Thermal Transformation of Sulfur Species and Hydrogen Sulfide Formation in High-Pyrite-Containing Lacustrine Type-II Shale during Semiopen Pyrolysis

Abstract

The shale of the seventh member of the Triassic Yanchang Formation in the Ordos Basin (abbreviated as Chang 7 shale) is the most prospective shale for in situ conversion process. In our previous study, a series of artificial maturation experiments were conducted to simulate the hydrocarbon generation–retention–expulsion process. However, during the experiments, a large amount of hydrogen sulfide (H₂S) was generated along with hydrocarbon products. Therefore, in this study, a combination of X-ray photoelectron spectroscopy, X-ray diffraction, and elemental analysis was used to reveal the thermal transformation of sulfur species and the formation process of H₂S. The results showed that kerogen decomposition and the corresponding products exert a strong influence on the transformation of organic sulfur and pyritic sulfur and, thus, on H₂S formation. Before the peak hydrocarbon-generating stage (<0.6% Rₒ, 340 °C), the H₂S yield was very low, primarily originating from organic sulfur in the kerogen. During the peak hydrocarbon generation and secondary cracking stage (0.6–1.24% Rₒ, 340–400 °C), the H₂S content showed an noticeable increase, and because the bulk atomic Sₒᵣg/C ratio remained relatively constant, the main source of H₂S changed from organic sulfur to inorganic sulfur. Kerogen decomposition, pyrite decomposition, and thermochemical sulfate reduction (TSR) reactions contributed to H₂S formation, whereas secondary pyrite formation consumed H₂S. When the hydrocarbon generation potential of kerogen was almost exhausted, H₂S exhibited an abnormally sharp increase, and little or no secondary pyrite was formed. The sulfur generated by pyrite decomposition partly formed H₂S and partly incorporated into the organic matrix of kerogen. Hydrogen radicals generated by kerogen decomposition and secondary oil cracking are proposed as the controlling factor in the initial pyrite decomposition of the Chang 7 shale under the present pyrolysis experimental conditions.