Abstract:

The mechanism of helium accumulation is unique and more complex than that of petroleum systems. Since the discovery of helium-rich hot springs in Tanzania in 1967, no large-scale helium fields have been identified. To establish the mechanism of helium charging and accumulation in the Rukwa Rift Basin, this study conducted research in five key areas using geochemical, seismic, drilling, and logging data: analyzing the helium and carrier gas content at the surface, calculating helium production in the craton basement, examining the helium release process, establishing the helium charging mechanism, and identifying favorable helium zones.The findings reveal that the Mesozoic-Cenozoic rifts play a critical role in controlling helium migration and accumulation within the basin. Surface helium content ranges from 1.0% to 10.2%, with carrier gases including nitrogen, carbon dioxide, and methane. The ³He/⁴He isotope ratio ranges from 0.039 to 0.053 R_a, indicating that helium originates from both crustal and mantle sources. Helium generation in the basement and sedimentary layers is estimated at 367 billion cubic meters. The release of helium from the peripheral rift of the Tanzanian Craton involves five key processes: generation, release, migration, charging, and overflow.Three helium accumulation models have been identified in the Rukwa Basin: nitrogen-helium desorption, coalbed methane-helium extraction, and carbon dioxide-helium extraction. Nitrogen, an inorganic gas of deep origin, is characterized by accumulation from homologous sources, with helium desorption from water being the primary helium charging mechanism in the basin. Coalbed methane, primarily sourced from the depositional center of the basin, accumulates independently of helium. Carbon dioxide, an inorganic gas of metamorphic origin, shares a symbiotic accumulation mechanism with helium similar to that of coalbed methane.The most favorable helium-rich zones in the basin are located in the Basin Margin Fault Closures (BMFCs). In the overlying soil of these traps, helium content is 35% higher than the background value. To date, 12 BMFCs have been identified, with unrisked prospective geological helium resources estimated at 5.74 billion cubic meters, accounting for 64.6% of the undiscovered resources in the basin. These zones represent favorable exploration targets in rift basins. The results of this study enhance the understanding of helium accumulation mechanisms in rift basins and provide valuable guidance for optimizing helium exploration in favorable areas.

Key words: seep, BMFC, desorption, extraction, helium source, accumulation, potential