Organic matter accumulation, paleoenvironment and kerogen structure of the Middle Jurassic Dameigou Formation, Qaidam Basin: Evidence from organic petrography and geochemistry

Abstract

The organic matter in terrestrial sediments provides not only a record of geological events but is also as a vital source of fossil fuels. Its enrichment is fundamentally governed by the interplay of complex depositional environments and diverse climatic conditions. This study presents an integrated investigation utilizing major element oxides, trace elements, rare-earth elements, organic petrography, pyrolytic analysis, and infrared spectroscopy to systematically evaluate the provenance, tectonic setting, paleoclimate, weathering conditions, and organic matter accumulation of the Middle Jurassic Dameigou Formation, as well as to elucidate the chemical composition of kerogen. Geochemical parameters, particularly the Th/Sc vs. Zr/Sc ratios, indicate the absence of recycled sediments throughout the studied succession. Geochemical discrimination diagrams, particularly the Th/Sc vs. Zr/Sc and La/Th vs. Hf plots, provide compelling evidence for felsic volcanic rocks serving as the predominant source material.The interpretation of the tectonic setting as an active continental margin is constrained by multiple discrimination diagrams, such as the La–Th–Sc and Th–Co–Zr/10 ternary systems. The depositional sequence exhibits distinct vertical variations in organic matter composition and paleoenvironmental conditions. Unit 1 is characterized by terrestrially derived macerals (vitrinite and inertinite), while units 2 and 3 show a progressive transition to liptinite-dominated assemblages, primarily composed of lamalginite. Multiple chemical weathering proxies, specifically the Chemical Index of Alteration, Weathering Index, and Sr/Cu ratios, demonstrate that paleoclimatic conditions evolved from warm/humid environments characterized by intense chemical weathering towards cooler conditions with diminished continental weathering intensity. Molecular-level characterization using μ-FTIR and CP-Py-GC/MS reveals distinctive organic matter compositions: unit 1 is dominated by aromatic structures (73.0–83.5 %), while unit 2 shows predominant aliphatic compounds (41.6–72.9 %). The decrease in aliphatic compounds to 39.2 % in unit 3 correlates with increased water salinity, suggesting environmental control on kerogen composition. The δ¹³C values exhibit stratigraphic variations, with comparatively higher values in Unit 1 (−33.01 ‰ to −25.70 ‰) compared to lower values in Units 2 and 3 (−41.56 ‰ to −28.79 ‰), reflecting the higher abundance of terrestrial maceral components in Unit 1 and proximal Unit 2 deposits. These findings provide crucial insights into the depositional evolution and organic matter accumulation mechanisms in the study area.