Abstract:

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 δ¹³C₁ distribute between -2.673% and -4.360%. The values of δ¹³C₂ are from -2.253%～2.580%. The values of δ¹³C₃ 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.

Key words: fluid inclusion, natural gas, carbon isotope, gas source, Ordos