The methods of pyrolysis and vaccum ball grinding are used to obtain carbon dioxide

methane and other gasses. Then

the carbon isotope values of theses gases can be assayed by a higher sensitive apparatus(Delta S). CO resulting from the pyrolysis can affect the assaying of isotope value whereas the method of vacuum application can't change nature of gasses

therefore

gases extracted by the vacuum ball grinding can reflect the nature of true ancient current and it's more reliable for assaying isotopic value. This method has been used to isotopic assay hydrocarbon gas in inclusions from the Ordovician reservoir calcite veins in central gas field of Ordos basin. The results show that the values of δ

13

C

1

distribute between -2.673% and -4.360%. The values of δ

13

C

2

are from -2.253%～2.580%. The values of δ

13

C

3

are from -2.253%～2.580%. The characteristics of gas from the fluid inclusion show that the gas generated from coal is very different from the gas in gas fields. It is speculated by the author that there would have been at least two stages for the formation of the low Paleozoic natural reservoirs in central gas field of Ordos basin

coal type gas was probably dominant at the earliest stage.