionation range.The fractionation range is largewhen the maturity is lower with methane generated contains light carbonisotopic composition;the isotopic fractionation is decreased

when the methanecontains obviously heavier Carbon isotopic composition.The range of carbonisotopic fractionation depends also on different primary textures of organicmatters

giving rise to methane with its own properties.The isotopic differencein methane and kerogen can manifest the maturity of organic matters.2.The carbon isotopic composition of methane in gas generated from crudeoil become heavier with the increase of cracking of crud oil.The differencein carbon isotope of methane and crude oil can be used to distinguish theoil-cracked gas and the associated one.3.Because of either the emergence of additional gas derived from thelater maturity of organic matters in source rocks or carbon isotopic exchangeunder high temperature

the natural gas affected by high temperature mayproduce an anomalous phenominon of δ

13

C

1

>

δ

13

C

2

.4.The manifestation of carbon isotopic composition of methane from coalseams is quite unique

for the less metamorphic coal might bring about ahigher δ

13

C

1

value

while the highly metamorphic coal species might even giverise to lower δ

13

C

1

value.The factor which exercises influence on this valueis more or less complicated.So it can't be used for direct correlation.