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.
petrography and pore structure characterization of low-mature and gas-mature marine organic-rich mudstones: Insights into porosity controls in gas shale systems, Marine and Petroleum Geology (2019),
This
work investigates the relation between the molecular composition
of the organic matter (OM), hydrocarbon generation, and porosity of
artificially matured mudstones from the Kimmeridge Clay formation.
Anhydrous thermal maturations, geochemical characterization (gas chromatography–mass
spectrometry, gas chromatography with a thermal conductivity detector),
scanning electron microscopy observations, and nitrogen adsorption
measurements were carried out. The results were compared to the calculated
OM porosity after maturation. Our results reveal that samples richer
in phytoplanktonic OM generated more abundant oils enriched in alkanes
and lighter aromatic hydrocarbons (i.e., biphenyls and naphthalenes)
that are more able to fill the adjacent pores of the mineral matrix
during maturation. Both the calculated and the observed OM porosity
increases during maturation, but the measured rock pore volume shows
a non-linear evolution related to the amount of gas generated and
the ensuing ability of the rock to preserve the pores. The secondary
cracking of highly oil-prone samples generated larger amounts of C1–C5 gases but lower pore volumes, less well
preserved. OM composition and its ability to generate oil and gas
seem therefore to affect the pore volume. These variations are nevertheless
small compared to the effect of thermal maturity, which remains the
major process controlling the evolution of porosity. During gas generation,
high C2–C5 concentrations are generated
compared to methane. The short thermal maturation duration of our
experiments may have delayed the conversion of C2–C5 hydrocarbons to methane. In addition, slight differences
in the concentrations of saturated and aromatic components and markedly
different pore volumes and pore size distributions exist between artificially
and naturally matured rocks. The conditions of artificial maturations
may thus impact the thermal transformations of OM, emphasizing the
necessity to investigate the role of the artificial maturation kinetics
on OM and porosity.
Influence of experimental temperature and duration of laboratory confined thermal maturation experiments on the evolution of the porosity of organic-rich source rocks
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
<p>Light Non-Aqueous Phase Liquids (LNAPLs) are one of the most important sources of soil and groundwater contamination worldwide. When they infiltrate through the unsaturated zone, part of the LNAPLs remains trapped by capillary forces. The others accumulate above the top of the water table, forming a floating &#8216;free&#8217; phase able to generate a long-term dissolved LNAPL plume that durably alters the quality of the water resource. Seasonal variations in the groundwater level lead to significant vertical spreading of these light petroleum hydrocarbon contaminants at the capillary fringe, favoring their release into the air and groundwater. In the climate change context, the IPCC predicts an intensification of these groundwater level variations over the next century in response to variations in rainfall intensity and frequency, whose effects are increased by the use of water resources. This context may strongly impact the mobilization of these organic contaminants and their release to the environment. To study these phenomena, it is, therefore, essential to better understand the impact of the groundwater level fluctuation patterns on the LNAPLs mobilization processes. To this end, an original experimental system combining indirect geophysical (complex electrical conductivity, permittivity), in-situ physical-chemical (pH, Eh, temperature), and geochemical measurements was developed at the GISFI station (Hom&#233;court, France). This device allows the assessment and the comparison of the amount and nature of LNAPLs release into the atmosphere and water from contaminated soil during two groundwater level fluctuations scenarios: one corresponding to the &#8216;actual&#8217; rainfall pattern based on regional climate records; the other based on the predictions of the most extreme IPCC scenario. This study will be conducted at different scales (laboratory decametric columns and 2 m<sup>3</sup> lysimeters) and on soils of different geological complexity. The remobilized hydrocarbons will be collected via suction cups and gas collection chambers as the groundwater table fluctuates and will be regularly analyzed (GC-MS, FTIR). The complementarity of the monitoring methods aims to provide a better understanding of the fate of these organic pollutants at contaminated sites and the evolution of the associated environmental risks in the coming years, under the expected effect of climate change. Preliminary results concerning the hydrocarbon pollution migration through the unsaturated zone and the distribution of the LNAPL will be presented to illustrate the capacity of this new instrumental system.</p><p>This work is partly funded by the DEEPSURF project "Lorraine Universit&#233; d&#8217;Excellence", ANR-15-IDEX-04-LUE".</p>
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