Removal of organic compounds is a common first step in surface characterization of inorganic materials. To explore the surface reactivity of environmental samples such as rocks, soil, and sediments, it is essential to know if they are pure or covered with organic material. Our purpose was to evaluate the influence of some common cleaning procedures on the surface composition of inorganic solids and to investigate if such treatments result in completely clean mineral surfaces. We used Soxhlet solvent extraction and chemical oxidation to remove crude oil from samples that we synthesized to represent oil bearing sandstone. After each cleaning step, we tracked surface composition using X-ray photoelectron spectroscopy. We demonstrated that (i) it was impossible to completely remove the organic material with any of the methods, (ii) the oxidation treatment dissolved soluble minerals, and (iii) surface composition was dominated by clay particles adhering to the larger grains and thus was responsible for a significant fraction of the residual adsorbed organic compounds. The most important conclusion is that the pore surface composition, i.e. the clay fraction not the bulk matrix composition, ought to be used for thermodynamic and kinetic calculations to predict pore fluid composition and evolution during transport through pore networks.