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 δ¹⁵N-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.