Deep crustal source for hydrogen and helium gases in the São Francisco Basin, Minas Gerais, Brazil.

The intracratonic São Francisco Basin (E Brazil) contains the sedimentary record of multiple basin-cycles younger than 1.8 Ga. Spanning the states of Minas Gerais, Bahia and Tocantins, it is bounded in the west, east and north by Brasiliano/Pan-African orogenic systems. To the northeast, the basin is limited by a Neoproterozoic intracratonic deformation zone, while the southern border is defined by an erosional unconformity of Proterozoic strata over Archean/Paleoproterozoic basement rocks.

In most areas, the crystalline basement of the basin is only identified on seismic sections and the composition remains unknown, but is thought to be composed of TTG suites (tonalite-trondhjemite-granodiorite), Archean igneous rocks, greenstonebelt successions, and Paleoproterozoic metasedimentary rocks. Hydrocarbon exploration in the basin over the last two decades has revealed that gases rich in hydrogen are common in the basin and often restricted to tight, low-permeability, Meso to Neoproterozoic reservoirs. Early analyses of some of these gases revealed the presence of significant concentrations of native H₂, commonly associated with helium, nitrogen and light hydrocarbons. Surface seeps of gases containing some of these components are also common across the basin.

A project is underway to investigate the source, generation mechanisms, and possible migration paths of H₂-rich gases at key localities within this basin. Analyses of gas sampled at the surface, from the head of exploration wells drilled in the São Francisco basin, confirm high concentrations of H₂ (up to ~20%) and He (>1%), with the associated methane-dominated hydrocarbons and nitrogen. Stable isotope data suggest an abiotic origin for the methane, while He isotopes reveal a strong crustal signature (³He/⁴He < 0.02 R/R_A). Neon isotope data suggest the presence of an Archaean crustal component in the gases, indicating that a component of the gas has likely originated from the underlying crystalline basement, or within Archaean-derived sedimentary rocks.

This new data reinforces the potential for crystalline Archaean rocks to be a significant geological source of He and H_2. Furthermore, these preliminary results seem to support prevailing theory that subsurface H₂ production may ultimately play an important role in abiotic methane generation.