Sulfur Differentiation in Organic-Rich Shales and Carbonates via Open-System Programmed Pyrolysis and Oxidation: Insights into Fluid Souring and H2S Production in the Bakken Shale, United States

The amount of organic sulfur influences the kinetic behavior of kerogen upon thermal maturation as well as the crude quality and sourness. There has always been an interest to specify the amount and different forms of sulfur in source and reservoir rocks. In this study, we analyzed 28 samples from the Bakken Formation having different maturities and 5 samples from other shales containing type I, type IS, or type HS kerogen using the Rock-Eval 7S instrument. Total sulfur data were compared to those from LECO SC-632 and CHNS elemental analyzer, with excellent to very good correlation, respectively. The sulfur index is a fast and reliable method to differentiate between type I and type II kerogens against their S-rich counterparts. Organic sulfur in the Bakken is not massively associated with S2 but with the refractory component of total organic carbon (S4). Thus, not all total organic sulfur (TOS) is responsible for the early generation of liquid hydrocarbons in organic-rich shales. Also, TOS should not be used as an indicator of kerogens being type S. Residual S-org in the Bakken increased with maturity, and a considerable amount was available to generate H2S at relatively low temperatures via aquathermolysis, a process that occurs in the Greater Permian Basin. High concentrations of residual TOS in the Bakken (average of 2.3 wt % on whole-rock basis) meet the minimum of 1.4 wt % S (on a kerogen basis) required for H2S production. This suggests a possible correlation between TOS and high H2S production in Williston and other basins.

Automated summary

This study analyzed the organic sulfur content in samples from the Bakken Formation, showing that organic sulfur is mainly associated with the refractory component of total organic carbon rather than S2, indicating it is not the sole indicator for early liquid hydrocarbon generation and should not be used as an indicator for type S kerogens.