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Surface Energy Effects on Formation and Preservation of Microrhombic Calcite Fabrics and Porosity

Abstract

Surface energy affects both nucleation and growth of small crystals. This study evaluates the theory behind several proposed mechanisms for porosity preservation by surface-energy control related to crystal and pore-size variations. These theories are tested with microrhombic calcite fabrics in the Pawnee Field (Cretaceous limestone reservoir, Bee Co, TX). Ostwald ripening, crystal growth, and size-selective nucleation will be evaluated. Examined microrhombic calcite fabrics do not have the size distribution expected by Ostwald ripening. Therefore, alteration of crystal and pore-size distribution by Ostwald ripening after calcite precipitation is not a major influence on microrhombic calcite and associated micropore fabrics. Emmanuel et al. (2010) proposed that surface energy selectively preserves small pores by reducing Surface-area normalized growth rate into small pores. The smallest possible stable pore has a critical radius that is controlled by degree of supersaturation and the surface energy. Smaller pores enlarge to the critical radius by dissolution. Larger pores cement until they also approach the critical radius. Observed mean crystal size and size distributions could be explained by this model, but dissolution-enlarged micropores are absent and pore-size variation is too large to be consistent with this theory. This mechanism may help form microrhombic calcite, but it is not responsible for its preservation. Nucleation controls burial cementation by controlling where and how many crystals grow in pores. Burial calcite nucleation is also controlled by surface energy and supersaturation. Because surface area per pore is small for small pores, calcite crystals are less likely to nucleate in a small pore, and if they do, they occlude only the small volume of the pore. Microrhombic calcite fabrics at Pawnee field are most consistent with selective porosity preservation during burial by nucleation. These concepts can be used to predict settings where porosity between microrhombs is expected and how this porosity is preserved during early and late burial. Supersaturation controls both nucleation and growth, so supersaturation history controls formation and preservation of porosity associated with microrhombic calcite.