Formation mechanism of thiadiamondoids: evidence from thermochemical sulfate reduction simulation experiments with model compounds

Abstract

High concentrations of thiadiamondoids together with elevated H2S are indicative of the occurrence and extent of thermochemical sulfate reduction (TSR). However, the obscure formation mechanism and accumulation timing of thiadiamondoids restricts their application in TSR evaluation. In this study, TSR simulation experiments with MgSO4 and three model compounds, namely adamantane, 1-methyladamantane, and diamantane, were performed in gold tubes in high pressure reactors. The gaseous hydrocarbons and liquid products generated from the model compounds by TSR were identified and quantified. According to the H2S yields from the three model compounds, the susceptibility to TSR of three model compounds is 1-meththyladamantane > adamantane > diamantane. Four types of compound groups with diamondoid structure were detected in the liquid products: from high to low abundance these are diamondoidyl ketones, diamondoidyl alcohols, diamondoidthiols and thiadiamondoids, respectively. Thiadiamondoids can be generated from C0-alkylated diamondoid species by TSR. During the TSR process, the concentrations of both diamondoidyl alcohols and diamondoidthiols first increased within the maturity range 0.89–1.09 Easy%Ro and then decreased at maturity > 1.09 Easy%Ro. The rapid decrease in concentration of diamondoidthiols does not correspond to increasing thiadiamondoids, suggesting that diamondoidthiols may not be reaction intermediates for thiadiamondoids in laboratory experiment conditions. In the TSR simulation experiments, slowly increasing concentrations of diamondoidyl ketones correspond to slowly increasing concentrations of thiadiamondoids. For the synthesis mechanism of thiaadamantanes derived from thiaadaamantane-4,8-dione, we propose that the diamondoidyl ketones could be reaction intermediates in thiadiamondoid formation. The thiadiamondoids in petroleum could have two different formation mechanisms: (1) low-temperature cationic carbon ion rearrangement in diagenesis to early catagenesis stages, in which thiadiamondoids are derived from the diagenetic products of polycyclic thiols in the source rock; and (2) a free sulfur radical mechanism that occurs in the TSR process at high temperature in the middle catagenesis to metagenesis stages. During the TSR process, free sulfur radicals attack the diamondoidyl ketones generated in the first stage of TSR and open the cage structure. Then the carbonyl carbon atom is replaced by a sulfur atom, followed by cyclization to form thiadiamondoids. Based on the H2S yields in the TSR simulation experiments with diamondoid model compounds, most thiadiamondoids formed in the second stage of the autocatalyzed TSR process within the maturity range of 1.09–1.21 Easy%Ro.