Mechanisms of helium differential enrichment and helium-nitrogen coupling: A case study from the Weiyuan and Anyue gas fields, Sichuan Basin, China

The helium (He) contents of the Weiyuan and Anyue gas fields in the Sichuan Basin (China) differ significantly despite their similar helium-nitrogen (N-2) coupling characteristics, that is, the positive correlation between the N-2 and He contents, and the negative correlation between the nitrogen/helium (N-2/He) ratio and depth. To determine the factors influencing the differential enrichment of He resources and He-N-2 coupling in the Weiyuan and Anyue gas fields, this study analysed the compositions of natural gases within the study area and established a computational model for evaluating the dissolution and exsolution amount of He and N-2 in the pore water. The results indicated that: the uplift of the basement granite was crucial for the exsolution and enrichment of dissolved helium, and it was also the main reason for He-N-2 coupling. The differential uplift that occurred in the Leshan-Longnvsi palaeo-uplift during the Himalayan period further promoted the differential enrichment of He resources, thus also impacting the N-2/He ratio in the study area. The distinct quantities of N-2 and He exsolution during the uplift directly affected the negative correlation between the N-2/He ratio and the depth. Within the same depth range of the uplift, the average and variations of the N-2/He ratio in the crust-derived helium-rich natural gas reservoir were inversely proportional to the in situ molar partial pressure of He. The uplift of the crystalline basement is an important mechanism that causes the enrichment of helium resources and the coupling of helium-nitrogen in the crust-derived helium-rich natural gas reservoir, which is of great significance for understanding the variation of N-2/He ratio and exploration of He resources.