Gaseous hydrocarbons generated during pyrolysis of petroleum source rocks using unconventional grain-size: implications for natural gas composition

Open-system pyrolysis experiments were performed on a suite of immature to marginally mature source rocks to investigate the influence of kerogen type on primary gas composition and the effect of grain size on gas expulsion c haracteristics. The pyrolysis of rock powders confirmed that hydrogen-rich kerogens yielded wetter gases than did hydrogen-poor kerogens. Gases detect ed from the pyrolysis of rock chips were drier than those from powders of e quivalent samples. This was due to two processes: the retention and seconda ry cracking of higher molecular weight pyrolysis products and the preferent ial expulsion of methane from the rock matrix. These two effects, one chemi cal the other physical, could be distinguished using a novel approach invol ving multi-step pyrolysis of rock chips followed by on-line crushing of the residues. The enrichment of methane in natural gas attributed, by earlier workers, to be a consequence of fractionation during secondary migration (p ost-expulsion) has been proven to be real also during expulsion from source rocks at least for pyrolysis conditions. (C) 2000 Elsevier Science Ltd. Al l rights reserved.