Sulfur isotopic analysis derived from hydrogen sulfide in oilfield: evaluation of isotopic fractionation of precipitation in the form of sulfides

Sulfur isotopic analysis is an essential biogeochemical tool in environmental studies. Like carbon and nitrogen, oxidized and reduced sulfur species are usually controlled by biological and thermochemical processes. In oil and natural gas exploration,  unwanted sulfur gases are often byproducts of extraction, production, and refining processes. Depending on its concentration, hydrogen sulfide (H₂S) can pose complications oil and gas operations since it may contaminate and affect the quality of the produced oil and natural gas. Hydrogen sulfide may be formed by different processes: thermal cracking of kerogen, bacterial metabolism (BSR), and thermochemical pathway (TSR). Isotopic analysis (δ³⁴S) and the H₂S concentration allow identifying the process that generated this gas. H₂S sampling for δ³⁴S determination is an important step to track possible sulfur isotopic fractionation. An efficient means of sampling is the precipitation of sulfur in the form of sulfide, an inert solid that is easy to handle and transport. In this study, we evaluated the sulfur isotopic fractionation related to the precipitation of Ag₂S, ZnS, and CuS from a H₂S gas. Hydrogen sulfide was produced by reacting ammonium sulfide and phosphoric acid in a Falcon tube attached to a gas sensor. The produced H₂S was pumped through three vials with the following solutions: zinc acetate, silver nitrate, and copper nitrate. Upon bubbling the H₂S in the solutions, silver sulfide, zinc sulfide, and copper sulfide precipitates formed in each vial, respectively. The composition of the precipitates was determined by XRD. A fourth experiment was conducted precipitating zinc sulfide following by reaction with silver nitrate, resulting in the precipitation of silver sulfide. For sulfur isotope analyses, an aliquot of 0.5 mg of each sulfide was analyzed using International Atomic Energy Agency (IAEA) S1 and internal CdS as reference materials.  The δ³⁴S of the ammonium sulfide was 12.7‰ and of the sulfide precipitates was: 7.5‰ for Ag₂S; 8.1‰ for ZnS; 7.9‰ for the CuS; and 7.7 for the Ag₂S of the fourth experiment.  In general, the sulfur isotopic fractionation between the ammonium sulfide and the precipitates varied between 4.6 and 5.2‰. One possible cause for this fractionation could be explained by the production reaction of H₂S or even by the incomplete consumption of H₂S during the sulfide precipitation reactions.