In-situ combustion (ISC) is a successful method with great potential for the production of heavy oil. Application of ISC is limited, however, because the process is complex and not well understood. A significant open question for ISC is the formation of coke or "fuel" in correct quantities that is sufficiently reactive such that combustion is sustained. We study ISC from a laboratory perspective in one-meter long combustion tubes that allow monitoring of the progress of the combustion front using X-ray computed tomography (CT). Two crude oils with API gravities of 12 and 9 are studied. Images of oil movement and banking in situ are obtained through appropriate analysis of the spatially and temporally varying CT numbers. Combustion tube runs are quenched prior to front breakthrough at the production end thereby permitting a post mortem analysis of combustion products and in particular of the fuel (coke and coke-like residues) just downstream of the combustion front. Fuel is analyzed using both scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). XPS and SEM results are useful to identify the shape, texture, and elemental composition of fuel in the X-ray CT images. The SEM and XPS results aid in differentiation among combustion tube results with significant and negligible amounts of clay minerals. Initial results indicate that clays increase the surface area of fuel deposits formed and this aids combustion. Additionally, comparisons are made of coke-like residues formed during experiments under an inert nitrogen atmosphere and coke-like residues from in-situ combustion. Study results contribute to an improved mechanistic understanding of ISC, fuel formation, and the role of mineral substrates in aiding or impeding combustion. CT imaging permits inference of the width and movement of the fuel zone in situ.