Pyrite morphologies in Bazhenov Formation source rocks: diversity, systematic classification, sulfur isotopic composition and insights into sedimentation and diagenesis processes

Pyrite is a common authigenic mineral in organic-rich source rocks, which is diverse in morphology and carries valuable genetic information about geological processes. This study proposes comprehensive genetic morphological classification of pyrite aggregates in organic-rich shales of the Bazhenov Formation – one of the world largest source rock located in the Western Siberia. Scanning electron microscopy was used to analyze over 530 samples from 17 wells, identifying various types of pyrite such as framboids (with corresponding subtypes based on orderliness of the internal structure, crystallographic shape of microcrystals, infilling of internal structure, and integrity), euhedral crystals, and pyrite pseudomorphosis. The obtained sulfur isotopic composition for framboidal pyrite, which turned out to be the lightest (δ³⁴S vary in the range from −55.38 ‰ to −26.02 ‰), is indicative of anoxic conditions on seafloor. Euhedral pyrite develops in diagenesis under limited sulfate supply, reflecting slow growth in pore-water depleted in ³²S (δ³⁴S values range from +15.05 ‰ to +26.49 ‰). Described morphological types cover the diversity of pyrite forms in other similar geological objects around the world, which allows pyrite to be used as a geochemical proxy for paleoenvironmental reconstructions and hydrocarbon exploration, enhancing understanding of sulfur cycle and basin evolution.