Morphology and sulfur isotopes of pyrite in Chang73 shale: Indications for redox conditions and enrichment of organic matter and hydrocarbons

Shale oil is abundant in lacustrine organic-rich source rocks within the Chang7₃ (Ch7₃) submember of the Triassic Yanchang Formation in the Ordos Basin, China. However, there remains a lack of understanding regarding the coexistence of pyrite with organic matter and hydrocarbons in the Ch7₃ submember. This study systematically investigates organic geochemistry, elemental geochemistry, pyrite sulfur isotopes (δ³⁴S_py), and size distributions of pyrite framboids to reconstruct bottom-water redox conditions and elucidate its influence on organic matter enrichment and the role of pyrite in organic matter and hydrocarbon enrichment. Pyrite-based parameters, including DOP_M proposed in this study (mineral-based degree of pyritization, ranging from 0.53 to 1.00 with an average of 0.84), size distributions of pyrite framboids (ranging from 1.10 μm to 31.54 μm with an average of 8.80 μm), and δ³⁴S_py (ranging from −0.5 ‰ to 6.1 ‰ with an average of 3.2 ‰), indicate that the bottom-water redox conditions during deposition of the Ch7₃ submember were primarily dysoxic–oxic, frequently accompanied by anoxic–euxinic. Good correlations between redox parameters (pyrite framboids’ size distributions, δ³⁴S_py, Fe_T/Al, DOP_M, and enrichment factors of U and Mo) and total organic carbon (TOC) contents suggest that anoxic bottom-water conditions were a significant, although not exclusive, controlling factor for organic matter enrichment in the Ch7₃ submember. Moreover, correlations between pyrite-related parameters (pyrite framboids' size distributions and δ³⁴S_py) and TOC, chloroform bitumen “A”, and Rock-Eval parameters, respectively, suggest that smaller diameter and narrow size distributions of pyrite framboids as well as low δ³⁴S_py values indicate organic matter and hydrocarbon enrichment and great oil potential.