Understanding the origin of non-hydrocarbons in natural gases is crucial in the exploration and production cycle. High nitrogen (N2) contents are occasionally observed during bottom-hole samples collection. A novel geochemical approach, including gas compositions, and stable carbon and nitrogen isotopes, was used to assess the origin of high N2 content collected from several bottom-hole samples. The results suggest that the measured samples are likely to be contaminated from the pressurized N2 cylinder used for gas lifting and sampling. The values of the nitrogen isotopic compositions (δ15N) of the analyzed samples with high N2 concentrations are similar to the pressurized N2 cylinder used for gas lifting and sampling. The contamination in pressurized N2 compensated cylinder is due to N2 leakage from the N2 side to the sample side of the chamber while conducting downhole fluid sampling. Our mass-balance calculation suggests that the contamination percentages (i.e., amount of leaked N2) from the pressurized N2 cylinder into the samples is up to 70 mol%. One of the major causes of high N2 content in natural gas is the occurrence of thermochemical sulfate reduction (TSR). The measured chemical and carbon isotopic compositions of the hydrocarbons indicate the absence of thermochemical sulfate reduction in the analyzed samples. The findings of this study provide geochemical evidence that help to solve problems while sampling and reducing the investment risks encountered with high N2 content during exploration and production cycles.

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