THIS DOCUMENT IS A NON-PEER REVIEWED PREPRINT SUBMITTED TO EARTHARXIV Mechanistic insights into sulfur rich oil formation, relevant to geological carbon storage routes. A study using APPI-P FTICR-MS(/MS) analysis

7 Sulfur incorporation into sedimentary organic matter has a key role in carbon preservation in the 8 geosphere. Such processes can inform strategies for human timescale carbon storage to mitigate 9 climate change impacts and thus more detailed knowledge of sulfur incorporation into biomass 10 species is needed. Until recently, detailed chemical characterization of sulfurized organic matter 11 was only possible by analyzing individual building blocks obtained after desulfurization 12 reactions. In this study, Fourier transform ion cyclotron resonance mass spectrometry (FTICR13 MS), with atmospheric pressure photoionization in positive ion mode (APPI-P) was used to 14 investigate the chemical composition of sulfur rich crude oils and to obtain mechanistic insights 15 into the sulfur incorporation reactions happening during early diagenesis. Contrary to the current 16 body of knowledge, APPI-P FTICR-MS data show that sulfurized lipids (with up to 6 sulfur 17 atoms) occur as free molecules in these oils, rather than within a macromolecular network linked 18 by (poly)sulfide bridges. In contrast to Peace River (Canada) oils, the thermally immature Rozel 19 Point (USA) and Jianghan Basin (China) oils show a carbon number preference in sulfurized 20 species resembling biogenic precursor molecules, which highlights the importance of S-bound 21 molecules as geochemical proxies for early diagenetic processes. This study indicates that sulfur 22 incorporation reactions involve the formation of S-cyclic structures in which the double bond 23 equivalent is ≥ the number of S atoms, which can then aromatize to thiophenes depending on the 24 thermal stress level they experience. FTICR-MS/MS experiments suggest the occurrence of 25 intermolecular sulfur incorporation reactions, but only as a mechanism secondary to 26 intramolecular sulfur addition. The FTICR-MS/MS fragmentation studies also indicate the 27 aliphatic nature of class SR1R species and sulfurized alkenones, the preference of cycle-forming 28 sulfur incorporation in steroid structures, and the occurrence, to a minor extent, of thiol/thioether 29