In shale oil and gas exploration and production, unexpected mineral scale formation, which has been treated as a cancer of this industry, can cause severe formation damages and production reductions due to the much lower permeability and smaller porosity in shale formations. In Changqing Oilfield, calcite and siderite mineral scaling commonly occurred and caused significant production loss. To mitigate and avoid such mineral scale formation, it is critical to identify its root cause. However, due to the complicated working conditions in the field, it is difficult to figure out the source of scaling ions and the scaling mechanisms only via field testing. In this study, we developed a new workflow that couples both experimental testing and thermodynamic model simulations to identify the source of major scaling ions (e.g., Ca2+, Fe2+) and the scaling mechanisms. The experimental results show that both tubing corrosion and formation rock dissolution contribute to the source of ferrous ion. Using ScaleSoftPitzer, one of the most widely used scale risk prediction software, we simulated the scaling risks of calcite and siderite changing with production time via mixing the fracturing fluids and formation fluids at different proportions. Based on the solution compositions of the mixed fluids, the software can predict the scaling risks of different minerals at field conditions. The model simulation results show that the commingling of fracturing fluids and reservoir fluids can lead to the gradual transition of major scaling minerals from siderite to calcite when fluid composition gradually varies while production continues. This study provides a template following which field engineers can determine the root cause of mineral scaling in the field and lay down theoretical foundations for scaling mitigation strategies.