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Impact of Oil-Prone Sedimentary Organic Matter Quality and Hydrocarbon Generation on Source Rock Porosity: Artificial Thermal Maturation Approach.

ABSTRACT: 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.

SUBMITTER: Cavelan A

PROVIDER: S-EPMC7301563 | biostudies-literature | 2020 Jun

REPOSITORIES: biostudies-literature

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Impact of Oil-Prone Sedimentary Organic Matter Quality and Hydrocarbon Generation on Source Rock Porosity: Artificial Thermal Maturation Approach.

Cavelan Amélie A Boussafir Mohammed M Le Milbeau Claude C Laggoun-Défarge Fatima F

ACS omega 20200604 23

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 calcula ...[more]

PMID: 32566868

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Project description:The diversity of microorganisms active within sedimentary rocks provides important controls on the geochemistry of many subsurface environments. In particular, biodegradation of organic matter in sedimentary rocks contributes to the biogeochemical cycling of carbon and other elements and strongly impacts the recovery and quality of fossil fuel resources. In this study, archaeal diversity was investigated along a salinity gradient spanning 8 to 3,490 mM Cl(-) in a subsurface shale rich in CH(4) derived from biodegradation of sedimentary hydrocarbons. Shale pore waters collected from wells in the main CH(4)-producing zone lacked electron acceptors such as O(2), NO(3)(-), Fe(3+), or SO(4)(2-). Acetate was detected only in high-salinity waters, suggesting that acetoclastic methanogenesis is inhibited at Cl(-) concentrations above approximately 1,000 mM. Most-probable-number series revealed differences in methanogen substrate utilization (acetate, trimethylamine, or H(2)/CO(2)) associated with chlorinity. The greatest methane production in enrichment cultures was observed for incubations with salinity at or close to the native pore water salinity of the inoculum. Restriction fragment length polymorphism analyses of archaeal 16S rRNA genes from seven wells indicated that there were links between archaeal communities and pore water salinity. Archaeal clone libraries constructed from sequences from 16S rRNA genes isolated from two wells revealed phylotypes similar to a halophilic methylotrophic Methanohalophilus species and a hydrogenotrophic Methanoplanus species at high salinity and a single phylotype closely related to Methanocorpusculum bavaricum at low salinity. These results show that several distinct communities of methanogens persist in this subsurface, CH(4)-producing environment and that each community is adapted to particular conditions of salinity and preferential substrate use and each community induces distinct geochemical signatures in shale formation waters.

Dataset Information

Impact of Oil-Prone Sedimentary Organic Matter Quality and Hydrocarbon Generation on Source Rock Porosity: Artificial Thermal Maturation Approach.

Publications

Impact of Oil-Prone Sedimentary Organic Matter Quality and Hydrocarbon Generation on Source Rock Porosity: Artificial Thermal Maturation Approach.

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