Abstract
This study is focused on the mineralogical, chemical, and isotopic characterization of pyrites from the rocks of the Bazhenov Formation (Upper Jurassic–Lower Cretaceous organic-rich shales, Western Siberia, Russia). Scanning electron microscopy (SEM) revealed pyrites of different morphologies: small and large framboids, small crystals, and large euhedral crystals; all morphotypes were usually combined into aggregates. Isotope ratio mass spectrometry (IRMS) and secondary ion mass spectrometry (SIMS) showed that small framboids and microcrystalline pyrite are isotopically light, with δ34SCDT varying from −55 to −20‰. Large framboids and euhedral crystals of pyrite are isotopically heavy with δ34SCDT up to +26‰. Both morphology and δ34S were suggested to be controlled by the redox conditions and sedimentation regime. The abundance of small framboids suggests that pyrite sedimentation occurred under anoxic conditions; the presence of the large framboids and euhedral crystals of pyrite suggest the accumulation of sediments occurred at suboxic conditions, possibly in the presence of oxygen.
Highlights
At reduced or oxygen-depleted conditions, isotopically light pyrite could crystallize above the sediment-water boundary or in the upper layer of sediments having an excess of marine sulfate
Four types of pyrite were identified in the sediments of the Bazhenov Formation of Siberia: small framboids, aggregation of small crystals, large framboids, and large euhedral crystals
Large framboids and euhedral crystals of pyrite are isotopically heavy with δ34 SCDT up to +26‰
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The Bazhenov Formation of West Siberia is one of the largest source rock formations in the world. It has been actively studied for more than two decades, and interest in understanding this formation remains high. The potential of unconventional hydrocarbon reservoirs is defined by the amount and quality of organic matter, which controls the genesis of hydrocarbons [1]. Organic matter is actively accumulated and well preserved in stagnant basins lacking oxygen, i.e., processes that occur at low oxygen (suboxic), oxygenfree (anoxic), and hydrogen sulfide (euxinic) conditions [2,3,4]
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