Reconstructing the basin thermal history with clumped isotope

Due to the lack of effective paleo-geothermometers, it has often been difficult to reconstruct the thermal history of marine carbonate strata, resulting in unreliable conclusions. As emerging paleo-geothermometers, the clumped isotope of carbonate rocks could directly provide the temperature information without relying on the assumed isotopes of paleo-fluids, which has shown excellent potential in the study of the thermal history of carbonate reservoirs. To exclude the interference from late diagenesis, this study examined multiple carbonate samples under minor diagenetic alteration, which were selected from several wells in the Tarim Basin and Sichuan Basin to examine the clumped isotope (measured as Δ47 values). Considering the temperature range and acid fractionation factors of various clumped isotope formulae, the Deflieses formula was selected to calculate the temperature of clumped isotope. By analyzing the experimental results, the meaning of Δ47 temperature in the naturally evolved limestones and dolomites, the reordering rules of 13C–18O solid-state bond in calcite clumped isotope, and the effects of thermal history on clumped isotope in different lithologies have been comprehensively discussed herein. The Shun-Ka area located in the center of the Tarim Basin has been subjected to simple tectonic movements and continuous subsidence, because of which the strata of this area are currently experiencing the highest burial depth and temperature, which provide ideal conditions for studying the influence of temperature on Δ47 values. The measured Δ47 values of 17 samples from the Shun-Ka area were in the range 0.443‰–0.634‰, and the calculated temperatures of the clumped isotope were in the range 49.9–201.7°C. The Δ47 temperatures of these samples were higher than their diagenetic temperatures, indicating that Δ47 was affected by the burial process with the reordering of solid-state 13C–18O bonds. Based on the relationship between the Δ47 temperature and borehole temperature in the Shun-Ka uplift of the Tarim Basin, it was suggested that the "blocking temperature" of 13C–18O solid-state bond reordering in the naturally evolved calcites would not be higher than 120°C and the "equilibrium temperature" would not be lower than 160°C, which was consistent with the foreign laboratory results. Furthermore, to compare the effect of clumped isotope in different lithologies, another 11 dolomite samples were selected from the central uplift of the Sichuan Basin for analyzing the clumped isotope. The measured Δ47 values of dolomites from the Sichuan Basin were in the range 0.423‰–0.537‰, and the calculated temperatures of the clumped isotope were in the range 105.7–233.5°C. Similarly, it is possible that the warming effect associated with increased burial depth resulted in 13C–18O solid-state bond reordering and a higher temperature, which was the result of both diagenesis and burial heating. To investigate the effects of thermal history on clumped isotope, the first-order approximation model was used to simulate the clumped isotope values based on the thermal history reported in previous research. The initial diagenesis temperature was set to 20°C, and it was assumed that the Δ47 temperatures of samples were only affected by diagenesis temperature and thermal history. Results revealed that the Δ47 temperature calculated using the first-order approximation model of limestone samples from the Yingshan Formation of the Tarim Basin exhibited good agreement with the measured temperature, but the calculated Δ47 temperature in the dolomites considerably differed from the actual measured temperature. This conflict could have occurred due to various thermodynamic properties (such as frequency factors and activation energy) and the complex diagenesis process between limestones and dolomites. Therefore, the first-order approximation model could be used more accurately to describe the effect of the basins thermal history on 13C–18O solid-state reordering in calcites. However, further research should be conducted on dolomites to build an inversion model focusing on the thermal history of the basin and clumped isotope.