Carbonate clumped isotopes: a new tool to assess carbonate cementation in clastic sediments

Abstract

Carbonate cements in clastic rocks capture a geochemical record of burial processes. As such, much of our understanding of clastic diagenesis and subsurface microbial activity is influenced by studies of carbonate cemented horizons and concretions. Furthermore, their presence can have a significant impact on clastic reservoir quality. However, the formation of cement bodies is poorly understood. A part of this problem is that studies investigating carbonate cements have historically been limited by the δ18Ocarbonate being dependent on both the carbonate precipitation temperature and δ18Oporewater. Often, these variables cannot be constrained and one or the other has to be assumed to derive an interpretation, with significant implications for understanding the fluid history and timing or diagenetic processes. This PhD study applies a new geochemical palaeothermometry technique, "carbonate clumped isotopes", to constrain the temperature of the cement precipitation. With this, the δ18Oporewater can be back-calculated and the formation burial history of changes in δ13C and δ18Oporewater can be constrained. This thesis contains three studies using the clumped isotope technique to investigate carbonate cementation. The first examines outcrop carbonate cements in the Mancos Shale, Colorado, where changes in δ13C and δ18Oporewater are captured over a temperature range of 33 – 117 °C and placed into a burial history. The second study then examines a variety of cements and concretions from different formations around the world to determine the geochemical conditions necessary for cement precipitation, through trends in δ13C, δ18Oporewater and temperature. This reveals that the temperatures of cementation appears to be strongly controlled by the optimal temperature of activity for subsurface microbes. The final study then applies the technique to subsurface carbonate cements in cores from the Bruce Field, UK North Sea. This places carbonate cementation and the δ18Oporewater into the burial history of one of the reservoir formations. The results of this PhD demonstrate the usefulness of carbonate clumped isotopes for understanding clastic carbonate cementation burial histories, in outcrop and the subsurface. Furthermore, they provide an insight into the temperatures at which microbial processes occur during diagenesis and more generally, the chemical conditions common to carbonate cementation.