Abundance and origin of helium in the Lower Paleozoic gas: A case study from the Daniudi field of the Ordos Basin, central China

Abstract

Natural gas from ancient strata in sedimentary basins tends to be enriched with helium (He). Lower Paleozoic gas in the Daniudi field (DNDF) of the Ordos Basin in central China provides a good case study to reveal He generation and accumulation mechanisms in deep strata. Understanding He accumulation in the Daniudi field is crucial because the increasing global demand for He requires the identification of new sources in existing natural gas reservoirs. Here, the geochemical characteristics of Lower Paleozoic gas from the DNDF are investigated, aiming to reveal the concentration, origin, and controlling factors of He accumulation. Natural gas in Lower Ordovician Majiagou Formation (O₁m) reservoirs is He-depleted (0.0126 %–0.0187 %, mean 0.0153 %). The ³He/⁴He and corresponding R/Ra ratios range from 3.12 to 3.95 × 10⁻⁸ and 0.022 to 0.028, respectively, indicative of a crustal origin of He. The He concentration was diluted by alkane gases, as indicated by a negative correlation between He content and the CH₄ content, as well as the dryness coefficient; however, it was not affected by the degree of thermal maturity or the type of organic matter in the source rocks, as indicated by a lack of correlation between He content and the carbon isotopic ratios of CH₄ and C₂H₆. The N₂/He ratio of O₁m gas is generally in a range from 1.14 to 7.65, which is markedly lower than the ratio (19–50) in natural gas with He commercial production value (He% ≥ 0.1 %). The He content in O₁m gas is negatively correlated with the content of total dissolved solids (TDS) in formation water; the low He content may be attributable to the high TDS content. The pressure in Paleozoic strata of the DNDF has decreased since the Late Cretaceous due to continuous regional uplift, resulting in decreased He solubility in the formation water and the exsolution of previously dissolved He into gas pools.