Yields of H2S and gaseous hydrocarbons in gold tube experiments simulating thermochemical sulfate reduction reactions between MgSO4 and petroleum fractions

The aim of this work was to assess the ability of four representative fractions (saturated, aromatic, polar and asphaltene) of crude oils to participate in thermochemical sulfate reduction (TSR) and thus to evaluate their pyrolysis behaviors, which is of great relevance to the stability of reservoir hydrocarbons. Experiments simulating reactions between oil fractions and magnesium sulfates (anhydrous MgSO4 and hydrated MgSO4·7H2O) at a constant temperature of 420 °C and under a pressure of 50 MPa were conducted over a period of 24 h using a gold tube confined system. The yields of non-hydrocarbon and hydrocarbon gases and their δ13C values were measured and compared in order to reveal the main control factors on TSR under practical geological conditions. In pure blank pyrolysis experiments with oil fractions cracking without TSR, different pyrolysis rates were seen for the four petroleum fractions with a sequence of saturated > asphaltene > polar ≫ aromatic. Minimal yields of C1–C5 hydrocarbons in the aromatic fraction can mainly be attributed to the greater thermal stability of aromatic compounds, which was also reflected by their positive δ13C signatures. ddition of anhydrous MgSO4 to the saturated fraction, a small quantity of H2S was generated, indicating that only a weak TSR occurred in this series. Upon addition of hydrated MgSO4·7H2O to the four petroleum fractions, high yields of H2S were generated, indicating that water plays an important role in the occurrence of TSR. The presence of nitrogen and/or oxygen containing heteroatomic compounds in the polar fraction decreases the yields of H2S when subjected to MgSO4 TSR. Similar yields of H2S from the saturated, aromatic and asphaltene fractions revealed that TSR occurring in the aromatic fractions is unlikely to be associated with the opening of benzene rings but closely related to availability of hydrogen atom in the alkyl moiety. dition of MgSO4·7H2O, TSR can significantly enhance the yield of methane, also reflected in an increase in its δ13C value. Besides methane, the other gaseous hydrocarbons (C2, C3) in the MgSO4·7H2O series also showed some less negative δ13C values relative to the blank series, irrespective of the fraction type. Relative to the other fractions (saturated, polar and asphaltene), low yields of C1–C5 gaseous hydrocarbons derived from the aromatic fraction could be attributed to the greater stability of the benzene ring and indicate that paraffinic oils may be more susceptible to thermal or thermochemical alterations than aromatic oils under the conditions of real subsurface reservoirs.