Hydropyrolysis of algae, bacteria, archaea and lake sediments; insights into the origin of nitrogen compounds in petroleum

Abstract

Nitrogen compounds are ubiquitous in fossil fuels and yet our understanding of their origins in the geosphere is limited. In this study, high hydrogen pressure pyrolysis was performed on sample material representing potential contributors to sedimentary organic matter (algae, bacteria and archaea) and sediments representing early diagenetic accumulations from Lake Pollen (Norway) and Priest Pot (UK). Previous workers demonstrated the structurally conservative nature of high hydrogen pressure pyrolysis in that the technique maximizes the yields of covalently bound hydrocarbon biomarkers from organic matter without adversely affecting their stereochemistry ( Love et al., 1995). Release of covalently bound biomarkers in high yields from kerogen via catalytic hydropyrolysis. In this study, the types and distributions of organic nitrogen compounds in the hydropyrolysates were characterised under similar conditions to such experiments where biomarker hydrocarbons undergo minimal rearrangement. Compounds identified by gas chromatography–mass spectrometry included alkyl-substituted indoles, carbazoles, benzocarbazoles, quinolines and benzoquinolines. Indoles are present in all hydropyrolysates, suggesting a common origin. A potential source of indoles is represented by tryptophan which was shown to degrade through a series of alkylated intermediates to indole. Carbazole, quinoline and benzoquinoline were also found in the hydropyrolysates of algae, bacteria and archaea. The presence of these petroleum-related nitrogen compounds in hydropyrolysates generated from biomass suggests an early origin for petroleum nitrogen compounds. A potential source of naturally occurring nitrogen compounds such as that in the alkaloids has yet to be realised. Benzocarbazoles were absent from hydropyrolysates of algae, bacteria and archeae, but present in those from recent sediments, suggesting their presence may be related to processes occurring during early diagenesis at, or immediately below, the sediment–water interface. In sediments from Lake Pollen, changes in the benzocarbazole ratio [ a]/([ a] + [ c]) ratio coincides with the interval described as a transition from fjord to lake environment, suggesting that benzocarbazoles are sensitive to changes in depositional environment and may have potential to act as a marker for environmental conditions.