Microbial and thermochemical controlled sulfur cycle in the Early Triassic sediments of the Western Canadian Sedimentary Basin

Abstract

Pyrite is one of three main sulfur reservoirs and one of the biggest fluxes in the global sulfur cycle. The sulfur isotopic signature of hydrogen sulfide, pyrite and their parent sulfate is widely used as a proxy for tracking sulfur cycle variations in diagenetic environments. The Early Triassic Montney Formation in the Western Canadian Sedimentary Basin is characterized by distinct regional variations in pyrite abundance, type, sulfur isotopic signature and H 2 S concentrations in natural gas. Two main types of framboidal and crystalline pyrite were identified to have formed during various stages of diagenesis. The wide range of δ 34 S pyrite values (−34.4 to + 57.8‰ V-CDT) demonstrates that the sulfur cycle in the Montney Formation is governed by both microbial and thermochemical processes. The comparison of δ 34 S of the produced-gas H 2 S with pyrite, anhydrite and solid bitumen of the Montney, and underlying and overlying formations, suggests a mixture of dominantly in situ and minor migrated H 2 S with thermochemical sulfate reduction origin in the Montney Formation. The large diagenetic variations in pyrite types and δ 34 S pyrite values suggest a lack of direct biogeochemical connection to the sulfur cycle and emphasize the importance of careful petrographic observations and micro-scale isotopic analysis of sedimentary units to accurately reconstruct paleoenvironmental conditions. Supplementary material: Methodology and δ 34 S results are available at https://doi.org/10.6084/m9.figshare.c.5272484