Thermal transformations of organic and inorganic sulfur in Type II kerogen quantified by S-XANES

Abstract

Sulfur X-ray Absorption Near Edge Structure Spectroscopy (S-XANES) was used to quantify the thermal transformations of organic and inorganic sulfur forms in pyrite-containing Type II kerogens and kerogen chars after open system pyrolysis for a series of well-defined times and temperatures. These results are compared to identical experiments conducted on the same kerogens treated to be pyrite-free. No significant differences were found in the thermal transformations of organic sulfur between the pyrite-containing and pyrite-free kerogen. The loss of aliphatic sulfur forms occurs early (equivalent Ro<1.5%) while there is a relative increase in thiophenic sulfur over aromatic sulfide at higher maturity (Ro>1.5%). In pyrite-containing kerogens, the complete conversion of pyrite (FeS2) into pyrrhotite (Fe1−xS) occurs by a laboratory equivalent Ro=1.5%, followed by the conversion of pyrrhotite into troilite (FeS) at higher maturity. The availability of hydrogen from hydrocarbons generated from kerogen accelerates the initial decomposition of pyrite resulting in pyrrhotite and H2S evolution. H2S evolution up to Ro=2.4% corresponds to the sulfur loss associated with pyrrhotite and troilite formation and aliphatic sulfur loss, indicating that sulfur from pyrite results almost exclusively in H2S evolution with no significant incorporation and retention of sulfur into the organic matter of the kerogen chars under the current set of open system pyrolysis conditions.