Daniel Dessort

C(14a)-homo-26-nor-17α-hopanes, a novel and unexpected series of molecular fossils in biodegraded petroleum

A series (C27–C35) of pentacyclic triterpene hydrocarbons, the C(14a)-homo-26-nor-17α-hopanes, has been characterized in a biodegraded petroleum by gas chromatography–mass spectrometry and NMR structural determination of the C29 and C30 members; these biological markers reveal an as yet unknown transposition of the hopane skeleton.

Synthesis and NMR characterisation of novel highly cyclised polyprenoidhydrocarbons from sediments

We report here on the identification of two novel hexacyclic alkanes (C33 and C35) occurring in bitumen. The C35 compound 1 was identified by comparison with a standard obtained by synthesis involving a biomimetic proton-induced extensive cyclisation of an acyclic heptaprenoid. This cascade cyclisation allows the formation of eleven asymmetric centres present in the natural compound in only one step. The C33 analogue 2 was identified by NMR studies after isolation from the saturated hydrocarbon fraction of a bituminous rock. Both compounds are “orphan” molecular fossils of biological lipids of unknown origin formed by the extensive cyclisation of higher regular polyprenoids.

New Bio-Indicators for Long Term Natural Attenuation of Monoaromatic Compounds in Deep Terrestrial Aquifers

Deep subsurface aquifers despite difficult access, represent important water resources and, at the same time, are key locations for subsurface engineering activities for the oil and gas industries, geothermal energy, and CO2 or energy storage. Formation water originating from a 760 m-deep geological gas storage aquifer was sampled and microcosms were set up to test the biodegradation potential of BTEX by indigenous microorganisms. The microbial community diversity was studied using molecular approaches based on 16S rRNA genes. After a long incubation period, with several subcultures, a sulfate-reducing consortium composed of only two Desulfotomaculum populations was observed able to degrade benzene, toluene, and ethylbenzene, extending the number of hydrocarbonoclastic–related species among the Desulfotomaculum genus. Furthermore, we were able to couple specific carbon and hydrogen isotopic fractionation during benzene removal and the results obtained by dual compound specific isotope analysis (𝜀C = -2.4‰ ± 0.3‰; 𝜀H = -57‰ ± 0.98‰; AKIEC: 1.0146 ± 0.0009, and AKIEH: 1.5184 ± 0.0283) were close to those obtained previously in sulfate-reducing conditions: this finding could confirm the existence of a common enzymatic reaction involving sulfate-reducers to activate benzene anaerobically. Although we cannot assign the role of each population of Desulfotomaculum in the mono-aromatic hydrocarbon degradation, this study suggests an important role of the genus Desulfotomaculum as potential biodegrader among indigenous populations in subsurface habitats. This community represents the simplest model of benzene-degrading anaerobes originating from the deepest subterranean settings ever described. As Desulfotomaculum species are often encountered in subsurface environments, this study provides some interesting results for assessing the natural response of these specific hydrologic systems in response to BTEX contamination during remediation projects.

A Scenario for the Creation of H2S Heterogeneities in Acid Gas Reservoirs in Contact with an active Aquifer: a Simulation Study

Compositional heterogeneities of H2S have been noticed in many sour gas reservoirs. Its occurrence is an important factor of economic depreciation. Thus, the knowledge of its content and distribution is a critical parameter when planning field development. The paper aims at exploring the role of an active aquifer in the creation of H2S heterogeneities in high H2S-bearing gas reservoir. Indeed, under conditions of pressure and temperature of typical reservoirs, H2S is far more soluble than hydrocarbons and other gases. A preferential leaching of H2S (e.g. versus CH4) over time is thus possible. This mechanism is controlled by: (1) Differential solubility of gases, which change the relative amounts of each gas near the gas-water contact (GWC); (2) Contact with an active aquifer, which can export the dissolved gases thus enhancing dissolution on the long-term; (3) Diffusional transport in the gas phase, which transfers the compositional anomalies farther from the gas-water contact; (4) Geological parameters (type of aquifer, permeability heterogeneities) which can modify the transport scenario. To illustrate and quantify this process, we show the results of numerical simulations, performed with the two-phase transport and geochemical software Hytec. First, a very schematic reservoir with a composition considered uniformly distributed within the reservoir, has been simulated to quantify the leaching of H2S. The results highlight the potential role of the active aquifer, which can leach the gases and export them outside the reservoir. In a second phase, the effect of geological parameters on the H2S heterogeneity development was studied: additional simulations were performed on geometries closer to natural cases. The amount of leached H2S depends strongly on the geometry: the larger the GWC area, the larger the amounts leached.

Role of hydrodynamism in compositional heterogeneities in acid gas reservoir

Acid gases (H2S and CO2) compositional heterogeneities have been noticed in many sour gas reservoirs. Their occurrence is an important factor of economic depreciation. Thus, the knowledge of the acid gases distribution is a critical parameter for the design of field development. The mechanisms to explain compositional heterogeneities of acid gas in a reservoir are various. The paper aims at exploring the role of an active aquifer in contact with an initial high H2S content reservoir. The major mechanisms may be controlled by: * Differential solubility of gases which can change the relative amounts of each gas near the contact; * Active aquifer solubilization and transport which can export dissolved gases thus enhancing dissolution on the long-term; * Diffusional transport in the gas phase which can transfer the compositional anomalies farther from the gas-water contact. To test the influence of several parameters on the efficiency of the acid gases leaching, simulations on basic geometries have been performed with the diphasic transport and geochemical software Hytec. The simulation results show a major role of the occurrence of horizontal impermeable barriers yields to sharp heterogeneities, including a decrease in acid gas near the contact, while farther areas H2S concentration remain unaffected.