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Attempt Using Carbonate Clumped Isotopes to Reconstruct the Thermal History of Sichuan Basin


The ability to reconstruct the thermal history of a basin is essential in modeling reservoir and source rock quality. In carbonate rocks, conventional thermal indicators could not be used: apatites and vitrinite are hard to be found. The clumped isotope palaeothermometer is a promising technique for constraining the thermal history of basins. Carbonate clumped isotope thermometry is based on the preference of 13C and 18O to form bonds with each other. At elevated temperatures such bond ordering is susceptible to resetting by diffusion of C and O through the solid mineral lattice. This type of bond reordering has the potential to provide a basis for thermal history of basins. In this study we test if carbonate clumped isotope thermometry could be used to explore the thermal histories. The study reservoir is Cambrian strata in Sichuan basin, southwest China, which is deeply-buried sedimentary carbonate hydrocarbon reservoirs. It provides ideal test cases as samples from industry wells are available over a relatively wide burial depth range of ~3000-5000 meters and the calcites and dolomites are relatively homogeneous. The Carbonate components of Cambrian strata in Sichuan basin, buried to 5000m depth, yield statistically indistinguishable clumped isotope temperatures(TΔ47), ranging from 121 to 189°C. We predicted TΔ47 by first-order kinetic models of C-O reordering based on assumed T-t histories. If it was coupled with the measured TΔ47 of carbonate samples, the thermal history may be effective. Application of these models, suggests that the temperature of Cambrian strata in Sichuan basin had reached more than 200°C in Cretaceous, and then decreased to present temperature. This result coheres with former research. The 40-50°C cooler clumped isotope temperatures measured can be explained by recrystallization and cementation during shallow burial combined with a greater inherent resistance to solid-state reordering. Applications of clumped isotopes to recover basin thermal history is currently more challenging because of the lack of credible C-O reordering model. But Clumped isotope thermometry give us a new approach to determine burial history based on geochemistry, mineralogy, petrology, and modeled isotope-exchange reactions by solid-state diffusion.