Molecular and isotopic compositions of hydrocarbon gases generated from anomalously 13C-enriched sapropelic sedimentary organic matter in lacustrine basins

Abstract

For lacustrine basins, the enrichment of 13C in sedimentary organic matter is closely related to the growth of plankton biological productivity. Consequently, there are many cases where the sapropelic organic matter is more 13C-enriched than the humic organic matter. However, the geochemical characteristics of the pyrolysis products produced by 13C-enriched lacustrine sapropelic organic matter are less discussed. Herein, two lacustrine sapropelic kerogen samples with remarkably different carbon isotopic ratios (–23.2‰ and –28.4‰) and a coal sample (–25.6‰) were performed by a closed and dry pyrolysis system at temperatures ranging from 300 °C to 600 °C. The results indicated that the carbon isotopic compositions of hydrocarbon gas products were significantly affected by the initial carbon isotopic composition of their sources. At similar simulated temperatures, the carbon isotopic compositions of methane generated from the 13C-enriched sapropelic sample and the coal sample were more consistent. However, owing to the difference in their gas-generation mechanism, the natural gas generated from the coal sample had a more 13C-enriched isotopic composition of ethane (δ13C2) and a higher dryness, particularly at higher maturity levels. The hydrogen isotopic composition of methane (δ2HC1) was markedly enriched in deuterium as the thermal maturity increased and may have been independent of the genetic type and salinity. Therefore, we used the “Dryness vs. δ2HC1” and “δ13C2 vs. δ2HC1” diagrams to confirm the genetic type of natural gas in lacustrine basins.