Electromagnetic waves as a viable means of introducing heat energy to reservoirs to allow for transmission of heavy or extra-heavy oil have been gaining prominence and notoriety in recent years due to its applicability to a wide variety of reservoirs. However, how reservoir properties affect the electromagnetic wave penetration is not well defined. This study investigates the impact of different reservoir rock and fluid combinations on the electromagnetic wave penetration and also introduces the dependency of dielectric properties on pressure. Several different reservoir rock samples (quartz rich, carbonate rich) with varying lithology and porosity were used in this study. The contribution of the fluid type was investigated by saturating the cores with water as well as measuring the responses on dry cores as a control. Air and water saturated rock samples were irradiated electromagnetically at varying frequencies (200 MHz to 6 GHz) under pressure. Frequency dependent dielectric properties were measured for each sample utilizing a coaxial dielectric probe and a vector network analyzer. Dielectric constant (ε′), loss index (ε″), and loss tangent of test mediums were utilized to generate the penetration depth of each sample as a function of frequency. The loss index and dielectric constant comprise the complex permittivity which is the foundation for microwave absorbance and penetration depth. Penetration depth is highly frequency dependent and exhibits an exponential decay where as the wave travels further into the sample more energy is gradually absorbed by the material and thus the energy content of the wave continually diminishes. With lower frequencies, higher penetration depth was obtained for all samples where less energy has been dissipated and absorbed by the formation. The utilization of both water and air represent both a very effective absorber of microwaves as well as a material transparent to microwaves respectively. Therefore, the dry cores (air saturated) realized greater penetration depths as less attenuation occurred due to the transparent nature of the saturating fluid. The quartz rich sandstone achieved lower penetration depths than the limestone core utilized which is indicative of greater capability of the sandstone samples to absorb microwave energy. Reservoir properties will affect the dielectric response of the material and so it becomes necessary to account for the presence of pressure due to overburden while taking laboratory measurements. The pressurized samples for both the sandstones were found to cause disparity between the control experiments when saturated with water. Introducing pressure of the water saturated sandstone samples effectively lowers the loss tangent resulting in a decreased capability to absorb microwave energy.