Reservoir production monitoring using marine controlled-source electromagnetic (mCSEM) has been studied recently because it is sensitive to resistivity changes resulting from variations in hydrocarbon saturation. However, mCSEM for sequestration monitoring has scarcely been investigated, although the mCSEM method is advantageous for monitoring injection and migration. To investigate the feasibility of mCSEM monitoring for sequestration, we conducted numerical experiments of representative injection models at a deep brine aquifer in a shallow sea. By using a modified secondary field method, we effectively address the airwave problem occurring when mCSEM is applied to a target beneath a shallow sea. We demonstrate the modified secondary field method can restore high frequency band data, which is beneficial for the detection of the movement of injected . Furthermore, using a modified scattered field approach for 2.5D forward modeling, we achieve very high accuracy, which is essential for the simulation of electromagnetic fields generated by injection in a deep brine aquifer. The mCSEM response, which is enhanced by the modified secondary field method, shows small but measurable changes in a given pseudorealistic sequestration scenario. The mCSEM responses differ for horizontal and vertical injections. These results include the feasibility of applying mCSEM to sequestration monitoring. Optimum operating frequency bands and source-receiver geometries for a brine aquifer model beneath a shallow sea are proposed based on the results of the numerical experiments. Moreover, we suggest the necessity of various types of data acquisition for the monitoring of a plume based on analyses of multiple components of electric and magnetic fields.