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Experimental Insights on Mineralogical Stabilization in Carbonates

Abstract

Limestone microporosity impacts hydrocarbon reservoirs worldwide. The majority of micropores are hosted between diagenetic low-magnesium calcite (LMC) microcrystals, which exhibit a multitude of textures. Although these microcrystal textures correlate with porosity and permeability, little attention has been given to their diagenetic origins. Case studies generally lack the necessary control to make unequivocal interpretations. A few experimental studies also exist, but have not been methodical in their approach. In this study, stabilization experiments are used to systematically investigate various controls on microcrystal texture. In experiments using aragonite and HMC reactants, temperature (25-215 °C), fluid chemistry, reactant texture, mineralogy, and degree of stabilization were evaluated. Despite a duration of 1 year, experiments at 25 °C produced no LMC. In experiments at ≥70 °C, reaction rates increase with temperature. Despite the temperature range, all products exhibit the granular-euhedral texture, comparable to microcrystals observed in ancient limestones. The crystal size of LMC products, however, decreases with increasing temperature. Reactant texture was evaluated in experiments using single crystal aragonite, modern aragonite ooids, and modern Scleractinian corals pulverized to obtain a specific size fractions. All LMC products exhibit bimodal crystal size distributions. Ooids and corals yielded polyhedral LMC microcrystals, whereas the single crystal aragonite yielded granular-euhedral LMC crystals. Fluid chemistry was evaluated by reacting aragonite at 100 °C in distilled water, a CaCl2 spiked fluid, synthetic seawater, and seawater spiked with CaCl2 to reduce the Mg/Ca in the fluid. The CaCl2 spiked fluid produced polyhedral crystals similar to the “rounded” microcrystals previously cited as evidence of dissolution. No LMC was produced even after 7 months in seawater or CaCl2 spiked sea water. This supports previous work suggesting that Mg inhibits LMC formation. After 1 year HMC (goniolithon powder) reacted in CaCl2 spiked fluids at 25 °C, converted to 50% LMC. The LMC products are characterized by anhedral crystals suggesting that Mg is a control on morphology. No LMC formed in distilled water, seawater, or modified seawater in any of the experiments. These observations further complicate the hypothesis that LMC microcrystals form in marine settings, which are known to have an abundance of Mg.