Natural H₂ in <scp>K</scp>ansas: Deep or shallow origin?

Abstract

Abstract A geochemical study of gas coming from three wells in northeastern Kansas supplements previous studies from the 1980s and points to a persistent regional phenomenon of H 2 production. In 2008, a new well showed, just after drilling, a free gas phase with more than 80 mole % of H 2 , followed by water production associated with gas. This gas is mainly composed of N 2 , He, H 2 , and occasionally CH 4 , with changing proportions through time. A drastic decrease in H 2 at the well was observed since the aquifer is produced, along with occasional recharges in H 2 evidenced notably in the early phases of gas sampling. We demonstrate that this evolution of gas composition is closely associated to the well completion story. Accordingly, two distinct origins of H 2 are proposed: (1) deep crustal H 2 : water reduction associated to iron oxidation in the Precambrian basement; (2) reactions occurring in the tubing, primarily attributed to high contents of reduced iron and/or dissolved organic carbon (DOC = 4.1 mg L −1 ) in the water. The low δD values averaging −760‰ are attributed to a low temperature process, possibly a re‐equilibration with water. Furthermore, the suggested origins are supported by the observed gas associations: (a) deep crustal H 2 with radiogenic gases ( 4 He and 40 Ar) and metamorphic N 2 (δ 15 N averaging +2.5‰); (b) surficial H 2 with methane produced in the sedimentary aquifer and the tubing by methanogenic organisms.