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TitreMethane-derived authigenic carbonates from active hydrocarbon seeps of the St. Lawrence Estuary, Canada
AuteurLavoie, D; Pinet, N; Duchesne, M; Bolduc, A; Larocque, R
SourceMarine and Petroleum Geology vol. 27, no. 6, 2010 p. 1262-1272,
Séries alt.Secteur des sciences de la Terre, Contribution externe 20090076
ÉditeurElsevier BV
Documentpublication en série
Mediapapier; en ligne; numérique
ProvinceRégion extracotière de l'est
Lat/Long OENS-70.0000 -67.0000 49.3333 48.0000
Sujetshydrocarbures; capacité de production d'hydrocarbures; méthane; méthane hydraté; roches sédimentaires; roches carbonatées; carbonates; combustibles fossiles; géologie marine
Illustrationslocation maps; sections; seismic sections; photographs; photomicrographs
ProgrammeSécurité de l'approvisionnement énergétique du Canada
Résumé(disponible en anglais seulement)
Nearly 2000 pockmarks with diameters ranging from a few tens of meters up to 700 m are present on the seafloor of the St. Lawrence Estuary in eastern Canada. Coring of some pockmarks resulted in the recovery of various-sized and shaped carbonate concretions in a predominantly silty mud matrix. Petrographic and geochemical data on four authigenic carbonate concretions are reported as well as data from shell material in the unconsolidated sediment. Video observations and echo-sounder images indicate that the sampled pockmarks are actively gas venting. The video images show significant look-alike microbial mats in areas where gas is venting. The carbonate concretions are primarily made up of carbonate cements with varying percentage of shell fragments, micrite particles and fine-grained clastics. Orthorhombic crystal morphology and diagenetic fabrics including isopachous layers and botryoids characterize the aragonite cement. Oxygen isotopes ratios for the cement crusts do not record any thermal anomaly at the site of precipitation with delta18OVPDB ratios (+3 permil) in equilibrium with cold (5 °C) deep marine waters, whereas significant negative delta13CVPDB ratios (-9.9 to -33.5 permil) for cement and shell material within concretions indicate that the carbonates largely derive from the microbial oxidation of methane. The delta13CVPDB ratios of aragonite shells (-2.7 to -5.6 permil) taken from unconsolidated sediments at some distance from the concretions/vents show variable dilution of HCO3- with negative delta13CVPDB ratios derived from microbial oxidation of methane with isotopically normal (0 permil) marine bicarbonate. These results are in agreement with other lines of evidence suggesting that pockmarks formed through the recent and still active release of gas from a reservoir within the Paleozoic sedimentary succession.