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 C1-C5 gaseous hydrocarbons. This is because of the difficulty in simultaneously separating C1-C5 efficiently. By using a GasPro capillary column to separate C1-C5 gaseous hydrocarbons and using polystyrene as external standard, we explore a quantitative Py-GC flame ionization detector (FID) method to study the C1-C5 pyrolysis products released from kerogens (coals) by primary cracking. Our study indicates that there is a good linear relationship between peak area of C1-C5 on Py-GC FID and kerogen sample weight. Based on the above methods, the yields of C1-C5 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 C1-C5 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 °C, 1000 °C and 1200 °C suggests the generation mechanism of C1 is different from that of C2-C5. C2-C5 are mainly generated by the release of alkyl precursors while aromatization and condensation of the kerogen structure may also be an important source of C1 at high maturity, especially for type III kerogens (vitrinite rich coals). It seems to be a reliable way to study the generation mechanisms of C1-C5 by kerogen primary cracking and also the distribution of short alkyl chains within kerogen structure.