Experimental and theoretical study on the formation of volatile sulfur compounds under gas reservoir conditions

Abstract

Volatile organic sulfur compounds (VOSC) are found in petroleum natural gas and condensates at trace levels. The abundance and δ34S values of VOSC were proposed as a proxy for thermal processes such as oil and gas generation during thermal maturation, thermochemical and microbial sulfate reduction, migration and expulsion. The understanding of VOSC formation and degradation mechanisms is needed to refine the use of δ34S values of VOSC as proxies for thermal processes over geological timescales in the subsurface. We therefore conducted pyrolysis experiments at 360 °C for 4, 12 and 72 h using pentane and H2S or pentanethiol as model compounds to study the formation and degradation pathways of VOSC and associated variations in δ34S values. The main products of these experiments are C1-C4 alkanes along with a variety of thiols and thiophenes, the most dominant VOSC formed. Most thiols were in equilibrium with H2S after 4 h based on their δ34S values. Thiophenes were first 34S depleted relative to H2S and only reached equilibrium at the 72 h experiment. The products and 34S fractionations in the pyrolysis experiment of pentanethiol at 360 °C and 12 h were similar to those of the experiment with H2S and pentane at the same conditions. This similarity suggests that pentanethiol is an intermediate product in the formation of aromatic VOSC during pyrolysis of pentane and H2S. Benzo- and dibenzothiophenes (BTs and DBTs) were formed in the liquid phase and their 34S depleted values relative to H2S indicated that they had not reached equilibrium. Ab-initio calculations of the thermodynamic properties of thiols, thiolanes, thiophenes BTs and DBTs were used to explain the relative abundance of products in the system and predict their abundance and S isotopic signature at reservoir conditions. The thermodynamic data suggests that at equilibrium, only small quantities of thiols and even smaller quantities of thiolanes can exist under our experimental conditions. Unlike thiols, the free energy of formation of thiophenes, BTs and DBTs is negative under these conditions indicating that the formation of these compounds is thermodynamically favored. The result suggests that the δ34S values of thiols are controlled by a very rapid equilibrium isotopic effect (EIE). On the other hand, aromatic sulfur compounds can preserve the δ34S value of the kinetic isotopic effect (KIE) associated with their formation for longer. However, under typical petroleum reservoir conditions, the formation of aromatic VOSC in the gas phase from short alkanes (<C5) and H2S is not thermodynamically preferred and thus their presence is expected to be either as kinetic products or due to influx and charge from a different source where the reactions were thermodynamically favorable. Thus, the abundance and Δ34S between H2S and VOSC can be used as a new approach to evaluate charge history within sulfur containing reservoirs.