CaCO 3 scaling at 120 degrees C and formation water flow along well tubing during production, is modelled using tube blocking experiments utilizing Na-K-Ca-Cl-HCO 3 -H 2 O brines. All tests produced scale assemblages dominated by aragonite, vaterite (< or =45%) and some calcite. Vaterite is a rare form of CaCO 3 so its occurrence is particularly significant. Vaterite precipitation was initiated by rapid CaCO 3 supersaturation, driven by lower CaCO 3 solubility at 120 degrees C, combined with high relative concentrations of HCO (super -) 3 in the brine (HCO (super -) 3 /Ca = 12). Few accounts of oil field scaling focus on carbonate mineralogy. The formation of mixed-carbonates in this study emphasizes that more attention should be paid to mineralogical characterization of carbonate scale in relation to production conditions, formation water chemistry and inhibitor selection. This could lead to improvements in both scale prediction and inhibitor efficiency.