Constraining paleothermal histories of Mesozoic basins through clumped isotope analysis of fine-grained carbonates

Abstract

Understanding burial diagenesis is important for reconstructing basin histories, economically important for hydrocarbon maturation, and scientifically interesting for unravelling the bio- geneic and abiogenic reactions that occur during burial. Observing diagenetic processes which have occurred in the subsurface is difficult due to the increasing depth and temperature over geological time. Therefore, having paleothermometers plays a key part in providing evidence for diagenetic processes that have occurred in the subsurface. Carbonate clumped isotopes (∆47) is a paleothermometry susceptible to re-setting via post-depositional diagenetic processes. Al- lowing this tool to be utilised to reconstruct tectonic trends, diagenetic histories and fluid migration. In this thesis I have applied carbonate clumped isotopes to fine-grained sedimen- tary rocks, which are the most abundant clastic rocks in sedimentary basins and therefore are important for capturing subsurface processes. Fine-grained sedimentary rocks also typically have lower permeability, therefore different processes occur in these sediments in comparison to coarsers sedimentary rocks. In this thesis, the primary aim was to apply carbonate clumped isotopes in fine-grained sedimentary rocks in order to test the robustness of this method on the reconstruction of thermal histories. I first demonstrated how the burial and exhumation of an immature organic-rich fine-grained sedimentary rocks can be determined by measuring the car- bonate clumped isotopes. I then showed how this methodology could be extended by measuring carbonate clumped isotopes of subsurface mature samples that had generated hydrocarbons. I demonstrated how hydrocarbon generation effects the ability of carbonate clumped isotopes to reset to maximum burial temperatures within fine-grained sediments. Finally, I examined the role different lithologies, carbonates poor verses carbonate rich, have on the ability of carbonate clumped isotopes to reset to burial temperatures. To conclude in this thesis applying carbonate clumped isotopes to fine-grained sedimentary rocks in order to reconstruct the thermal history provides a valuable constraint when applied to immature to oil mature samples and to carbon- ate rich samples. However, during gas generation carbonate clumped isotopes are susceptible to resetting to high transient peak temperatures that are no longer representative of burial temperatures. Care must also be taken using this technique in fine-grained sedimentary rocks that contain low carbonate content and high phyllosilicate content. The lower the carbonate content the less readily the carbonate minerals resets to the burial temperatures and therefore may not represent minimum estimates of maximum burial temperatures.