The genesis and significance of pyrite in the Ordovician Majiagou Formation in the central and eastern Ordos Basin

The geochemical data of the gas sourced from the Majiagou Formation indicate low organic content and high thermal maturity, which make the evaluation of the hydrocarbon generative potential difficult. However, the low organic abundance and high thermal maturity of the Majiagou Formation make it difficult to evaluate its hydrocarbon generation potential. The study of the genesis of pyrite can provide a better understanding of a source-rock depositional environment and its evolution. In this paper, we report on a significant amount of pyrite linked to the hydrocarbon source in the Majiagou Formation. Light and electron microscopy were used to observe the morphology of the pyrite, while sulfur, carbon and oxygen isotopes were used to investigate the geochemical characteristics of pyrite and the surrounding rock matrix. The results showed that macroscopic stellate pyrite filled cracks and is found in veins or along the laminar surface, as well as agglomerated pyrite; microscopically, pyrite is dominated by autotypic pyrite in the form of cubes, pentagonal dodecahedrons and columns. The $δ$³⁴S_CDT ranged from 4.6 to 27.5‰, $δ$¹³C_PDB values ranged from −3.4 to −2.3‰ and $δ$¹⁸O_PDB values ranged from −10.5 to −7.3‰. Our findings, combined with the geological background and thermal history of the region, suggest that the pyrite in the Majiagou Formation was formed by thermochemical sulfate reduction, which indicates that large-scale hydrocarbon generation events have occurred. The rocks of the Majiagou Formation had the ability to generate hydrocarbons. Our research can be utilized as supporting material for the exploration and development of natural gas fields in the Ordos Basin.