Geological conditions and enrichment patterns of helium reservoir in Yancheng Formation, Huangqiao area, Subei Basin

The helium (He) content in the natural gas of the Neogene Yancheng Formation in the Huangqiao area of Subei Basin ranges from 1.05% to 1.40%, which is generally higher than the industrial grade, indicating a crust and mantle composite type of helium resource. However, current research on the main controlling factors of helium enrichment and the evaluation of exploration targets from this source is relatively weak. Therefore, the study systematically analyzed the helium genesis, source, and trapping characteristics of typical helium-rich gas reservoirs in the Huangqiao area. It focused on the influence of mantle-derived faults on the transport and accumulation of helium-rich gas reservoirs and the trapping conditions within the Yancheng Formation. An enrichment model for helium-rich gas reservoirs in the Yancheng Formation of the Huangqiao area was proposed. The results show that: (1) The Nanxinjie deep fault in the Huangqiao area was active during the sedimentation of the Yancheng Formation, resulting in upwelling of mantle-derived materials, accompanied by volcanic activities and migration of CO2, N2, and helium along the fault to shallow layers. (2) The burial depth of the Yancheng Formation in the Huangqiao area ranges from 370 to 400 m. The thickness of the bottom sandstone of the Yancheng Formation is about 40 m, capped with 10 to 40 m of mudstone. The reservoir and cap combination is favorable for enrichment, and the helium-rich layers pinch out from east to west. (3) The Yancheng Formation in the Huangqiao area exhibits a monoclinal structure as a whole, with the bottom interface displaying angular unconformity with the underlying strata. Through seismic calibration and interpretation, the lithological and stratigraphic composite trap area of the helium-rich gas layer was outlined. The helium-rich gas reservoir of the Yancheng Formation in the Huangqiao area is characterized by fault-communicated mantle transport, sandstone reservoirs, mudstone cap layers, as well as lithological and stratigraphic composite traps for enrichment. This understanding is significant for the evaluation of favorable helium exploration targets in the Huangqiao area.