What Pulsating H2 Emissions Suggest about the H2 Resource in the Sao Francisco Basin of Brazil

Lawrence Cathles,Alain Prinzhofer

doi:10.3390/geosciences10040149

Abstract

Proterozoic sedimentary basins very often emit natural hydrogen gas that may be a valuable part of a non-carbon energy infrastructure. Vents in the Sao Francisco Basin in Brazil release hydrogen to the atmosphere mainly during the daylight half of the day. Daily temperature and the regular daily tidal atmospheric pressure variations have been suggested as possible causes of the pulsing of H2 venting. Here, we analyze a ~550 m-diameter depression that is barren of vegetation and venting hydrogen mainly at its periphery. We show that daily temperature changes propagated only ~1/2 m into the subsurface and are thus too shallow to explain the H2 variations measured at 1-m depth. Pressure changes could propagate deeply enough, and at the depth at which the cyclic variations are measured hydrogen concentration will have the observed phase relationship to atmospheric pressure changes provided: (1) the pressure wave is terminated by geologic barriers at about 25% of its full potential penetration distance, and (2) the volume of gas in the vents is very small compared to the volume of gas tapped by the venting. These constraints suggest that there is a shallow gas reservoir above the water table under the ~550 m-diameter barren-of-vegetation depression. The 1D-analytical and finite-element calculations presented in this paper help define the hydrogen system and suggest the further steps needed to characterize its volume, hydrogen flux and resource potential.

Highlights

Hydrogen gas may become an important part of the zero-carbon energy economy because its combustion produces only water vapor and no CO2
Pressure changes could propagate deeply enough, and at the depth at which the cyclic variations are measured hydrogen concentration will have the observed phase relationship to atmospheric pressure changes provided: (1) the pressure wave is terminated by geologic barriers at about 25% of its full potential penetration distance, and (2) the volume of gas in the vents is very small compared to the volume of gas tapped by the venting
This paper analyzes whether pressure- or temperature-driven air flow can explain the temporal variations in hydrogen concentration measured at 1 m depth along the perimeter of a 550 m diameter, largely barren depression in the Sao Francisco Basin in Brazil

Summary

Introduction

Hydrogen gas may become an important part of the zero-carbon energy economy because its combustion produces only water vapor and no CO2. That there are natural sources of hydrogen that could be an important resource (Ward, 1933 [1]; Goebel et al, 1984 [2]; Newell et al, 2007 [3]; Sherwood-Lollar et al, 2014 [4]; Prinzhofer and Deville, 2015 [5]; Guélard et al, 2017 [6]; Prinzhofer et al, 2018 [7]). Studies so far have focused on the chemistry of the seeps that have been discovered worldwide (Larin et al, 2014 [8]; Zgonnik et al, 2015 [9]; Deville and Prinzhofer, 2016 [10]; Prinzhofer et al, 2019 [11]) and the origin of the hydrogen (Larin, 1993 [12]; Gilevska, T., 2007 [13]; Milesi et al, 2015 [14]; Vacquand et al, 2018 [15]; Truche et al, 2018 [16]). The many seeps that have been found indicate the operation of an active hydrogen system, and transient accumulations of hydrogen at relatively shallow depth are known

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Discussion

Conclusion