Relative reactivity of saturated hydrocarbons during thermochemical sulfate reduction

Tazhong and four other well-known TSR-altered petroleum reservoirs from around the world were analyzed for their molecular and carbon isotopic compositions of saturated hydrocarbons, thiaadamantanes concentrations or H2S contents to determine the relative stability of saturated hydrocarbons during thermochemical sulfate reduction (TSR). We find that TSR does not preferentially deplete tricyclic terpanes and 17α, 21β hopanes with the “exposed” tertiary carbon atoms compared to 18α (H)-22, 29, 30-trisnorhopane (Ts) and C29Ts that have no tertiary carbon atoms, as proposed by Xia and Gong [35] . However, pristane (Pr) and phytane (Ph) seem to have been preferentially oxidized over normal C17 and C18 alkanes, respectively, resulting in decreasing Pr/nC17 and Ph/nC18 ratios with increasing extent of TSR for the Tazhong and Nisku oils and condensates. iC4 appears to be preferentially depleted relative to nC4 during early TSR stage. At later TSR stage both nC3 and nC4 are oxidized more readily than iC4 in the Mobile Bay Norphlet Formation condensates. Thus, we tentatively propose that relative oxidization order during TSR may start with long chain iso-alkanes followed by n-alkanes, then cycloalkanes, C2-C4 alkanes and finally methane; however, it seems that oxidization of these compounds overlaps significantly.