On-line hydropyrolysis gas chromatography-mass spectrometry (HyPy-GC-MS) for kerogen-bound biomarkers

Abstract

Flash pyrolysis gas chromatography-mass spectrometry (Py-GC-MS) is seldom used to release bound biomarkers, because its hydrogen-poor pyrolysis conditions usually result in a low yield of bound biomarkers and high concentrations of olefins. In this study, by using a Py-GC-MS system and immature kerogen/coal samples, we investigated the effects of heating rate (flash versus 30 degrees C/min), carrier/reaction gas (He versus H-2), hydrogen pressure, and catalyst [(NH4)(2)MoO2S2] on the yields and distributions of bound biomarkers. Also, the bound biomarkers obtained by different pyrolysis conditions were compared with those from catalytic offline hydropyrolysis (HyPy) and free biomarkers from extracted organic matter (EOM). We propose a new technology of hydropyrolysis-gas chromatography-mass spectrometry (HyPy-GC-MS) to be used to release bound biomarkers. HyPy-GC-MS using low-pressure H-2 at slow heating rate can increase the yield of bound biomarkers by 3-9 times, minimize the cracking of the hopanes side chain, better preserve the original sterane and terpane distributions, and have characteristics of lower maturity, compared with conventional flash Py-GC-MS without H-2. Additionally, the steroidal and terpenoid yields by HyPy-GC-MS using low-pressure H-2 at slow heating rate from immature kerogens were 3-6 times that by HyPy. Compared with EOM, biomarker maturity parameter derived from HyPy-GC-MS may not fully represent the actual maturity of samples. Higher hydrogen pressure can significantly facilitate the hydrogenation of unsaturated hydrocarbons into saturated hydrocarbons, but it can also decrease the yields of pyrolysates with higher carbon numbers (i.e., >C-25), leading to a change in the biomarker ratios related to carbon numbers (e.g., the proportion of C-27-C-28-C-29 alpha alpha alpha 20R steranes and TT23/H-30 ratio)