Teoh, Chia Pei (2021-07). Dolomite and high-magnesium calcite: Case studies from laboratory, field, and computer simulation studies. Doctoral Dissertation.
Thesis
Dolomite (CaMg(CO3)2) is a diagenetic mineral occurring throughout geological time which replaces the precursor limestone via dolomitization. Dolomitization can alter rock properties such as porosity, permeability, and geochemistry; it requires a source of magnesium, a limestone precursor, and a hydrodynamic drive to move fluids through the limestone precursor. High-magnesium calcite (HMC) is of relevance as it can potentially satisfy the first two requirements of dolomitization on its own. This dissertation examines the relationship between HMC and dolomite using high-temperature dolomite synthesis experiments, powdered X-ray diffraction, scanning electron microscopy, petrographical observations, geochemical analyses, digital outcrop models, and fluid flow simulations. In high-temperature dolomite synthesis studies, we find that neither the rate of dolomitization nor dolomite stoichiometry correlate with HMC magnesium concentrations. However, mean dolomite crystal size has a negative relationship with HMC magnesium concentration. In field studies in Bonaire, selective dolomitization is observed within partially dolomitized samples, with the micrite matrix most likely to be dolomitized first, followed by coralline red algae, and subsequently all other allochems. Dolomitization also appears to have initiated from coralline red algae grains rather than by external fluids. Flow simulation results reveal an exponential relationship between water breakthrough times and flow rates versus dolomite proportions. Additionally, the arrangement of the dolomite bodies (aligned vs. disjoined) has very similar fluid flow behavior across a wide range of dolomite proportions. Sensitivity of flow behavior to the geological models is strongly dependent on dolomite permeability relative to precursor limestone. These studies have the following implications: 1) The observed correlation between coralline red algae abundance and global dolomitization events in the Neogene is not caused by reactant magnesium concentration, but likely reflects changes in species abundance and microstructure. 2) There is a more complex paragenetic sequence for the Seru Grandi clinoforms than previously known. 3) Having a good understanding of the dolomitization mechanism and dolomite body geometries reduces uncertainty in dolomite distributions and petrophysical properties in the subsurface, which allows for better fluid flow simulations.
