Noble gases and nitrogen in natural gases from Gujarat, India

Abstract

Isotopic and elemental ratios have been measured in three natural gas samples from Gujarat, India. The 3He/ 4He ratio is radiogenic for Kalol- 166 and Kalol- 183, while a mantle component is clearly indicated for Motwan- 2. Although Ne is atmospheric in these Kalol gases, for Motwan- 2, a clear excess 21Ne due to 18O(α,n) 21Ne is present. All three gases have radiogenic 40Ar and air values for 36Ar/ 36Ar. Krypton is atmospheric in composition for all three gases. Xenon in Kalol- 183 is purely atmospheric, but for Kalol- 166 a pure fission Xe from 238U is also present. Xenon from Motwan- 2, on the other hand, shows excess 129Xe, 131Xe, 132Xe, 134Xe and 136Xe which do not match either 238U or 244Pu fission yields. The higher yields of 131Xe and 132Xe are most likely a result of preferential migration or leaching of their radioactive precursors from their host rock. But the 129Xe excess could not be due to such preferential effects, and represents a mantle component. Both 3He and 129Xe excesses can be accounted for by the presence of ∼20% mantle component. The δ 15N-values for the three gases Kalol- 166, Kalol- 183, and Motwan-2 are (in ‰) +9.27, −12.56 and −11.48, respectively. They indicate a sedimentary source combined with effects due to migration.