A method to quantify C1-C5 hydrocarbon gases by kerogen primary cracking using pyrolysis gas chromatography

Flash pyrolysis (Py) in combination with gas chromatography (GC) and mass spectrometry are often used to elucidate the structure of geomacromolecules such as kerogens, coals, asphaltenes and humic acids by analyzing their low molecular weight pyrolysis products. Most previous work restricts itself to C6+ pyrolysis products with only a few reports on the quantitative measurement of individual C-1-C-5 gaseous hydrocarbons. This is because of the difficulty in simultaneously separating C-1-C-5 efficiently. By using a GasPro capillary column to separate C-1-C-5 gaseous hydrocarbons and using polystyrene as external standard, we explore a quantitative Py-GC flame ionization detector (FID) method to study the C-1-C-5 pyrolysis products released from kerogens (coals) by primary cracking. Our study indicates that there is a good linear relationship between peak area of C-1-C-5 on Py-GC FID and kerogen sample weight. Based on the above methods, the yields of C-1-C-5 gaseous hydrocarbons released from various kerogen types were quantitatively studied. The results indicate that kerogen type plays a key role in controlling the compositions of C-1-C-5 gaseous hydrocarbons released by kerogen primary cracking under pyrolysis conditions. Type III kerogens (vitrinite rich coals) seem to have more abundant short alkyl chains linked to aromatic nuclei. Sequential flash pyrolysis at 800 degrees C, 1000 degrees C and 1200 degrees C suggests the generation mechanism of C-1 is different from that of C-2-C-5. C-2-C-5 are mainly generated by the release of alkyl precursors while aromatization and condensation of the kerogen structure may also be an important source of C-1 at high maturity, especially for type III kerogens (vitrinite rich coals). It seems to be a reliable way to study the generation mechanisms of C-1-C-5 by kerogen primary cracking and also the distribution of short alkyl chains within kerogen structure. (C) 2014 Elsevier Ltd. All rights reserved.