Effect of organic matter composition on source rock porosity during confined anhydrous thermal maturation: Example of Kimmeridge-clay mudstones

International audienceTo investigate how slight differences in the total particulate organic matter (OM) assemblage and maceral types of marine mudstones influence the development and the evolution of OM porosity throughout the same formation, laboratory gold-tube anhydrous confined thermal maturation was applied on low-mature organic-rich marine Kimmeridge clay mudstones (Yorkshire, UK). Organic petrography (palynofacies and maceral analysis), SEM observations, global and molecular geochemical characterization and low-pressure nitrogen adsorption measurements were performed to examine, to our knowledge for the first time, the role of the OM composition and properties on the evolution of porosity during thermal maturation. Evidence from organic petrography and geochemical analyses showed that organic-rich Kimmeridge clay samples containing a higher relative proportion of brown amorphous organic matter (AOM), known to be derived essentially from the cell walls of microalgae, exhibit a different pore evolution compared to samples containing a higher proportion of oil-prone orange AOM derived primarily from preservation by natural sulfurization of phytoplanktonic organic components naturally rich in lipidic compounds. OM conversion of orange AOM-rich samples has led to the production of higher amounts of low viscous oil and abundant gaseous hydrocarbon that are more favorable to the formation of abundant but small OM micropores and mesopores in the condensate/wet gas zone and less abundant large modified mineral pores in the dry gas zone. The secondary cracking of the heavier oil produced by brown-AOM-rich OM has led to the formation of a greater quantity of CO2 and an abundant organic-rich residue more favorable to the formation of larger mesopores. Moreover, the more abundant thick laminar bituminite macerals forming a continuous ductile network particularly prone to compaction and the higher amounts of oil/bitumen generated during their conversion may have led to a higher pore collapse after oil/gas generation and migration, responsible for the destruction of part of the secondary pore network observed on some samples. Despite their slightly lower oil-prone potential, brown AOM-rich samples have thus developed larger mesopores during thermal maturation, resulting in higher pore volumes in the dry gas zone. Thus, the ability of marine mudstones to develop pores during maturation do not increase proportionally with the OM oil prone quality. These results show that variations in the individual particulate OM assemblage of a similar type II kerogen can significantly influence the amount of oil and gas generated during thermal maturation throughout the same formation, resulting in different pore evolution models. This reveals the importance of clearly identifying the composition of the original particulate OM assemblage to better predict the amount of bitumen, oil and gas generated during thermal maturation and the associated pore development