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

Abstract

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 IIS 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 Sorg 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.

CO2-Enhanced Oil Recovery Mechanism in Canadian Bakken Shale

“…Sulfide (i.e., pyrite) and sulfate are other major sulfur species in the samples and range from 0.5 to 3.4 wt.% and 0.1 to 0.7 wt.%, respectively (Table 1). The sulfur index (SI) for all samples ranges from 23 to 30, which indicates a Type II marine kerogen (Figure 2b) similar to Bakken Formation in the US (e.g., Carvajal-Ortiz et al [34]). Although the organic matter in the studied samples is marginally mature, the S1 peak is relatively high in all four samples, which means there is free oil in the samples to be recovered (Table 1).…”

“…The temperature program is similar to the basic programmed pyrolysis method [33]. It starts at 300 • C iso-temperature for 3 min, followed by a 25 • C/min ramp to 652 • C, but also includes an extra oven step during the pyrolysis stage using a sulfur oven to oxidize the evolved gases into SO 2 at 840 • C. The samples are then transferred to the oxidation oven with an extended analysis time during the oxidation stage with a 20 • C/min ramp from 300 to 1200 • C for decomposition of sulfate moieties [32,34]. The SO 2 gas released during both pyrolysis and oxidation is measured in real-time by an ultraviolet (UV) detector.…”

Early porosity generation in organic-sulfur-rich mudstones

“…It started at a 300 °C iso-temperature for 3 min, followed by a 25 °C/min ramp to 652 °C, but with an extra oven step during the pyrolysis stage, which is a sulfur oven where the evolved gases are oxidized into SO 2 at 840 °C. Samples were then transferred to an oxidation oven with an extended analysis time during the oxidation stage and a 20 °C/min ramp from 300 to 1200 °C for decomposition of sulfate moieties 80 , 82 . The SO 2 gas released during both pyrolysis and oxidation stages was measured in real time by an ultraviolet (UV) detector.…”

Simulation of Thermal Maturity in Kerogen Type II Using Hydrous and Anhydrous Pyrolysis: A Case Study from the Bakken Shale, United States

“…Based on the currently available information, this is the first study that examines the role of water in the alterations of macerals in the Bakken Formation and alterations in the maturation, GC, and GC-MS parameters of this well-known shale play. For example, Carvajal-Ortiz et al, 25 used opensystem pyrolysis only for the purpose of examining the fluid souring and H 2 S production in the Bakken Formation. In another study conducted by Jin et al, 26 the Bakken Shale play was subjected to HP to merely study oil-generation kinetics.…”