Oil-shale (OS) self-heating retorting (SHR), which can spontaneously increase retorting temperature in the absence of external heat provision, is a low-energy-input retorting process but without the cost of loss in the oil yield. However, because OS decomposition is extremely complex, the sources of the heat used to form SHR are still unclear. In this work, the semicokes obtained from SHR and anaerobic retorting (AR) are analyzed and compared for finding a possible heat source. The results show that a noticeable amount of carbon remains (CRs) are produced and left in the semicoke byproduct during AR, but CRs can be utilized in SHR via in situ exothermically reacting with oxygen during retorting, which serves as a source of the heat used to form SHR. By 500 degrees C, AR produces similar to 3000 J more CRs byproduct than SHR (for per gram of OS); that is, SHR extra utilizes similar to 3000 J heat for replacing external energy input. This heat is equivalent to as much as similar to 3 times of the heat needed by completing a 550 degrees C AR process if only the heat adsorbed by OS is considered (i.e., if you do not consider the heat adsorbed by the reactor, the heat lost by radiating outside the reactor, etc.). This work shows the remarkable contribution of CRs to forming SHR, which can be considered as a way of in situ heat generation for decreasing external energy input in the retorting.