Since most easy oil has been produced, there is an increased interest in enhanced oil recovery methods, e.g., in-situ oil combustion. Oil combustion in its current form was first patented in 1953, by R.L. Smith and K.M. Watson. It has not seen widespread use, because controlling the process is considered very difficult, and explosion hazards and corrosion in wells made thermal oil recovery by employing steam, which was also developed in the fifties, more popular. Nevertheless, a steady stream of research continued to be done throughout the years. Experimental and modeling work largely focused on the so-called longitudinal behavior, which describes the process in the form of a wave-train related to a propagating combustion zone. In practice, however, the override of injected air implies the existence of reactive and transport processes perpendicular to the main direction of gas flow. Therefore, this paper embarked upon an experimental study of oil combustion processes occurring below and perpendicular to the overriding gas stream. A T-shaped quartz-reactor is used, of which the vertical part is filled with a mixture of sand and oil. An electrical resistance wire was used to heat up the interface of the overriding gas-stream and the oil-saturated sand column. The composition of the effluent gas was recorded using a gas-chromatograph. The temperature in the vertical part is recorded using thermocouples. From the produced data, a theoretical model is suggested describing the reactions and transport phenomena occurring perpendicular to the main gas-flow direction. Both low temperature oxidation and high temperature oxidation are observed. Air is transported through the sand by thermally driven natural convection and diffusion. Coke formation is observed. From the coke formation observed when using pure hexadecane it is inferred that a mechanism exists that forms coke directly from hydrocarbon components that are in the gas-phase. An experiment using glass beads as a porous medium instead of sand proves that this mechanism also takes place in the absence of potential catalysts. The amount of coke and the amount of transport of oxygen to the combustion zone can both limit the continuation of the combustion. Which is the case depends on what oil is used.