Pyrolysis of kerogens in the absence and presence of montmorillonite—II. Aromatic hydrocarbons generated at 200 and 300°C

Abstract

Kerogen from organic-enriched sediment (Type II Cretaceous black shale and Type III Rocky Mountain coal) has been used to conduct anhydrous pyrolysis experiments (2–100 h at 200 and 300°C, in the presence and absence of montmorillonite. Naphthalenes, phenanthrenes, and their alkyl derivatives, as well as aromatic steroidal hydrocarbons were determined. Tri-methyl phenanthrene and di-methyl phenanthrene are the most common phenanthrenes in the pyrolysates of both black shale kerogen and Rocky Mountain coal kerogen. There is no significant relationship between the phenanthrenes and triaromatic steroidal hydrocarbons. The data suggest that triaromatic hydrocarbons are thermally generated during pyrolysis from the intermediate formed polar components, asphaltene and kerogen by partial break-up of the polymer network. Montmorillonite did not enhance aromatization in either kerogen, but had a catalytic effect on cracking and breakdown of the aromatic steranes, forming either phenanthrene or small molecular-wieght hdyrocarbons. Monoaromatic steranes were only present in the 200°C pyrolysate, and nearly disappeared in the high (300°C) temperature pyrolysates. Nevertheless, the decrease of monoaromatic steroidal hydrocarbons does not always correlate well with the increase of triaromatic steranes. It appears that during long pyrolysis times, degradation of steranes is more important than the aromatization process. These steroidal aromatic hydrocarbons are degraded to form smaller saturated or aromatic compounds. In addition, because of the apparent destructive effect of high temperatures and montmorillonite clays during pyrolysis, the most valid application of the mono- and tri-aromatic sterane parameters is for relatively low maturity source rocks with carbonate mineral matrix or other matrices of low catalytic activity.