Thesis of Pauline DODINET

Defense of thesis Pauline DODINET - laboratory LOG

Abstract : 

Sulfurization of organic matter is a process reputed to lead to the preservation of sedimentary organic matter over geological time scales. If it takes place in sediments containing poorly degraded and chemically reactive organic matter, sulfurization may a priori prevent the activity of methanogenic organisms and the generation of biogenic methane. In order to test this hypothesis, recent sediments from the Guaymas Basin (Mexico) were selected with the following objectives: (i) to study the diagenesis of organic matter and in particular its sulfurization, (ii) to study the impact of sulfurization on methane production in sediments. Rock-Eval 7S ® (RE7S) analysis was mainly used. This is a rapid thermal method that quantifies different forms of sulfur and carbon (Corg, Cmin, Sorg, Smin, Ssulfate) without chemical preparation. This technique was developed for sedimentary rocks and its implementation in this work required adaptations for the analysis of recent sediments, in particular for the quantification of sulfur species.
Through the analysis of numerous standards, several difficulties were highlighted when analyzing recent sediments at RE7S.These include an overestimation of sulfur compounds released at the start of the temperature program, interactions between sulfur effluents and organic matter, interactions at higher temperatures between sulfur effluents and carbonates, and difficulties in quantifying gypsum. Apart from the sulfur species released at the start of the program, the sulfur speciation obtained after correction by the processing software nevertheless appears satisfactory overall.
The application of RE7S analysis to Guaymas basin sediments revealed a predominantly marine OM. During burial, this is degraded by bacterial sulfate reduction, followed by methanogenesis. The deepest samples are at the entrance to the oil window. Despite different contexts, in the majority of sites studied, sulfidation of organic matter is maximal near the sulfate-methane interface, but decreases in deeper sediments. These observations are confirmed by the specific analysis of kerogen prepared from selected samples.
The sulfur-containing OM formed in the sediments of the Guaymas Basin loses its sulfur at greater depths. This “desulfurization” of organic matter is particularly noticeable in the biogenic methane production zone. These observations suggest that, in the case of the Guaymas Basin, sulfidation of organic matter has little impact on methanogenesis.

Keywords : Sedimentary organic matter,depositional environment,H2S,biogenic methane,geothermal fluids,basin modeling