Liberation of hydrogen-containing gases during closed system pyrolysis of immature organic matter-rich shales

Abstract

The liberation of hydrogen sulfide (H2S), molecular hydrogen (H2) and hydrocarbon gases as well as CO2 from three immature carbonaceous rocks has been studied by Micro-Scale Sealed Vessel (MSSV) pyrolysis in the temperature range from 350 to 600°C at a heating rate of 1°C/min. The samples analyzed were the lacustrine Miocene Condor Shale from Australia (CON-1) and two marine marlstones (HBY-1 and HBY-2) from Southern Lebanon, with different contents of total sulfur (TS; 0.38–1.6%), total organic carbon (TOC; 3.7–11.6%) and total inorganic carbon (TIC; 2.8–9.3%). The Condor Shale has a high siderite (FeCO3) content (20.3%) and 0.2% pyrite (FeS2). The total iron (Fe) content is 10.5% and the kerogen is sulfur-lean. The iron (Fe) contents of the HBY samples are below 1% (0.9% and 0.3%, respectively) and insignificant amounts of pyrite are present, indicating a predominance of organic sulfur (sulfur-rich kerogen) with Sorg values of >1%. The low Rock-Eval Tmax values support the presence of sulfur-rich, labile kerogen.CO2 was the major pyrolysis gas released from the CON-1 over the entire pyrolysis temperature range while no H2S was detected at any temperature level. The molar H2 yields tend to exceed the CH4 yields up to 475°C while at higher pyrolysis temperatures CH4 is the predominant hydrogen-containing product gas. For the HBY-1 and HBY-2 samples H2S and CO2 are the major product gases up to 450°C. Up to this temperature H2S is also the predominant hydrogen-bearing gas component. Subsequently methane and ethane yields exceed the molar H2S yields. At higher temperatures an increasing proportion of hydrogen is released as H2. Ultimately the molar H2 yields approach or even exceed the H2S yields from the sulfur-rich HBY samples.At pyrolysis temperatures<450°C the sulfur-rich kerogens liberate more hydrogen as H2S than as CH4 and H2. At high temperatures hydrogen release as H2 and H2S is less significant. These results indicate that, besides the amounts of molecular hydrogen released, the liberation of H2S should be taken into account for a comprehensive H mass balance of organic sulfur-rich source rocks. Approximately 50% of the hydrogen in sulfur-rich kerogen is released as CH4 during cracking at high temperatures (600°C).