Impact of natural weathering on source rocks: ESI FT-ICR MS evidence from Triassic Chang 7 outcrop profile in Tongchuan City, Southern Ordos Basin (China)

Abstract

Atmospheric, water, and biological interactions with source rocks exposed at or near the surface due to tectonic uplift can result in both the loss of quantity and variability in the chemical composition of organic matter (OM) in source rocks during natural weathering, especially in extractable organic matter (EOM). Most of the research work on weathering has focused on characterization of the changes of saturated and aromatic hydrocarbon fractions in EOM at molecular level. However, few studies have reported how weathering affects polar compounds in EOM. In this work, a series of source rock samples with varying degrees of weathering were obtained from the Yishicun Profile of Triassic Chang 7 member in Tongchuan City of the Ordos Basin, providing an opportunity to study the weathering mechanisms of EOM in weathered source rocks. Gas chromatography-mass (GC-MS) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were used to analyze the chemical composition and variations in the characteristics of EOM. Asphaltenes, resin fractions, and the slightly oxygenated heteroatomic species (O1–O3) identified by positive- and negative-ion ESI showed a clearly pronounced enrichment trend during weathering due to the oxidation of saturated and aromatic hydrocarbons (biodegradation and photo-oxidation). While saturated, aromatic fractions and the highly oxygenated heteroatomic species (O5–O6) showed a declining trend. Additionally, it was also observed that the highly oxygenated heteroatomic species with low carbon number (CN) showed a more significant loss trend than their high CN counterparts during weathering. This may be due to depletion by surface water leaching due to the high water solubility of these compounds. Therefore, both the oxidation and surface water leaching can play important roles in regulating the OM chemical composition of source rocks during natural weathering.