Genetic source, migration and accumulation of helium under deep thermal fluid activities: A case study of Ledong diapir area in Yinggehai Basin, South China Sea

Based on the geochemical parameters and analytical data, the heat conservation equation, mass balance law, Rayleigh fractionation model and other methods were used to quantify the in-situ yield and external flux of crust-derived helium, and the initial He concentration and thermal driving mechanism of mantle-derived helium, in the Ledong Diapir area, the Yinggehai Basin, in order to understand the genetic source, migration and accumulation mechanisms of helium under deep thermal fluid activities. The average content of mantle-derived He is only 0.001 4%, the ³He/⁴He value is (0.002–2.190)×10⁻⁶, and the R/R_a value ranges from 0.01 to 1.52, indicating the contribution of mantle-derived He is 0.09%–19.84%, while the proportion of crust-derived helium can reach over 80%. Quantitative analysis indicates that the crust-derived helium is dominated by external input, followed by in-situ production, in the Ledong diapir area. The crust- derived helium exhibits an in-situ ⁴He yield rate of (7.66– 7.95)×10⁻¹³ cm³/(a·g), an in-situ ⁴He yield of (4.10–4.25)× 10⁻⁴ cm³/g, and an external ⁴He influx of (5.84–9.06)×10⁻² cm³/g. These results may be related to atmospheric recharge into formation fluid and deep rock-water interactions. The ratio of initial mole volume of ³He to enthalpy (W) is (0.004–0.018) ×10⁻¹¹ cm³/J, and the heat contribution from the deep mantle (X_M) accounts for 7.63%–36.18%, indicating that deep hot fluid activities drive the migration of mantle-derived ³He. The primary helium migration depends on advection, while the secondary migration is controlled by hydrothermal degassing and gas-liquid separation. From deep to shallow layers, the CO₂/³He value rises from 1.34×10⁹ to 486×10⁹, indicating large amount of CO₂ has escaped. Under the influence of deep thermal fluid, helium migration and accumulation mechanisms include: deep heat driven diffusion, advection release, vertical hydrothermal degassing, shallow lateral migration, accumulation in traps far from faults, partial pressure balance and sealing capability.