Three types of pathways (degradation-recondensation, natural sulphurization and selective preservation) are commonly considered for the formation of kerogen dispersed in sedimentary rocks. A fourth pathway has been recently put forward, however, from studies on Recent marine sediments, the so-called sorptive protection mechanism. This pathway is based on the adsorption of otherwise labile organic compounds onto minerals, thus preventing their diagenetic degradation and promoting their subsequent condensation into kerogen. The main results of the present study are derived from a combination of microscopic and pyrolytic methods applied on a Cenomanian kerogen. They provide (i) evidence, on an ancient material, for a crucial role of the mineral matrix both in organic matter (OM) preservation during kerogen formation and in kerogen stability once formed, (ii) indications that the dominant protective process likely involves physical protection by minerals, resulting from alternation of organic and clay nanolayers of approximately 100 nm in thickness, rather than OM adsorption as molecular monolayers and (iii) observations of the relatively poor stability of an isolated kerogen, contrary to the inertness commonly assumed for fossil macromolecular organic matter.