Application of clumped isotope solid-state reordering to thermal evolution: A case study of Ordovician strata in Ordos Basin

Understanding the thermal evolution of sedimentary basins is critical to hydrocarbon accumulation, as it influences basin development, hydrocarbon generation, migration, and the formation of source rocks and reservoirs. While paleothermometry, primarily applied to organic matter and heavy minerals, has traditionally been the standard method for reconstructing basin-scale thermal histories, marine carbonate strata lack conventional paleothermometers, posing a significant challenge. Clumped isotope analysis, however, offers a promising alternative, leveraging temperature-dependent ¹³C-¹⁸O bond reordering influenced by lattice defects. Recent advancements in modeling approaches—such as first-order approximation (Passey et al., 2012), transient defect models (Henkes et al., 2014), paired reordering/diffusion models (Stolper et al., 2015), and continuous first-order reaction models (Hemingway et al., 2021)—have broadened the applicability of clumped isotopes across diverse geological contexts. However, applying clumped isotope solid-state reordering models without constrained thermal history paths may lead to significant discrepancies in simulation outcomes. To improve accuracy and reduce uncertainty, this study integrates fluid inclusion microthermometry, U-Pb dating, and vitrinite reflectance (R_o) to jointly constrain thermal history paths.

The Ordos Basin, a major hydrocarbon-bearing region in northwestern China, has undergone complex tectonic and depositional transformations, particularly during the Caledonian Orogeny, which obliterated sedimentary records from the Late Ordovician to Early Carboniferous periods. To address the challenges of reconstructing its thermal history, this study combines clumped isotope thermometry, U-Pb dating, fluid inclusion analysis, petrography, X-ray diffraction, and carbon-oxygen isotope analysis. Clumped isotope reordering simulations in calcite cements, constrained by in situ U-Pb dating and fluid inclusion microthermometry, reveal Ordovician paleotemperatures of 180–190°C during the Cretaceous. Similarly, reordering simulations in micritic matrices, supported by R_o and fluid inclusion microthermometry, indicate paleotemperatures of 170–200°C during the Caledonian, a period characterized by deep burial and accelerated source rock maturation. These findings provide critical insights into the thermal history of Ordovician strata in the Ordos Basin, offering valuable guidance for hydrocarbon exploration and advancing our understanding of early hydrocarbon generation processes.

Additionally, this study examines core samples from different depositional environments within a single well, utilizing petrography, Sr isotope, and trace element analyses. Variations in Δ47 clumped isotope values among dolomites from distinct depositional settings suggest that factors such as paleo-salinity and microbial sulfate reduction (MSR) significantly influence Δ47 values. By incorporating clumped isotope kinetic models, this study also investigates the impact of microbial activity, pH, and temperature on ancient dolomite formation. These findings provide a theoretical framework for further research into the formation mechanisms of ancient dolomites in sedimentary strata.