Evaluating the nature of binding of the organic phase (called kerogen) with clays and other minerals in gas and oil shale is of critical importance for developing optimal processes for hydrocarbon extraction and recovery. Here we report the results of an ab initio reactive study of a gaseous oil/clay interface, which consists of grafting fragments of sp³ methane and sp² benzene onto the basal surface of illite. Methane and benzene fragments were selected as they correspond to the simplest monomers of the immature (aliphatic sp³ carbon atoms) and mature (aromatic sp² carbon atoms) stages of the kerogen macromolecule while illite is encountered in many shale formations. We find that the methyl or phenyl radicals always bond upright to one of the outmost Si atoms of the clay phase, while the H atom attaches to the inner Al–OH group forming a water molecule. The covalent attachment of the radicals to the oxygen atoms of the clay surface is always less favorable than to the silicon atoms. The energetics of these grafted adducts are discussed along with the maturation state of the shale. An unanticipated signature of the covalent Si–C binding is the formation of hypervalent Si complexes, characterized by elongated bonds at the oil/clay interface. We complement our atomistic modeling by simulating IR spectra of the two covalently grafted interfaces (methyl–illite and phenyl–illite) which fingerprint elongated Si–C bonds in the frequency range 1100–1250 cm^–1. This novel finding should have a large impact on the understanding of fracking conditions in shales through this tight- and loose-bonded organic/inorganic interface.