Hydrocarbons in subsurface reservoirs are generally found to be compositionally graded, with fluids deeper in connected and equilibrated reservoirs being relatively enriched in asphaltenes. These gradients result from effects such as gravity, entropy, and solubility. However, it is unclear if those same effects lead to gradients in the detailed molecular composition of asphaltenes. Here, we investigate the sulfur chemistry of asphaltenes from a reservoir with a large gradient in asphaltene content. Measurements of the sulfur content from combustion as well as measurements of sulfur speciation from K-edge X-ray absorption near edge structure (XANES) spectroscopy find no significant difference in the composition of the asphaltenes. Thus, different locations within this reservoir contain oils with different asphaltene concentrations, but the asphaltenes from throughout the reservoir all have the same sulfur chemistry. This result suggests that gradients in asphaltene content can be successfully modeled with the simplifying assumption that the asphaltene molecular composition is not graded in connected and equilibrated reservoirs.