Objective  Significant breakthroughs have been achieved in the exploration of bauxite gas reservoirs in the Taiyuan Formation, Longdong area, Ordos Basin. Bauxites are generally considered to hold great helium generation potential due to their relatively high uranium (U) and thorium (Th) contents. However, there remains a lack of quantitative studies on their actual contribution to helium abundance in natural gas in the area.

Methods To determine dominant helium source rocks and mechanisms behind helium enrichment, this study investigated the bauxite gas reservoirs within the Taiyuan Formation in the Longdong area. Through analyses of the composition, carbon isotopes, and noble gas isotopes of helium-bearing natural gas, the origins of natural gas and helium in the reservoirs were identified. Based on the conventional genetic approach, this study developed a quantitative method for calculating the contribution of sedimentary effective helium source rocks to relative helium abundance in bauxite gas reservoirs. Accordingly, the contributions of bauxite reservoirs and hydrocarbon source rocks to helium abundance were assessed, and the mechanisms underlying helium enrichment in the reservoirs were explored.

Results  In the Longdong area, natural gas in bauxite gas reservoirs exhibits helium volume fractions ranging from 0.06% to 0.25% (average: 0.12%) and ³He/⁴He ratios varying from 2.27×10⁻⁸ to 3.86×10⁻⁸, indicating a typical crustal origin of helium. Theoretical calculations indicate that bauxite reservoirs and the Upper Paleozoic coal-measure hydrocarbon source rocks contribute relative helium abundance (280‒801) × 10⁻⁶ and 75×10⁻⁶, respectively to natural gas. In the case where sedimentary effective helium source rocks serve as the only helium source, the maximum helium content in natural gas was calculated at 876×10⁻⁶, which is lower than the threshold for helium-rich natural gas (He content > 0.1%). Therefore, bauxite reservoirs and coal-measure hydrocarbon source rocks possess a limited helium supply capacity, failing to form helium-rich natural gas independently. In contrast, basement granites exhibit huge volumes and can release large amounts of helium under geological conditions such as tectonic uplift, thereby serving as major helium source rocks in the Longdong area.

Conclusions In the Longdong area, helium enrichment in bauxite gas reservoirs is collectively controlled by multiple factors, including tectonic activity, fault development, and the natural gas accumulation process. The helium enrichment pattern of the reservoirs is characterized by a multi-source helium supply predominated by basement granites and supplemented by sedimentary effective helium source rocks.