THAI, or "Toe-to-Heel Air Injection," is an integrated horizontal well process for in situ recovery and upgrading of heavy oil and tar sands bitumen. During extensive studies of the process at the University of Bath, involving more than 50 three-dimensional combustion cell tests, the process has repeatedly demonstrated robust and stable operation. However, the answer to the question: "Why does oxygen breakthrough into the toe of the horizontal well not occur," has not been fully developed. This is now all the more important since the THAI process is about to be tested in the field. In order to maintain stable propagation of the combustion front, sufficient fuel needs to be available ahead of the front. This is fundamental to the in situ combustion (ISC) process, whether in its conventional form, or THAI. When the combustion front approaches close enough to the horizontal producer well, heavy residue can drain into the well. This residue, or coke material, provides a gas seal, preventing the injected air from channeling through to the well. This paper presents post-mortem results of two THAI experiments, in which the horizontal well was cut-open to reveal the extent of heavy oil residue/coke deposition. The visual evidence is supported also by a numerical simulation of the experiment, showing the distribution of coke and oxygen through the oil layer.