Molecular hydrogen (H2) and light hydrocarbon gases generation from marine and lacustrine source rocks during closed-system laboratory pyrolysis experiments

Abstract

The pyrolytic liberation of molecular hydrogen (H2) and methane (CH4) from lacustrine and marine shales has been investigated in closed system (Microscale Sealed Vessel, MSSV) non-isothermal heating experiments up to final temperatures between 400°C and 600°C and a heating rate of 1°C/min. The H2 yields in the closed system were much lower than in previous open system pyrolysis tests while CH4 yields were much higher. Total gas yields increase continuously from 400 to 500°C while at higher temperatures different evolution patterns are observed. Methane yields invariably increase with temperature for all shales. H2 yields depend strongly on the void space (degree of filling) of the MSSV tubes. This is taken as an indication for temperature- and partial pressure-dependent reactions of hydrogen atoms/radicals or molecules (hydrogenation) with both, original OM and primary bitumen. Pyrolytic yields of H2 and CH4 correlate positively with TOC but TOC-normalized total yields of both gases at 600°C show neither a clear positive nor negative correlation with Rock-Eval hydrogen index or vitrinite reflectance. The immature Condor Shale with very low sulfur content exhibits the highest TOC-normalized H2 yields (>2mmol/g TOC) and the lowest hydrocarbon yields, both, in terms of CH4 (<8mmol/g TOC) and the Rock-Eval Hydrogen Index (HI). The effects of (organic or inorganic) sulfur and the formation of H2S on molecular hydrogen liberation in MSSV pyrolysis are the objective of the ongoing investigations.