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 17 alpha, 21 beta hopanes with the "exposed" tertiary carbon atoms compared to 18 alpha (H)-22, 29, 30-trisnorhopane (Ts) and C-29 Ts 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 C(17 )and C-18 alkanes, respectively, resulting in decreasing Pr/nC(17 )and Ph/nC(18) ratios with increasing extent of TSR for the Tazhong and Nisku oils and condensates. iC(4) appears to be preferentially depleted relative to nC(4) during early TSR stage. At later TSR stage both nC(3) and nC(4) are oxidized more readily than iC(4) 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, C-2-C-4 alkanes and finally methane; however, it seems that oxidization of these compounds overlaps significantly.