Pore Structure of Organic Matter in the Lacustrine Shale from High to Over Mature Stages: An Approach of Artificial Thermal Simulation

[Image: see text] The pore structure of organic matter (OM) is affected by the maturity and type of OM, and these exert an important control on the development of OM pores. However, it is often challenging to distinguish between autochthonous and secondary OM. In this study, different types of OM were distinguished by means of morphology and the form of its occurrence under scanning electron microscopy (SEM) observation. Pyrolyzed samples were used to obtain pore structures of autochthonous and secondary OM from a lacustrine shale from high to over mature stages. The SEM observations indicated that the secondary OM is porous, whereas the autochthonous OM is nonporous. Further, nitrogen adsorption data showed that nanopores in the autochthonous OM are blocked in non-extracted samples and reoccurred after extraction, whereas nanopores in the secondary OM remain open from high to over mature stages. Moreover, the pore structure, i.e., pore type and pore size, differed for autochthonous and secondary OM in the lacustrine shale. For instance, nanopores in the autochthonous OM were micropores (smaller than 1.5 nm); nanopores in the secondary OM were mesopores (ranging from 5 to 20 nm) and micropores. Nanopores in the autochthonous OM were formed at reflectance values (Ro) of 1.50%, but they were slightly destroyed at the over mature stage. Nanopores in the secondary OM were formed at 2.00% Ro and were preserved. Therefore, the development of OM nanopores in the lacustrine shale was synchronously affected by the type of OM and maturity. Furthermore, nanopores in the secondary OM were more developed than those in the autochthonous OM; thus, the OM nanopores at the high mature stage belonged to the autochthonous OM, and those at the over mature stage mainly belonged to the secondary OM. Besides, the nanopores in the autochthonous OM are easily blocked. Therefore, the secondary OM is important to the development of OM nanopores in the lacustrine shale at the over mature stage.