Re-Os Geochronology of Ediacaran Bitumen from Northeast Sichuan Basin: Implications for Thermal Cracking and Hydrothermal Alteration in Petroleum Systems

Abstract

This study explores the timing and processes of hydrocarbon evolution in the Sichuan Basin, focusing on pyrobitumen and sphalerite from the Dengying Formation. By utilizing Re-Os geochronology, we aim to refine the precise timing of hydrocarbon generation, migration, and alteration in this area, with particular attention to the effect of hydrothermal fluids. The Re-Os study of pyrobitumen provided isochron ages of 166 ± 26 and 183 ± 39 Ma, which are interpreted as representing the pyrolysis of ancient hydrocarbon reservoirs and subsequent pyrobitumen generation, potentially associated with thermochemical sulfate reduction (TSR). These findings correspond to the timing of large-scale mineralization events in the Sichuan-Yunnan-Guizhou region, reinforcing the value of Re-Os geochronology in high thermal maturity bitumen. Additionally, the study underscores the impact of hydrothermal alteration on the Re-Os system, as sphalerite samples showed lower Re-Os concentrations than bitumen. This suggests that the Re-Os isotope composition of petroleum may reflect inputs from both source rocks and hydrothermal fluid interactions. The research emphasizes the necessity for precise sampling and understanding of isotopic systematics to minimize variability in data and ensure reliable geochronological interpretations. Overall, this study enhances our understanding of hydrocarbon system evolution and provides key insights into the time-frame of critical events in hydrocarbon systems.

Keywords

Re-Os geochronology / solid bitumen / hydrothermal alteration / Sichuan Basin / petroleum geology

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