Résumé
Kerogen evolution in organic-rich sedimentary rocks, by its time scale and complexity, is a particularly challenging process for atomistic modeling and simulations. Yet, such modeling approaches, provided that appropriate simulation techniques are used and reasonable simplifications are made, can unravel the most significant physicochemical mechanisms underlying oil and gas generation and the evolution of the kerogen structure and properties. Here, we combine reactive molecular dynamics and the replica-exchange molecular dynamics simulation technique to simulate a simple model of type I organic matter, described as a mixture of five unsaturated fatty acids as a simplified model of algae, along its maturation pathway, from diagenesis to late metagenesis. We describe fluid production, from the oil to gas windows, and kerogen evolution in terms of the structure and properties. We show that the formation of permanent microporosity within the kerogen matrix is intimately correlated to both the fluid production and the rigidification of the kerogen skeleton occurring in the gas window, i. e., at low O/C and H/C atomic ratios. We show that because of their pristine molecular structures with a more or less long tail made of alternating single and double carbon–carbon bonds, fatty acid molecules do decompose into alkanes of various lengths, up to octane in our simulations, by contrast to lignin and cellulose that only produces short alkanes, mostly methane.