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

Abstract

To 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 2 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.

Impact of Fe(III) (Oxyhydr)oxides Mineralogy on Iron Solubilization and Associated Microbial Communities

“…Mineralogical morphologies of all Fe (oxyhydr)oxides were characterized using a scanning electron microscope (SEM) and Brunauer, Emmett and Teller (BET) surface area measurement (determined by multipoint BET N 2 adsorption) (Brunauer et al, 1938). Specific surface areas were determined from N 2 adsorption isotherms in the best linear range (with a minimum of 15 points) between the relative pressure P/Po 0.03 and 0.33 (Cavelan et al, 2019).…”

Impact of thermal maturity on the concomitant evolution of the ultrafine structure and porosity of marine mudstones organic matter; contributions of electronic imaging and new spectroscopic investigations

“…Significant variations of the content of these organic components exist in the studied KCF series (Boussafir and Lallier-Vergès, 1997;Herbin et al, 1995;Lallier-Vergès et al, 1995). The studied sample was selected to contain a relatively high total organic carbon content (TOC = 10.3 wt.%, Hydrogen index= 557 mg HC/g TOC, Cavelan et al, 2019a) and high contents of brown and orange AOM, which are the predominant OM components in the KCF. Note that for the rest of the manuscript, the terms from palynofacies observations (orange, brown and black amorphous organic matter (AOM) are used to refer to the corresponding OM compounds rather than those from the maceral composition (such as Bituminite) because some of these OM compounds are diffused in the mineral matrix and do not correspond to any macerals (i.e black and brown AOM) that could be named.…”

“…Table 1. Summary of the origin and the main characteristics under reflected light microscopy, SEM and TEM of the organic components identified by previous works (Boussafir et al, 1995b(Boussafir et al, , 1995a(Boussafir et al, , 1994Cavelan et al, 2019a;Tribovillard et al, 2001) in the studied area of the KCF and their relative abundance in OM of the studied sample (from Cavelan et al, 2019a Structured components with, sometimes, a "shredded" texture.…”

“…summarized in Table 1. The relative quantities of each of these constituents in the studied sample were previously determined by Cavelan et al, (2019a) and presented in Table 1. Before this study, the different macerals present in the studied rock were identified under reflected light and UV excitation; the result is shown in Figure 1.…”

“…Before this study, the different macerals present in the studied rock were identified under reflected light and UV excitation; the result is shown in Figure 1. For a more detailed description of palynofacies, maceral composition and the calculation of the abundance of each component, see Cavelan et al, (2019a).…”