We have developed an all-new ultradeep boundary detection method for reservoir mapping using borehole electromagnetic (EM) logging while drilling (LWD) measurements. The new method is based on hybrid dipole antennas, which combine the magnetic dipole (MD) transmitter with an electric dipole (ED) receiver. This takes advantage of the strong signal strength of the crosscoupling electric component and its slow attenuation property. Thus, the detection capability improves significantly. Moreover, the induced signal at the ED antenna usually enlarges with decreasing transmitter-to-receiver (T-R) spacing, which has balanced the tradeoff between the depth of detection (DoD) and the tool size. A practical geosteering tool configuration is then proposed by introducing a new open-loop half-circle antenna. Numerical results demonstrate that the new method is feasible for ultradeep boundary detection. By using the coaxial opened half-circle antenna and a coaxial MD transmitter, the DoD can be extended to tens of meters, up to 16 times larger than the T-R spacing under favorable conditions. The new method is a breakthrough in geosteering techniques and it will benefit the wellbore placement optimizations and reservoir evaluation.