Helium, nitrogen and hydrogen are continually generated within the deep continental crust1,2,3,4,5,6,7,8,9. Conceptual degassing models for quiescent continental crust are dominated by an assumption that these gases are dissolved in water, and that vertical transport in shallower sedimentary systems is by diffusion within water-filled pore space (for example, refs. 7,8). Gas-phase exsolution is crucial for concentrating helium and forming a societal resource. Here we show that crustal nitrogen from the crystalline basement alone—degassing at a steady state in proportion to crustal helium-4 generation—can reach sufficient concentrations at the base of some sedimentary basins to form a free gas phase. Using a gas diffusion model coupled with sedimentary basin evolution, we demonstrate, using a classic intracratonic sedimentary basin (Williston Basin, North America), that crustal nitrogen reaches saturation and forms a gas phase; in this basin, as early as about 140 million years ago. Helium partitions into this gas phase. This gas formation mechanism accounts for the observed primary nitrogen–helium gas discovered in the basal sedimentary lithology of this and other basins, predicts co-occurrence of crustal gas-phase hydrogen, and reduces the flux of helium into overlying strata by about 30 per cent because of phase solubility buffering. Identification of this gas phase formation mechanism provides a quantitative insight to assess the helium and hydrogen resource potential in similar intracontinental sedimentary basins found worldwide.
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All data are previously published and available in ref. 7, with data tables uploaded to an open-source repository at https://doi.org/10.5281/zenodo.7267734. Source data are provided with this paper.
The code for the numerical model developed for this paper can be accessed on an open-source repository at https://doi.org/10.5281/zenodo.7271773.
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This study was supported by funding from China Scholarship Council, the Oxford Department of Earth Sciences, the Natural Sciences and Engineering Research Council of Canada, and the NERC Centre for Doctoral Training in Oil and Gas. C.J.B. and B.S.L. are fellows of the CIFAR Earth 4D Subsurface Science and Exploration programme.
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Extended data figures and tables
Extended Data Fig. 1 Examples of diffusive concentration profiles of helium and nitrogen with the diffusion-only model.
The diffusive concentration profiles of helium (a,b) from the diffusion-only model are shown for a basement flux of 0.8 × 10−6 mol 4He m−2 yr−1 and 1.6 × 10−6 mol 4He m−2 yr−1. The concentration profiles presented are modelled results considering several aquifer flushing events in the past million years (natural recharge and anthropogenic flooding)7. For each helium flux value, the nitrogen profiles modelled are with N2/4He basement flux ratios of 13, 25 and 50, respectively (c,d) with the diffusion-only model. The horizontal lines are the boundaries of lithological units. The dotted black lines are the solubility of nitrogen over a range of salinities (labelled to the right of each line). From right-most to the left, the lines correspond to salinities of 0, 10, 25, 50, 100, 150, 200, 250 and 300‰ respectively. The solid red lines represent the saturation limit for different formation waters estimated from salinity maps produced by ref. 43. If the predicted nitrogen concentration for a particular 4He flux and basement N2/4He exceeds solubility (that is, the coloured concentration line is higher than the solid red line), a nitrogen gas phase is predicted. When helium basement flux is 0.8 × 10−6 mol 4He m−2 yr−1, gas is predicted to form only in the basal units when N2/4He is 50. As demonstrated in the figure, when helium basement flux is 1.6 × 10−6 mol 4He m−2 yr−1, gas is predicted to develop in the bottom unit when N2/4He is 25 and in units from Cambrian up to Devonian when N2/4He is 50.
Extended Data Fig. 2 Gas composition obtained from diffusion and exsolution model over a range of basement helium and N2/4He flux.
The plots present the modelled (a) 4He/20Ne ratios, (b) 4He concentration, (c) N2/4He ratios in the gas phase and gas–water ratio (Vg/Vl) in the Cambrian Deadwood Formation. The shaded areas are constrained by the lower and upper limits of the samples. By overlapping plots (a) (b) and (c), plot (d) demonstrates the best-estimated combinations of 4He basement flux and N2/4He ratios to be4He basement flux of 0.8 × 10−6–1.6 × 10−6 mol 4He m−2 yr−1 and basement N2/4He ranges between 30 and 50.
Supplementary Video 1
Visualization of timing and magnitude of gas-phase formation. The video shows an example of the diffusion and exsolution model run with a He flux of 1.6 × 10−6 mol 4He m−2 yr−1 and a N2/4He flux ratio of 50 run for 500 Ma with 0.1-Ma time resolution. Sedimentary units are added throughout geological time7. The left and middle panels show the concentration profiles of 4He and N2 simulated from the diffusion–exsolution model, respectively. The red dashed line represents the solubility limit of N2 for different sedimentary units as the model progresses. The right panel shows the gas–water ratio through geological time simulated from the diffusion–exsolution model. With these parameters, a gas phase may have formed in the Deadwood Formation as early as about 140 Ma.
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Cheng, A., Sherwood Lollar, B., Gluyas, J.G. et al. Primary N2–He gas field formation in intracratonic sedimentary basins. Nature 615, 94–99 (2023). https://doi.org/10.1038/s41586-022-05659-0
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