oa Organic-Rich Rocks: Sinks, Not Sources, of Natural Hydrogen

Organic-rich rocks of high thermal maturity have been proposed as an alternative H2 source based on three primary arguments: (1) residual hydrogen atoms in kerogen during the late dry gas window, (2) H2 release during specific laboratory pyrolysis experiments, and (3) H2 occurrence in some organic-rich formations. This study explains that, although H2 may dominate under some lab conditions with ambient pressure and temperatures exceeding 500°C, under sedimentary and metamorphic pressure-temperature conditions, CH4 is thermodynamically favorable. A kinetically viable pathway hardly exists for H2 to surpass CH4 as a dominant product during late thermal maturity. Throughout thermal evolution, organic matter may actively consume natural H2 sourced from serpentinization, as observed in several gas fields. Thus, the assumption of an H2-generating stage in organic-rich rocks post-CH4 generation in the dry gas window is unsupported.