⁴⁰Ar/³⁶Ar Characteristics of Natural Gases and Its Geological Significance in the North Margin of Qaidam Basin

Key words: 
• ⁴⁰Ar/³⁶Ar /
•   /
• natural gas /
• Carboniferous

Abstract: He and Ar isotopic compositions of fourteen natural gas samples from the north margin of Qaidam Basin were analyzed. The relation of gas and source was also correlated using 40Ar/36Ar ratio. It is found that the 40Ar/36Ar values of natural gases in the area distribute in the range from 951 to 1 712, averaging 1098, with most values higher than 40Ar/36Ar values of the Jurassic natural gases from other Chinese basins. The research shows that there were not mantle derived fluid with high 40Ar/36Ar values in the region, and an aging effect of reservoirs could not result in the phenomena. Therefore, the higher 40Ar/36Ar values are mainly caused by the aging effect of source rocks. By calculation, the age of gas source rocks distributes between 164.7 and 460.8 Ma, averaging 345.1 Ma, showing that the most might be from Carboniferous. This conclusion is in  agreement with the fact that there are widely distributed Carboniferous source rocks and crude oils derived from the Carboniferous. The discovery of the Carboniferous gases in the area indicates that Carboniferous is a new exploration target, developing a new exploring area in the north margin of Qaidam Basin.