Hydrogen sulfide (H2S) is an undesired by-product of oil and gas production in both conventional and unconventional plays. Its random occurrences in various locations of Bakken petroleum system (BPS) wells and secular variations in concentrations have been poorly understood by oil producers trying to minimize H2S in their wells. The goal of this work was to provide insight into one of the potential mechanisms that caused H2S generation in the BPS. This insight will enable more effective H2S mitigation and management strategies. Many operators consider the Madison petroleum system as the primary source of H2S, which is coproduced from the Bakken reservoir as a result of out-of-zone well completions into the Madison reservoirs. This work focuses on quantifying the H2S generation potential of the Madison Group and Bakken Formation source rocks during the maturation process.
Organic-rich rock samples representing geographical areas with various levels of H2S concentration in the Williston Basin of western North Dakota (Divide, Mackenzie, Dunn, and Bottineau Counties), stratigraphic units (Bakken and Madison), and a wide range of maturities were collected to investigate cracking of kerogen during source rock maturation as a possible mechanism of H2S generation in the BPS. Analysis of the sulfur content and type using the Rock-Eval 7S method did not indicate elevated contents of total sulfur and total organic-linked sulfur in organic-rich rocks selected for this study, refuting an initial hypothesis of Type IIS kerogen presence in the BPS or the overlying Upper Lodgepole/Lower Mission Canyon source rock. In particular, the kerogen from the highest H2S-producing areas of Divide County did not show an elevated total organic sulfur content in the Bakken Formation or Madison Group. However, obvious differences in these source rocks have been found in the 1) distributions of saturate biomarkers, 2) concentration of sulfur-containing benzothiophenes, and 3) contents of carbonate material and pyrite-linked sulfur.
Low- and high-temperature pyrolysis experiments of organic matter proved that the source rocks included in this work have immense potential to generate significant concentrations of H2S via thermal breakdown. However, δ34 of the pyrolysis gas did not match those measured in the H2S in a sizable group of samples collected from producing BPS wells, suggesting that thermal cracking of the organic matter of the source rocks was not the only contributor to H2S generation in the BPS. It is likely that the current presence of H2S in the Bakken reservoir is due to other mechanisms of H2S generation, such as thermochemical sulfate reduction (TSR) or/and chemical reactions triggered by injected well stimulation fluids.
