Maturity-Driven Generation and Transformation of Acidic Compounds in the Organic-Rich Posidonia Shale as Revealed by Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Abstract

The Posidonia Shale in the Hils syncline in North-West Germany represents a natural maturity sequence from 0.48 to 1.45% vitrinite reflectance (R0). In this study, the molecular composition of the acidic compounds of six Posidonia Shale samples with a different maturation level has been determined using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) combined with electrospray ionization (ESI) in the negative ion mode. The changes in the distribution of elemental and compound classes as well as of double-bond equivalents (DBE) and carbon numbers at the different maturity levels are described in detail. Aromatic N1, N1O1, and N1S1 compounds are preferably formed during ongoing maturation, while the amount of O2 compounds decreases. With increasing thermal stress, condensation and aromatization of the acidic NSO compounds increases. In the sample of highest maturity (R0 1.45%), which has been shown to have a relevant shale gas potential, a significant enrichment of nonalkylated and methylated highly aromatic N1, N1O1, and N1S1 compounds has been observed, which can be the result of thermally induced side chain cracking of compounds with a higher alkylation degree. The other cracking products would be short chain hydrocarbons contributing to a significant amount to the overall gas potential of the Posidonia Shale.