Qin Shengfei
Research Institute of Petroleum Exploration and Development, PetroChina Company Limited, Beijing 100083, China
Abstract:
Helium is a strategically scarce noble gas widely applied in aerospace, semiconductors, medical nuclear magnetic resonance and other critical fields. Globally, industrial helium is dominantly produced as a by‑product from helium‑bearing natural gas, mostly crust‑derived helium generated by radioactive decay of uranium and thorium in crustal rocks. Trace uranium and thorium occur in various rock types, which can produce minor amounts of helium. During natural gas accumulation, helium generated via radioactive decay of uranium and thorium in source rocks is expelled together with hydrocarbon‑bearing gas derived from source rocks, migrates into reservoirs, and entraps helium produced by uranium‑thorium decay within reservoirs, ultimately forming helium‑bearing gas reservoirs. Generally, source rocks contain higher uranium and thorium concentrations than reservoirs; hence helium in gas reservoirs is predominantly sourced from source rocks, with minor contributions from reservoirs. Strong dilution by natural gas commonly results in low helium concentrations in gas reservoirs, hindering helium enrichment.
The formation of helium‑rich gas reservoirs requires supplementary helium supply from additional helium sources. Large ancient basements such as granites constitute the primary helium sources for helium‑rich gas accumulations. Ancient basements feature great age, huge volume and long decay duration, yielding substantial helium. Helium generated is dissolved in ancient formation water. Under conditions of fault connectivity and tectonic uplift, ancient formation water migrates toward gas reservoirs and releases helium into gas accumulations, elevating helium content and even forming helium‑rich reservoirs. The core prerequisite for helium‑rich reservoir formation is the migration of dissolved helium from formation water within ancient basements into gas reservoirs, which demands moderate‑to‑low hydrocarbon supply intensity for target gas reservoirs. Furthermore, overly tight sealing conditions and excessively high pressure coefficients are unfavorable, because groundwater carrying helium cannot readily migrate upward into overlying gas reservoirs. These geological prerequisites for helium enrichment deviate from conventional selection criteria for natural‑gas exploration.
Given the substantial discrepancies in geological conditions and accumulation processes between helium enrichment and natural‑gas formation, conventional natural‑gas exploration strategies cannot be directly adopted for helium exploration. Helium enrichment is independent of genetic types of natural gas and gas‑reservoir categories. The major helium‑source rocks for helium‑rich accumulations cannot be simply judged by uranium‑thorium contents in rocks. Moreover, it is incorrect to assume that superior caprocks are mandatory for helium enrichment merely because helium has low molecular weight and high diffusivity.
The helium‑targeted exploration philosophy is summarized as follows: target peripheries of hydrocarbon‑generating centers, ancient craton margins with moderate‑to‑low hydrocarbon‑generating intensity, intra‑basin paleo‑uplifts and inter‑basin collision zones; avoid high‑abundance natural‑gas sweet spots and search for helium‑rich resources in areas with low natural‑gas resource abundance, covering conventional, unconventional and non‑hydrocarbon gas reservoirs. Prioritize zones where ancient basements are coupled with tectonic uplift, taking ancient granite‑dominated basements as core helium sources. Give preference to regions with well‑developed basement faults that connect basements and reservoirs to guarantee upward migration pathways for water‑dissolved helium. Focus on tectonic belts subjected to intense post‑Himalayan uplift; tectonic uplift provides driving force for formation‑water migration and facilitates degassing and enrichment of water‑dissolved helium.
Keywords: helium; enrichment factor; helium‑rich gas reservoir; exploration philosophy
