Coupling clumped isotope thermometry and XRD analysis in the study of shallow-burial dolomite diagenesis

Abstract

Dolomitization is a common reaction in carbonate hydrocarbon reservoirs. The volume of hydrocarbons in place and their recovery can be affected by the presence of dolomite bodies that can increase or decrease the reservoir porosity and permeability. At the beginning of the reservoir burial history (< 1 km), dolomitization is likely caused by seawater, that brings Mg into the reservoir. The location and size of the dolomite bodies are challenging to determine because they are controlled by the paleoflow of seawater in the subsurface. The flow pattern depends on unknown parameters, including the seawater temperature and chemistry. The seawater temperature and chemistry are usually inferred from indirect geochemical methods, using proxies measured in dolomites, such as δ18O, δ13C, the Mg/Ca ratio, and recently, clumped isotopes (D47). These proxies can be modified by fluid/rock interactions after dolomitization, a process called diagenesis. If the impact of fluid/rock interactions on the proxies is not well characterized, the reconstruction of ancient seawater temperature and chemistry is biased. The aim of this thesis is to examine the impact of diagenesis on shallow-burial dolomite clumped isotope composition, and therefore evaluate the reliability of D47 in dolomite studies. First, new mass balance models were developed in Matlab in order to quantify the impact of dissolution/precipitation diagenetic processes on D47, as well as δ18O, δ13C, and the Mg/Ca ratio. The outputs of the mass balance models were compared with a real case study, the dolomites of the Marion Plateau (NE Australia). Marion Plateau dolomites were sampled from three IODP holes and the isotopic composition was measured, as well the stoichiometry, ordering, luminescence, crystal size and crystal orientation. The results show that the dominant diagenetic process is rock-buffered recrystallization at low water/rock ratio, evidenced by a correlation between D47 and δ18O. Recrystallization starts from the seafloor and is only associated with an increase of stoichiometry at 700 mbsf. Recrystallization is driven by the dolomite Ca-excess and poor degree of ordering, and enhanced in fine-crystalline dolomites due to their high surface area with the micropores. These findings demonstrate that D47 is the most sensible proxy to diagenesis and that caution must be taken if the aim is to reconstruct the initial conditions of dolomitization. However, clumped isotopes help to gain insight into the diagenetic history of shallow burial dolomites, and potentially many other early diagenetic minerals, for which other temperature proxies such as fluid inclusions are not always available.