Assessing the Helium Potential of Variscan Batholiths: Insight From Corsica Island

Abstract

Most of the current helium (He) reserves originate from fortuitous discoveries, mainly made during oil and gas exploration in sedimentary basins. As helium generation depends on U and Th α-decay, old geological provinces gather key ingredients for high He accumulation. However, numerous He-rich springs have also been documented in much younger rocks, such as Variscan granites (320–250 Ma). These latter discoveries question the current exploration guidelines and require revisiting some of the longstanding paradigms. Here, is investigated He migration along a major fault rooted in the Corso-Sardinian batholith (France). Two thermal springs, Caldanelle and Guagno-Les-Bains, show significant outgassing activities of crustal sourced He with concentrations up to 1.45 vol% and flow rates of 110 m³ STP ⁴He/year. Besides He, the gas phase is dominated by N₂ (≈98 vol%) and minor CH₄. Based on a survey employing multidisciplinary methodologies, it is revealed that (a) Variscan rocks represent efficient ⁴He source rocks, (b) the main source of He comes from the underlying Eo-Variscan basement, (c) A deeply rooted fault and dense fractures networks drain the He, (d) the helium loss is limited, (e) faults and fractures may act as partial traps, and finally (f) the presence of an efficient trap could promote a He-rich reservoir with high flux but low reserves. In that sense, young post-orogenic granites represent promising helium plays. The geological context in which Caldanelle and Guagno-Les-Bains are embedded is ubiquitous in European Variscan batholiths. This case study is therefore intended to serve as a guide for helium exploration and to provide insights into helium behavior within a Variscan geological context.