oa Chemical Structure Evolution of Lacustrine Type-Ii Kerogen Under Semi-Open Pyrolysis as Investigated by 13C Nmr and Ft-Ir

This study aim to investigate the structural changes of a lacustrine Type-II kerogen, by taking the seventh Member of the Triassic Yanchang Formation in the Ordos Basin (Chang 7 Shale) as an example. Integrated analysis including elemental analysis, programmed pyrolysis (Rock-Eval), FT-IR, and 13C NMR were conducted on the unheated and post-pyrolysis rocks and kerogens.

Results show that before the peak hydrocarbon-generating stage (Ro < 0.6%), defunctionalization of oxygen-containing functional groups, which generates water, CO2 and NSOs, occurs simultaneously with the detachment of short aliphatic side chains and small aromatic rings, which generates volatile fractions (C6-C14, C15+ sat and C15+ aro), suggesting that the Chang7 kerogen is highly branched. During the peak hydrocarbon-generating stage (0.6–1.09 %Ro), the concurrent breakage of weak cross-link bonds and long aliphatic chains lead to the simultaneous peak generation of liquid hydrocarbons and NSOs, indicating the highly cross-linked features of the Chang7 kerogen. After the peak hydrocarbon-generating stage (Ro > 1.09%), the reactions in the previous two stages produce a highly aromatic kerogen. The increased aromaticity of the highly matured kerogen is likely to be caused by aromatization or dehydrogenation of hydroaromatics and coking of aromatic bitumen, not by fusing or condensation of aromatic rings.